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Electrical Detection of Fast Reaction Kinetics in Nanochannels with an Induced Flow

Nanofluidic channels can be used to enhance surface binding reac-tions, since the target molecules are closely confined to the surfaces that are coated with specific binding partners. Moreover, diffusion-limited binding can be significantly enhanced if the molecules are steered into the nanochannels via either pressure-driven or elec-trokinetic flow. Monitoring the nanochannel impedance, which is sensitive to surface binding, has led to electrical detection of low analyte concentrations in nanofluidic channels within response times of 1-2 h [1]. This finding represents a ~54 fold reduction in the response time using convective flow compared to diffusion-limited binding [2]. At high flow velocities, the presented method of reac-tion kinetics enhancement is potentially limited by force-induced dissociations of the receptor-ligand bonds [3]. Optimization of this scheme could be useful for label-free, electrical detection of bio-molecule binding reactions within nanochannels on a chip.

Integration of Actuated Membranes in Thermoplastic Microfluidic Devices

PolyDiMethylSiloxane (PDMS) is a common material for fabrication of microfluidic devices. Elasticity provided by PDMS enables the creation of active devices that utilize pressurized membranes such as pumps and mixers. However, for structures requiring dimen-sional stability, rigidity, or disposability, plastics have the required properties [1]. Plastics can be manufactured using mass fabrication technologies such as injection molding and hot embossing with es-tablished bonding processes [2], but at the cost of sacrificing active device functionality. A new fabrication process combining plastic substrates with PDMS membranes enables the creation of active microfluidic devices inside dimensionally stable systems, merging the functionality of PDMS with established plastic fabrication tech-nologies.Irreversible bonding between PDMS and plastics for fluidics re-quires interfaces that can handle high pressure and harsh chemical environments. Hydrolytic stability under acidic or basic conditions is particularly important. Direct bonding between PMMA and PDMS has been explored [3], but interfaces withstood only 2.5 psi before failure. Surface modification of polycarbonate and PMMA surfaces with AminoPropylTriEthoxySilane (APTES) [4] has also been shown to enable PDMS plasma bonding [5], but no data on hydrolytic sta-bility was shown. To improve hydrolytic stability, two additional silanes were explored, BisTriEthoxySilylEthane (BTESE) and Bis(TriMethoxySilylPropyl)Amine (BTMSPA), for thin and thick primer coatings, respectively. Devices with PDMS membranes suspended over 25-µL fluid reser-voirs were fabricated in PC and PMMA to test interface robustness. For all devices, membrane ruptures occurred instead of delamina-tion at 60 psi, making the devices suitable for active valves. Blisters were then subjected to NaOH and HCl solutions from the PDMS side at 70 C for 2 hours, followed by pressure testing. Figure 1 shows that hydrolytic stability improves over APTES with addition of BTMSPA to the primer solution for thick coatings or BTESE for monolayer coatings. A test chip containing peristaltic pumps and mixers was then fabricated, and pump rate versus frequency was measured as shown in Figure 2.

Teflon Films for Chemically-inert Microfluidic Valves and Pumps

Like transistors in electronic microprocessors, microfluidic valves and pumps are the fundamental elements of logic and control in many lab-on-a-chip devices. Flexible elastomers make good can-didates for the moving parts in valves and pumps, and elastomers like polydimethylsiloxane (PDMS) have found widespread use in a variety of normally-open and normally-closed microfluidic valves. Unfortunately, the limited chemical compatibility of PDMS has complicated its use in many microfluidic applications. Many chemi-cals commonly used in organic synthesis readily swell PDMS devices or dissolve PDMS oligomers from the elastomer. Small hydrophobic molecules readily partition into and out of bulk PDMS, complicating the determination of their on-chip concentration. Some reusable glass microfluidic devices must be equipped with removable, dis-posable valves because the PDMS valves would be destroyed by the harsh acid used to clean the device before reuse. For these reasons, a large variety of interesting and useful chemistries may be unsuitable for use in native PDMS devices.We have developed a simple alternative method for fabricating Teflon monolithic membrane valves and pumps in glass micro-fluidic devices [1]. We have found that inexpensive, commercially-available fluorinated ethylene-propylene (FEP) Teflon films can be bonded between etched glass wafers to form chemically-inert monolithic membrane valves and pumps. Both FEP and polytetra-fluoroethylene (PTFE) are comprised entirely of carbon and fluorine and are similarly inert. But while PTFE is opaque and must be cut or skived to make rough thin sheets, FEP is transparent and available as a smooth, uniform thin film. Chemical compatibility data from nearly 50 years of use as a commercial product show that FEP is re-sistant to virtually all chemicals except “molten alkali metals, gas-eous fluorine, and certain complex halogenated compounds such as chlorine trifluoride at elevated temperatures and pressures.”[2] The resulting glass-FEP-glass devices are optically transparent and suit-able for imaging or fluorescence applications (Figures 1, 2). The FEP Teflon valves permit unimpeded (0.9 µL/s) flow while open and neg-ligible (< 250 pL/s) leakage while closed against 14 kPa fluid pressure. The FEP pumps can precisely meter nanoliter-scale volumes at up to microliter/second rates. The pumps also show excellent long-term durability with < 4% change in pumping rate after 13 days of con-tinuous operation. By combining ease of fabrication with extreme chemical inertness, these Teflon monolithic membrane valves and pumps enable research involving a vast array of chemistries that are incompatible with native PDMS microfluidic devices.

Nanofluidic System for Single-particle Manipulation and Analysis

Nanopores are versatile sensors for detection of single molecules and particles in solution. When a molecule passes through a nano-pore with a voltage bias applied across it, the resulting transient blockage of the nanopore yields a detectable current change that enables single-molecule sensing [1, 2]. Different molecules may[1, 2]. Different molecules may, 2]. Different molecules may2]. Different molecules may. Different molecules may Different molecules maymay exhibit different current blockage and duration profiles, which mayfferent current blockage and duration profiles, which may current blockage and duration profiles, which may, which may be used to characterize the molecules. However, the sensing ability of nanopores is often limited by the quick transit times of molecules through the nanopore that result in poor signal-to-noise ratio. To ToTo address this issue, we are developing a nanofluidic system to manip-ulate single particles and molecules that will enable multiple mea-surements on the same molecule (Figure 1). Nanofluidic channels will function as traps to localize the molecule in the system after its translocation (transit) through the nanopore. When the electric When the electricWhen the electric field across the nanopore is reversed, the same molecule will travel through the nanopore again. Feedback control will be used to reverse the applied voltage bias and thus ensure multiple translocations of a molecule through the nanopore. This technique will enable inte- This technique will enable inte-This technique will enable inte-gration of a signal over multiple translocations, thereby improving the signal-to-noise ratio. For proof-of-concept, translocation signals For proof-of-concept, translocation signalsFor proof-of-concept, translocation signals of DNA molecules through a PDMS (polydimethylsiloxane) nano-(polydimethylsiloxane) nano- nano-pore will be measured using a patch-clamp amplifier. Techniques of E-beam lithography and UV lithography are adopted to create aE-beam lithography and UV lithography are adopted to create aa master mold of the device consisting of nanopores and reservoirs.mold of the device consisting of nanopores and reservoirs.of the device consisting of nanopores and reservoirs.the device consisting of nanopores and reservoirs.nanopores and reservoirs. Soft lithography with PDMS will be used for rapid and reproducible PDMS will be used for rapid and reproducible fabrication of nanopores connecting two reservoirs (Figure 2) [3].nanopores connecting two reservoirs (Figure 2) [3]. [3].[3]. This approach will demonstrate a new paradigm in sensing by using nanopores, and it may enable an unprecedented level of character-ization of nanoparticles and biomolecules.

Microfluidic Systems for Continuous Crystallization

Microfluidic systems offer a unique toolset for discovering new crys-tal polymorphs and for studying the growth kinetics of crystal sys-tems because of well-defined laminar flow profiles and online opti-cal access for measurements. Traditionally, crystallization has been achieved in batch processes that suffer from non-uniform process conditions across the reactors and chaotic, poorly controlled mix-ing of the reactants, resulting in polydisperse crystal size distribu-tions (CSD) and impure polymorphs. This reduces reproducibility, increases difficulty in obtaining accurate kinetics data, and manu-factures products with inhomogeneous properties. The short length scale in microfluidic devices allows for better control over the pro-cess parameters, such as the temperature and the contact mode of the reactants, creating uniform process conditions across the reac-tor channel. Thus, these devices have the potential to generate more accurate kinetics data and produce crystals with a single morphol-ogy and a more uniform size distribution. In addition, microfluidic systems decrease waste, provide safety advantages, and require only minute amounts of reactants, which is most important when deal-ing with expensive materials such as pharmaceutical drugs. Figure 1 shows a microfluidic device used for crystallization and Figure 2 shows optical images of different polymorphs of glycine crystals grown in reactor channels. A key issue for achieving contin-uous crystallization in microsystems is to eliminate heterogeneous crystallization – irregular and uncontrolled formation and growth of crystals at the channel surface-- and aggregation of crystals, which ultimately clogs the reactor channel. We have developed a micro-crystallizer using soft lithography techniques that introduces the reagents to the reactor channel in a controlled manner, preventing heterogeneous crystallization and aggregation. We have also inte-grated an online spectroscopy tool for in situ polymorph detection. Our ultimate goal is to develop an integrated microfluidic system for continuous crystallization with the ability to control and detect the crystal morphology, as well as obtain kinetics of crystallization through online detection.

Massively-parallel Ultra-high-aspect-ratio Nanochannels for High-throughput Biomolecule Separation

Many bottom-up approaches have been used to build nano/meso-porous materials/filters with a good size control, but the integration of these systems into a microsystem format has been a challenge. Top-down nanofilter fabrications, on the other hand, suffered from small open volume and low throughput. For this paper, we devel-oped a top-down fabrication strategy for massively-parallel, regular vertical nanochannel membranes with a uniform, well-controlled gap size of ~50 nm and a depth up to ~40 µm, by using only stan-dard semiconductor fabrication techniques [1]. The vertical nanofil-ter membranes were fabricated into an anisotropic nanofilter array, which demonstrates the ability to integrate nanofilters and micron-sized channels/pores seamlessly. We demonstrated efficient con-tinuous-flow separation of large DNAs in a two-dimensional verti-cal nanochannel array device as shown in Figure 2. Compared with planar nanofilter systems [2], an important feature of our device is a sample processing rate as high as ~ 1 µL hour-1, and further im-provement of throughput can be achieved simply by upscaling the channel depths. These ultrahigh-aspect-ratio nanochannels have the advantage of large open volume, enabling high-throughput ap-plications.

Microfluidic Control of Cell Pairing and Fusion

Currently, several different methods have been used to reprogram somatic cells to an embryonic stem cell-like state. Nuclear transfer and fusion methods [1] use either oocytes or embryonic stem cells (ESCs) as a source of reprogramming factors. Recently, defined fac-tors have been identified that are capable of inducing pluripotency in somatic cells2 [2]. While all three approaches can be used success-fully for reprogramming, cell lines generated are not yet suitable for potential therapeutic applications in humans and many questions remain about the process of nuclear reprogramming.We have developed a microfluidic system in which thousands of ESCs and somatic cells (SCs) are properly paired and immobilized, resulting in a high number of one-to-one fusions that can be clearly identified for further studies [3]. The device consists of thousands of cell traps in a millimeter-sized area, accessed by microfluidic chan-nels (Figure 1). The traps consist of larger frontside and smaller back-side capture cups made from a transparent biocompatible polymer. Cells are loaded sequentially in a 3-step loading protocol enabling capture and pairing of two different cell types. The geometry of the capture comb precisely positions the two cells, and flow through the capture area keeps the cells in tight contact in preparation for fu-sion. Pairing efficiencies of ~70% are possible over the entire device (Figure 2).The device is compatible with both chemical and electrical fusion. The PEG-mediated fusion is initiated by flowing PEG past the cells for 3 minutes and then rinsing with warm media. With 4 doses of PEG, we have observed that 15 % of the traps contain cells that have exchanged fluorescent proteins, and 25 % of the traps contain cells whose membranes have reorganized. A control protocol done in a standard conical tube yielded only 6 ± 4 % fusion of the same fluo-rescent cells. Electrofusion is made possible by bonding the PDMS device to a glass slide with pre-patterned metal electrodes that are then connected to a standard fusion power supply. We have ob-served membrane fusion efficiencies up to 90% and can achieve > 50% properly paired and fused cells, based on exchange of fluores-cence, over the entire device. Control fusion protocols, performed using the same power supply with a commercial electrofusion chamber, obtained only 11 +/- 9 % fusion. We have demonstrated pairing and fusion of mESCs and mEFs and are currently using the device to explore fusion-based reprogramming.

BioMEMS for Modulating Stem Cell Signaling

The stem cell microenvironment is influenced by several factors in-cluding cell-cell, cell-matrix, and cell-media interactions. Although conventional cell-culture techniques have been successful, they provide incomplete control of the cellular microenvironment. To en-hance traditional techniques, we have developed several microscale systems for adherent cell culture of mouse embryonic stem cells (mESCs) while controlling the microenvironment in novel ways [1]. We are using stencil cell patterning and microscopic analytical tools to investigate cell-cell interactions, in particular the role of colony-colony interactions in self-renewal of mESCs. Since autocrine sig-naling in mESCs has not been thoroughly characterized, we validate our platform using a model autocrine cell line, A431 epidermoid carcinoma cells. By precisely controlling the colony size, spacing, and the medium replenishing frequency, we modulate the degree of colony-colony interactions (Figure 1). We are also using the Bio Flip Chip to investigate cell-cell signaling in mESCs. The chip is made from PDMS using replica molding, and it contains hundreds-to-thousands of microwells, each sized to hold either a single cell or small numbers of cells (Figure 1) [2]. A microscale cellular manipula-tion technique for cell-matrix interactions involves the patterning of specific protein signals around live mESC colonies in order to study the local effects of signal presentation. A photopolymerizable polymer (PEG-diacrylate) with attached proteins has been used to pattern structures around growing cell colonies in vitro, thereby exposing them to a very controlled microenvironment. Using these methods, we have patterned a known regulator of pluripotency, LIF, around mESC colonies and analyzed how far this signal propagated through the colony (Figure 1).To control cell-media interactions we have developed a two-layer PDMS microfluidic device that contains two sets of triplicate cham-bers, allowing implementation of different culture conditions on the same chip. The device incorporates a valve architecture modeled af-ter Irimia et al. [3], which enables different parts of the device to be fluidically isolated during different stages of the experiment. Using our system, we demonstrated that microfluidic perfusion can affect the soluble microenvironment. We showed that defined serum-free media (N2B27), sufficient for differentiating cells into neuronal pre-cursors in static culture [4], did not allow cells to proliferate or dif-ferentiate on-chip. On the same chip we cultured cells in N2B27 that had been supplemented with media containing cell-secreted factors from a static culture. In this media, we were able to restore growth and differentiation (Figure 2).

Microfabricated Slits in Series: A Simple Platform to Probe Differences in Cell Deformability

Change in cell stiffness is a characteristic of blood cell diseases, such as sickle cell anemia [1], malaria [2], and leukemia [3]. Often, increas-es in blood cell stiffness lead to loss of the cells’ ability to squeeze through capillaries, resulting in organ failure, coma, and ultimately death [4]. The goal of this project is to create a microfluidic device that can quickly and accurately screen, diagnose, and treat disorders involving cell deformability. We report the creation of a microfabri-cated device consisting of a series of 1-2 µm wide polymeric slits, as Figures 1 and 2 show. This device can potentially be used to screen and diagnose disorders involving cell deformability.The device fabrication process is depicted in Figure 1 and follows approaches similar to those in [5]. First, a 2-level negative PDMS stamp was made using soft lithography techniques from a silicon template, Figure 1a. A droplet of UV-sensitive prepolymer NOA 81 was stamped on a glass slide and exposed to UV, Figure 1b. Similarly, a droplet of NOA was stamped using a flat PDMS slab and exposed to UV on a PDMS cover sheet treated with oxygen plasma to improve the adhesion. After the stamps were peeled off , the two pieces were brought in contact and bonded by completing the crosslinking with a second exposure to UV, as in Figure 1C. Figure 1 details the device’s operation and results of fabrication.

Microfluidic System for Screening Stem Cell Microenvironments

Embryonic stem cell (ESC) differentiation is a potentially powerful approach for generating a renewable source of cells for regenerative medicine. It is known that the microenvironment greatly influences ESC differentiation and self-renewal. Most biological studies have aimed at identifying individual molecules and signals. However, it is becoming increasingly accepted that the many kinds of signals inmany kinds of signals in of signals in the ESC microenvironment interact in a synergistic and antagonistic manner based on their temporal and spatial expression, dosage, and specific combinations. This interplay of microenvironmental factors regulates the ESC fate decisions to proliferate, self-renew, differen-tiate, and migrate. Despite this complexity, the systematic study of stem cell cues is technologically challenging, expensive, slow, and labor-intensive. Here we propose to develop a high-throughput mi-crofluidic based system that overcomes many of these challenges. We will subsequently analyze the resulting high-throughput system in elucidating specific aspects of mesodermal and endodermal dif-ferentiation in a systematic manner. A simple microfluidic screening device consisted of fluidic chan-nels, control channels, and poly(ethylene glycol) (PEG) microw-ells has been developed (Figure 1). A microfluidic screening device was fabricated by multi-layer soft lithography technique [1]. The fluidic channel made by positive photoresist (AZ 4620) is 10-µm--thick pattern with a round shape and the pneumatically actuatedpattern with a round shape and the pneumatically actuatedwith a round shape and the pneumatically actuated control channel fabricated by negative photoresist (SU-8 2150) is a 40-µm-thick pattern. To obtain a round profile of a fluidic channel, pattern. To obtain a round profile of a fluidic channel,. To obtain a round profile of a fluidic channel, the positive photoresist (AZ 4620) was reflowed at 200°C for 120 sec after development. A replica of the fluidic channel was obtained by spin-coating poly(dimethylsiloxane) (PDMS) at 1700 rpm for 1 min followed by baking at 70 °C for 1.5 hours. This process resulted in a 20-µm-thick PDMS membrane containing the fluidic channel. The crossing of the control channel over the fluidic channel formed the on-chip barrier valve. We used ES-green fluorescent protein (GFP) cells that can express Octamer-4 (Oct4), a homodomain transcrip-tion factor. The ES-GFP cells were seeded into a fluidic channel and localized within PEG microwells in a flow-based microfluidic screening device (Figure 2). The ES cells were well docked and pat-terned within a microwell, while cells that were not localized within a microwell were flowed into a reservoir. The ES cells expressed by Oct4 (green) maintained self-renewal during media perfusion (0.3 µl/min). The ES cells docked within a microwell showed high cell viability (> 90%).

Self-assembly of Cell-laden Microgels with Defined 3D Architectures on Micro-patterned Substrate

Most living tissues are composed of repeating units on the scale of hundreds of microns; these units are ensembles of different cell types with well-defined three-dimensional (3D) microarchitectures and tissue-specific functional properties (i.e., islet, nephron, or si-nusoid) [1]. To generate engineered tissues, the recreation of these repeating structural features is of great importance in enabling the resulting tissue function. Here, we tried to self-assemble cell-laden microscale hydrogel (microgel) units as 3D tissue constructs with defined architecture by using hydrophobic/hydrophilic interactions. By micro-contact printing [2], we patterned the glass slides with spe-cific hydrophobic and hydrophilic regions [3]. We hypothesized that the hydrophilic microgels tend to stick to the hydrophilic patterns, while not on the hydrophobic patterns. Therefore, we could control the architecture of the microgel assembly by creating different hy-drophilic patterns.To achieve microcontact printing, we first created different SU 8 pat-terns on the silicon wafer based on the photomask by using stan-dard photolithography. The SU 8 patterns were transferred to the PDMS mold, which was soaked with the hydrophobic ink. The ink was printed with specific patterns on the glass slide by microcon-tact printing. Afterwards, the slides were covered in DPBS (~600 µL of DPBS) containing microgels (approximately 1500 gel units each patterned slide). After a few minutes, the slides were tilted over, to allow the liquid to drain off the slide. Microgels remained on only the hydrophilic glass surfaces, as predicted. Below are images of mi-crogel assembly on a 1600-um square pattern on the glass slide.

High-throughput Study of Cell-ECM Interactions in 3D Environment Using Microwell Arrays

The extracellular matrix (ECM) is critical in developing an integrat-ed picture of the role of the microenvironment in the fate of many cells. A two-dimensional (2D) microarray method was reported for cell-ECM interaction study [1]. These 2D approaches can be comple-mented by three-dimensional (3D) approaches such as embedding cells within ECM gels [2]. However, 3D microarray methods are diffi-cult to develop due to difficulties such as ECM array fabrication and nanolitre liquid handling. To overcome these difficulties, microwell array and robotic spotting may be useful.In this study, we develop an approach using a microarrayer (Piezorray) and microwell arrays for cell-ECM interaction study with high throughput. The microwell array was fabricated with soft li-thography (Figure 1). The diameter of a microwell was 400 µm with a pitch of 600 µm. In total, 2100 microwells were fabricated on a single slide with numbers and alphabets in between for identifica-tion (Figure 2A). As a proof-of-concept experiment, it was shown that dye solutions can be printed accurately into these microwells preloaded with collagen solution (Figure 2B). For future study, we will print the ECM component in a combinatorial manner into the microwell array preloaded with cells in prepolymer solutions. Then, the mixtures will be UV-crosslinked to immobilize the ECM mixture inside each isolated microwell for cell-ECM interaction study.

Amplified Electrokinetic Response by Concentration Polarization near Nanofluidic Channel

Due to a strong electrokinetic response inside an ion-depletion re-gion created by concentration polarization, the velocity of non-equi-, the velocity of non-equi- the velocity of non-equi-librium electroosmotic flows (EOF) inside the ion-depletion zone can be 10 times faster than any equilibrium EOFs. [1, 2] Fast fluid. [1, 2] Fast fluid, 2] Fast fluid] Fast fluid Fast fluid vortices were generated at the anodic side of the nanochannel due to the non-equilibrium EOF. The vortex flow speed was estimated to be usually about 1000 µm/sec, which is about ~10X higher than that of primary EOF under the same electrical potential, and was propor-tional to the square of applied voltage, as shown in Figure 1(a). At the steady state, we can clearly observe the two counter-rotating vorti-ces beside the nanochannel, as Figure 1(b) shows. In the dual-sided nanochannel device, since the ions were depleted through both walls, the four independent vortices were formed in the four divided regions, as shown in Figure 1(c). One can independently suppress the convective part of the phenomena by decreasing the microchan-nel thickness. As Figure 1(d) shows, the size of the vortex in the dot-ted circle was approximately 2µm, which corresponded to the depth of the microchannels. We also observed that, once the particles pass the depletion zone and entered the downstream low concentration zone, they travel 25 times faster than in the buffer zones, as Figure 2 shows. These results indicate that the concentration polarization (depletion) can be utilized to make efficient and novel electrokinetic pumps and fluid switching devices, at an efficiency that has never been demonstrated.

Micropipette Interfaces for Lab-on-a-Chip Systems

We have developed a simple to use, pipette-compatible, integrated fluid injection port to interface closed microfluidic chambers for ap-plications such as cell culture or microchamber PCR that are sensi-tive to external contamination. In contrast to open systems where fluid can be easily loaded into wells or flow-through microfluidic systems where interfacing involves bridging millimeter scale tubing with micrometer scale channels [1], filling closed chambers requires either first applying vacuum or venting the chamber. We have fab-ricated a pipette interface that automatically vents and seals upon insertion and removal of a pipette tip that can be directly integrated into fluidic devices.The injection port is composed of a deformable elastomer nipple, compression housing, and flow and vent channels that interface with the fluid chamber. A schematic of the components is shown in Figure 1a and photographs are shown in Figures 1b through 1e. When the elastomer nipple (Figure 1c) is inserted in the compression hous-ing (Figure 1d), the slit of the elastomer nipple is sealed closed, iso-lating the fluidic chamber from the external environment. Insertion of the pipette tip into the slit (Figure 1e) causes the nipple to deform, which opens the venting channel to the air while the pipette tip seals against the fluid flow channel. Actuation of the pipette plunger forces fluid into the chamber while air is vented around the pipette tip. Removing the tip reseals the port to prevent external contamina-tion. The seal can withstand at least 15psi of backpressure. The integrity of the injection port seal against bacterial contamina-tion was tested using the device shown in Figure 2, which comprised eight closed chambers of 150µL in volume interfaced with an inte-grated injection port. By visual inspection and plating, the sealed ports prevented contamination while the negative controls were clearly contaminated.

Multiplexed Proteomic Sample Preconcentration Chip Using Surface-patterned Ion-selectiveChip Using Surface-patterned Ion-selective Using Surface-patterned Ion-selective Membrane

We report a new method of fabricating a high-throughput proteine report a new method of fabricating a high-throughput protein preconcentrator in poly(dimethylsiloxane) (PDMS) microfluidic chip format. We print a submicron-thick ion-selective membrane on the glass substrate by using standard patterning techniques. By simply plasma-bonding a PDMS microfluidic device on top of the printed glass substrate, we can integrate the ion-selective mem-brane into the device and rapidly prototype a PDMS preconcentrator without complicated microfabrication and cumbersome integration processes. The PDMS preconcentrator showed a high preconcen-tration efficiency with a factor as high as ~104 in just 5 min., which was 12x higher than our previous PDMS preconcentrator fabricated by junction gap breakdown [1]. Moreover, we have demonstrated a [1]. Moreover, we have demonstrated a. Moreover, we have demonstrated a fabrication of 10 single preconcentrators in an array format which increased the preconcentrated volume by 3 orders of magnitude compared to our previous result obtained with the silicon nanoflu-idic preconcentrator [2]. The ability to build a massively parallel ar- [2]. The ability to build a massively parallel ar-. The ability to build a massively parallel ar-ray using this technique is significant in terms of the integration of our preconcentrator to an external sensing unit such as mass spec-trometer. In addition to a shorter preconcentration time, the array can offer a sufficient amount of the concentrated sample volume to transfer it to an external sensing unit. Due to this capability, we ex-pect a high potential of our PDMS preconcentrator chip as a signal enhancement tool for a mass spectrometer to detect low-abundance proteins and peptides. Furthermore, the PDMS microfluidic format of this device would allow the integration of preconcentrator into many different BioMEMS platforms, including cellular BioMEMS devices.

Improving the Sensitivity and Binding Kinetics of Surface-based Immunoassays

Immunoassays are currently among the most widely used diagnos-tics tools in the healthcare industry. The usage of current immuno-assays is limited by the availability of good antigen-antibody pairs, time-consuming incubation, and sensitivity limits. In particular, the sensitivity and binding kinetics are limited by the usually low concentration of molecules that we are trying to detect. One of the most common methods to overcome the limitations of sensitivity is by adding a post-binding amplification step, meaning that signals get enhanced after molecules are bound to capture antibodies. This method helps improve the sensitivity of the assay, but it fails to re-duce the time required for the sensor to reach an equilibrium value because the low concentration of molecules still takes the same time to saturate the sensor. An alternative to post-binding amplification is pre-binding amplification. By increasing concentration of mol-ecules of interest prior to their capture by antibodies, pre-binding amplification improves both the sensitivity of the sensor and kinet-ics of binding. Our lab has developed and integrated a nanofluidics-based con-centrator and has successfully integrated the device with an immu-noassay [1]. The principle behind the concentrator, electrokinetic trapping, is a space-charge induced phenomenon that can be fine-tuned using external voltage controls. After application of appropri-ate voltages, a charge-depletion zone forms near the nanochannels and excludes all charged species. If the analyte-containing fluid is continuously moved into this zone, the analytes would accumulate and their concentration increase (Figure 1). The concentrator can be combined with an on-chip assay for improved assay sensitivity. In our lab, a 1,000-fold increase in sensitivity of assays has been demonstrated with fluorescent proteins in simple buffers (Figure 2). Currently, efforts are underway to adapt the system for use with non-natively fluorescent proteins in a more complex background such as serum. Development of a surface-coating method and a pre-concentration scheme for non-natively fluorescent proteins is cur-rently the main focus of this project.

Mass-based Readout for Agglutination Assays

Agglutination assays based on nanometer- and micrometer-sized particles were originally inspired by natural agglutination of cells [1] and provide a simple, rapid means for diagnostic testing. There are several commercial examples of agglutination assays used for clini-cal diagnostics applications. These assays are typically straightfor-ward to administer and provide fast response times. Techniques for measuring agglutination include turbidity, dynamic light scattering, and UV – Vis spectroscopy. In some cases, particle-counting tech-niques such as flow cytometry and image analysis can improve sen-sitivity by quantifying small aggregates that are produced during the initial stages of aggregation, allowing a reduction of the required in-cubation times. Additionally, particle-counting enables gathering of more specific information about the agglutination distribution in a population, rather than reliance on the average agglutination infor-mation typically obtained by ensemble measurement techniques. Furthermore, microfluidic approaches for particle-counting can re-duce the required sample volume from milliliters to microliters and enable integration with sample treatment steps. We have developed a non-optical alternative for particle counting in which early-stage aggregation is quantified by measuring mass with the suspended microchannel resonator (SMR) [1]. In SMR detec-tion, each aggregate is weighed in real-time by measuring transient changes in resonant frequency as it flows through the vibrating mi-crochannel (Figure 1). Using a model system of streptavidin-func-tionalized microspheres and biotinylated antibody as the analyte, we obtain a dose-response curve showing particle agglutination over a concentration range of 630 pM to 630 nM (Figure 2). We show that the results are comparable to what has been previously achieved by image analysis and conventional flow cytometry.

Measuring the Mass, Density, and Size of Particles and Cells Using a Suspended Microchannel Resonator

Nano- and micro-scale particles and colloidal solutions are central to numerous applications in industrial manufacturing, nanotech-nology, and the life sciences. We demonstrate the measurement of mass, density, and size of cells and nanoparticles using suspended microchannel resonators (SMRs) [1]. The masses of individual par-ticles are quantified as transient frequency shifts while the particles transit a microfluidic channel embedded in the resonating cantile-ver. Mass histograms resulting from these data reveal the distribu-tion of a population of heterogenously sized particles. Particle den-sity is inferred from measurements made in different carrier fluids, since the frequency shift for a particle is proportional to the mass difference relative to the displaced solution (Figure 1). We have char-acterized the density of polystyrene particles, Escherichia coli and human red blood cells with a resolution down to 10-4 g/cm3.The SMR’s particle measurement capabilities are a valuable comple-ment to light scattering and other particle sizing methods currently used in numerous industrial and research applications. Of particular note is the SMR’s ability to directly measure the mass/density of in-dividual particles with high precision and accuracy. These capabili-ties provide a counterpoint to optical “ensemble” techniques such as laser diffraction, which are sensitive to the shape and optical properties of the target particles, and which for some samples are prone to artifacts and irreproducibility. In its current incarnation, the SMR excels for particles from ~ 50 nm to ~ 10 µm. Future im-provements in mass resolution may allow measurement of particles down to the ~10-nm scale. The SMR’s ability to measure particle density is unique among particle size analyzers and may be applied to applications such as the measurement of porosity and capacity of drug-loaded microspheres; the characterization of engineered porous silica used in coatings, slurries, and optoelectronics; and ex-amination of the structure of submicron-sized particles.

Making it Stick: Convection, Reaction and Diffusion in Surface-based Biosensors

The past decade has seen researchers from a diverse range of disci-plines develop and apply novel technologies for biomolecular de-tection, at times approaching hard limits imposed by physics and chemistry. In nearly all types of biomolecular sensors, the diffusive and convective transport of target molecules to the sensor can play as critical a role as the chemical reaction itself in governing binding kinetics and, ultimately, performance. This is particularly true as ever-smaller sensors are developed to interrogate ever-more-dilute solutions. Yet rarely does an analysis of the interplay between diffu-sion, convection and reaction motivate experimental design or data interpretation.We have developed a physically intuitive and practical understand-ing of analyte transport for researchers who develop and employ bi-osensors based on surface capture [1]. Using a model sensor embed-ded within a microfluidic channel (Figure 1), we explore the quali-tatively distinct behaviors that can result (Figure 2), develop rules of thumb to quickly determine how a given system will behave, and derive scaling relations that give order-of-magnitude estimates for fundamental quantities of interest, such as fluxes, collection rates, and equilibration times. We pay particular attention to collection limits for micro- and nano-sensors and highlight unexplained dis-crepancies between reported values and theoretical limits.

Iso-dielectric Separation of Cells and Particles

The electrical properties of cells and particles offer insight into their composition and structure as well as provide an intrinsic handle upon which separations can be based. Over the past several decades, dielectrophoresis (DEP) [1], electrorotation [2], and impedance spec-troscopy [3] have been used to characterize the electrical properties of cells. Not surprisingly, these techniques – in particular, DEP - have also proven effective for cell sorting [1]. One significant barrier in de-veloping effective electrical sorts of cells, however, is our relatively poor understanding of cells’ electrical properties and how they vary under different environmental conditions. Better understanding of how phenotype and genotype manifest themselves through the electrical properties of a cell under different environmental condi-tions is crucial for developing new screens. Towards this end, we have created a separation method – iso-dielectric separation, or IDS – that separates continuous streams of cells and particles according to their intrinsic dielectric properties [4, 5]. Iso-dielectric separation uses dielectrophoresis (DEP) and a medium with spatially varying conductivity to sort cells according to their ef-fective conductivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [6]. The IDS leverages many of the advantages of microfluidics and equilibrium gradient separation methods to create a device that is continuous-flow, capable of par-allel separations of multiple (>2) subpopulations from a heteroge-neous background, and label-free. Additionally, because IDS offers analog separation of cells and particles according to their intrinsic properties, it can be also be used as a platform to characterize par-ticles. We have demonstrated the separation and characterization of particles ranging from polystyrene beads, to the budding yeast Saccharomyces cerevisiae, to mouse pro B cells (Figure 2), represent-ing three orders of magnitude in particle volume (~1-1000 μm3) and conductivity (~0.001–1 S/m).

Sub-cellular, Precision, On-chip Immobilization, Imaging, Manipulation, and Sorting of Small Animals

Today, pharmacological drug and genetic screens require use of in-vitro cell cultures due to the absence of high-throughput technolo-gies for studying whole animals. However, isolated cells do not rep-resent truly the physiology of live animals, and many multi-cellular processes cannot be screened using cell cultures alone. Although small-animal studies have significantly impacted cellular biology and continue to do so, the lack of techniques for rapid and high-throughput observation and manipulation of sub-cellular features in live animals has significantly limited the use of small-animal as-says for drug/genetic discoveries. We have recently invented and developed the first technologies to conduct critical high-through-put drug/genetic studies on whole animals at cellular resolution at unprecedented speeds [1, 2]. These technologies can greatly acceler-ate drug discovery using small animals for target identification and validation as well as compound mode-of-action screens.Using microfluidic large-scale-integration techniques, we enable sub-cellular precision high-throughput screening of C. elegans, a small semi-transparent nematode that is a powerful model organ-ism for studying a wide variety of biological phenomena. We have created a whole-animal sorter (Figure 1a), which makes use of single and multiple suction channels and an additional control layer to isolate and immobilize a single animal from a group. A microfluidic valve is opened at the input, and the circulating animals enter the sorter. A single small suction channel held at a low pressure is used to capture a single worm, and the remaining animals are washed away. The single worm is then partially immobilized in a straight configuration using multiple aspiration channels on the opposite site of the sorter. The aspiration immobilizes animals only partially, and it is not sufficient to completely restrict their motion. In order to fully immobilize the animals, we create a seal around them that re-stricts their motion completely. This is done by using a flexible seal-ing membrane that separates a press-down channel from the flow channel underneath. The press-down channel can be rapidly pres-surized to expand the thin membrane downwards. The membrane flexes on top of the captured animals, wrapping around them and forming a tight seal that completely constrains their motion in a lin-ear orientation (Figure 1b). Although the animals are constrained by the PDMS membrane from the top and bottom, they still have access to liquid media via the multiple aspiration channels on the side. The stability of the immobilization is comparable to that achieved using anesthesia, which allows imaging using high-magnification optics (Figure 1c) as well as the use of advanced techniques including fem-tosecond microsurgery and multi-photon imaging, both of which we have demonstrated on-chip [2]. The ability to flow worms at a high density, combined with the high actuation speed of the valves, means that animals can be isolated and immobilized for analysis very quickly and sorted based on highly complex phenotypes.

High-throughput pI-based Fractionation of Biological Samples in Microfluidic Chip for Massthroughput pI-based Fractionation of Biological Samples in Microfluidic Chip for Masshroughput pI-based Fractionation of Biological Samples in Microfluidic Chip for Massbased Fractionation of Biological Samples in Microfluidic Chip for Massased Fractionation of Biological Samples in Microfluidic Chip for Mass Spectrometry

We have developed a microfluidic chip for pI (isoelectric point)-e have developed a microfluidic chip for pI (isoelectric point)-have developed a microfluidic chip for pI (isoelectric point)- microfluidic chip for pI (isoelectric point)-microfluidic chip for pI (isoelectric point)-based fractionation of peptides and proteins as a sample preparationfractionation of peptides and proteins as a sample preparation of peptides and proteins as a sample preparation step for mass spectrometry (MS). The sorting chip with its multiplesorting chip with its multiple chip with its multiple with its multiple outlets allows continuous-flow binary sorting of the proteomic sam-s continuous-flow binary sorting of the proteomic sam- continuous-flow binary sorting of the proteomic sam-continuous-flow binary sorting of the proteomic sam- binary sorting of the proteomic sam-ples into positively and negatively charged molecules without usings into positively and negatively charged molecules without using into positively and negatively charged molecules without using without using any carrier ampholytes (Figure 1a). When coupled with pH titration,. When coupled with pH titration, When coupled with pH titration,with pH titration, pH titration, two fractionation steps enable us to isolate molecules within a pre-fractionation steps enable us to isolate molecules within a pre-steps enable us to isolate molecules within a pre-enable us to isolate molecules within a pre-us to isolate molecules within a pre-to isolate molecules within a pre-isolate molecules within a pre-molecules within a pre-pre-determined pI range from proteomic sample mixtures [1]. The pIpI range from proteomic sample mixtures [1]. The pIfrom proteomic sample mixtures [1]. The pI proteomic sample mixtures [1]. The pIsample mixtures [1]. The pI [1]. The pI. The pIpI information of the isolated molecules that is not provided by theof the isolated molecules that is not provided by thehat is not provided by theat is not provided by the is not provided by thenot provided by thethe standard ion-exchange chromatography can lead to a substantialcan lead to a substantial lead to a substantial reduction of peptide sequencing time in shotgun proteomics [2].peptide sequencing time in shotgun proteomics [2].sequencing time in shotgun proteomics [2]. in shotgun proteomics [2]. [2].The electrical junction inside the sorting chip was created simplyas created simply created simplycreated simply by patterning multiple submicron-thin hydrophobic layers on glasspatterning multiple submicron-thin hydrophobic layers on glassmultiple submicron-thin hydrophobic layers on glass-thin hydrophobic layers on glassthin hydrophobic layers on glasslayers on glass on glassglass substrate prior to plasma bonding with the PDMS chip (Figure 1b).to plasma bonding with the PDMS chip (Figure 1b).plasma bonding with the PDMS chip (Figure 1b). bonding with the PDMS chip (Figure 1b).bonding with the PDMS chip (Figure 1b). To demonstrate the sorting capability, we used three pI markers, 10.3, 8.7 and 6.6, in 20mM phosphate buffer solution with pH 8.4. As Figure 1c) shows, three bands were clearly visible in presence of1c) shows, three bands were clearly visible in presence of) shows, three bands were clearly visible in presence of an electric field of 200 V/cm. We could also separate two different of 200 V/cm. We could also separate two different200 V/cm. We could also separate two different00 V/cm. We could also separate two different. We could also separate two differentWe could also separate two differente could also separate two different proteins, GFP and R-Phycoerythrin, differing by only 0.5 pI units, into two streams (Figure 1d). In addition, we demonstrated the high-1d). In addition, we demonstrated the high-). In addition, we demonstrated the high-In addition, we demonstrated the high-addition, we demonstrated the high-, we demonstrated the high-we demonstrated the high- the high-he high-throughput capability of the device by processing raw samples at 1at 1 µL/min, which is sufficient for downstream, standard biomolecule, which is sufficient for downstream, standard biomolecule assays such as MS.We validated the two-step sorting result of a peptide mixture, pI 9.7,e validated the two-step sorting result of a peptide mixture, pI 9.7, pI 7.2 and pI 5.1, into three different fractions with the MALDI-MS. into three different fractions with the MALDI-MS.with the MALDI-MS. As Figure 2 shows, pI 7.2 (falling between pI 6-8) could be isolated shows, pI 7.2 (falling between pI 6-8) could be isolated, pI 7.2 (falling between pI 6-8) could be isolated from the mixture. The test of the device with more complex samplesThe test of the device with more complex samples such as human serum will ultimately demonstrate its potential in sample preparation for mass spectrometry.. Its successful develop-ment will have a significant impact on MS-based bioanalysis.

Microfabricated Devices for Sorting Cells Using Complex Phenotypes

This research involves the development of sorting cytometer archi-tectures for genetic screening of complex phenotypes in biological cells. Our approaches combine the ability to observe and isolate individual mutant cells within surveyed populations. In this work we merge the benefits of microscopy and flow-assisted cell sorting (FACS) to offer unique capabilities in a single platform. Biologists will leverage this flexibility to isolate cells based upon imaged dy-namic or intracellular responsesOur most recent electrical approach to image-based sorting [1] com-bines microfabricated weir structures and their efficient single-cell capture mechanics with negative dielectrophoretic (n-DEP) actua-tion (Figure 1). In these designs, we “pin” individual cells in desig-nated on-chip locations using “capture cups” formed from a pho-topatterned silicone polymer [2]. Negative DEP forces then operate as a switch to unload targeted subgroups of the weirs and prevent site-specific loading altogether in arrayed weir grouping. This func-tionality enables the placement of multiple cell types in organized single-cell patterns on a common substrate, permitting new screen-ing and response assays for cell-cell signaling dynamics. With this platform, manipulations prove feasible in standard cell-culture me-dia, thus avoiding cell health concerns associated with comparative p-DEP approaches. We have also continued developing our optical approach to image-based cell sorting. In this approach, cells are captured in a 10,000-site silicone microwell array. Following imaging, we use an infrared laser to levitate and thus sort cells out of microwells. Over the past year we have demonstrated the ability to purify cell populations up to >150× as well as sort cells based upon a localization-based pheno-type [3].Additionally, we are investigating the effects of DEP manipulation on cell physiology using a microfabricated, high-content screening (HCS) platform that applies electrical stimuli to cells and monitors the resulting subcellular molecular responses via automated fluo-rescence microscopy. The platform consists of a chip with individu-ally addressable arrayed electrodes and peripheral support electron-ics (Figure 2). We seed cells onto the chip and then expose them to a variety of electrical stresses. By monitoring the response of the cells via a fluorescent reporter cell line, we can assess how cells respond to the electric fields.

Inkjet Stimulation of Neurons

Electrical excitation is the standard method for stimulating neural tissue [1]. Although widely used, it is not the most efficient method. We have been investigating the use of potassium ions as a method of stimulating neural tissue. The use of ionic stimulation allows for a more biocompatible and low-power method of stimulation. Initial in-vitro experiments on rabbit retina show that a modest increase (~10mM) of extracellular potassium ion concentration elicits neural responses.Our initial experiments were performed by pressure ejection of KCl using a multi-barrel glass pipette and performed on the epi-retinal side of the retina [2], as in Figure 1. However, the final envisioned device will be situated in the sub-retinal space. Furthermore, a dif-ferent in-vitro experimental platform needs to be designed to over-come the limitations of the existing setup. Furthermore, a device that allows for ejection of very small volumes (pL compared to nl) and also allows for accurate estimation of volumes ejected would greatly enhance the development of a prosthetic device using this concept. An additional advantage of ionic stimulation over electri-cal stimulation, as an investigation tool for neuroscience, is that ionic stimulation does not induce a stimulus artifact that allows for simultaneous recording from multiple neurons. Thus, it would be advantageous to build an ionic stimulation plat-form that has the capability of array stimulation. Inkjet printing technology naturally lends itself to this endeavor and is the platform of choice for our device. Figure 2 illustrates the scheme . However, our initial experiments using thermal inkjet technology met with failure for reasons including inkjet head construction and cham-ber size. Simple experiments performed using a piezoelectric inkjet head showed more promise and we are currently building a custom in-vitro stimulation platform using a piezoelectric inkjet head con-trolled by custom electronics and using a software platform based on LabView.

Flexible Multi-site Electrodes for Moth Flight Control

Significant interest exists in creating insect-based Micro-Air-Vehicles (MAVs) that would combine advantageous features of in-sects—small size, relatively large payload capacity, navigation abil-ity—with the benefits of MEMS and electronics—sensing, actua-tion and information processing. In this work, we have developed a flexible electrode array that provides multi-site stimulation in the moth’s abdominal nerve cord. These flexible multi-site electrodes (FMEs) are implanted into moth (Manduca Sexta) pupae and direct-ly interface with the central nervous system (CNS) of the moths for flight control.The FMEs are composed of two layers of polyimide with gold sand-wiched in between and have 4 – 8 stimulation sites (Figure 1). The FMEs have a split-ring design that allows the FME to encircle the nerve cord, and the electrodes on the FME are on flexible tabs that protrude into the split ring and can bend back to make good con-tact with the nerve cord. The split-ring and tab design makes the FME adaptable to a wide range of nerve cord diameters, maintain-ing good contact as animals undergo metamorphosis and the nerve cord diameter increases. These FMEs were inserted into pupae as early as 7 days before the adult moth emerges and could stimulate pupae and adult moths. In pupae, we observed abdominal flexion using square wave pulses of ≥4 volts at various pairs of the stimulation sites, and similar behav-ior was observed in tethered adult moths. The electrode implanta-tions and stimulation experiments were performed by our collabo-rators at the University of Arizona and University of Washington, respectively. Finally, in loosely tethered flight, we have used this ab-dominal ruddering to cause the normally hovering moth to change its abdominal angle, leading to a change in flight direction (Figure 2). This demonstrates our ability to create MEMS-based electrodes that can be implanted in pupae, directly interface with the CNS, and enable control of insect flight.

Protein Separation by Free-flow Isoelectric Focusing

Disposable, inexpensive microfluidic devices have the potential to become a robust new tool for proteomic research involving difficult proteins and protein complexes. In this work, a preparative scale free-flow IEF isoelectric focusing (FF-IEF) device was designed, in-vestigated, and optimized. Prior work on micro FF-IEF has described devices with volumes in the range of 1-2 μL [1] and a flow rate of sub-microliters per minute. A larger FF-IEF device was developed to address the needs of molecular biologists working with samples of milligrams in mass and milliliters in volume. Earlier work [1] with IEF simulations has confirmed the advantages of using non-rectilinear channel geometries. Here we present a triangular-shaped prepara-tive IEF device fabricated by soft lithography in PDMS and having 24 outlets. The triangular design facilitates the development of the pH gradient with a corresponding increase in separation efficiency and decrease in focusing time. The unique design of a triangular separation channel required the electric fields across the central channel to be optimized. After the shaping of the PDMS prior to the device binding, a functionalized polyacrylamide gel region at the bottom of the device was selec-tively controlled to adjust the ratio of the applied potential across the separation channel (Figure 1). At the device depth of 160 mm, the electric fields of as high as over 300 V/cm could be achieved. To further investigate the separation of the protein complex mixture on the microdevice, whole cell lysate of U20S was applied and separat-ed under denaturing conditions. To validate the performance of the free-flow IEF separation, selective fractions representing the acidic, neutral, or basic region were run on a traditional 2D gel. As Figure 2 shows, effective isolation of acidic (blue), neutral (green), and basic (orange) proteins from the whole cell lysate was achieved. High-mo-lecular-weight proteins were retained by FF-IEF (shown in the blue box), but they are mostly missing from the 2D gel separation. Thus using the IEF device is an advantage for biologists interested in high-molecular-weight proteins, which presently are difficult to iso-late with conventional IEF-strip 2D gel techniques. The devices can process complex biological samples and fractionate whole cell lysate at rates between 10-30 uL/min while providing greater separation of traditionally difficult proteins. These findings show the promise of inexpensive, disposable microfluidic FF-IEF devices in proteomics research.

Multiplexed Comet Assay for DNA Damage and Repair

The use of DNA damage as a biomarker with predictive value for cancer and other diseases requires the development of a robust as-say that enables routine assessment of DNA damage levels in human samples. Many applications, such as toxicity testing and epidemio-logical studies, require an assay that is capable of testing many con-ditions or many samples in parallel. To this end, we are developing a high-throughput version of the comet assay, a well-known assay for DNA damage. The basic principle of the assay is that undamaged DNA is supercoiled and highly compact, whereas damaged DNA is composed of relaxed loops and fragments and is more mobile when electrophoresed in an agarose gel. Our assay offers many distinct advantages over other DNA damage assays, including a high level of sensitivity and the ability to detect multiple damage types. The assay can also be implemented as a measure of DNA repair kinet-ics. Despite the assay’s apparent benefits, it has been underutilized because of poor reproducibility, both from laboratory to laboratory and from user to user, and the time- and labor-intensive process of performing the assay. The major goal of this project is to overcome this assay’s limitations, such as its low throughput and poor repro-ducibility, to create a multiplexed assay for DNA damage and repair. The goal is a new tool that will be useful in a broad range of clinical, epidemiological, and experimental settings.Cell micropatterning enables spatial encoding of assay conditions, and it vastly improves spatial utilization of chips over the case with randomly placed cells. Both of these features are critical to a truly multiplexed assay platform. We have implemented single-cell mi-cropatterning using microwells1-2 fabricated directly into agarose gel (Figure 1). A negative relief mold of the microwells is fabricated us-ing photolithography of SU-8 on a silicon substrate. Molten agarose is applied to the mold and allowed to solidify, resulting in an agarose gel with patterned microwells. A cell suspension with 1− 2 ×106 cells/ml is applied to the gel, and cells are allowed to settle into the wells by gravity. Afterwards excess cells are rinsed, leaving only cells contained in the microwells. Finally, another agarose layer is applied to encapsulate the cells and to contain the DNA during the comet assay. Examples of microarrayed comets appear in Figure 2. The size of the well is a tunable parameter, which allows us to control the number of cells trapped in a single well. Additionally, this method places the cells in the same focal plane, which facilitates automated imaging, and it gives control over the cell microenvironment. We are currently developing a method for applying multiple chemical dam-aging agents to a single comet chip. With 100% filling efficiency, 200-µm cell spacing requires only 4 mm2 for 100 cells/condition, which would allow 200 conditions on a single comet chip (20 × 50 mm2 imaged area). Combining a platform for applying multiple con-ditions with the existing comet chip would provide the first truly multiplexed assay for DNA damage.

Microfluidic Devices for Studying Early Response of Cytokine Signaling

This study presents the design, fabrication, and characterization of a microfluidic device (as shown in Figure 1) integrated with cell cul-ture, cell stimulation, and protein analysis as a single device towards efficient and productive cell-based assay development. In particu-lar, it demonstrates the feasibility of culturing human cancer cells in microliter-volume reactors in batch and fed-batch operations, stimulating the cell under well-controlled and reproducible condi-tions at early stages, and detecting the protein signals with an im-munocytochemical (In-Cell Western) assay. These microfluidic devices take advantage of microfabrication tech-niques to create an environment suitable for cell culture, biome-chanical and biochemical stimulation of cells, and protein detection and analysis. The microfluidic approach greatly reduces the amounts of samples and reagents necessary for these procedures and the re-quired process time compared with their macroscopic counterparts. Moreover, the technique integrates unit operations, such as cell cul-ture, stimulation, and protein analysis, in a single microchip. The microfluidic technique presented in this study correlates the space in the microchannels with the biological process time (cell stimulation time). Thus, a single experiment in one microfluidic device is capable of generating a multiple experimental complete temporal cell response curve, which otherwise would have required multiple experiments and manual assays by standard microwells and pipetting techniques. The developed method also provides high time-resolution and reproducible data for studies of cell signaling events, especially at early stages. These cell signaling events are dif-ficult to investigate by conventional techniques. This study reports the development not only of a cell population analysis method, but also of a single-cell detection and analysis technique to explore cell-to-cell variations. In this study, a new mi-croscope stage holder was designed and machined, and an auto cell counting algorithm was developed for single-cell analysis. This sin-gle-cell method provided data on cell-to-cell variations and showed that the average cell signaling profiles were consistent with those by population-based analysis. The integration of single-cell imaging and microfluidic-enabled measurements shows promise as a tech-nique for exploring cell signaling with single-cell resolution.

Micromechanical Actuators for Insect Flight Mechanics

This project aims to develop MEMS actuators to aid in the study of insect flight mechanics. Specifically, we are developing actua-tors that can stimulate the antennae of the crepuscular hawk moth Manduca Sexta. The possible mechanosensory function of anten-nae as airflow sensors has been suggested [1], and recent discoveries of our collaborators reveal that mechanosensory input from the an-tennae of flying moths serves a similar role to that of the hind wings of two-winged insects, detecting Coriolis forces and thereby medi-ating flight stability during maneuvers [2]. Early evidence suggests that mechanical stimulus of the antennae may enable flight control. In addition, the crepuscular hawk moth Manduca Sexta has a wide wingspan (~110 mm) and is capable of carrying at least one quarter of its own weight. Thus, studying the flight of Manduca Sexta by at-tachment of microsystems seems plausible. The goal of our project is to design and fabricate micromechanical actuators, which will be mounted onto the moth antennae. Our collaborators will study the flight control mechanism by mechanical stimulation.Our first step was to fabricate “dummy” silicon rings for our biolo-gist collaborators for implant experimentation. Due to the nature of the moth antennae, ring-beam-ring construction was designed and fabricated, like a “shackle,” to meet the mounting requirements [3]. Our current work focuses on integrating actuators onto the mount-ing kit. A piezoelectric-bender and piezoelectric-stack are consid-ered the actuator (Figure 1). Live testing is also done while the moth is resting or flipping its wings (Figure 2). The moth apparently re-sponds to the mechanical stimulus under both circumstances by swinging its wings and abdomen. Future work will refine the actua-tor design and quantitatively analyze the moth’s reaction to the me-chanical stimulation, which might lead to successful flight control of the moth.

Biomimetically Inspired MEMS Pressure Sensor Assays for Passive Underwater Navigation

A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the later-al line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings [1, 2]. The canal sub-system of the organ can be described as an array of pressure sensors [3]. Interpreting the spatial pressure gradients allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby ob-jects [2]. It also aids schooling [5]. Similarly, by measuring pressure variations on a vehicle surface, an engineered dense pressure sensor array allows the identification and location of obstacles for naviga-tion (Figure 1). We are demonstrating proof-of-concept by fabricat-ing such MEMS pressure sensors by using KOH etching techniques on SOI wafers to construct strain-gauge diaphragms.The system consists of arrays of hundreds of pressure sensors spaced about 2 mm apart on etched silicon and Pyrex wafers. The sensors are arranged over a surface in various configurations (Figure 2). The target pressure resolution for a sensor is 1 Pa, which corresponds to the noiseless disturbance created by the presence of a 0.1-m-radius cylinder in a flow of 0.5 m/s at a distance of 1.5 m. A key feature of a sensor is the flexible diaphragm, which is a thin (20 μm) layer of silicon attached at the edges to a silicon cavity. The strain on the diaphragm due to pressure differences across the diaphragm is mea-sured. At this stage, the individual MEMS pressure sensors are being constructed and tested.In parallel to the construction of a sensor array, techniques are be-ing developed to interpret the signals from a dense pressure array by detecting and characterizing wake structures such as vortices and building a library of pressure distributions corresponding to basic flow obstructions. In order to develop these algorithms, experi-ments are being performed on coarse arrays of commercial pressure sensors.

Piezoelectric Micro-power-generator: MEMS Energy-arvesting Device for Self-powered Wireless Monitoring Systems

A novel thin-film lead zirconate titanate Pb(Zr,Ti)O3 (PZT) MEMS energy-harvesting device is designed and developed for powering autonomous wireless sensors. It is designed to harvest energy from parasitic vibrational energy sources and convert it to electrical en-ergy via the piezoelectric effect [1-4]. The new pie-shaped design always generates positive tension on the PZT layer and then positive charge output throughout vibration cycles. It produces mono-polar-ity output charge without using any additional bridge rectifier cir-cuitry, which will be a huge cost saving for commercial production of scaled-up products. Contrary to the high-Q cantilever designs, the new design has a low-Q, doubly anchored beam design, which provides a wide bandwidth of operational frequency. This will en-able more robust power generation even if the frequency spectrum of the source vibration varies unexpectedly. Furthermore, the beam shape is optimized to achieve uniform strain throughout the PZT layer [5]. In this new design, the whole thickness of the silicon wafer is used as the proof mass to increase the power of the generator. The fab-rication includes CVD of 10-micron-thick oxide, followed by spin-coating, patterning, wet-etching, and annealing a thin ZrO2 layer as the diffusion barrier layer, followed by three layers of PZT. The top interdigitated electrodes are patterned by the lift-off method out of gold. A long BOE etching through the oxide followed by a DRIE of silicon from the wafer’s back finalizes the device structure and re-leases the beams and proof mass (Figure 1). The SEM images of the released multi-beam cantilever beam design with a common heavy proof mass (an improved version of type-I PMPG) and a pie-shaped device with a center proof mass (type-II device) are imaged using scanning electron microscope (SEM) as shown in Figure 2.

MEMS Vibration Harvesting for Wireless Sensors

The recent development of “low-power” (10’s-100’s of μW) sensing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of individu-al wireless nodes, has created a need for a new kind of power source. Embeddable, non-life-limiting power sources are being developed to harvest ambient environmental energy available as mechanical vibrations, fluid motion, radiation, or temperature gradients [1]. While potential applications range from building climate control to homeland security, the application pursued most recently has been that of aircraft structural health monitoring (SHM).This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy, compared to other transduction principles. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam and plate structures have been developed to predict the electrical and me-chanical performance obtainable from ambient vibration sources. The optimized models have been validated by comparison to prioroptimized models have been validated by comparison to prior models have been validated by comparison to prior published results [2] and verified by comparison to tests on a mac-ro-scale device [3]. A non-optimized, uni-morph beam prototype3]. A non-optimized, uni-morph beam prototype]. A non-optimized, uni-morph beam prototype (Figure 1) has been designed and modeled [4-5]. Dual optimal fre-4-5]. Dual optimal fre-]. Dual optimal fre-quencies with equal peak power and unequal voltages and currents are characteristic of the response of such coupled devices when op-erated at optimal load resistances (Figure 2). Design tools to allow device optimization for any given vibration environment have been developed for both geometries. Future work will focus on fabrica-tion and testing of optimized uni-morph and proof-of-concept bi-morph prototype beams. This work will include system integration and development, including modeling the power electronics.

A Muscle-inspired Cellular Piezo Actuator

A muscle-inspired linear actuator that combines many piezoelectric micro-actuator “cells” into a single functional collection is designed and fabricated via a folding assembly technique. A triplet of individ-ually contractive MEMS actuator cells is designed and fabricated in series and three triplets are assembled by folding them out-of-plane around gold ribbon hinges. A triplet demonstrates peak unblocked displacement of 15.24µm, which is about 30 times amplification of the PZT strain at 10V stimulus. The loaded displacement measure- at 10V stimulus. The loaded displacement measure-at 10V stimulus. The loaded displacement measure-ments predict the 9.21µN blocking forces for the single triplet. Since the motion of the end effecter is linear and in plane, the device is arrayable in series. The use of flexible gold ribbon hinges and the folding method out-of-plane allows assembly of strings of actuators around the hinges [1].The final goal of this study is to array these actuators massively in se-ries and in parallel to make a linear actuator like an artificial muscle bundle. An improved folding assembly method such as a stacking assembly will be used to assemble hundreds of discrete piezoelec-tric MEMS actuators. This muscle-like actuator can be attached di-rectly to the skeletal structure without tendon wires and additional transmission mechanisms, simplifying micro-robot systems.

A System for Measuring Micro-scale Contact Resistance

Designing devices utilizing micro-scale electrical contacts requires a precise knowledge of the relationship between contact force and contact resistance [1]. This relationship must be obtained experimen-tally because traditional contact theory does not always hold at the micro-scale, particularly at very low contact forces [2]. Additionally, this relationship has been shown to change with repeated cycling. The changes in this relationship are linked to physical changes of the contacts [3]. A system has been developed that measures the relationship between contact force and contact resistance for flat-on-flat micro-scale electrical contacts and also permits the contacts to be observed intermittently during testing [4]. This system is com-posed of two separate coupons, each containing a metal trace of the contact material and three KOH-etched pits. The coupons are as-sembled by placing stainless steel ball bearings into the KOH-etched pits of the bottom coupon and then placing the pits of the top cou-pon over the balls. An integrated flexure on the top coupon allows the metal traces to be brought into and out of contact, as shown in Figure 1. This kinematic coupling configuration allows the coupons to be assembled and reassembled with a repeatability on the order of a few microns [5]. During testing the metal traces are brought into contact while a load cell measures force and an integrated Kelvin structure measures contact resistance, as Figure 2 shows. This type of measurement has previously been used to measure the contact resistance of carbon nanotubes [6]. The contact surfaces are then separated and observed with an SEM. The cycle of repeated mea-surements and observation of the contact surface can be used to quantify the relationship between contact resistance and contact force and describe how this relationship and the physical attributes of the contact surface change with cycling.

A MEMS-Relay for Make-Break Power-Switching Applications

We present a horizontal-displacement, electrostatically-actuated, MEMS relay for make-break power switching applications. The relay features {111}-plane silicon-etched electrical contacts. Experimental relays exhibit a minimum total on-state contact resistance of 130 mΩ, a response time of 750 µs, a theoretical electrical isolation in excess of 1 kV (tested to 450 V with available equipment), and a cur-rent-carrying capacity of 800 mA. The MEMS relay has been hot-switched in excess of 105 cycles without signs of performance deg-radation [1].The relay, shown in Figure 1, is composed of four double-parallelo-gram flexures (1) that serve as bearings, eight pairs of engaging and disengaging electrostatic “zipper” actuators (2), one moving {111} contact (3), and a pair of static {111} contacts (4a, 4b). The {111}-plane contacts [2] offer several advantages over traditional MEMS-relay metal contacts: they provide large travel, on the order of 70 µm, which exceeds the 30 µm required to withstand contact erosion and the 10 µm required to prevent arcing while operating in air at atmo-spheric pressure; the oblique contact geometry introduces contact wipe, which is known to enhance the contact reliability; and the contact geometry allows for an enhanced metallization process that provides low on-state contact resistance. The relay is etched in (100) Si through a combination of KOH etching and DRIE using nested masks. After evaporation of a gold seed-layer on the contacts using a shadow mask, the silicon is bonded to a glass substrate. Next, the contacts are plated with a 10-µm-thick copper and a 2-µm-thick pal-ladium-cobalt film. The device is released by dicing and packaged in a pin grid array for testing.During testing, voltages and currents were continuously monitored as the relay cycled, and the instantaneous total contact resistance was computed, as shown in Figure 2. The load current and voltage were increased until the relay showed any signs of temporary con-tact-sticking during any actuation cycle throughout the test. The maximum hot-switched current achieved without any signs of con-tact sticking was 800 mA with a resistive load and 350 mA with a 1 mH inductive load. While operating at or below these currents, the MEMS relay was hot-switched in excess of 105 cycles without signs of performance degradation. While the device operated at higher currents than the threshold, the sticking phenomenon was found to occur sporadically and to be reversible. Once stuck, the contacts recovered after the relay was cycled with the load disconnected.

Fabrication and Testing of a Fully-Integrated Multiwatt TurboGenerator

There is a need for compact, high-performance power sources that can outperform the energy density of modern batteries for use in portable electronics, autonomous sensors, robotics, and othersensors, robotics, and other, robotics, and other applications. The current research aims to produce a fully-integrat- The current research aims to produce a fully-integrat-ed, synchronous permanent magnet microturbogenerator capable of generating 10 W DC output power using compressed air as its energy source. Past conference abstracts by Yen, et al. [1, 2] focused on the theoretical design as well as core fabrication procedures and techniques. Presently, all the silicon die fabrication is complete, andis complete, and complete, and the magnetic components are being integrated onto the die in prep-aration for power generation testing.While the magnetic integration is in progress, efforts are underway to separately test and qualify the gas-lubricated bearings that will support the magnetic rotor to very high speeds. To make the tests relevant, they are conducted on silicon dies similar to the final gen-erator dies, with the only differences being the lack of surface wind-ings and a laminated magnetic stator. Figure 1 shows a bearing rig die enclosed in an acrylic package, as well as the metal tubulations and o-rings used to bring nitrogen into the die.Three sets of bearing rig tests are currently planned. A light rotor made purely of silicon and shown in Figure 2 will be used to assess the nominal imbalance, defined as the distance between the geo-metric and mass center of the rotor, introduced by the fabrication process. This rotor has approximately half the mass of the magnetic rotor, so a solder-filled rotor twice as heavy will be tested next to determine whether the bearings perform well with a massive rotor. After these two sets of experiments are complete, the magnetic ro-tor, which has permanent magnets and a soft magnetic back iron embedded in it, will be characterized. Because the silicon die can be easily opened along its eutectic interface [2], it is anticipated that the magnetic rotor can be removed from the die after testing and reused for the generator die.

MEMS Micro-vacuum Pump for Portable Gas Analyzers

There are many advantages to miniaturizing systems for chemical and biological analysis. Recent interest in this area has led to the cre-ation of several research programs, including a Micro Gas Analyzer (MGA) project at MIT. The goal of this project is to develop an in-expensive, portable, real-time, and low-power approach for detect-ing chemical and biological agents. Elements entering the MGA are first ionized, then filtered by a quadrupole array, and sensed using an electrometer. A key component enabling the entire process is a MEMS vacuum pump, responsible for routing the gas through the MGA and increasing the mean free path of the ionized particles so that they can be accurately detected.A great deal of research has been done over the past 30 years in the area of micro pumping devices [1, 2]. We are currently developing a displacement micro-vacuum pump that uses a piezoelectrically driven pumping chamber and a pair of piezoelectrically driven ac-tive-valves; the design is conceptually similar to the MEMS pump reported by Li et al. [3]. We have constructed an accurate compress-ible mass flow model for the air flow [4] as well as a nonlinear plate deformation model for the stresses experienced by the pump parts [5]. Using these models, we have defined a process flow and fabricat-ed five generations of the MEMS vacuum pump over the past years and are currently working on improving the overall design. Figure 1 shows a schematic of the pump. For ease in testing we have initially fabricated only layers 1-3 and have constructed a testing platform which, under full computer control, drives the pistons and monitors the mass flows and pressures at the ports of the device. The lessons learned from the first four generations of the pump have led to numerous improvements. Every step from the modeling, to the etching and bonding, to the testing has been modified and improved along the way. The most recent fifth generation pump test data ap-pears in Figure 2. Figure 2a shows the measurements of the vacuum being generated in an external volume (5.6cm3) by the micropump operating at 2Hz. The pump was able to reduce the external volume pressure by 163 Torr. Figure 2b shows the micropump-generated flow rate as a function of pumping frequency (driven in a 6-stage cycle by a controlling microprocessor to move the gas from the input to the output). The performance of this pump compares very well with that of other similar scaled micropumps in the literature. Next, we plan to fabricate and test an improved overall design and develop a final set of models to fabricate any future micropumps to the de-sired specifications.

Batch-fabricated Linear Quadrupole Mass Filters

In recent years, there has been a desire to scale down linear quad-rupoles. The key advantages of this miniaturization are the por-tability it enables and the reduction of pump power needed due to the relaxation on operational pressure. Attempts at making mi-croelectromechanical systems-based linear quadrupoles met with varying degrees of success [1-3]. Producing these devices involved some combination of precision machining or microfabrication and downstream assembly. For miniature quadrupole mass filters to be mass-produced cheaply and efficiently, manual assembly should be removed from the process.A purely microfabricated quadrupole mass filter consisting of a pla-nar design and a rectangular electrode geometry has been made. Rectangular rods were utilized since they are most amenably shaped for planar microfabrication. This deviation from the conventional round rod geometry required optimization and analysis. After we minimized unwanted effects through various simulations, we pro-posed a design (Figure 1), conceived a process flow, and fabricated the Micro-Square Electrode Quadrupole Mass Filter (MuSE-QMF) (Figure 2). The process requires the bonding of five silicon wafers and the use of deep reactive ion etching to pattern the features. It is a relatively simple process, furthering the case for mass-production of these devices.This non-conventional design will introduce non-linear resonances that manifest as peak splitting in the mass spectrum. Reported work involving linear quadrupoles operated in the second stability region shows improved peak shape without these splits [3]. It is believed that operating this device in the second stability region will provide a means to overcome the nonlinear resonances introduced by the square electrode geometry. Successful implementation of this de-vice will lead into arrayed configurations for parallel analysis and aligned quadrupoles operated in tandem for enhanced resolution.

MEMS Ejector Pumps Driven by MEMS Steam Generators

Vacuum pumping of gases at the MEMS scale is an ongoing chal-lenge; MEMS pumps typically have pressures far above and pumping rates far below those of their macroscale counterparts. To meet this challenge, we are creating high-mass-flow-rate MEMS steam-ejec-tor pumps that are driven by MEMS-based steam generators. Ejector pumps scale favorably to the MEMS scale because the entrainment of the flow to be pumped by the driving fluid takes place over a much shorter distance in a narrow, millimeter-scale channel than in a wide macroscale channel [1]. However, delivering driving fluid from a compact source remains a significant challenge. Our solution to this challenge is MEMS steam generators that decompose hydro-gen peroxide with a liquid catalyst in order to produce the ejectors’ driving fluid. The creation of MEMS steam generators enables the creation of systems of high-performance MEMS pumps. One important objective of this work is the design, fabrication, and demonstration of the MEMS steam generator to drive the pumps. The generator decomposes hydrogen peroxide using a homoge-neous (liquid) catalyst to produce steam, which is then accelerated through a nozzle to the high velocities required for effective pump-ing. Hydrogen peroxide is selected for its availability and environ-mental friendliness. The use of a liquid catalyst eliminates common problems of catalyst poisoning and aging, and the system is sized and designed to minimize thermal losses and enable complete de-composition of the peroxide.Our work to date has primarily focused on the design and modeling of the steam generator and its interface with the pumps. Conceptually, the MEMS steam generator consists of a microscale mixer, a reactor, and a converging-diverging nozzle to accelerate the exiting flow, as shown in Figure 1. One or more steam generators would be coupled to a MEMS ejector as shown in Figure 2. Liquid H2O2 is mixed with the catalyst in the generator’s “engulfment” mixer [2] and then in-jected into the reactor, where the peroxide decomposes into steam and oxygen gas. The mixing timescale is much less than the reac-tion timescale, so that the reaction and vaporization take place in uniformly-mixed fluid inside the reaction chamber. The gaseous products are then accelerated to supersonic velocities through the converging-diverging nozzle. Models predict adequate thermal management and high performance for the generator-driven MEMS pumping system. The research now focuses on the realization and experimental demonstration of the MEMS steam generator to drive the MEMS pumping system.

Micro-Reaction Technology for Energy Conversion

The development of portable-power systems remains an important goal, with applications ranging from the automobile industry to the portable electronics industry. The focus of this work is to develop microreaction technology that converts fuels – such as light hydro-carbons and their alcohols-- to hydrogen for use in solid oxide fuel cells or directly into electricity. Developing high-efficiency devices requires addressing difficulties in high temperature operation: spe-cifically, thermal management, material integration, and improved packaging techniques. In addition, recent work has included efforts to harness energy rejected to the environment as heat.The microreactor designed for combustion has been improved, resulting in longer residence times within the reactor. This longer residence time ensures full combustion of propane fuel over a plati-num catalyst. The channels within the reactor are etched using wet potassium hydroxide, which is the most economical etch technique available. The reactor remains suspended via thin-walled glass tubes, reducing conductive heat losses and allowing the reactor to operate at high temperatures. The tubes are brazed to the micro-reactor using a thermally-matched glass braze technique that was developed in-house. The coupling of two reactors has allowed for combustion to occur in one and ammonia cracking in the other, re-sulting in autothermal hydrogen generation.A combined reforming/separation device has been developed and demonstrated. Specifically, the hydrogen generation unit combines a 200-nm-thick palladium-silver film with a methanol reforming catalyst, e.g., LaNiCoO3. The catalytic combustion unit employs a platinum catalyst. Both units are formed in a silicon wafer by bulk silicon micromachining techniques. The energy generated in the combustion unit is efficiently transferred to the hydrogen produc-tion unit in the thermal conduction of silicon support. With a modi-fied brazing technique, the reactor is thermally insulated from its environment. The system has been demonstrated to purify hydro-gen at elevated pressures (up to 2 atm). Joint combustion/purifica-tion of the system has also been demonstrated, in which combus-tion and reforming occur simultaneously with the purification of the resulting hydrogen.Recent work has also included efforts to harness waste heat in the form of electrical energy. Thermophotovolatics (TPV) cells are be-ing integrated to harness radiation energy. Work is also ongoing to integrate thermoelectric (TE) devices to harness waste heat through intimate contact of the TE device with the microreactor.

Microfabricated Thin-film Electrolytes and Electrodes for Solid Oxide Fuel-cell Electrodes

Micro-solid oxide fuel cells (SOFCs) are currently under intense in-vestigation for portable power applications, such as notebook com-puters and mobile phones [1, 2]. While thin-film nanostructured solid electrolytes result in lower cell losses due to ohmic resistance, grain boundaries may serve as fast diffusion pathways for cations, resulting in poorer long-term stability. The effects of grain bound-ary chemistry and interdiffusion on ionic transport have yet to be systematically investigated. To explore the relationship between performance and stability, CeO2 thin films were grown by pulsed laser deposition (PLD), as shown in a transmission electron micrograph (TEM) in Figure 1 [3]. Thin dif-fusion sources of NiO and Gd2O3 were deposited, and samples were annealed in the temperature range of 700-800˚C to in-diffuse the Ni cations heterogeneously along the grain boundaries. Confirmation of diffusion along the grain boundaries was achieved via time-of-flight secondary ion mass spectrometry (ToF-SIMS). After modifi-cation, the diffusion source was removed by a wet etching process, and Pt microelectrodes were prepared via a photolithographic lift-off process. The electrical conductivity was measured by impedance spectroscopy and two-point DC techniques, and it decreased 10x following grain boundary in-diffusion. These results are being mod-eled by examination of changes in charge-carrier profiles induced by the in-diffusion in the space charge region adjacent to the bound-ary.

Chemical Synthesis with Online Optimization in Microreactor Systems

Microreactors are powerful instruments for scanning and optimiz-ing chemical reactions due to their enhanced heat and mass trans-fer, reduced reaction volume, and the ability to run several experi-ments in parallel. Applying fabrication principles that have been developed for integrated circuits, such as lithography, deep reactive ion etching (DRIE), oxidation, anodic bonding, and electron beam metal deposition, microreactors can be designed to accommodate a comprehensive set of chemistries. In addition to the study of chemi-cal reactions under these enhanced conditions, such as high tem-perature and high pressure, use of other process components such as mixers, heat exchangers, and phase separators can be incorpo-rated on a chip to provide a multifunctional microreactor (Figure 1). Previous work in our group has focused on exploiting these benefits in order to determine optimal reaction conditions (e.g., tempera-ture, pH, etc.) quickly, as well as to accurately evaluate the reaction kinetics for chemical syntheses related to pharmaceutical and fine chemistry sectors.[1, 2]Microreactors can also be integrated with physical sensors to pro-vide online measurement of process variables. Pressure sensors can be used to determine liquid flow rates, and temperature sensors are readily integrated by using thin film resistors or by incorporating a thin thermocouple. The progress of the chemical reaction can be monitored on-chip through UV/Vis, infrared, or Raman spectros-copy. Incorporating these measurements with traditional feedback control and optimization algorithms enables the optimization pro-cedure to be completely automated. Such an ‘intelligent’ microreac-tor system was applied experimentally for a multi-step reaction, the oxidation of benzyl alcohol by chromium trioxide to benzaldehyde with further oxidation to benzoic acid, to determine the conditions that maximize the yield of the intermediate, benzaldehyde. In a multi-parameter (e.g., reaction temperature and reagent flow rates) optimization approach, the system performed approximately 30 ex-periments in a completely automated fashion to determine the opti-mal yield of 82 – 84% (Figure 2).

Novel Synthesis of Polymeric Nanoparticles for Drug Delivery Applications Using Microfluidic Rapid Mixing

The development of smart targeted nanoparticles (NPs) that can deliver drugs at a sustained rate directly to specific cells may pro-vide better efficacy and lower toxicity for treating many diseases. For these applications, control of the NP properties such as size and polydispersity is of utmost importance for the particles’ end thera-peutic effects. Here we report the use of rapid microfluidic mixing using hydrodynamic flow focusing to control self-assembly of poly-meric NPs. Self-assembly occurs through nanoprecipitation, a pro-cess that involves dilution of a block copolymer from a solvent to an anti-solvent resulting in the precipitation of NPs [1]. We demon-strated that through the rapid mixing of precursors with anti-solvent (i.e., water), the particle size could be tuned and more homogeneous NPs could be synthesized. This work is the first implementation of nanoprecipitation on a microfluidic platform. The PDMS microfluidic devices were used to synthesize PLGA-PEG NPs by mixing PLGA7400-PEG3500 in acetonitrile (50 mg/ml) with water (anti-solvent). Hydrodynamic focusing was achieved by con-trolling flow rates with syringe pumps. Figure 1 shows the polymer stream being focused by two water streams as well as a TEM image of the resulting NPs. Figure 2 shows the change in PLGA-PEG particle size as mixing time is varied. Mixing time (~ 1-10 ms) can be tuned by changing the flow ratio of the solvent to anti-solvent. These re-sults agree with the idea that self-assembly of block copolymers into NPs by nanoprecipitation yields smaller particles as mixing time is decreased [2].This work demonstrates that microfluidic synthesis of polymeric nanoparticles with rapid mixing allows for tuning of NP size and dis-tribution through control of flow rates. These results lay the founda-tions of a microfluidic platform for controlled synthesis of NPs that may result in improved performance in drug delivery applications.

Design, Fabrication, and Testing of Multilayered, Microfabricated Solid Oxide Fuel Cells (SOFCs)

Microfabricated solid oxide fuel cells were investigated for portable power applications requiring high energy densities [1]. The thick-ness of the electrolyte, the travel length of oxygen ions, was reduced down to ~150nm. The tri-layers (yttria-stabilized zirconia (YSZ) as an electrolyte and platinum-YSZ cermet as cathode/anode) were sputter-deposited on a silicon wafer, and then they were released as square plates by KOH-etching the silicon through patterned silicon nitride masks on the back side. High intrinsic and extrinsic (thermal) stresses due to fabrication and operation (25-600oC) [2], respective-ly, require careful thermomechanically stable design of µSOFCs. First, material properties of the ultra-thin YSZ were characterized experimentally and found to be significantly different than those of bulk YSZ [3]. Second, based on the obtained properties, maximum stresses in the plates at 625°C were analyzed using non-linear von Karman plate theory [4]. The stresses showed three regions with sidelength variation: un-buckled regime, buckled regime with high stresses, and post-buckling regime with lower stresses (see Figure 1). The µSOFCs were fabricated in the post-buckling regimes with ~80-~180-µm sidelength and total ~450-nm thickness. With the plates buckled as shown in Figure 2, the µSOFCs produced power output of 0.008mW/cm, lower than the expected power from their electrochemical test. Given the high-performance predicted for the underlying nano-structured ultra-thin electrolyte, anode, and cath-ode layers, additional studies are needed to improve specimens and test setup and to assess µSOFCs’ long-term operational stability.

Microscale Singlet Oxygen Generator for MEMS-based COIL Lasers

Conventional chemical oxygen iodine lasers (COIL) offer several important advantages for materials processing, including short wavelength (1.3 µm) and high power. However, COIL lasers typically employ large hardware and use reactants relatively inefficiently. This project is creating an alternative approach called microCOIL. In microCOIL, most conventional components are replaced by a set of silicon MEMS devices that offer smaller hardware and improved per-formance. A complete microCOIL system includes microchemical reactors, microscale supersonic nozzles, and micropumps. System models incorporating all of these elements predict significant per-formance advantages in the microCOIL approach [1].Initial work is focused on the design, microfabrication, and dem-onstration of a chip-scale singlet oxygen generator (SOG), a micro-chemical reactor that generates singlet delta oxygen gas to power the laser. Given the extensive experience with micro-chemical reactors over the last decade [2], it is not surprising that a microSOG would offer a significant performance gain over large-scale systems. The gain stems from basic physical scaling; surface-to-volume ratio in-creases as the size scale is reduced, which enables improved mixing and heat transfer. The SOG chip being demonstrated in this project employs an array of microstructured packed-bed reaction channels interspersed with microscale cooling channels for efficient heat re-moval [3]. To date the device has produced oxygen concentrations of 1017 cm-3, yields approaching 80%, and molar flowrates in excess of 600x10-4 moles/L/sec [4]. The yield and molar flowrates indicate a significant improvement over the macroscale SOG designs.

Templated Assembly by Selective Removal

Templated assembly by selective removal (TASR) is an effective technique for site-selective multi-component assembly at the nano- and micro-scales. In this project, the TASR approach has been cre-ated, demonstrated and quantitatively modeled; work to expand the technology and exhibit practical applications is now underway. The TASR approach offers great promise for assembling arbitrary (not necessarily periodic) systems of multiple different types of na-noscale components, such as electronics and biological or chemical sensing devices. It also offers a path to a new kind of shape- and size-selective chromatography.TASR employs a combination of chemistry, surface topography and controllable ultrasonically-induced fluid forces to assemble diverse sets of objects selectively from fluid into designated sites on a 2D surface [1]. Figure 1 shows a schematic layout of the process set-up. The components and the substrate, after undergoing chemical sur-face modification by a coating of an adhesion promoter, are placed in a fluid environment for the assembly process and megahertz fre-quency ultrasound is applied to the fluidic bath. Competition be-tween the chemical adhesive effects and fluidic removal forces takes place in which adhesive forces emerge as stronger for components in a well-matched site. The selectivity is based on the degree to which the component to be assembled matches the shape and dimensions of the surface topography at that location. Figure 2 is an optical mi-crograph showing the successful assembly of 600 nm and 2 mm di-ameter silica microspheres using TASR. Experiments are now being conducted to extend the technique to a variety of different materials such as biological cells, polymers and nanorods which vary mark-edly not only in their physical configuration and properties but also in their chemical interaction with the substrate onto which they are to be assembled. Thus, TASR can be used as a low-cost nanofabrication method with the ability to create complex, arbitrary patterns. We are also investigating the extension of TASR to a shape- and size-sensitive separation mechanism enabling the fabrication of a filtering device with chromatography applications. Present work focuses on the ex-tension of TASR to smaller size scales, a diverse set of component shapes and materials, and improved template fabrication tech-niques with the goal of demonstrating numerous practical applica-tions enabled by this approach.

Transplanting Assembly of Individual Carbon Nanotubes to MEMS Devices

The biggest challenge in integrating nanostructures to MEMS is how to handle and assemble individual nanostructures. We demonstrate a novel assembly method for fabricating CNT-tipped atomic force microscopy (AFM) probes at a high rate and controllable quality via integrating the CNT into MEMS. Its key idea is to grow individual CNTs on a separate substrate and to transplant a well-grown CNT to the target location on a MEMS cantilever (Figure 1). This assem-bly concept transforms the scale of CNT assembly from nano-scale to micro-scale, which enables even manual assemble of individual CNTs in a deterministic way. An array of CNTs is grown from the nickel (Ni) nano-dots defined on Si substrates using electron-beam lithography followed by metal de-position and lift-off processes. Each CNT is embedded into a MEMS scale polymer block that serves as a CNT carrier. A double polymeric layer encapsulation process with SU8 (top) and PMGI (bottom) en-ables an easy release from the substrate and a deterministic length control of the CNT tip. Manual assembly of a polymer block to the end of a tipless AFM cantilever forms a CNT-tipped AFM probe. No laborious weeding, trimming, or welding process was required, and the transplanting assembly technique enables reliable assembly of CNT tips on various AFM cantilevers. The exposed CNT tip normal to the sample surface is 1.5 µm long, which corresponds to the thick-ness of the bottom layer (Figure 2, top). The scanning results over a grating with 3-µm pitch and 100-nm-deep vertical trenches shows that our CNT-tipped AFM probe scans the vertical trenches close to their vertical walls. The scanning on a biological sample (filament actins) demonstrates the potential of a CNT-tipped AFM probe for use with soft biological samples (Figure 2, bottom). This technology makes readily feasible massive parallel assembly, which will be pursued in the future.

Surface Micromachining via Digital Patterning

Conventional microelectromechanical systems (MEMS) fabrication relies heavily on the semiconductor manufacturing paradigm. While this model is well-suited for planar devices such as integrated cir-cuits, it is drastically limited in the design and fabrication of three-dimensional devices such as MEMS. From a commercial viewpoint, this paradigm also poorly fits MEMS because the lower market de-mand makes it harder to offset the high production costs. Ridding MEMS fabrication of its reliance on such techniques may introduce several advantages, namely a wider base of substrate materials as well as decreased manufacturing costs.Our project investigates severing MEMS fabrication from the tradi-tional paradigm via digital patterning technologies. We have previ-ously shown how MEMS can be used for the direct patterning of small molecular organics [1]. Using similar concepts, we have shown that surface micromachining can also be achieved.In 2007-2008, we identified a viable material set for our surface mi-cromachining process’ sacrificial and structural layers: poly-meth-ylmethacrylate (PMMA) and silver nanoparticles. To account for surface non-uniformity of the deposited PMMA, we employed sol-vent vapors to effectively lower the polymer’s glass transition tem-perature and cause reflow at room temperatures [2]. To limit surface wetting and increase material loading of the silver nanoparticles, we deposited a PMMA reservoir to contain the silver nanoparticle solu-tion (Figure 1). Free-standing cantilevers were fabricated (Figure 2), confirming that these techniques can be used for a surface micro-machining process.The next stage will be to fabricate additional MEMS structures and test the silver nanoparticle’s mechanical properties. These proper-ties will be used to design and fabricate a demonstration system based on our surface micromachining process. Subsequent stages will consist of creating a library of digital fabrication processes so that entire MEMS devices can be fabricated without the use of semi-conductor manufacturing techniques.

Vertical Growth of Individual CNTs/CNFs as Building Blocks for Functional Nano-devices

We grow the vertically aligned single-strand CNT/CNF with thegrow the vertically aligned single-strand CNT/CNF with the the vertically aligned single-strand CNT/CNF with thewith the plasma-enhanced chemical vapor deposition (PECVD) machine-enhanced chemical vapor deposition (PECVD) machineenhanced chemical vapor deposition (PECVD) machine machine we developed [1]. We found that ammonia (NHdeveloped [1]. We found that ammonia (NHWe found that ammonia (NHe found that ammonia (NHammonia (NH (NH3) gas etching is one etching is one of the key process parameters in growing vertically aligned CNTs/ in growing vertically aligned CNTs/vertically aligned CNTs/ aligned CNTs/s//CNFs. The NHs. The NH. The NH3 gas etches Ni catalyst layers to form nanoscale islands while the NH3 plasma etches deposited amorphous carbon.A 30-nm-thick Ni layer is deposited on top of a 25-nm-thick titaniumNi layer is deposited on top of a 25-nm-thick titaniumis deposited on top of a 25-nm-thick titaniums deposited on top of a 25-nm-thick titaniumtop of a 25-nm-thick titanium25-nm-thick titanium-nm-thick titaniumnm-thick titanium-thick titaniumthick titanium layer where CNT/CNF forest can grow vertically. For individual where CNT/CNF forest can grow vertically. For individual. For individual CNT/CNF growth at deterministic locations, 100~200-nm-sized growth at deterministic locations, 100~200-nm-sized, 100~200-nm-sized-nm-sizednm-sized-sizedsized nano dots were made by the E-beam lithography process (Raith,ere made by the E-beam lithography process (Raith, made by the E-beam lithography process (Raith,the E-beam lithography process (Raith,E-beam lithography process (Raith, process (Raith, (Raith, SEBL). The individual CNT/CNF growth requires shorter NHThe individual CNT/CNF growth requires shorter NHindividual CNT/CNF growth requires shorter NH growth requires shorter NH shorter NHshorter NH3 etch-ing time than is needed for a large-area forest growth. We obtained. We obtainedWe obtainede obtained a well-grown array of vertically aligned individual CNTs/CNFs withwell-grown array of vertically aligned individual CNTs/CNFs with-grown array of vertically aligned individual CNTs/CNFs withgrown array of vertically aligned individual CNTs/CNFs witharray of vertically aligned individual CNTs/CNFs withvertically aligned individual CNTs/CNFs withindividual CNTs/CNFs with CNTs/CNFs withs/CNFs with/CNFs withs with with 5~10 µm in length (Figure 1). High-resolution transmission electronµm in length (Figure 1). High-resolution transmission electronm in length (Figure 1). High-resolution transmission electronin length (Figure 1). High-resolution transmission electronlength (Figure 1). High-resolution transmission electronHigh-resolution transmission electron microscopy (HRTEM) images show fishbone structures with mul-tiple layers parallel to the etched surface of a Ni dot and the spac-and the spac-the spac-ing between the layers is measured as 0.34 nm, which confirms that they are stacked graphene layers (Figure 2). (Figure 2).In this research, we obtained vertically aligned individual CNTs/n this research, we obtained vertically aligned individual CNTs/s//CNFs on predefined location. We found that NH3 time in gas states on predefined location. We found that NH3 time in gas state on predefined location. We found that NH3 time in gas statepredefined location. We found that NH3 time in gas state location. We found that NH3 time in gas state is one of important parameters which affect in growing CNTs by PECVD. These individual CNTs/CNFs will be excellent candidatess as building blocks for functional nano-devices such as an AFM tip,building blocks for functional nano-devices such as an AFM tip,s for functional nano-devices such as an AFM tip, for functional nano-devices such as an AFM tip,an AFM tip,AFM tip, photovoltaic cell, super capacitor, and so on.

High-pressure, High-temperature, Continuous Micro-flow Synthesis of Narrow Size-distribution Quantum Dots

We have developed a high-pressure, high-temperature continuous-flow Silicon-Pyrex microreactor for the synthesis of CdSe quantum dots (QDs). The microreactor consists of a 400-µm-wide and 250-µm-deep channel with a 0.1- m-long mixing zone maintained at room temperature and a 1-m-long reaction zone heated up to 350°C. The two zones are separated by a thermally isolating halo etch that allowed for a temperature gradient of over 250°C. High-pressure modular compression fluidic connections are realized by compress-ing the microreactor between two stainless steel parts using silicone O-rings. In this configuration, the set-up allows reaching high pres-sure (up to 15 MPa) and temperature (up to 350 °C in the heated sec-tion). The entire set-up (Figure 1) is first pressurized from the inlet to the outlet using a pressurized nitrogen gas cylinder. Thereafter, the nitrogen valve is closed and the two precursor solutions are de-livered independently using a high pressure syringe pump, insuring good control of the flow rate. Applying pressure allows the use of more conventional solvents like hexane, instead of high-boiling-point solvents (squalane) used previously [1]. One can even reach the low-viscosity supercritical fluid phase of hexane (Tc = 234.7 °C and pc = 3.03 MPa, 20 < η < 70 µPa.s). In contrast to viscous single-phase flow reactors, the supercritical fluid flow approach enables narrow distribution of residence time, factors which have strong influence on the ultimate QD size distribution, as well as higher nucleation rates. Cadmium and selenium precursor solutions are delivered sep-arately in the cooled mixing region and are thereafter allowed to re-act in the heated region. The use of supercritical hexane has a strong effect on the size distribution of the QDs and consequently the Full Width at Half Maximum (FWHM) of the emission peak (Figure 2). The size distribution for QDs synthesized in hexane, 4 - 5% (FWHM: 25 - 26 nm), is much smaller than for those synthesized in liquid squalane, 9 - 11% (FWHM: 45 - 49 nm).

Modeling of Pattern Dependencies in Hot Embossing Processes

The embossing of thermoplastic polymeric layers has proved to be both a lithographic technique with exceptional lateral resolu-tion and a promising approach to high-volume microfabrication. Understanding of the mechanics of hot embossing is well developed thanks to experimentation and meticulous finite-element model-ing [1], but it is not practicable to extend such approaches to the feature-rich embossed patterns of real devices. What is needed is a computationally efficient simulation technique that can predict the fidelity of an embossed topography, given an arbitrary stamp layout and a chosen embossing temperature, pressure, and loading dura-tion. Previous attempts to develop efficient embossing simulators have modeled the polymer as a Newtonian fluid [2], an assumption that neglects the elasto-plastic and rubbery behavior that is present between the glass-transition and melting temperatures of popular embossing materials such as polymethylmethacrylate (PMMA). We present a highly computationally efficient way of simulating the deformation of a polymeric layer when embossed with an arbitrarily patterned stamp [3]. Our approach takes a discretized stamp design and iteratively finds the distribution of stamp–polymer contact pressure that is consistent with the stamp’s remaining rigid while the polymer deforms. We model the polymer in its rubbery regime as a perfectly elastic layer with a temperature-sensitive Young’s modulus; we find the overall embossed topography by convolving the pressure distribution with the response of the polymeric sur-face to unit pressure applied in one cell of the discretized region. This topography is assumed to be “frozen” in place by cooling be-fore unloading. The simulation is implemented in Matlab and the convolution uses Fast Fourier Transforms so that we can complete simulations containing ~106 elements within a few minutes using a standard desktop computer. We can additionally represent plastic flow of the polymer during embossing by scaling the point-load-re-sponse function and performing a time-stepped simulation.

Inexpensive Metrology Approaches for Process Variation in Polymeric MEMS

Polymeric materials, often inexpensive, tough and transparent, are attractive for manufacturing micro- and nano-fluidic devices. Here we describe three projects to develop tools for monitoring polymer-ic microstructure production. The first uses diffraction to identify dimensional defects in embossed thermoplastic components. A col-limated laser beam is shone through a component whose micro-embossed structure includes a specially designed holographic test pattern that spatially modulates the phase of the transmitted light (Figure 1). A far-field diffraction pattern is formed, yielding infor-mation about the embossed topography without requiring precise alignment of, or contact with, the manufactured part [1].The second project uses moiré interference to study distortions of hot-embossed polymeric substrates. The only apparatus required is a desktop image scanner and a precisely printed reference grid. The reference grid and a substrate embossed with a grid of the same tar-get pitch are placed on the scanner and rotated by hand until moiré fringes are seen. At least two scans are made, each with a different relative reference–part rotation. These rotations are extracted from the images and, together with the moiré fringes’ orientations and spacings, reveal the part’s distortions.Thirdly, we are designing a way of testing the toughness of bonds between polymeric layers. The UV/ozone- and plasma-activated bonding of polymeric layers is appealing because, unlike with ther-mal fusion bonding, microstructures at the interface remain intact [2]. However, the lack of a simple bond test method has impeded the development of these processes. Our approach is to pattern one of the layers with one or more steps to ~1 µm deep. The bonding pro-cess is then performed and, immediately after bonding, the layers peel apart locally around each step (Figure 2). The lengths of the re-sulting cracks are measured with optical microscopy, revealing the bond’s toughness. Interfacial cracks are usually shorter than a mil-limeter, meaning that these test structures can be interspersed with functional devices.

Relationship between Pad Properties and CMP Planarization

Chemical mechanical polishing (CMP) is a key technology in semi-conductor and micromachining processes. In previous work, our group proposed semi-empirical and physically-based die-level CMP models to understand and optimize the dielectric planarization process [1]. In this work, we seek to understand how planarization model parameters relate to specific pad properties. In particular, we are interested in how pad bulk stiffness and pad surface properties affect both within-chip planarization and step-height reduction or planarization efficiency. Our recent work has investigated pad hardness effects on polish-ing performance by fitting experimental data from the polishing of patterned wafers and by extracting model parameters related to effective pad Young’s modulus and height distribution of surface as-perities. We polished wafers with the same pattern-density arrays and the same initial oxide thickness using four pads with different hardness (standard, high, low and very low), and measured the ox-ide thickness and step-height evolution during the process.From the data fitting and model prediction, we conclude that the standard hardness pad achieves faster planarization and has better linear step-height removal for a longer time than the other pads, as shown in Figure 1. All pads have a pattern-density dependency ef-fect; however, the stiffer pads show less oxide thickness variation across the chip. Figure 2 shows the evolution of step height at the test point on a 50% pattern density array boundary next to a 10% layout pattern density area. This edge planarizes faster than the ar-ray center point because of the lower effective pattern density. We also see that the very low hardness pad has substantially different step-height removal behavior than the other pads, indicating that the pad surface asperity height distribution may be substantially dif-ferent for this pad. Current work is seeking to make direct pad physi-cal measurements in order to verify the relationship between both pad surface (asperities) and bulk effective modulus and the resulting planarization performance.

Cascaded Mechanical Alignment for 3D MEMS Assembly

The fabrication of MEMS devices relies, for the most part, on tools and processes developed originally for the fabrication of electronic chips in the IC industry. However, in contrast to electrical circuits, functional micro-electro-mechanical systems need features that are three-dimensional (3D) in structure. To capitalize on the well-de-veloped techniques and equipment of 2D-patterning technologies, we have developed a method to create 3D MEMS devices by folding, aligning and latching 2D micro-fabricated films.The folding process is relatively well developed. Various methods for bending films out-of-plane have been demonstrated, including thermal contraction [1], stress gradients [2], surface tension [3], and external magnetic forces [4]. However, aligning the folded segments and latching them, while maintaining the structural integrity of the MEMS devices, remain challenges.We have designed, fabricated, and tested a mechanical alignment mechanism that enables the precise angular positioning of 2D mem-branes to form 3D structures. The alignment system is based on a cascaded set of triangular protrusions on the target segment and rhombic holes on its corresponding aligning segment. Upon fold-ing, the protrusion-hole pairs start to engage sequentially, starting with the pair closest to the fold. The alignment progresses in a zip-per-like manner, allowing a large range of correction as well as high alignment accuracy (Figure 2). We have demonstrated our align-ment mechanism by assembling a corner-cube retro reflector. The alignment system’s accuracy was within 16 mrad and the measured range of correction was 0.38 rad [5]. We have also demonstrated the ability to simultaneously align multiple segments at different angles (Figure 2).

Direct Printing of PZT Thin Films for MEMS

In 2007-2008, we reported a new method for depositing piezoelec-tric thin films via thermal ink jet (TIJ) printing of a modified PZT sol-gel [1]. Direct printing of lead zirconate titanate (PZT) thin films eliminates the need for photolithographic patterning and etching, allows for controlled deposition over non-planar topographies, and enables the deposition of films with varied thickness. We developed conditions of deposition and crystallization for high-quality PZT thin films via thermal inkjet printing, including solution chemistry, printing conditions, and thermal processing parameters. The inks developed for this work were based on a commercially available PZT sol gel. Dilution of the sol is required to control the evaporation rate and characteristic dimensionless numbers of the ink, and our work included a jetability study of various solution chemistries. This study resulted in an ink that can be jetted reliably and is made up of 50% isopropanol, 30% 2-methoxyethanol, %15 A6 sol-gel, and 5% ethylhexanoic acid. This work also investigated factors that control the droplet size and the contact angle of the PZT ink deposited on a Pt substrate. The edge roughness of deposited lines was controlled to +/- 10µm. We further investigated the effect of droplet size, spac-ing, ink boiling point and substrate temperature on the deposited film uniformity. Figure 1 demonstrates the effect of substrate tem-perature on the film topography. Films between 100-500nm in thickness with a variation of less than 40nm were produced (Figure 1b). A capacitor test device was fabricated with approximately 400 nm of printed PZT between two platinum electrodes. The bottom electrode was 200 nm Pt/20nm Ti/200 nm SiO2/Si. The capacitor area was 6.25·10-4 cm2. Finally, it was determined that the modified ink requires a prolonged drying step to remove added solvents, and pre-dryed films showed a drastically improved polarization perfor-mance (Figure 2).

Printable Microfluidic Valves Composed of Thermosensitive Hydrogels

A method for fabricating compact microfluidic valves using ther-mal inkjet printing is presented. Poly(N-isopropylacrylamide) (poly(NIPAAm)) is a temperature-sensitive hydrogel that shrinks when heated above a Lower Critical Solution Temperature (LCST) (~32°C). With the swelling behavior of poly(NIPAAm) as a flow con-trol mechanism, a compact microfluidic valve has been designed and fabricated. The proposed valve provides a series of benefits over conventional microfluidic valves such as the “Quake” valves [1], in that they allow for the use of single-layer PDMS microchannels. Additionally, the need for a bulky external pump is eliminated by localized electromagnetic heating if the hydrogel valve.The design of the proposed valve is composed of an SU-8 microwell into which the prepolymer NIPAAm solution is printed (Figure 1). The well contains micro-anchors to ensure that the hydrogel always shrinks downward in order to prevent any unintended blocking of the flow channel. After the prepolymer solution with photoinitia-tors has been printed into the wells, it is polymerized using ultra-violet light. Finally, the PDMS channel is placed above the well. This channel contains discontinuities at the location of the valves, which block the flow when the hydrogel is in a swollen state. When the poly(NIPAAm) valves are heated above the LCST, the hydrogel plug shrinks and allows flow (Figure 2). The amount by which the gel plug shrinks depends on monomer concentration and UV expo-sure energy. By fabricating the wells on a separate substrate from the channels, users can use the same valve substrate with a variety of different fluidic circuit designs. Device geometry was chosen using CFD to minimize pressure drops across the valve.

Integration of Printed Devices and MEMS

As part of an overall effort on Non-Lithographic Technologies for MEMS and NEMS, we are de-veloping processes for the integration of printed MEMS and devices. The goal of this project is to demonstrate the power of a printed technology for microsystems. We have already developed a surface micromachined cantilever technology that utilizes silver as a structural material and a novel organic spacer. Further, we have developed a family of both inorganic and organic devices that can ulti-mately be printed. As an initial demonstration, we are building a MEMS capacitive accelerometer that integrates the silver surface micromachined proof mass and spring with a capacitive sense circuit fab-ricated using organic FETs.

The MIT-OSU-HP Focus Center on Non-lithographic Technologies for MEMS and NEMS

This center is part of a set of centers on MEMS/NEMS fundamen-tals supported by DARPA. The MIT-OSU-HP Focus Center aims to develop new methods for fabrication of MEMS and NEMS that do not use conventional lithographic techniques. The Center leverages the leading expertise of MIT and OSU in MEMS and printed devices, with the printing expertise of HP. The Focus Center is organized into four primary areas: tools, materials and devices, circuits, and dem-onstration systems.In the area of tools, we are leveraging the existing thermal inkjet (TIJ) technology of HP and augmenting it with specific additional features, which expand the palette of available materials for print-ing. We are developing materials and devices over a broad spectrum from active materials and photonic and electronic materials to me-chanical materials. In the circuits area, we are studying the behavior of the devices that can be realized in this technology with the goal of developing novel circuit architectures. Lastly, we intend to build several “demonstration” systems that effectively communicate the power of the new technologies that will emerge from this center. In the past year, the center has succeeded in demonstrating a number of the key “building blocks” for a fully printed system. Specifically, we have created printed transistors, printed optical elements (light emitters and photodetectors), printed active materials (piezoelec-trics), and a printed MEMS structure (micro-cantilever). Looking forward, we will begin efforts to integrate some of these building blocks.

Inkjet-printed Quantum Dot and Polymer Composites for AC-driven Electroluminescent Devices

We introduce a technique for the reliable deposition of intricate, multicolored patterns using a quantum dot (QD) and polymer com-posite and demonstrate its application for robust AC-driven displays with high brightness and saturated colors. The AC electrolumines-cent (AC EL) devices are a well-established technology [1]. Their rel-atively simple fabrication and long operating lifetimes make them desirable for large-area displays; however, a major challenge with AC EL remains finding efficient and stable red phosphors for mul-ticolored displays. Colloidally synthesized QDs are robust, solution-processable lumophores offering tunable and narrowband photolu-minescence across the visible spectrum [2]. By integrating QDs into an AC EL device, we demonstrate patterning of saturated red, green, and blue pixels that operate at video brightness.The concept behind the device operation is optical downconver-sion: red and green QDs absorb blue electroluminescence from phosphor grains and then emit at longer wavelengths. The device, pictured schematically in Figure 1, is fabricated with a layer-by-lay-er approach that is compatible with flexible substrates. A QD and polyisobutylene (PIB) solution is printed on conductive indium tin oxide (ITO) using a Hewlett Packard Thermal Inkjet Pico-fluidic dis-pensing system (TIPs). Figure 2a shows examples of the intricate and multicolored patterns possible. The electroluminescent phosphor paste (ZnS:Cu powder in a transparent binder from Osram-Sylvania) is deposited uniformly over the sample using a disposable mask and doctor-blading to define the device area. Top contacts are made with conductive tape from 3M. This basic device structure is assembled and tested entirely under atmospheric conditions. When an AC voltage waveform is applied across the device, we measure spectrally pure QD emission in the red and green and ~100 Cd/m2 brightness. Photographs of the red, green, and blue pixels of a working, AC-driven device appear in Figure 2b. The Commission International d’Eclairage (CIE) coordinates of the pixels device define a color triangle that is comparable to the International Telecommunication Union HDTV standard.

Milli-watt Energy-harvesting from Low-frequency Vibrations

This project is part of the Hybrid Insect Microelectromechanical System MEMS (HI-MEMS) program sponsored by the Defense Advanced Research Projects Agency (DARPA). The main objective of this program is to establish the interface between adult neural systems and appropriate computational and MEMS systems. Here, insects are the first test bed, and they will be directed to fly to spe-cific locations in real time via remote control. In order to support the flight-control systems, a local energy-harvesting power system is re-quired on the moth. The energy-harvesting system has two ports: the mechanical port and the electrical port. Mechanical power is in-put from moth motion at the mechanical port, and electrical power is output for general consumption at the electrical port. Internal to the harvester between the two ports are an electromechanical en-ergy converter (generator) and the power electronics. In the past 12 months, a 0.1-mW bench-top electromechanical energy converter, which extracts power from low-frequency vibration, was designed and fabricated. Figure 1 shows the harvester.The electromechanical energy converter has two major components: a resonator with moving magnets and a coil. The magnets serve as a proof mass, and as the resonator vibrates, the magnets sweep past coils through which power will be harvested. In collaboration with the Daniel Group at the University of Washington, we determined the resonating frequency to be 25 Hz by tracking the three-dimen-sional inertial motion of a moth and taking the Fourier transform of the moth’s motion. Figure 2 shows a snapshot of the moth carrying a resonator during flight. The coils are wound on a plexiglass form, such as that shown in Figure 1; future flight-qualified windings will be made with flexible printed-circuit technology. The electrome-chanical energy converter was tested on a shaker table, which simu-lates the vibration of a moth, and 0.1 mW of time average power was extracted from the output of the series coil connection.We are now optimizing a more compact advanced energy-harvester that has flexible printed-circuit windings, neodymium iron bo-ron magnets. Simulations indicate that 1-mW energyharvesting is achievable at a cost of 0.27g. Harvester components including the magnets and windings have been designed and are under fabrica-tion. Currently, we are beginning the analysis and design of the power electronic circuit. The first pass will focus on switched-ca-pacitor power electronics, and the next milestone will be testing the advanced energy-harvester on a shaker table and developing power electronics compatible with radio micro- fabrication.

Development of a Tabletop Fabrication Platform for MEMS Research, Development, and Production

A general rule of thumb for new semiconductor fabrica-tion facilities (Fabs) is that revenues from the first year of production must match the capital cost of building the fab itself. With modern fabs routinely exceeding $1 billion to build, this rule serves as a significant barrier to entry for research and development and for groups seeking to commercialize new semiconductor devic-es aimed at smaller market segments and requiring a dedicated process. To eliminate this cost barrier, we are working to create a suite of tools that will process small (~1”) substrates and cumulatively cost less than $1 million. This suite of tools, known colloquially as the 1” Fab, offers many advantages over traditional fabs. By shrinking the size of the substrate, we trade high die throughputs for significant capital cost savings, as well as substantial savings in material usage and energy con-sumption. This substantial reduction in the capital cost will drastically increase the availability of semiconduc-tor fabrication technology and enable experimentation, prototyping, and small-scale production to occur locally and economically. To implement this suite of 1” Fab tools, our current research has primarily been focused on developing a 1” Fab deep reactive ion etcher (DRIE). DRIE tools are used to create highly anisotropic, high aspect-ratio trenches in silicon—a crucial element in many MEMS processes that will benefit from a 1” Fab platform. In 2015-2016 we completed the development of the tool, and in this past year, our focus has been on optimizing its design for manufacturability. We ultimately demonstrated the manufacturability of the tool by setting up a satellite laboratory in Beijing, China with our research collaborators at the General Research Institute for Nonferrous Metals (GRINM). (See Figure 1 for a photo of the system set up in China). Our GRINM colleagues are helping develop etch recipes and providing feedback on the operation of the tool. We have also been working with the Perreault group at MIT to develop a low-cost, resistance-compression-based impedance matching network for use with this DRIE system and other plasma-based processing toolsIn addition to the optimization of the DRIE tool, we are currently developing novel PECVD and magnetron sputtering tools. In the PECVD research, we are exploring the use of inductively coupled plasma sources and non-pyrophoric mixtures of silane gas for Si-based film depositions. For sputtering, we are looking at novel techniques for creating low cost multi-layer film stacks. These two new systems will leverage the pre-existing 1” Fab modular infrastructure and will be fully compatible with the common base assembly that was developed for the 1” Fab DRIE system, as shown in Figure 2.

Development of in-situ Depth Profiler for Real-Time Control in a Deep Reactive Ion Etcher

Standard process development for micro and nanofab-rication etching cycles rely on open-loop trial and error testing of recipes to achieve optimal etch depths and uniformities. This strategy is non-optimal for research and fabrication of novel devices where one-of-a-kind experiments can not justify lengthy process develop-ment times. As an alternative, we are developing an in-situ depth measurement device for real-time feed-back of etch depth and uniformity. This will facilitate far shorter process development times, ideally enabling the desired etch to be achieved on the first process run.Many system constraints make this very difficult and preclude the use of existing technology. We are pursuing an optical measurement approach based on a parallelized confocal design. The measurement must be done at a distance of around 8” through an aperture of around 2” in diameter, significantly limiting the numerical aperture. We are currently investigating the fundamental resolution limits of a confocal depth measurement under these conditions. We expect the dominant noise source to be laser speckle which will result from coherent illumination of the rough surface left by the plasma etching process. Calculations and simulations indicate that the confocal depth measurement is significantly corrupted by this speckle noise, severely limiting the depth resolution to around 100 μm. The desired depth resolution is around 1 μm which should be achievable if the specke noise could be removed. By measuring and characterizing the statistical properties of the rough surface’s height distribution, we hope to remove the speckle noise and significantly improve the achievable depth resolution.

Resonant Body Transistor with MIT Virtual Source (RBT-MVS) Compact Model

High-Q mechanical resonators are crucial components for filters and oscillators that are essential for radio frequency and analog circuits. It is highly desirable for resonators to scale to GHz-frequencies and beyond to meet today’s challenging requirements in terms of speed and data rates. Furthermore, aggressive scal-ing requirements call for monolithic integration with complementary metal-oxide semiconductor (CMOS) circuits to allow for a smaller footprint and reduced parasitics and power consumption. Micro-electro- mechanical (MEM) resonators represent a potential solution for frequency and footprint scaling, along with monolithic integration in CMOS.A resonant body transistor (RBT) is a MEM resonator with a field-effect transistor (FET) incorporated into the resonator structure. The FET is intended for active sensing of the mechanical vibrations through piezoresistive modulation of the channel mobility. RBTs also rely on electrostatic internal dielectric transduction for actuation, by means of metal–oxide–semiconductor capacitors (MOSCAPs). Such sensing and actuation enable these devices to easily scale to multi-GHz frequencies while being compatible with CMOS manufacturing technologies.Compact modeling for these devices is essential to gain a deeper insight into the tightly coupled physics of the RBT while emphasizing the effect of the different parameters on the device performance. It also grants circuit designers and system architects the ability to quickly assess the performance of prospective RBTs while minimizing the need for computationally intensive coupled-multiphysics finite element method (FEM) simulations.The RBT compact model is developed as a set of modules, each representing a physical phenomenon. Mechanical resonance, FET sensing, MOSCAP driving, and thermal modules are the most notable. The modules are interconnected through a set of nodes (namely, mechanical nodes and a thermal node) to represent the coupling between the different physics. This modular approach enables the seamless expansion of the RBT model either by incorporating new physics, adding driving or thermal sources, or mechanically coupling multiple RBTs together. A modified version of the MIT Virtual Source (MVS) model is used to implement both the electrostatic driving (as a MOSCAP) and the piezoresistive active FET sensing. The full model is developed in Verilog-A and available on nanohub.org.

Shielded, Flexible, and Stretchable Tactile Pressure and Shear Sensors Based on Deformable Microwave Transmission Lines

Tactile sensors and skins aimed at replicating the human sense of touch are an active topic of research with numerous potential applications in areas includ-ing robotics, healthcare, and prosthetics. Current skin technology is limited by mechanical fragility, complex fabrication, and the need for large numbers of connec-tions to external electronics. We have developed a new sensing technology based on microwave transmission lines that address these challenges.The pressure sensor (Figure 1) consists of a shielded flexible and stretchable 3-mm-thick transmission line constructed with conductors made of stretchable conductive cloth and a dielectric made of silicone rubber. Where pressure is applied, the dielectric deforms causing a change in the local characteristic impedance of the line. We have developed an algorithm that can reconstruct the deformation of the line as a function of position, based on the terminal impedance of the line measured across a wide frequency range (30 MHz to 6 GHz). This algorithm can also correct for resistive loss in the transmission line. To demonstrate this sensor, three different pressure deformations were applied at each of three locations, and the responses were combined to create Figure 1. Due to the shielding, the sensor performs correctly even when tied in a knot (with updated baseline subtraction).We have also developed a shear sensor (Figure 2) capable of measuring deformation due to applied pressure, and separately, deformation due to the force applied parallel to the surface of the sensor. This device consists of two independent transmission lines, which are constructed so that pressure causes equal impedance change but shear causes unequal change, allowing pressure to be differentiated from shear. Shear sensors are rare in the field of tactile skins; this technique, requiring only two connections, has promise for inexpensive and simple wide-area flexible and stretchable pressure and shear sensors.

Micro-Engineered Pillar Structures for Pool-Boiling CHF Enhancement

Increasing the performance of phase-change heat transfer phenomena is key to the development of next-generation electronics as well as power gener-ation systems and chemical processing components. Surface-engineering techniques could be successfully deployed to achieve this goal. For instance, by engineer-ing micro/nano-scale features, such as pillars, on the boiling surface, it is possible to attain 100% enhance-ment in pool boiling critical heat flux (CHF). Research-ers have been working on several CHF enhancing micro- and nano-structured surfaces for years. However, due to the complexity of CHF phenomena, there is still no general agreement on the enhancement mechanism. An investigation of the effect of micropillar height on surface capillary wicking and the associated pool- boiling CHF enhancement has been conducted. Several silicon micropillar structures have been fabricated using MTL photolithography and DRIE facilities.The surfaces were characterized using MTL’s scan-ning electron microscope (SEM), as shown in Figure 1a. The surfaces were then characterized by measuring the capillary wicking rate as presented in Figure 1b. A mechanistic capillary wicking estimation has been provided and compared with experimental wicking results (Figure 1c). Finally, the performance of such structures was characterized through traditional pool boiling experiments (Figure 1d). The results demonstrate the benefits of wicking promoted by these structures in terms of CHF enhancement. The microstructured surfaces fabricated at MTL have also been tested in pool-boiling with an electric field applied to replace for low gravity in space applications. A further increase in CHF has been observed due to the application of the electric field, on both flat and micro-structured silicon heaters. Notably, the combined use of passive (micro-structured surfaces) and active (electric field) CHF enhancement techniques has produced the maximum CHF enhancement.

An Ultra-Thin Nanoporous Membrane Evaporator

Evaporation is a ubiquitous phenomenon found in nature and widely used in industry. Fundamental un-derstanding of the interfacial transport during evap-oration remains limited to date as it is generally chal-lenging to characterize the heat/mass transfer at the interface level, especially when the heat flux is high (> 100 W/cm2). In this work, we were able to accurate-ly monitor the temperature of the liquid-vapor inter-face, reduce the thermal-fluidic transport resistance, and mitigate the clogging risk due to contamination. This was done with an ultra-thin (≈ 200-nm thickness) nanoporous (≈ 130-nm pore diameter) membrane evap-orator, Figure 1 a, b, and c. At a steady state, we demon-strated high heat fluxes across the interface (≈ 500 W/cm2) with pure evaporation into an air ambient over a total evaporation area of 0.20 mm2. In the high flux regime, we showed the breakdown of Fick’s first law of diffusion and the importance of convective trans-port caused by evaporation itself (Figure 2). The pres-ent work improves the fundamental understanding of evaporation and paves the way for applications of high flux phase change devices.

Thin-Film Evaporation from Nanoporous Membranes for Thermal Management

Performance and lifetime of emerging electronics are often dictated by the ability to dissipate heat generated in the device. In fact, a number of advanced electronics can generate heat fluxes exceeding 1000 W/cm2, such as gallium nitride high electron mobility transistors, and pump lasers. To put that in context, the heat flux of a typical electric stovetop is more than 100x less. The large heat fluxes generated in these devices, coupled with the negative impact on the device’s performance, has created the need for new thermal management techniques. Thin-film evaporation from nanopores has emerged as a promising candidate by reducing the thermal transport resistance across the liquid film while simultaneously providing capillary pumping. The combination of low resistance and large capillary pumping allows large heat fluxes to be dissipated with minimal temperature rise in the device.In this work, we study the dependence of evaporation from nanopores on a variety of geometric parameters, including pore diameter, membrane porosity, and the location of the meniscus within the pore. Anodic aluminum oxide membranes were used as an experimental template. A bi-philic treatment was used to create a hydrophobic section of the pore to control meniscus location. This membrane was sealed in a text fixture shown in Figure 1. Heat was supplied to the membrane, and the resulting temperature was monitored. We demonstrated different heat transfer regimes and observed more than an order-of-magnitude increase in dissipated heat flux by confining fluid within the nanopore, as seen in Figure 2. Similar tests were run systematically varying pore diameter, porosity, and meniscus location within the pore. We were able to show that pore diameter had little effect on evaporation performance at these pore diameters due to the negligible conduction resistance from the pore wall to the evaporating interface. The dissipated heat flux scaled linearly with porosity as the evaporative area increased. Furthermore, it was demonstrated that moving the meniscus as little as 1 μm into the pore could decrease performance significantly. The results of this study provide a better understanding of evaporation from nanopores and provide guidance in future high heat flux thermal management device design.

Suppressing High-Frequency Temperature Oscillations in Microchannel Heat Sinks with Surface Structures

Thermal management of high performance electron-ic devices such as gallium nitride (GaN) power ampli-fiers and solid-state lasers is critical for their efficient and reliable operation. Two-phase microchannel heat sinks are attractive for thermal management of high heat flux electronic devices, yet flow instability, which can lead to thermal and mechanical fatigue, remains a significant challenge. Much work has focused on long-timescale (~seconds) flow oscillations, which are usually related to the compressible volume in the loop. However, the rapid growth of vapor bubbles, which can also cause flow reversal, occurs on a much shorter timescale (~tens of milliseconds). While this high-frequency oscillation has often been visualized with high-speed imaging, its effect on the instantaneous temperature has not been fully investigated due to the typical low sampling rates of the sensors. We propose to suppress this high-frequency temperature oscillation using surface microstructures that promote capillary wicking during flow boiling. We fabricated microchannels with micropillar arrays on the bottom heated surface (Figure 1). The geometries of the micropillars were optimized based our previously developed numerical model that maximizes the capillary flow. We then investigate the temperature response as a result of the high-frequency flow oscillation in microchannel heat sinks with smooth and microstructured surfaces with a measurement data acquisition rate of 1000 Hz. For smooth surface microchannels, the fluid flow oscillated between a complete dry-out and a rewetting annular flow due to the short-timescale flow instability, which caused high-frequency and large amplitude temperature oscillations (10 °C in 25 ms, Figure 2a). In comparison, hydrophilic surface structures on the microchannel promoted capillary flow, which delayed and suppressed dry-out in each oscillation cycle, and thus significantly reduced the temperature oscillation (Figure 2b) at high heat fluxes. This work suggests that promoting capillary wicking via surface structures is a promising technique to reduce thermal fatigue in high heat flux, two-phase, microchannel thermal management devices.

EWOD Actuation of a Vertical Translation and Angular Manipulation Stage

Adhesion and friction during physical contact of sol-id components in microelectromechanical systems (MEMS) often lead to device failure. Translational stag-es that are fabricated with traditional silicon MEMS typically face these tribological concerns. Meanwhile, electrowetting, a phenomenon whereby the contact angle of a fluid can be changed with an applied volt-age, allowing control of droplet shape, has had a limited role in MEMS applications. We show through modeling and experimental demonstration that the electrowet-ting-on-dielectric (EWOD) technique has the potential to eliminate solid-solid contact during MEMS stage operation by actuating via deformable liquid droplets placed between the stage and base to achieve stage dis-placement as a function of applied voltage (Figure 1).Our EWOD stage is capable of linear spatial manipulation with resolution of 10 μm over a maximum range of 130 μm and angular deflection of approximately ±1°, comparable to piezoelectric actuators (Figure 2). We demonstrate with our model that a higher intrinsic contact angle on the EWOD surface can further improve the translational range, which was validated experimentally by comparing different surface coatings. The capability to operate the stage without solid-solid contact offers potential improvements for applications in micro-optics, actuators, and other MEMS devices.

Additively Manufactured Miniature Diaphragm Vacuum Pumps

Miniaturized pumps supply fluids at precise flow rates and pressure levels in a wide variety of microfluidic sys-tems. In particular, microfabricated positive displace-ment pumps that exploit gas compressibility to create vacuum have been reported as a first pumping stage in non-zero flow, reduced-pressure miniaturized systems, such as mass spectrometers. Compared to standard microfabrication, additive manufacturing offers the advantages of rapid prototyping, larger displacements for better vacuum generation and larger flow rate, freeform geometries, and a broader material selection while attaining minimum feature sizes on par with mi-crofluidic systems (out-of-plane features in the 10-300-μm range and in-plane features in the 25-500-μm range). In addition, a number of 3-D printing techniques make possible the definition of leak-tight, closed channels or cavities, sometimes involving a second sacrificial material that is removed after printing. Using polyjet 3-D printing technology with 42-μm XY pixelation and 25-μm layer height, a single-stage vacuum pump design with active valves and a total pumping volume of 1 cm3 with 5% dead volume was implemented (Figure 1a). Devices were printed in the acrylate based, UV curable photopolymer TangoBlack Plus® (Shore 27A) in one piece (Figure 1b) or in two halves for ease in removing the sacrificial material. The pumps were pneumatically actuated and consistently pumped down a 1 cm3 volume from atmosphere to 330 Torr in under 50 seconds operating at 3.27 Hz (Figure 2); from the data, the effective flow rate of the device is estimated at 8.7 cm3/min.The compression chamber diaphragms exhibited lifetimes approaching 20,000 cycles, while the valves’ membranes have not leaked after >1-million cycles. Current work focuses on increasing the diaphragm lifetime, reducing the ultimate pressure, and improving the mass flow rate vs. pressure pump characteristics.

Evaluation of Lost-Wax Micromolding for Additive Manufacturing of Miniaturized Metallic Vacuum Components

In contrast to traditional subtractive methods, additive manufacturing (AM) is a process of joining materials layer by layer to generate solid structures from comput-er-aided design (CAD) data. Benefits of AM include the reduction of the raw materials required to make the part, fast manufacturing speed, versatility, and adaptabili-ty. Furthermore, AM has the potential to enable novel designs that could not be fabricated with conventional machining practices and to enhance the capability of true 3-D micromanufacturing. Standard 3-D printing of metallic parts is done via selective laser sintering, where a coherent photon beam is used to create a solid from the melting of metal powders. However, the printed struc-tures are coarse and porous with profusely outgassing surfaces and have electrical conductivity and mechanical strength less than those of the bulk material. Therefore, there is a need for better AM technologies to fabricate vacuum-compatible miniaturized metallic structures. In this project, we are exploring lost-wax micromolding as an alternative AM technology for metal parts. Wax masters printed via stereolithography were duplicated in sterling silver by encasing the master in a ceramic mold, removing the wax by melting it, and filling-in with metal the cavities left within the mold after wax removal; finally, the parts are extracted from the mold and polished. An array of pillars (Figure 1) with diameter varying from 350 μm to 500 μm and height from 400 μm to 950 μm was created to characterize feature size repeatability (Figure 2). We found close agreement between the intended and cast heights for cylinders 400 μm to 750 μm tall; however for taller cylinders, the measured values are smaller than expected, and the standard deviation is also larger. This might be related to the way high aspect-ratio pillars with a small diameter solidify during casting. Further work will focus on completing the exploration of this technology to print solid, pore-free metal parts including characterization of physical properties such as roughness, thermal diffusivity, and vacuum outgassing.

3-D Printed Multiplexed Electrospinning Sources for Large Production of Nanofibers

Electrospinning is a versatile process that creates ul-trathin nanofibers via electro-hydrodynamical jetting. Electrospun nanofibers are used in a wide variety of bio-medical (i.e., tissue healing/scaffolding, drug delivery), energy (i.e., electrodes, solar cells), and microsystem applications (i.e., sensors, batteries). Even though elec-trospinning is the only technique capable of generating nanofibers of arbitrarily length using a wide variety of feedstock, the throughput of an electrospinning emit-ter is very low, making difficult the use of these fibers in commercial products. Multiplexing the emitters, i.e., implementing arrays of emitters that work in parallel, is an attractive approach to increase the throughput of electrospinning sources without sacrificing the quality of the fibers generated. Microfabricated multiplexed electrospinning sources that achieve uniform opera-tion at low voltage and large emitter density have been reported. However, these devices do not really solve the problem well as they are made with standard microfab-rication, which is expensive and time-consuming. In this project, we are exploring stereolithography (SLA) to create disposable electrospinning sources capable of high-throughput generation of fibers. In SLA, UV light is focused on a photopolymer while 3-D layers are created through crosslinking, making it possible to print complex three-dimensional structures. The SLA process has several advantages over competing approaches such as a higher resolution, higher quality surface, higher customization, and the creation of watertight imprints. Devices with emitters with 300-µm internal diameter have been created (Figure 1). Measured per-emitter vs. flow rate characteristics using a PEO solution demonstrates that the arrays operate uniformly. Current research focuses on maximizing the throughput of the sources by emitter multiplexing, exploring approaches for charging up the emitted jets to produce thinner fibers, and in collecting and characterizing aligned PEO nanofibers using a drum as a collector system for tissue engineering applications (Figure 2).

Atmospheric Microplasma-Based 3-D Printing of Metallic Microstructures

State-of-the-art additive manufacturing techniques for metallic microstructures cannot yet deliver the feature resolution, electrical conductivity, and material choice flexibility needed for high-performance micro-circuits. Further, many current and proposed additive manufacturing approaches for fine-geometry metal features require high-temperature post-processing and restrict the substrate material. We aim to develop a mi-croplasma-based sputtering system able to direct write a wide range of materials onto any substrate. We have modeled, designed, and constructed a first-generation system that sputters gold onto a substrate. By manip-ulating the metal at the atomic level, we retain the re-sistivity of bulk metal, and by sputtering the metal, we eliminate the need for post-processing or lithographic patterning. We use a microplasma to sputter metal at atmospheric pressure, obviating the need for a vacuum. Our microplasma generator uses electrostatic fields to focus the imprints. With a suitable electrode arrangement, we can shape electrostatic fields that will guide the ionized fraction of the working gas towards a localized spot on the substrate. The directed ions will collide with other gas atoms and, crucially, with sputtered metal atoms from the sputtering target. The net force due to these collisions will indirectly guide the metal atoms towards the desired part of the substrate. This indirect electrostatic focusing not only mitigates the inherent spread of the sputtered material caused by collisions at atmospheric pressure, but also enables feature definition. In the absence of collisions, the printed line will be wider than the sacrificial cathode. By focusing the sputtered material, we achieve imprints significantly narrower than the cathode. This precludes the need to machine sacrificial electrodes as small as our desired printed lines.Our microplasma head has a central target wire acting as the cathode, surrounded by four electrodes (Figure 1), two biased at a positive voltage (relative to the grounded target) to form the plasma, and the other two biased at a negative voltage to focus the plasma. By both pulling and pushing the plasma, COMSOL simulations predict imprints orders of magnitude narrower than the cross section of the target wire (Figure 2).

MEMS Electrohydrodynamic High-Throughput Core-Shell Droplet Sources

Coaxial electrospraying is a microencapsulation tech-nology based on electrohydrodynamic jetting of two immiscible liquids that allows precise control with low size variation of the geometry of the core-shell particles it generates. Coaxial electrospraying is a very promising microencapsulation technique because (i) it is easy to implement, (ii) it can operate at room tem-perature and at atmospheric pressure, (iii) it does not require a series of steps in the encapsulation process, (iv) it can generate compound droplets with narrow size distribution, and (v) it can be used to encapsulate a great variety of materials of interest to biomedical and engineering applications. State-of-the-art coaxial elec-trospray sources have very low throughput because they have only one emitter. Consequently, coaxial elec-trosprayed compound particles are compatible with only high-end applications and research. An approach to increasing the throughput of a coaxial electrospray source without affecting the size variation of the emitted compound microparticles is to implement arrays of coaxial emitters that operate in parallel. However, no miniaturized coaxial array sources have been reported, probably due to the inherent three-dimensionality of the emitter geometry and the hydraulic network required for uniform array operation, which is at odds with the planar nature of traditional microfabrication. In this project, we demonstrated the first MEMS multiplexed coaxial electrospray sources in the literature. Miniaturized core-shell particle generators with up to 25 coaxial electrospray emitters (25 emitters·cm-2) were fabricated via digital light projection/stereolithography (DLP/SLA, Figure 1), which is an additive manufacturing process based on photopolymerization of a resin that can create complex microfluidics. The characterization of emitter arrays with the same emitter structure but different array size demonstrates uniform array operation. The core/shell particles produced by these additively manufactured sources are very uniform (Figure 2); the size distribution of these compound microparticles can be modulated by controlling the flow rates fed to the emitters.

High Current Density Si-field Emission Arrays (FEAs)

Silicon field emitter arrays (FEAs) are excellent cold cathodes that have not been fully exploited due to the nonzero tip radius distribution causing lower utilization of the arrays. This discrepancy in emitter tips causes sharper tips to burn out (by Joule heating) before duller tips, and therefore the maximum current achievable is small. In this work, we focus on achieving high current density Si FEAs, by integrating high-aspect ratio Si nanowires as to limit the supply of electrons and hence saturate the maximum current to avoid the burn-out of the sharper tips. Si nanowires of height ~10 µm and 100-200-nm diameter limit the current and improve reliability through velocity saturation and the pinch-off of majority carriers. To prevent charge injection and minimize the gate-substrate capacitance, a 2-µm-thick SiO2 insulator is added, and the Si nanowires are embedded in a conformal dielectric matrix consisting of Si3N4 and SiO2. High current densities are achieved as the nanowires (current limiter) are integrated with each field emitter, thereby preserving a high density of operational emitters (~108 emitters/cm2) without burning out. These Si FEAs have also been shown to provide consistent current scaling of array sizes from a single emitter to 25,000 emitters, low voltage (VGE < 60 V ), high current density ( J > 100 A/cm2), and long lifetime ( > 100 hours at 100 A/cm2, > 100 hours at 10 A/cm2, and > 300 hours at 100 mA/cm2). Compared to conventional Si FEAs operating without a current limiter, the device architecture shown here demonstrate a current density improvement of > 10 folds and low turn-on voltage (8.5 V). Cold cathodes based on Si-FEAs incorporating a current limiter have high potential in applications ranging from X-ray imaging, RF amplifiers, and THz sources to deep UV sources, ion sources, and neutron sources.

Field Emission from Silicon Tips Embedded in a Dielectric Matrix

Field emitter arrays (FEAs) are a class of cold cathodes with promising potential in a variety of applications requiring high current density electron sources. How-ever, FEAs have not yet achieved widespread usage because of fundamental challenges that limit their reliability in systems. Field emission from conduct-ing surfaces requires high fields and pristine surfaces; these surfaces are vulnerable to adsorption-desorption processes by residual gas molecules, leading to emis-sion current fluctuations and tip erosion. Moreover, electron transport through insulators often leads to im-pact ionization and dielectric breakdown. This project explores electron emission from field emitter tips that are embedded in a dielectric matrix, specifically silicon dioxide, as a potential approach to address reliability problems in classical field emitters.In the project, arrays of silicon emitter tips that are individually regulated by silicon nanowires are being fabricated. The silicon nanowires have diameters between 100-200 nm and heights of 10 µm, resulting in an aspect ratio of 50-100:1. The emitter tips typically have radii of 5 nm with a log-normal distribution and a density of 108 tips/cm2. Further, the silicon nanowires function as current limiters that improve reliability by preventing premature tip burn-out due to Joule heating, thermal runaway, and cathodic arcs. Chemical mechanical polishing (CMP) was used to form the self-aligned gates. The silicon tips formed by oxidation sharpening are embedded in a dielectric matrix and are not released. A diagram of the structure is shown in Figure 1.

A Silicon Field Emitter Array as an Electron Source for Phase Controlled Magnetrons

Magnetrons are a highly efficient (>90%), high-pow-er vacuum-based microwave source. In a magnetron, free-electrons in vacuum are subject to a magnetic field while moving past open metal cavities, resulting in resonant microwave radiation to be emitted. Current state-of-art magnetrons use a heated metal filament to thermionically emit electrons into vacuum continu-ously and are not addressable. This work seeks to re-place the heated metal filament as a source of electrons with silicon field emitter arrays in order to improve the efficiency and increase the power, especially when sev-eral sources are combined. Silicon field emitter arrays, schematic shown in Figure 1, are devices that are nor-mally off and are capable of high current densities plus spatial and temporal addressing. These arrays consist of a many sharp tips made of silicon sitting on long sil-icon nanowires that limit the current of the electron emission. Electrons from the silicon tip tunnel into a vacuum as a result of the high electric field of the ap-plied bias on the polysilicon gate. Pulsing the electric field applied on the gate can turn the arrays on and off. The proposed use of silicon field emitter arrays in a magnetron will allow injection locking and hence phase control of magnetrons. Phase-controlled mag-netrons have multiple applications in areas where high- power microwave sources are desired.

Ion Electrospray Thrusters for Microsatellite Propulsion

Ion electrospray propulsion systems (iEPS) are high specific impulse, low thrust, and extremely scalable devices; these characteristics make them excellent candidates for propulsion systems on microsatellites, which require some small amount of maneuverability primarily for station-keeping. Like other ion engines, they utilize an electrostatic potential to accelerate charged particles across a gap to relatively high veloci-ties to generate thrust. Utilizing ionic liquids – a special class of molten salts that do not evaporate in vacuum, thanks to their negligible vapor pressure – drastically increases the propellant density and obviates the need for a stage in which the propellant is first ionized, thus further reducing mass and volume requirements. The thrusters themselves are extremely simple in that they are passively fed through capillary action and require no moving parts. However, high electric fields, on the order of 109 V m-1, are required to extract ions from the liquid. This entails careful fabrication of the porous emitter substrates, which feature an array of roughly five hundred tips, patterned into the surface via laser ablation. By providing a sharp tip, the electric field is effectively intensified to the point that ions can be ex-tracted, through a sharpening effect similar to coronal discharge. The thrusters are constructed from several component parts. The frames are made via microelectromechanical (MEMS) processing: a silicon base layer, an insulating glass layer, and finally a top silicon layer with alignment features to correctly locate the tip array is etched and then anodically bonded. To those frames a porous substrate is affixed after being shaped and polished. A tip array is patterned into the substrate via laser ablation. Next, a silicon electrode grid, also fabricated with MEMS processing, is bonded to the frame so that the grid holes are aligned to the tip array, completing the emitter. The emitters are then bonded onto tanks that passively transport propellant to the emitter. The tanks are mounted on electronic power supply/control boards, creating a finished engine that may be integrated into a spacecraft. Four small satellites (CubeSats) equipped with these thrusters have already been launched into space. Our team is currently working on a new project, set to launch during Q1 of 2018.

Enhanced Water Desalination in Electromembrane Systems

Currently, reverse osmosis (RO) is considered the leading technology for desalination, and the operational efficien-cy of RO has been significantly improved over the last two decades with a thorough energy analysis. On the other hand, electrical desalination can be more advan-tageous in certain applications due to the diversity of allowed feed conditions, operational flexibility, and the relatively low capital cost needed (the size of a system is generally small). Yet, electromembrane desalination techniques such as electrodialysis (ED) have not been modeled in full detail, partially due to scientific challeng-es involving the multiphysics nature of the process.In addition, while current ED relies on bipolar ion conduction (Figure 1b), removing one pair of a cation and an anion simultaneously, one final but most important point is that desalination achieved by means of an anion exchange membrane (AEM) and a cation exchange membrane (CEM) should be considered separately and independently (Figure 1a). Based on the intrinsically different ion transport near AEM and CEM, our group previously presented a novel process of ion concentration polarization (ICP) desalination (Figure 1b), which can basically enhance the amount of salt reduction, by examining unipolar ion conduction through both experiments and numerical modeling. In our studies, we investigate the effects of embedded microstructures on mass transport enhancement; these microstructures affect the electrical energy efficiency of an ED system for its current application of brackish (low salinity) water desalination (Figure 1c); we also explore the technical and economic feasibility of the ICP desalination for potential applications in the emerging field of high-salinity brine desalination (Figure 1d).

Digital Optical Neural Networks for Large-scale Machine Learning

Artificial intelligence is becoming ubiquitous in our society; specifically, artificial deep neural networks (DNNs) have enabled breakthroughs in image classification, translation and prediction. The recent adoption of DNNs in a wide variety of fields is largely due to algorithms with improved accuracy that leverage more compute power and larger datasets. However, throughput and energy efficiency are currently limiting the further expansion and adoption of DNNs. We have proposed optical neural networks (ONNs), which we have theoretically shown to achieve low-energy, high-throughput DNN processing. Our latest results include a proof-of-concept demonstration of a digital ONN with little drop in classification accuracy on the MNIST dataset (-0.6% on a custom, fully-connected, 3-layer network, due to optical crosstalk). In this scheme, we use optics for passive digital data fan-out and routing. Owing to the length-independence of energy and latency in optical data transmission, we find that the digital ONN may enable more efficient DNN hardware. This work showcases the promise of ONNs as a new computing paradigm, which is required to unlock the full potential of DNNs.

Charge-carrier Recombination in Halide Perovskites

The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique among semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge-carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba splitting), and (4) photon recycling (Figure 1). We scrutinize the evidence for each case and the implications of each process for carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that shallow trap states and the possible indirect nature of the bandgap (e.g., Rashba splitting), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.

In-situ Gamma Radiation Damage on SiC Photonic Devices

In this report, we demonstrate real-time, in-situ anal-ysis of radiation damage in integrated photonic devic-es. The devices, integrated with an optical fiber array package and a baseline-correction temperature sensor, can be remotely interrogated while exposed to ionizing radiation over a long period without compromising their structural and optical integrity. We also introduce a method to deconvolve the radiation damage respons-es from different constituent materials in a device. The approach was implemented to quantify gamma radia-tion damage and post-radiation relaxation behavior of SiO2-cladded SiC photonic devices. Our findings sug-gest that densification induced by Compton scattering displacement defects is the primary mechanism for the observed index change in SiC. Additionally, post-radia-tion relaxation in amorphous SiC does not restore the original pre-irradiated structural state of the material. Our results further point to the potential of realizing radiation-hard photonic device designs taking advan-tage of the opposite signs of radiation-induced index changes in SiC and SiO2. The devices fabricated following CMOS-compat-ible protocols are symmetrically cladded with PECVD SiO2. In device packaging, the as-fabricated devices were packaged with optical fiber arrays (SQS Vlaknova Optika) using ultraviolet-curable epoxy (Masterbond UV15TK) as the bonding agent. Fibers with an incident angle of 15° were first active aligned to the on-chip grating couplers to maximize the transmitted power. Epoxy was applied onto the chip to securely bond the fibers to the chip. The active alignment was repeated after epoxy application to ensure optimal coupling. The epoxy was then cured through flood UV exposure. We monitored the device resonance peak position and Q-factor as gamma radiation progressed. The refractive index and absorption coefficient change of a-SiC core and a-SiO2 cladding were extract and plotted in Figure 1. As indicated in the graph, we clearly observe an op-posite in signs of index change in these two materials, suggesting the potential of realizing radiation-hard photonic devices.

Variation-aware Compact Models for Yield Prediction of Coupled-resonator Optical Wave-guides

Silicon photonics is a growing design platform due to all the potential applications and enhancements it can offer. Among these attractive applications are the sig-nificant computing system performance gains that can be achieved by transferring information using optical rather than electrical signals. Achieving this optical transmission requires on-chip optical buffers. Coupled resonator optical waveguides (CROWs), which chain a number of ring waveguides together as in Figure 1a, can be used as buffers. However, CROWs are challenged by the spatial variations within die or across the wafer, as CROWs are large structures extending hundreds of microns to mil-limeters in length depending on the number of constit-uent rings. These variations can change the passband or, more importantly, may cause the CROW to fail if the spatial variations cause the resonances of the coupled rings to lose their alignment. Moreover, varying the ring (constituting a CROW) design requires regenerat-ing the S-parameters, which is computationally expen-sive and time-consuming, if many variants need to be considered for Monte-Carlo statistical simulations or during design optimization. This highlights the need for a variation-aware compact model. We develop a method and variation-aware com-pact models that can be used to simulate and predict the CROW behavior (S-parameters) against spatially correlated process variations in thickness and width. Figure 1b compares the simulated performance of a 28-ring CROW using S-parameters generated directly from FDTD simulation and using S-parameters gener-ated using the developed compact model. This parame-trized compact-model-generated S-parameters can be used to facilitate and speed up design optimization, run Monte-Carlo simulations, and predict yield. Figure 2 shows the yield prediction of CROWs satisfying a suf-ficient amplitude pass band (above -20dB), in response to width variation as a function of spatial correlation length ( ) and amplitude (σ). This compact model can serve as a building block for a variation-aware process design kit (PDK) for photonics.

Graphene-loaded Slot Antennas for Multispectral Thermal Imaging

Color cameras are ubiquitous in everyday life. However, most color imagers rely on color filter arrays (CFAs), resulting in most incoming light being filtered out instead of detected. More generally, for a filter-based imaging array with N different colors, only 1/N of the incoming light is actually used. While lossless spectral imagers are available, they rely on bulky optics such as diffraction gratings or interferometers to achieve spectral resolution, which is often undesirable. In the thermal IR wavelength range, the problem of filter loss is exacerbated by reduced sensor detectivity compared to visible light sensors. We propose an efficient and compact thermal IR spectral imager based on a metasurface consisting of sub-wavelength-spaced, differently-tuned antennas with photosensitive loads. The different antenna resonances combine to yield broadband optical energy transfer to the loads exceeding the 1/N efficiency limit of CFAs. In particular, we investigate slot antennas due to their unidirectionality and high efficiency compared to typical dipole antennas. We use graphene as our photosensitive load because its 2D nature makes it easily adaptable to this imager architecture. To aid in the design of these slot antenna metasurfaces, we establish a circuit model for the optical properties of the antennas and demonstrate consistency between this model and full-wave electromagnetic simulations. We also show simulations results demonstrating broadband ~36% free space to graphene coupling efficiency for a six-spectral-band metasurface. Finally, we demonstrate a fabrication process which yields slot antennas with smooth surfaces suitable for graphene transfer on top. This research represents the first steps towards compact, monolithic, and potentially CMOS-integratable mid-IR spectral imagers whose low bulk and low energy consumption suit them for deployment on small drones for remote sensing and free-space communication purposes.

Room-temperature Strong Light-matter Interactions in Hybrid Perovskites

State-of-the-art perovskite materials demonstrate photoluminescence quantum efficiencies (PLQE) above 90% due to low non-radiative recombination rates and unparalleled defect tolerance. The optoelectronic properties that have allowed perovskites to emerge as a leading active layer material in high-efficiency thin-film photovoltaics (PVs)–high absorption coefficient, small Stokes shift, high PLQE, solution processability, and chemical tunability – simultaneously situate perovskites to function superbly as a coherent quantum material. In this work, we explore perovskites as a platform for strong light-matter coupling to sustain all-optical operations. Although light is weakly interacting, it is possible to form interacting quasi-particles, called exciton-polaritons, that have characteristics of both light and matter. Traditionally, polaritons have been studied at cryogenic temperatures in all-inorganic semiconducting materials (e.g., GaAs heterostructures). Here, we study the room-temperature formation of exciton-polaritons with large Rabi splittings in semiconductor microcavities, using solution-processed 2D perovskites as self-assembled quasi-quantum well structures (Figure 1). Polariton formation is probed by angle resolved reflectivity and photoluminescence measurements through a k-space imaging setup. Enhanced polariton propagation is explored by microstructuring the microcavity to funnel polaritons generated in smaller cavity length regions to lower confined photon energy regions of longer cavity length. The realization of stable, facilely-fabricated room-temperature exciton-polaritons has the potential to revolutionize a wide range of devices, from PVs to low-threshold lasers to all-optical switches.

Amorphous Silicon Carbide for Nonlinear Integrated Photonics

Silicon carbide (SiC) has been actively researched in recent years as a platform for linear and nonlinear photonics due to its large bandgap, large refractive index, low thermo-optic coefficient, excellent mechanical and chemical stability, and large Kerr nonlinearity. We have demonstrated amorphous SiC waveguides with propagation losses as low as 3 dB/cm, which enable their application in integrated photonics. We have demonstrated amorphous SiC ring-resonators on SiO2 insulator substrate with an intrinsic quality factor as high as 1.6×105. The Kerr nonlinearity obtained at 1550-nm wavelength was 4.8 ×10-14cm2/W, which was the highest value reported in both crystalline and amorphous SiC material, making it a promising platform for CMOS-compatible nonlinear photonics.The amorphous SiC photonic devices were fabricated in the MIT.nano cleanroom facilities. A plasma-enhanced chemical vapor deposition (PECVD) system using a silane and methane reactive gas mixture was used to deposit an amorphous SiC thin film on a 6-inch Si wafer that had a 3-µm thermal oxide insulating layer. Electron beam lithography was used to pattern the SiC-on-insulator ring resonators. Fluorine chemistry was used to dry etch SiC using reactive ion etching. We characterized the optical properties of the amorphous SiC photonic devices in collaboration with Dr. Peng Xing and Professor Dawn Tan at the Singapore University of Technology and Design (SUTD) and achieved the largest quality factor among all crystalline and amorphous SiC materials tested to date. The Kerr coefficient of the amorphous SiC film was extracted by fitting the nonlinear Schrödinger equation. The Kerr nonlinearity measured in our amorphous SiC is almost one order of magnitude higher than that reported in the literature for crystalline and amorphous SiC. Nonlinear behavior was observed for the first time for a-SiC at the wavelength of 1550 nm, with a high incident pulse peak power.

High Sensitivity Mid-Infrared/Thermal Detectors

Infrared detectors that are fast, high-detectivity, and room-temperature-operable are needed to enable next-generation hyperspectral arrays. While photo conductor (pc) detectors can achieve high detectivity and ~100 ps time constants, pc detectors suffer from a narrow spectral range and must be cooled to cryogenic temperatures for efficient detection beyond ~ 4 µm wavelength. Thermal detectors, meanwhile, exhibit a flat detectivity response with respect to wavelength and can, ideally, reach a detectivity of 1.98*1010 cmHz1/2W-1 at room temperature. In this work we focus on two sensitive novel thermal detector architectures: (1) a nanogap based thermomechanical bolometer and (2) a pyroelectric gated field-effect transistor (FET ) biased in the subthreshold regime. The thermomechanical (thm) bolometer achieves high sensitivity by closing a ~1.3-nm gap as the surrounding materials expand due to infrared light absorption, resulting in an exponential increase in current. The suspended thm bolometer is made of two metal cantilever arms connected by a 5-nm-thick platinum wire (see Figure 1). The nanogap detectors are mechanically stabilized via a self-assembled monolayer (SAM). Early experimental results show temperature coefficient of resistance (TCR) values as high as 0.16 K-1, which is higher than the state-of-the-art ~ 0.1 K-1. Studies to characterize the noise of these devices, measure their response to laser illumination, and determine their detectivity are in progress.We are also exploring an additional low-power and sensitive bolometer design using subthreshold, pyroelectric gated thin-film transistors. When infrared light is absorbed, dipole charges in the pyroelectric material align and gate the transistor channel. We estimate that these devices can achieve TCR values of 0.6275/I0 K-1, where I0 is the bias current. The proposed device structure can be found in Figure 2. We are currently exploring the design space of Hf0.5Zr0.5O2 ferroelectric/pyroelectric FETs and optimizing them for 5 µm – 10 µm wavelength detection.

3D Integrated Photonics Platform with Deterministic Geometry Control

3D photonics promises to expand the reach of photonics by enabling both the extension of traditional applications to non-planar geometries and adding novel functionalities that cannot be attained with planar devices. However, current fabrication methods limit the range of available materials options (e.g. to low index contrast polymers for 3D printing) or device geometries (e.g. to curvilinear geometries that are inherently 2D in topology). As an application example, the much-needed ability to monitor stress in biological samples such as cell cultures and tissue models requires a platform that provides precise measurements at multiple, pre-defined locations in 3D, which none of the current fabrication methods for 3D integrated photonics can offer.In this work, we report a fully-packaged 3D integrated photonics platform with devices placed at arbitrary pre-defined locations in 3D using a fabrication process that capitalizes on the buckling of a 2D pattern. The final structure consists in several buckled strips joining two planar edge platforms, as shown on Figure 1a. Each strip may contain waveguides and waveguide-coupled components such as resonators. We show that our fabricated devices (see Figure 1b) precisely match theoretical shapes. Finally, we demonstrate the amenability of this platform for mechanical strain sensing, e.g. in 3D cell cultures, by calibrating its stress-sensing response. Our results indicate a strain measurement accuracy of 0.01%, for materials with a Young's modulus down to 300 Pa. A key benefit of our fabrication approach for 3D integrated photonics lies in the wide range of physical and chemical sensing applications of optical resonators, as well as the possibility to multiplex resonators spec-trally and spatially. Our platform is thus amenable to monitoring a variety of parameters at a large number of locations in a distributed sensor array, potentially enabling multifunctional sensing, mapping, and light delivery in the 3D space.

Single-element, Aberration-free Fisheye Metalens

Wide-angle optical functionality is crucial for implementation of advanced imaging and image projection devices. Conventionally, wide-angle operation is attained with complicated assembly of multiple optical elements. Recent advances in nanophotonics have led to metasurface lenses or metalenses, a new class of ultra-thin planar lenses utilizing subwavelength nanoantennas to gain full control of the phase, amplitude, and/or polarization of light. Here we present a novel metalens design capable of performing diffraction-limited focusing and imaging over an unprecedented > 170° angular field of view (FOV). Similar to a Chevalier landscape lens, our metalens design concept spatially decouples the metasurface and aperture stop, but positions them on a common, planar substrate (Fig. 1a). This optical architecture allows input beams incident at various angles (indicated with colored arrows in Fig. 1a) to be captured on distinct yet overlapping areas of the metasurface. The metasurface further forms the pencil-beams, in such a way, that all of the focal spots are positioned in the same image plane. We fabricated the metasurface using PbTe meta-atoms of rectangular and H-shaped blocks, which induce distinct phase shifts arising from the electric and magnetic resonant multipole modes (Figure 1b). The meta-atom library consisted of eight elements covering the 360° phase space with a discrete step of 45° for linearly polarized light at the mid-infrared wavelength of 5.2 σm. The implemented metalens produced diffraction limited focal spots when illuminated with a laser beam at the incident angles ranging from 0σ to 85σ. We further demonstrated that the metalens can perform aberration-free imaging of the USAF resolution charts over the entire FOV. Our metalens design concept is generic and can be readily adapted to other meta-atom geometries and wavelength ranges to meet diverse application demands. In the scope of this project, we also explored machine learning approaches to generate free-form metalens designs with improved performance.

Ultra-sensitive All-optical Membrane Transducers for Photoacoustics

Photoacoustic imaging (PAI) has attracted much attention over the past two decades for various biomedical imaging applications. However, it is surprising to note that this unique imaging modality has not yet spun out much in commercial applications. One of the key obstacles in this direction is the limited sensitivity of the currently available ultrasound transducers. Existing acoustic transducer technologies based on bulk PZT, piezoelectric, and capacitive micromachined ultrasonic transducers have a significantly low sensitivity in orders of 0.2 – 2.0 mPa/sqrt(Hz). This feature limits the imaging depth, reliability, and molecular sensitivity of the current PAI systems.Our research work explores on-chip CMOS-compatible all-photonic architecture to develop PAI systems with significantly improved sensitivities, improved detection limits, and reduced power consumption. Spiral-shaped silicon nitride waveguides realized on suspended silicon-oxide membranes designed to have a center frequency between 5 MHz to 10 MHz are used as Mach-Zender arms for highly sensitive ultrasound reception with <1 mPa/sqrt(Hz) noise equivalent pressure. Hence, this approach allows fast intensity-based acquisition as opposed to interferometric acquisition, thus allowing on-chip optical interrogation. A few previously reported attempts in this direction have been limited to a single sensor element. Here, we attempt to leverage the benefits of existing photonic-based signal conditioning schemes and adopt them to multiplex the ultrasound reception from multiple sensor elements. The presented transducer technology has a multitude of advantages. Ultra-high sensitivity combined with an all-optical implementation will allow easy scaling-up of the technology and miniaturization for wearable applications.

Large-scale Integration of Diamond Qubits With Photonic Circuits

Quantum technologies can potentially offer dramatic speed-up and enhanced security in information pro-cessing, communication, and sensing. Such tasks would require the scalable construction and control of a large number of quantum bits (qubits). Here, we report the fabrication and characterization of the largest integrat-ed artificial atom-photonics chip. Defects such as color centers in diamond behave like “artificial atom” (AA) spin qubits in that they can be controlled via light and microwaves and can maintain long coherence times. Scaling such systems requires (1) high-yield qubit fabrication, (2) efficient photonic wires to route and manipulate single photons, and (3) post-tuning capability to compensate for inhomogene-ities between different qubit modules. Rather than fabricating a low-yield monolithic system with these necessary requirements, we intro-duced the heterogeneous integration of “quantum mi-cro-chiplets” (QMCs) into an integrated photonics pro-cess. The QMC (Figure 1A) consists of AA qubits in a di-amond waveguide array, while the photonic integrated circuit (PIC) is an aluminum nitride (AlN)-on-sapphire platform. We used a pick-and-place process to transfer the QMCs in diamond to the AlN photonics chip with success probability over 90% (Figure 1B). As Figure 1C shows, the diamond and AlN modules meet at tapered waveguide interfaces for efficient photon routing from the diamond layer to the integrated photonics layer. Room temperature and cryogenic measurements reveal single-photon emission in all 128 integrated waveguide channels (Figure 2). Additionally, the emit-ters exhibit near-lifetime-limited linewidths, indicating high optical coherence of emitters in nanostructures. Finally, we demonstrated on-chip tuning of the qubit optical transitions via strain fields in the waveguide. Our platform paves the way for on-chip generation and manipulation of large entangled quantum states and demonstrates the scalability of optically active spin qu-bits in solids for quantum information processing.

Transmittance Enhancement at Graphene/Al Interfaces

When two metal films stack together forming “hete-ro-film,” it has been generally accepted that the effective transparency is lower than in the respective metal film as a result of the absorption accumulation. In this work, we investigated the counterintuitive transmittance en-hancement of graphene/aluminum hetero-films. Single layer graphene was first grown by chemical vapor depo-sition and transferred on SiO2 substrate. Subsequently, an aluminum coating with a thickness of 4-20 nm was produced by an e-beam evaporator with a target of 99.99% pure aluminum. We acquired the transmittance spectra of graphene/aluminum hetero-films using a UV-vis-NIR spectrophotometer. One interesting obser-vation is that transmittance increased in samples with graphene, indicating a novel physical or chemical inter-action between graphene and aluminum. For 4-nm Al film, graphene induced transparency enhancement at UV range of 200 to 300 nm. As film thickness increas-es to 8 nm, the transparency enhancement extends to a wider UV range of 10-660 nm. In a 12-nm sample, we observe an averaged 12% increase in transmittance for the wavelength range of 500-2500 nm in the sample with graphene, compared with a pure Al coating on the substrate. More surprisingly, similar transparency en-hancement is captured when Al film was deposited on the graphene film. Due to the counterintuitive observa-tion, we anticipate this work will benefit the communi-ty in fundamental understanding and reliable utiliza-tion of graphene and Al interactions.

Decomposed Representation of S-Parameters for Analysis of Silicon Photonic Variation

Silicon photonics offers great potential for monolithic integrated photonic and electronic components using existing integrated circuit (IC) fabrication infrastructure. However, methods to analyze the impact of IC process variations on performance of photonic components remain limited. Statistical models based on either simulations or experiments that quantify the effect of these variations are necessary to achieve high-yield manufacturing. To cope with the non-linearity in the S-parameters of photonic device components and circuits, non-linear parameter fitting is often used prior to statistical modeling, e.g., rational polynomial fitting of ring resonator responses. The traditional approach treats the amplitude and phase of the S-parameters separately in the fitting process; however, this method can be problematic when the behavior of the S-parameters becomes complicated under the variations since it neglects the strong correlation between amplitude and phase. For example, the seemingly complicated spike in group delay shown in Figure 1 is actually where a smooth S-parameter accidentally crosses the origin point.We present a novel representation of S-parameters that decomposes the complex-numbered S-parameters into several components, each having a simple response that does not require non-linear parameter fitting and that supports subsequent statistical analysis. We apply the proposed S-parameter decomposition method to Y-splitters with imposed line edge roughness (LER) variations. In contrast to the difficulty of the traditional amplitude-phase representation, the decomposed representation shows improvement in statistical modeling of variation ensembles, e.g., using principle component analysis (PCA) (Figure 2).The method can be extended to other photonic components and circuits with other process variations to help quantify the effect of process variations for statistical analysis and to help designers predict and optimize photonic component performance and yield.

DC-DC Converter Implementations Based on Piezoelectric Resonators

Power electronics play a vital role in the technologi-cal advancement of transportation, energy systems, manufacturing, healthcare, information technology, and many other major industries. Demand for power electronics with smaller volume, lighter weight, and lower cost often motivates designs that better utilize a converter's energy storage components, particularly magnetics. However, the achievable power densities of magnetic components inherently reduce as volume decreases, so further progress in converter miniaturiza-tion will eventually require new energy storage mech-anisms with fundamentally higher energy density and efficiency capabilities. This prompts investigation into piezoelectric en-ergy storage for power conversion; piezoelectrics have comparatively superior volume scaling properties. While piezoelectrics have been used extensively for sensing, actuation, transduction, and energy harvest-ing applications, their adoption in power conversion has been more limited. Converter designs based on single-port piezoelectric resonators (PRs) report limit-ed power and/or performance capability, but without investigation into the full realm of possible converter implementations.In this work, we conduct a systematic enumeration and downselection of practical dc-dc converter switch-ing sequences and topologies that best leverage PRs as their only energy storage components. In particular, we focus on switching sequences that facilitate high-effi-ciency behaviors (e.g., low-loss resonant charging/dis-charging of the PR’s input capacitance and all-positive instantaneous power transfer) with voltage regulation capability. To analyze and compare implementations, we demonstrate methods for mapping PR state trajec-tories across a switching cycle, imposing practical con-straints on PR behavior, evaluating PR utilization, and estimating PR efficiency. Effective use of the PR's resonant cycle enables these converter implementations to achieve strong ex-perimental performance with peak efficiencies >99%, even with presently commercially-available PRs. This suggests that these PR-based converters are promis-ing alternatives to those based on traditional energy storage. With further development, PR-based convert-ers may pave the way for high-performance converter miniaturization in applications spanning consumer electronics, biomedical implants, and flight.

High Capacity CMOS-compatible Thin Film Batteries

The miniaturization of sensors through advancements in low-powered MEMS devices in integrated circuits has opened up new opportunities for thin film microbatteries. However, many of the available thin film battery materials require a high-temperature process that necessitates additional packaging volume, which reduces the overall energy density of these batteries. Previous research with collaborators in Singapore demonstrated an all-solid-state materials system with high volumetric capacity that exclusively utilizes CMOS-compatible (i.e., room temperature) processes. This process allows integration of these batteries directly onto CMOS circuits, thereby achieving energy densities comparable to bulk batteries for applications in distributed power supplies and integrated autonomous microsystems (Figure 1). Additionally, the ability to deposit all components of the battery at room temperature makes it possible to fabricate these batteries on thin, flexible substrates that can be densely stacked to achieve a wide range of capacities without sacrificing their high energy density.We have successfully demonstrated a full thin film microbattery using Ge and RuO2 as anode and cathode materials, respectively, with LiPON as the solid-state electrolyte (Figure 2b). Although RuO2 has traditionally been used as an anode material, it has significantly higher volumetric capacity than typical cathode materials and sufficiently high electrochemical potential versus Ge to provide an output voltage of ~ 0.5V at a capacity of ~40 Ah/cm3 (Figure 2a).

State Estimation, Parameter Inference, and Observability Analysis of Electrical Distribution Networks

In modern electrical power systems, distribution networks facilitate the final step of power delivery to homes and businesses. Distributed energy resources (DERs) such as Tesla powerwalls and rooftop PV systems, automated sensing devices equipped with telemetry capabilities such as micro-Phasor Measurement Units (μ-PMUs) and smart meters, and active loads, which are capable of responding to real-time pricing signals, all significantly disrupt the standard operating procedures of distribution networks. One of the primary roadblocks to successful operation and control of these systems is the lack of network observability. Due to the significant cost and effort associated with sensor deployment in ultra-large distribution networks, system operators must alternatively leverage the physical model of the network and various measurement sets to reconstruct the so-called “state” (i.e., voltage equilibrium) of the network. State estimation, therefore, is a vitally important tool for distribution system operators. Because network parameter values span many orders of magnitude and sensors are critically under-deployed, the traditional state estimation problem is severely ill-conditioned and is seldom deployed in the field.Standard DSSE techniques rely on strong, yet potentially unjustified, regularization to combat the ill-conditioning of the problem. In this project, we represent the operation of a distribution system as a sequence of nonlinear maps that relate measurements, states, controller decisions, and operational performance. Using advanced uncertainty quantification techniques, we quantify the subspace of input perturbations whose response is practically “unobservable” at the output of each nonlinear map. These sensitivity results (which must be regathered each time state estimation is employed) guide the selection of appropriate regularization methods whose application can be probabilistically justified. We therefore carefully apply varying degrees of statistical regularization, such as Bayesian priors, and physics-based regularization to solve the state estimation problem. Further uncertainty quantification not only gauges the quality of the result, but also suggests optimal field testing and optimal placement of future deployed sensors to system operators. Despite regularization, the Hessian used to iteratively solve the state estimation problem can still exhibit severe numerical ill-conditioning. To overcome this numerical ill-conditioning, we are developing a set of computationally efficient and numerically robust methods to invert the Gauss-Network “gain” matrix. This solution utilizes a semi-explicit LU decomposition in conjunction with a matrix series expansion (i.e., Neuman expansion) and sequential applications of the so-called Woodbury matrix identity. Homotopy methods are used to scale the measurement variances and line lengths to decrease the number of iterations needed to converge on a final solution.

Maximizing the External Radiative Efficiency of Hybrid Perovskite Solar Cells

Despite rapid advancements in power conversion efficiency (PCE)in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. We review the historical progress in enhancing perovskite’s external radiative efficiency (ERE) and determine key strategies for reaching high optoelectronic quality. Specifically, we focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. If defects are strategically targeted, it is likely that the next set of record-performing devices with ultra-low voltage losses will be achieved.

Blade Coating of Perovskite Solar Cells Toward Roll-to-roll Manufacturing

High efficiency combined with transformative roll-to-roll (R2R) printability makes metal halide perovskite-based solar cells the most promising solar technology to address the terawatt challenge of the future energy demand. However, translation from lab-scale deposition solution processing techniques, such as spin coating, to large-scale R2R compatible methods has been a significant challenge due to fundamental differences in coating fluid dynamics and resulting drying and crystallization processes with the different coating methods. Here we address this challenge by developing processes and device architectures with high-speed (> m min-1) blade-coating (Figure 1A), which is R2R manufacturing compatible. We constructed solar cells with structure of Glass/FTO/SnO2/FA0.8MA0.16Cs0.04PbBr0.16I0.84/Spiro-MeOTAD/MoOX/Ag (Figure 1B), where the SnO2 is blade-coated at an environment of 49% relative humidity and with overall device thickness of less than 1 μm, excluding the glass substrate. We demonstrated a light-to-electricity conversion efficiency up to 17%, with open-circuit voltage of 1.112 V, short-circuit current of 22.12 mA cm-2, and fill factor of 69.1% (Figure 1C). The application of blade-coating of SnO2 has been a first step to show the potential of scaling highly efficient perovskite solar cells with transformative R2R compatible manufacturing techniques.

Solid State Batteries: Interfacial Degradation Between Solid Electrolyte and Oxide Cathodes

All-solid-state batteries (SSBs) promise safer and higher performance energy storage than the present liquid-electrolyte Li-ion batteries. Li7La3Zr2O7 and Li1+xAlxTi2−x(PO4)3 are promising solid electrolytes for Li-ion SSBs. The wide electrochemical window of Li7La3Zr2O7 enables usage of a Li metal anode and high-voltage oxide cathodes. This combination makes Li7La3Zr2O7 a promising candidate for a high-capacity battery cell. Li1.4Al0.4Ti1.6(PO4)3 has a high stability win-dow and excellent chemical stability against moisture, enabling large-scale production with minimal cost. However, the development of SSBs in both systems is hindered mainly due to the high cathode |electrolyte interfacial resistance, which impedes the Li-ion trans-fer and ultimately the durability and power density. Sintering, which is necessary to get good contact be-tween a cathode and an electrolyte, leads to the forma-tion of detrimental phases that are insulting for Li-ion transfer. Despite the importance of the issue, only lim-ited understanding of the interfacial chemistry exists so far. The lack of research comes from the challenges in investigating buried interfaces.We aim to advance the understanding and control over the stability of the cathode|electrolyte interfaces. We use model systems made of thin-film cathode layers on dense electrolyte pellets (Figure 1). This approach enables us to use surface-sensitive and non-destruc-tive techniques such as X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorp-tion fine structure (EXAFS) to study buried interfaces. Our findings show that interfacial degradation is high-ly dependent on the gas environment used in the sin-tering process (Figure 2). Annealing in O2 environment does not lead to formation of a detrimental phase at the interface. In contrast, annealing in air or in CO2 led to severe degradation. We attribute this to the forma-tion of Li2CO3 and delithiated phases at the interface. The findings from this project will lead to identifying suitable process parameters to develop a stable cath-ode-electrolyte interface with good electrochemical properties.

Techno-economic Assessment and Deployment Strategies for Vertically-mounted Photovoltaic Panels

Conventional schemes of panel mounting require horizontal space, on the order of 20,000 to 40,000 m2 per megawatt peak (MWp), prompting us to investigate new strategies for deploying solar panels. Mounting solar photovoltaic (PV) panels vertically to the sides of existing structures, such as facades of buildings, offers one such strategy. Vertically-mounted PVs take advantage of otherwise unused vertical real estate in the built environment, with minimal additional structural reinforcement costs and no need for additional land area use. Uniquely, the peak electricity generation time of west-facing vertically-mounted PV panels occurs closer to the hour of maximum consumer power demand, allowing increased electricity generation when the same PV panels, if conventionally mounted, would generate lower amounts of power.Keeping these advantages in mind, we identified a set of potential profitable markets in the United States (U.S.) and enumerated the technical challenges to expanding PV usage into these markets. We calculated the levelized cost of electricity (LCOE) for vertically-mounted PVs as a function of the azimuth panel; then using county-level resolution we estimated economic viability for these installations in the contiguous U.S. The LCOE calculations allow us to identify target specifications for vertical PV panels to be economically competitive when compared to the commercial grid electricity. We show that lightweight, flexible and bifacial form factors attainable with the next-generation PV technology can lead to installation cost reductions. We are developing roof-of-concept prototypes to validate our hypothesized deployment strategies.