Datasets:

jgammack

/

MTL-abstracts

like 0

A Versatile MEMS Quadrupole Platform for Portable Mass Spectrometry Using the First and Second Stability Regions

The Micro Gas Analyzer Program aims to develop portable, low-power, fast and low-false-alarm-rate gas analyzer technology for a wide range of applications. One of the subsystems of the gas analyzer is a mass filter. An array of micro-fabricated quadrupole mass filters is being developed for this purpose. The quadrupoles will sort out the ions based on their specific charges. Both high sensitivity and high resolution are needed over a wide range of ion masses, from 15 to 650 amu. In order to achieve this perfor-mance, multiple micro-fabricated quadrupoles, each operating at a specific stability region and mass range, are operated in paral-lel.The proof-of-concept device is a single, linear quadrupole that has a micro-fabricated mounting head with meso-scaled DRIE-pat-terned springs. The mounting head allows micron-precision hand assembly of the quadrupole rods [1] –critical for good resolution and ion transmission. The micro-fabricated mounting head can implement quadrupoles with a wide range of aspect ratios for a given electrode diameter. The springs can be individually actu-ated using spring tip handlers. The current version of the spring-head is able to interact with rods with diameters from 1588 µm down to 250 µm. The quadrupoles that have been implemented thus far span the aspect ratio range from 30 to 60. The choice of electrode diameter takes into account the dimensional uncer-tainties and alignment capabilities with respect to the expected resolution and transmission goals. Figure 1 shows an assembled MEMS quadrupole with 250-micrometer diameter rods. Figure 2 shows the experimental data of one of these quadrupoles us-ing FC-43 as a calibration compound, where a mass resolution of 2 amu and a full mass range of 650 amu are demonstrated, while using a 1.44 MHz RF power supply to drive the quadrupole with a constant-width circuit made by the Extrel company (Pitts-burgh, PA). To obtain better resolution, the MEMS quadrupoles have been driven with up to 4 MHz RF sources, resulting in 0.7 amu peak width. Also, the devices have been driven in the second stability regions to obtain 0.4 amu of peak width and smoother peaks. Current research efforts concentrate on developing RF power supplies of higher frequency and further exploration of the second stability region to obtain better performance.

First Principles Optimization of Mass-producible Microscaled Linear Quadrupoles for Operation in Higher Stability Regions

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. Various attempts at making microscaled linear quadrupoles met with varying degrees of success [1-2]. Producing these devices involved some com-bination of precision machining or microfabrication and down-stream 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 comprising a planar design and a rectangular electrode geometry is proposed. Quadrupole resolution is inversely-proportional to the square of the electrode length, thus favoring a planar design since electrodes can be made quite long. Rectangular rods are considered since that is the most amenable geometric shape for planar microfabri-cation. This deviation from the conventional round rod geometry calls for optimization and analysis. Electrode designs were pa-rameterized, and the potential fields were solved using Maxwell 3D (Figure 1). The fields were decomposed using a multipole ex-pansion to examine the higher-order coefficients (Figure 2). This process was used to minimize the significant high-order terms, thus optimizing the design and determining the ultimate limita-tions of the device.Higher-order field contributions arising from geometric non-ide-alities lead to non-linear resonances. These resonances manifest as peak splitting that is typically observed in quadrupole mass spectra. Reported work involving linear quadrupoles operated in the second stability region show improved peak shape without these splits [3]. It is believed that operating the device in the second stability region will provide a means to overcome the non-linear resonances introduced by the square electrode geometry. This study was conducted to justify a fully microfabricated, mass-producible, MEMS linear quadrupole mass filter. Successful im-plementation of such devices will lead into arrayed configurations for parallel analysis and aligned quadrupoles operated in tandem for enhanced resolution.

A Single-gated Open Architecture carbon nanotube Array for Efficient Field Ionization

Mass spectrometers require a suitable ionizer to be able to discern the chemical composition of the sample that they are analyzing. Traditional ionizers for gases use either chemical ionization (CI) or electron impact ionization (EI). In the latter case, electrons from thermionic sources produce ions by colliding with neutral molecules. More efficient carbon nanotube-based field emitted electron impact ionizers have been developed [1]. However, one of the drawbacks of electron impact ionization is that the sam-ple is transformed into fragmentation products. Several samples could have similar fragmentation spectra but be quite different compounds, with radically different properties (for example, one substance can be a poisonous agent while another is a harmless material). Therefore, an approach to reduce the fragmentation products would improve the informational power of the mass spectrometer.Field ionization soft-ionizes molecules, thus reducing the frag-mentation products. In the field ionization scheme, ions are cre-ated by directly tunneling electrons from the outer shell of neutral molecules by virtue of a very high electric field [2]. The electric field is produced by high aspect ratio field enhancers and the ap-plication of a large (up to 1 kV) bias voltage. Carbon nanotubes are ideal field enhancers because of their high aspect ratio and their reduced tip radius. A good field ionizer should work in the field-limited regime instead of the molecular flux-limited regime, where all the molecules that approach the high field region are thus ionized. In the case of the electron impact ionizers, a closed architecture is implemented because it is intended to protect the field enhancers from back streaming ions [3]. Therefore, an open architecture, where the field enhancers surround a through-hole, is a more suitable approach to produce field ionization. We plan to implement a single-gated field ionizer array with an open ar-chitecture. Figure 1 shows a schematic of the open architecture concept. Figure 2 shows a cross section of the device. Current research effort focuses on device characterization.

A Fully Micro-fabricated Planar Array of Electrospray Emitters for Space-propulsion Applications

Electrospray thrusters work by extracting ions or charged droplets di-rectly from a liquid surface using an electrostatic field and accelerating them in that field to produce thrust [1]. This method could lead to more efficient and precise thrusters for space propulsion applications. Emission occurs from sharp emitter tips, which enhance the electric field and constrain the emission location. The electrospray process limits the thrust from a single tip. To get into the millinewton range will require an array with tens of thousands of emitters. Batch micro-fabrication is well suited to making this array.We have designed, built, and tested a thruster made in silicon using deep reactive ion etching (DRIE) and wafer-bonding technology (see Figure 1). This thruster comprises two components. The emitter die has up to 517 emitters in a 0.75 cm2 area, formed using DRIE and SF6 etching, and is plasma treated so that liquid can be transported to the tips in a porous black silicon surface layer. The extractor die incor-porates the extractor electrode, a Pyrex layer for insulation, and the springs, which are used to reversibly clamp the emitter die [2]. This versatile assembly method allows the extractor die to be reused with multiple emitter dies and potentially with emitter concepts radically different from the one we have experimented with.Figure 2 shows data collected when firing the thruster with the ionic liquid EMI-BF4. Measurable emissions occurred for extraction volt-ages down to 700 V. The current collected on the extractor electrode was less than 3% of the emitted current over a wide operating range and often less than 0.1 %. Beam-divergence half-angles were between 15 and 30 degrees, depending on the operating conditions. Emitted currents of 500 nA/emitter were observed in stable operation, for ex-pected thrusts of 25 nN/emitter. Time-of-flight measurements prove operation in the ion emission regime, which is most efficient for pro-pulsion.

Carbon Nanotube Electron Sources for Space Propulsion Applications

Low-power, low-voltage, efficient field emission neutralizers for FEEPs [1], colloid thrusters [2], and other micro-propul-sion engines are attractive for nanosatellites because they do not use mass flowrate to operate, unlike more conventional neutralizing solutions such as hollow cathodes [3]. Electrons are field-emitted from the surface of metals and semicon-ductors by the application of a high electrostatic field. Field emitters use high aspect ratio structures to generate very high fields even when low voltages are applied. The ideal field en-hancing structure is a rounded whisker [4]. Micro-engineered field emission neutralizers would have smaller starting volt-ages, better area usage, and more uniform I–V characteristics, compared to macro/meso fabricated field emitter versions. Plasma-Enhanced Chemical Vapor Deposited (PECVD) Car-bon Nanotubes (CNTs) are rounded whiskers with 100 nm or less of tip radius and 13 µm or more tall. The adoption of CNTs as electron-emitting substrate has recently being shown to have advantages compared to Spindt emitters because of the higher aspect ratio of CNTs and their superior resistance to harsh environments. This research focuses on the devel-opment of a batch-fabricated MEMS neutralizer that uses PECVD CNTs as field enhancers (Figure 1). As a reference, a previously made Busek-MIT MEMS CNT device that uses a randomly oriented CNT matrix produced by Busek Co. (Natick MA) with a proprietary arc-based process yielded de-vices with Fowler-Nordheim emission, startup voltage as low as 100 V, and electron currents as large as 3.2 mA/cm2 with about 20% of gate current interception.

Carbon Nanotubes for Electrospray Nanofluidic Applications

Electrospray is the technique to soft-ionize liquids by applying a high electric potential to a liquid meniscus. The liquid menis-cus is deformed into a cone [1], and charged species are emit-ted from its apex. The emission can be solvated ions, charged droplets, or a mix of the two. This low-divergence charged species source can be used in diverse applications such as mass spectrometry, propulsion, printing, and etching. Our research group has successfully developed several multiplexed MEMS electrospray sources, mainly intended for space propulsion ap-plications. These devices include internal pressure-fed spouts that emit charged droplets [2] and externally surface tension fed spouts that emit solvated ions [3]. In all cases, the emit-ter field enhancers and the hydraulic impedance are provided using silicon-based structures. Furthermore, the devices use a 3D packaging technology that allows decoupling the process flows of the subsystems without loss in emitter density [4]. Consequently, it is possible to use radically different fabrica-tion techniques and materials to implement MEMS electro-spray arrays. This project intends to investigate the application of Plasma Enhanced Chemical Vapor Deposition Carbon Nanotubes (PECVD CNTs) in multiplexed electrospray sources. Two research directions are currently pursued: the use of CNTs as hydraulic impedance to ballast the emitter array (both in internal and external architectures) and the use of CNTs as emitter field enhancers. On the one hand, PECVD CNT for-ests can be custom tailored to match a desired morphology. On the other hand, PECVD CNTs have remarkable field en-hancing properties. Figure 1 shows a silicon-based externally fed electrospray linear emitter array that uses PECVD CNTs as hydraulic impedance, while Figure 2 shows the PECVD CNT forest grown on top of the silicon structures, using our group’s reactor. Current research is focused on exploring the wettability of CNT forests using different liquids, catalysts, and growth conditions. These results will be used to choose the proper nanostructure to be used in an externally fed MEMS electrospray head that will eventually include CNT-based field enhancers.

A High-density Electron Source that Uses Un-gated Transistors for Ballasting

Electrons are field emitted from the surface of metals and semiconductors when the potential barrier (work function) that holds electrons within the metal or semiconductor is deformed by the application of a high electrostatic field. Field emitters use high aspect ratio structures with tips that have nanometer dimensions to produce a high electrostatic field with a low ap-plied voltage. We are implementing two types of field enhanc-ers: carbon nanofibers (CNFs) and silicon conical tips (Figure 1). Spatial variation of tip radius results in the spatial variation of the emission currents and non-uniform turn-on voltages. Small changes in the tip radius result in huge changes in the current density because of the exponential dependence of the emitted current on the bias voltage, as described by the Fowler-Nordheim theory. If the emitters are ballasted, the spatial non-uniformity can then be substantially decreased. Furthermore, ballasting individual emitters prevents destructive emission from the sharper tips allowing higher overall current emission because of the inclusion of duller tips. Ballasting also results in more reliable operation. The use of large resistors in series with the field emitters is an unattractive ballasting approach because of the resulting low emission currents and power dis-sipation in the resistors. A better approach for ballasting field emitters is the use of un-gated field effect transistors that ef-fectively provide high dynamic resistance with large saturation currents. In the past our research group demonstrated the use of a MOSFET to ballast the emission of electrons from silicon tips [1]. We plan to implement vertical un-gated transistors in series to the field emitters to obtain spatial uniformity in the current emission and I-V characteristics of the array [2]. The ballast structure is an n-doped, single-crystal silicon column, patterned using Deep Reactive Etching, and thinned using wet oxidation. Figure 2 shows a cross section of the un-gated transistors consisting of a 1-million elements in 1 cm2. The field emitters are formed on top of the columns. Current ef-forts focus on device testing.

Nanoelectromechanical Switches and Memories

The ability to change shape is a compelling attraction of molecu-lar semiconductors. Compared to rigid inorganic materials, mol-ecules are soft and malleable, and their conformational changes are essential to the functionality of biological systems. Applica-tions of nano-electro-mechanical (NEM) molecular devices in-clude memories and transistors: Information can be stored in the conformation of molecules, potentially leading to very high den-sity memories, and molecular transistors that change shape under bias could exhibit sub-threshold slopes of << 60 mV/decade [1]. Indeed, as an example of the potential of NEMs, voltage-gated ion channels possess sub-threshold slopes of approximately 15 mV/decade [2].Although many materials are available for NEM applications, carbon nanotubes exhibit low resistance and good mechanical properties. In this project, we are constructing an NEM testbed. The proposed design for our relay is shown in Figure 1. Nano-tubes are directly grown at the bottom of an electron-beam de-fined trench etched in Si. Leaving tube growth to the final step gives us better control of the nanotube and removes the need for additional steps that are required for the removal of surfac-tants and organics from the surface of the nanotubes. Because the nanotubes are vertically oriented, we are able to take advantage of the smallest size feature of the carbon nanotube, its diameter, which enables us to create dense arrays of relays for applications such as memory or logic devices. The vertical orientation allows NEM structures with very large aspect ratios. Theoretical results [3] have shown that increasing the aspect ratio of a carbon nano-tube reduces the voltage needed to pull in the nanotube, thereby reducing the power consumption. Furthermore, because of the ability to easily functionalize the surface of nanotubes, we can functionalize the tube with multiple charges to lower the pull-in voltage even further.

Exciton Coupled Surface Plasmon Resonance Biosensor

The development of portable and cost-effective biological sen-sors promises benefits to medical care, pharmaceutical testing, and the detection of biological warfare agents. Electronic devices are compact and readily integrated into microfluidic substrates, making them a promising alternative to today’s bio-detection requirements. Our sensor design aims to exploit the sensitivity of surface plasmon resonance (SPR). Unlike conventional SPR sensors, the plasmon is detected in the near field using a thin film organic photovoltaic (PV). High absorption coefficients make organic semiconductors ideal candidates for the detection of surface plasmons. Organic materials are also easily deposited on microfluidics, enabling use of these devices outside the laboratory environment in a convenient portable package.In the initial demonstration, plasmon modes are excited in the top surface of the gold cathode by a p-polarized laser beam, when the horizontal component of the light wavevector matches the plasmon wavevector [1]. The plasmon is absorbed by the organic semiconductor and split into holes and electrons at the interface between the donor and acceptor layers composing the PV cell (Figure 1). The plasmon resonance measured indirectly as reflect-ed power and photocurrent (Figure 2) has a strong angular loca-tion dependence on the adjacent layer’s dielectric constant that is altered upon binding of bio-molecular species. The steep slope of the resonance enables sensitive detection as well as measure-ment of kinetic parameters of the binding event.

Micromechanical Substrates for Reconfigurable cell culture

We have previously demonstrated the use of microfabricated cell culture substrates (Figure 1) to implement reconfigurable cell cul-ture (Figure 2) [1]. Specifically, we studied interactions between liver hepatocytes and supportive stromal cells. We found that preservation of liver-specific function depended on signaling from the stroma. Specifically, signaling both through direct contact and through diffusible secreted factors was important. However, while the secreted factors needed to be maintained for the entire dura-tion of culture (2 weeks), direct contact was required only for an 18-hour period early in culture. In addition, the secreted factors were found to have a limited effective range of less than 400 µm. Through FEM diffusion modeling, we showed that a half-life on the order of hours would result in such short-range signaling.Currently, we are exploring the use of this platform in a variety of applications including identification of the signaling factors in hepatocyte co-culture, stabilizing liver endothelial cells in culture, toxicity models for drug testing, preconditioning of hepatocytes prior to encapsulation in a 3D gel, and patterning cells directly on the combs to study contact signaling mechanisms.

Aligned Multimask Patterning of Biomolecules and cells

Surface engineering of cell culture substrates has developed into a powerful tool for controlling multicellular organization at the micrometer scale. This new capability has brought valuable in-sight into the biological mechanisms by which the cellular micro-environment determines cell fate and function. However, studies requiring more complex tissue structures have been hindered by limitations in surface patterning. Typically, molecules that medi-ate cell attachment are patterned against a non-adhesive back-ground, allowing arrays of a single cell type to be formed with control of cell positioning and relative spacing. Alternatively, patterns composed of two different adhesive regions can be em-ployed to form patterned co-cultures of two different cell types, as long as one cell type selectively attaches to a specific region. How-ever, there have been a few examples where multiple attachment chemistries have been successfully combined with non-adhesive surfaces in a multicomponent pattern. This has prevented the re-alization of configurations in which cell-cell contact and spacing between different cell types are controlled.The use of photolithography with multiple aligned masks is well established for generalized multicomponent patterning, but it is often too harsh for biomolecules. We report a two-mask photo-lithographic process that is tuned to preserve bioactivity in pat-terns composed of covalently coupled polyethylene glycol (PEG), adsorbed extracellular matrix protein (e.g., collagen I), and ad-sorbed serum proteins (e.g., vitronectin). Thereby, we pattern two cell types—primary hepatocytes and 3T3 fibroblasts—demon-strating control over contact and spacing (20-200 µm) between the two cell types for over one week. This method is applicable to the study of intercellular communication in cell biology and tissue engineering.

Collective Hydrodynamics and Kinetics of Sickle Cell Vaso-occlusion and Rescue in a Microfluidic Device

The pathophysiology of sickle cell disease, the first to be implicat-ed with a genetic origin, is complicated by the multi-scale nature of the processes that link the molecular genotype to the organis-mal phenotype. Here, we show that it is possible to evoke, control and inhibit the vaso-occlusive crisis event in sickle cell disease us-ing an artificial microfluidic environment. We use a combination of geometric, physical, chemical and biological means to quantify the phase space for the onset of a jamming crisis, as well as its dis-solution, as shown in Figure 1.The microfluidic chip designed to independently vary the various parameters that control the onset of vaso-occlusion in a sickle cell crisis is shown in Figure 2. This device allows us to dissect and probe the hierarchical dynamics of this multi-scale process by manipulating the geometrical, physical, chemical and biologi-cal determinants of the process. The chip consists of a series of bifurcating channels of varying diameters that grossly mimics the geometry of vasculature. By controlling the physical pressure gradient across the chip, we can vary the kinetic time scale for transit of red blood cells. The channels are separated from a gas reservoir by a thin gas-permeable polydimethylsiloxane (PDMS) membrane. As the geometries are microscopic, gas diffusion is rapid and the oxygen concentration in the microchannels is gov-erned by the concentration in the gas reservoir. By changing the mixture of this reservoir, we control oxygen concentrations in the channels and hence the onset of microscopic hemoglobin polym-erization. By using blood with varying concentrations of HbS and different hematocrits, we can mimic the variability among individuals. This device was used to study the phase space of jamming governed by pressure, channel dimensions and oxygen concentration as shown in Figure 1. Our experimental study integrates the dynamics of collective processes at the molecu-lar, polymer, cellular and multi-cellular level; lays the foundation for a quantitative understanding of the rate limiting processes; provides a potential tool for optimizing and individualizing treat-ment; and serves as a bench test for dynamical drugs.

Plasma-activated Inter-layer Bonding of Thermoplastics for Micro- and nano-fluidic Manufacturing

Plasma-activated polymer–polymer bonding is a promising way of encapsulating micro- and nano-fluidic channels across large substrate areas, without the substantial distortion of channel ge-ometries that can plague thermally- and solvent-assisted bond-ing. The process involves treating the surfaces to be bonded with an oxygen or air plasma, and then pressing the surfaces together to allow an irreversible chemical bond to form [1]. A conve-nient method is desired for measuring the toughness of such a bonded interface. Simple crack-opening tests (whereby a blade prizes apart the two bonded layers and the length of the inter-layer crack determines the bond toughness [2]) are clumsy and hard to automate. We propose that built-in microscopic crack-opening test sites be distributed across manufactured substrates [3]. At each test site, a polymeric film bonded over a step in the substrate would peel back from the step after bonding, by a dis-tance depending on the toughness of the bond. The presence of a wedge-shaped air gap between the covering film and the substrate leads to visible interference fringes, the spacing of which can be used to extract the bond strength (Figure 1). Arrays of these in situ cracks might be imaged without removing the substrate from a production line and would allow us to monitor both substrate-to-substrate and cross-substrate bond toughness variation. Bond toughness and polymer layers’ surface energies are of par-ticular relevance in planning the fabrication of very shallow flu-idic channels whose widths, w, are much larger than their depths, h. The risk of channels’ collapsing during fabrication must be controlled. For channels with h ~ 1 µm or less that are fabricated with thermoplastics, we expect collapsing to occur through lo-cal deformation of the surrounding material rather than through plate-like bending of the cover plate [4]. Our analysis suggests that the pressure applied during bonding, together with the polymer–polymer interface energies that exist before and after plasma-activated bonding, will delineate, on a w/h against h plot, regions in which collapsing will and will not occur. We have dem-onstrated nanochannels fabricated from polymethylmethacrylate (PMMA) that are 80 nm deep and 10 µm wide and other chan-nels that are 110 nm deep and 20 µm wide (Figure 2).

Environmentally Benign Manufacturing of Three-dimensional Integrated circuits

Along with scaling down in size, novel materials have been in-troduced into the semiconductor industry to enable continued improvements in performance and cost as predicted by Moore’s law. It has become important now more than ever to include an environmental impact evaluation of future technologies, before they are introduced into manufacturing, in order to identify po-tentially environmentally harmful materials or processes and un-derstand their implications, costs, and mitigation requirements. In this project we introduce a methodology to compare alterna-tive options on the environmental axis, along with the cost and performance axes, in order to create environmentally aware and benign technologies. This methodology also helps to identify po-tential performance and cost issues in novel technologies by tak-ing a transparent and bottoms-up assessment approach. This methodology is applied to the evaluation of the MIT 3D IC technology in comparison to a standard CMOS 2D IC approach. Both options are compared on all three axes–performance, cost, and environmental impact. The “handle wafer” unit process in the existing 3D IC technology, which is a crucial process for back-to-face integration, is found to have a large environmental impact because of its use of thick metal sacrificial layers and high-en-ergy consumption. We explore three different handle wafer op-tions: between-die channel, oxide release layer, and alternative low-temperature permanent bonding. The first two approaches use a chemical handle wafer-release mechanism while the third explores solid liquid inter-diffusion (SLID) bonding using cop-per-indium at 200°C. Preliminary results for copper-indium bonding indicate that a sub-micron thick multi-layer copper-in-dium stack, when bonded to a 300-nm-thick copper film, results in large voids in the bonding interface primarily due to rough as-deposited films. Finally, we conduct an overall assessment of these and other proposed handle wafer technologies. The overall assessment shows that none but the oxide release layer approach appears promising; however, each process option has its strengths and weaknesses, which need to be understood and pursued ac-cordingly.

An Implantable MEMS Drug-delivery Device

A novel drug-delivery system based on MEMS technology is be-ing developed. This implantable microchip is capable of deliv-ering vasopressin, a known vasoconstrictor that can prevent or delay death by hemorrhagic shock [1]. The device is specially tailored to treat hemorrhagic shock in ambulatory settings and is intended for in vivo use as a micro-implant in the peritoneum for people in high-risk situations.The device has a modular design and is composed of three layers (shown schematically in Figure 1): a large reservoir layer, where the drug solution is stored; a membrane layer from where the drug is ejected; and a bubble-generating layer, where bubbles are formed. The reservoir layer is defined by drilling through a Pyrex 7740 wafer with a diamond bit. Wafer thickness and hole diameter can be modified to change reservoir capacity. The membrane layer is composed of silicon nitride membranes cover-ing through-holes etched by DRIE into a silicon substrate. Thin gold fuses can be patterned on the membranes to detect ruptures, which then shows as an open circuit. The bubble-generating layer is defined by micro-resistors, which can quickly and locally heat the contained fluid to generate bubbles. The pressure exerted by these bubbles causes rupture of the silicon nitride membranes and forces the contained solution out of the device.In vitro operation of the device has been demonstrated, as shown in Figure 2. Further developments of this device include reduc-tion of power consumption during activation, wireless activation, and adaptation of the device for a pen-size, transdermal delivery system. We believe that the ramifications of this MEMS-based drug delivery system can be useful for a vast number of medical applications.

High Speed Three-dimensional Scanner for in vivo non-invasive Optical Biopsy using Two-photon Microscopy

We have recently demonstrated the modeling, design, and micro-fabrication process of a millimeter-scale, high-speed endoscopic scanner that is to be integrated at the distal end of an endomicro-scope [1]. The scanner system consists of (1) an active Silicon op-tical bench (SOB), as shown in Figure 1, which constrains, aligns, and thermally actuates (1) mm-size optics (GRIN lens and prism) at 5 Hz and (2) a slim fiber resonator that excites the double-clad photonic bandgap fiber at ~1 kHz. The scanner system has a 7-millimeter device envelope with a range of 100 micrometers in X, Y and Z. The design of a two-photon endoscope requires scanning of focused light to create tissue images, and scanning actuator technology still proves to be a bottleneck for practical endoscope design. The performance (force-speed-stroke) criteria for the prototype endomicroscope design are generated based on clinical needs. The strict force, speed, and stroke requirements (~10 mN, 1 kHz, and 100 µm) call for a new method for ac-tuation. The low voltage requirement for future in vivo examina-tion/operation makes a new class of thermomechanical actua-tors (TMAs) a suitable candidate among other micro-actuation technologies.The two-photon imaging technique requires scanning of focused light to create tissue images. The endoscopic scanner may en-able the design and construction of a miniaturized two-photon microscopic system to image the surface and sub-surface cells (up to 200 microns depth) of internal tissues with sub-cellular reso-lution. The two-photon endomicroscope is designed to perform non-invasive, in vivo, optical biopsy, which has numerous benefits over excisional biopsy. For example, non-invasive optical screen-ing may decrease the number of excision biopsies required, and optical biopsy can provide more informed selection of excisional biopsy sites, minimizing incorrect diagnosis due to random sam-pling. This is useful for detecting cancer at an early stage among other diseases. The chevron TMAs on the SOB are optimized through the geometric contouring method [2] to provide enhanced force, displacement and reduced power consumption compared to common chevron actuators. This also allows the TMAs to be operated at lower temperature and thus makes the TMAs more suitable for precision actuation. Figure 2 presents an example of a contoured chevron TMA. Early models and experiments of the contour shaping method have confirmed that the maximum achievable thermal strain of a driving beam may be increased by 29%, the actuator stroke may be increased by a factor of 3 or more, and identical force or displacement characteristics may be achieved with 90% reduction in power. A new high-speed pulsing technique has also been investigated recently; it enhances the dynamic performance of the contoured TMAs [3]. Prelimi-nary simulation results indicate a 12% bandwidth increase, 30% stroke enhancement, and 70% power reduction. This technique, together with the geometric contouring method for TMAs, may potentially increase the bandwidth of the endoscopic scanner by a factor of 10 and therefore meet the functional requirements for a two-photon scanning endomicroscope.

Electromagnetically-driven Meso-scale nanopositioners for nano-scale Manufacturing and Measurement

Nanopositioners – be they nano-, micro-, or macro-scale in physi-cal size – enable us to move large or small parts with nanome-ter-level or better precision. They therefore set the limits on our ability to measure, understand, manipulate, and affect physical systems. Six-axis small-scale nanopositioners enable a combina-tion of faster speed and better resolution. They are therefore important in scientific and commercial applications where speed and small-dimensions are important: biological sciences, data storage and nanomanufacturing equipment and instruments [1-4]. Emerging applications in these fields will benefit from por-table, multi-axis nanopositioners that are capable of nanometer-level positioning over tens-of-microns at speeds of hundreds to thousands of Hertz. Toward this end, our work aims to create a meso-scale, high-speed, six-axis nanopositioner.The nanopositioner is designed to operate with a range-of-mo-tion of larger than 10 micrometers in the X-, Y- and Z- direc-tions, possess a natural frequency of 1 kHz, and exhibit better-than-10-nm resolution. The nanopositioning system, shown in Figure 1, is composed of three sets of micro-actuators. Within each set, micro-coils are suspended above a linear array of 1 mm3 permanent magnets via a silicon flexure system [5]. Each actua-tor is composed of two independent coils that apply in-plane and out-of-plane forces to the flexure. The actuator inputs are com-bined to control the stage position in six axes. Figure 2, which shows the actuator’s force output capability vs. coil footprint, was generated using a numerical model. The micro-coils consist of two stacked copper micro-coils that are electrically isolated by a layer of silicon dioxide. They are created by electroplating cop-per within silicon and photoresist molds. The flexures are etched using deep reactive-ion etching. The system will be applied to the high-speed and precise positioning of small parts such as probes and thin-films. The system is scheduled to be integrated into a bench-top scanning-probe microscope and within a nano-electro-discharge machining station [4].

Barcoded Microparticles for Multiplexed Detection

The detection of multiple targets in a single sample is important for many applications, including medical diagnostics, genotyp-ing, and drug discovery. The current approaches to multiplexing, such as planar arrays (such as DNA microarrays) and suspension (particle-based) arrays, require expensive or cumbersome means of encoding, decoding, or functionalizing substrates. Currently, commercially available approaches for multiplexed analysis are cost-prohibitive for high sample throughput, low-cost applica-tions such as bedside diagnostics. We have developed a method [1], based on multifunctional bar-coded particles, for the sensitive and accurate multiplexed detec-tion of biomolecules. Our method is unique in that (1) we can fabricate, encode, and functionalize particles in a single step, (2) the particles are composed of poly(ethylene glycol) hydrogel to increase both sensitivity and specificity, and (3) only a single fluo-rescent wavelength is required to decode the particles and quan-tify the corresponding targets. Using an efficient one-step method based on continuous-flow lithography, we synthesize micropar-ticles with multiple functional regions (Figure 1). Each particle bears a fluorescent dot-pattern barcode (capable of providing over a million unique codes) to identify the target(s) it is looking for and one or more spatially separated regions containing a probe where those targets can bind and be detected via fluorescence. In this way, particles from a library can be mixed and incubated in a single sample to simultaneously detect many targets, such as DNA oligomers (Figure 2). The detection of targets is not only sensitive but also extremely specific due to the porous and bio-inert nature of the hydrogel structure that allows target molecules to diffuse and bind deep into the transparent particle surfaces.

Single-molecule DnA Mapping in a Fluidic Device

The ability to controllably and continuously stretch large DNA molecules in a microfluidic format is important for gene-mapping technologies such as Direct Linear Analysis (DLA). We have recently shown that electric field gradients can be readily generated in a mi-crofluidic device and the resulting field is purely elongational. We have performed a single-molecule fluorescence microscopy analysis of T4 DNA (169 kbp), stretching in the electric field gradients in a hyperbolic contraction microchannel. In addition, we are able to se-lectively pattern a crosslinked gel anywhere inside the microchannel. With an applied electric field, DNA molecules are forced to reptate through the gel and they stretch moderately as they exit the gel. By placing a gel immediately in front of the hyperbolic contraction, we bypass “molecular individualism” and achieve highly uniform and complete stretching of T4 DNA. This device offers a new method to efficiently stretch DNA for single-molecule mapping studies.

DnA Dynamics in nanofluidic Devices

In dilute polymer solutions, the shape, motion, dynamic response, and solvent-interaction (HI) of single polymer molecules change when geometric constraints reach the length scales of the equi-librium polymer conformation. Our study seeks to understand these changes using double-stranded DNA as a model polymer and to utilize these confinement effects to tune the dynamic re-sponse of single molecules. This ability is useful in processes that rely on controlling the conformation of a biomolecule for analysis [1] or in the manipulation of molecules for separations and/or reactionsOur experiments [2-3] use thermally-bonded pyrex channels with heights ranging from 75 to 500 nm and widths of 150 µm. The Brownian motion of stained DNA molecules is observed us-ing epi-fluorescence microscopy. By following the time evolution of the center-of-mass and orientation of single molecules, we can obtain the diffusion coefficient (D) and longest relaxation time (τ1) of the polymer independently. We find that scalings with molecu-lar weight of both D and τ1 agree with a free-draining polymer model, indicating that, in contrast to bulk solution, HI is not im-portant in slit confinement at length scales comparable to the size of the molecule. We find that the relaxation time of the polymer increases with confinement, which promises easier manipulation of DNA conformations. Our results in well-defined nanofluidic devices may also provide insight into polymer behavior in the less-controlled confinement that occurs in concentrated polymer solu-tions. We are currently working to stretch DNA in confinement and to study the effects of confinement far from the equilibrium conformation of the polymer

Microfluidic Bubble Logic

Large-scale microfluidic integration promises to revolutionize the fields of biology and analytical chemistry. The “Lab-on-a-Chip” community has long sought the ability to precisely control very small volumes (nanoliters) of fluid packets . Current mechanisms for fluid routing depend on external control elements with no feedback, limiting scalability and integration. In [1] we describe Bubble logic, an all-fluidic universal logic family implemented in a two-phase microfluidic system. The presence or absence of a drop or a bubble represents a bit of information. Non-linear hydrodynamic interactions of these elements in microfluidic ge-ometries are exploited to build logic gates (AND, NOT), bistable memory (toggle flip-flop), ring oscillators, ripple counters, and synchronizers. This provides an on-chip internal flow control mechanism with all the properties of a digital logic family includ-ing gain, bistability, cascadability, feedback and synchronization. Since no external control elements are required, bubble logic can also find applications in diagnostic instrumentation in resource-poor settings, controlled drug delivery or computation in harsh settings. Previous attempts at an all-fluidic computation mechanism used inertial effects (significant only at high Reynolds numbers) or non-Newtonian fluids (like polymer blends). Bubble logic operates at both low Reynolds and capillary numbers, allowing us to reduce length scales further and thus operate in nanoliter or smaller re-gimes. Figure 1 depicts device geometries for universal AND-NOT logic gate and a toggle flip-flop. The devices are fabricated using soft-lithography in PDMS bonded to glass. Propagation time for the logic gate and toggle flip-flop is ~10ms. Figure 2 de-picts a ring oscillator consisting of three AND gates and a delay line with photomicrographs of the device in operation (recorded by a high speed video camera). We are currently working on in-tegrating bubble logic elements to build high-density, random-ac-cess chemical memories.

Perfused Multiwell Tissue culture Plates for Development of Drug and Disease Models

A new platform for three-dimensional hepatic tissue engineering has been developed. It is based on the conventional multiwell tis-sue culture plate format but it allows the tissue to be continuously perfused with cell culture medium [1]. The new capability is achieved by a microfluidic perfusion system that re-circulates cell culture medium between reactors and reservoir (Figure 1). It fea-tures a network of microfluidic valves and pumps integrated into the plate [2]. Flow pulsatility is controlled by fluidic capacitors. In order to measure performance of fluidic capacitors, fluid was pumped through a capillary and a high-speed video camera was used to track the end position of the fluid. Figure 2 compares the performance of measured and modeled capacitors. As predicted by the model, the 10-mm capacitor effectively filters fluid pulses and generates a nearly constant flow. Flow with this characteristic is critical during the initial cell attachment time-period.Phase contrast and fluorescent imaging, measurement of oxygen consumption, accumulation of taurocholic acid, gene expression profiling, and drug metabolism assays are used to characterize the performance of the 3D perfused cultures. Because the new system features a standard multiwell tissue culture plate footprint, it is readily amenable to numerous high-throughput assays com-patible with automated technologies commonly used in pharma-ceutical development. The system provides a means to conduct assays for toxicology and metabolism and can be used as a model for human diseases such as hepatic disorders, exposure-related pathologies, and cancer.

A Patterned Anisotropic nanofilter Array for continuous-flow Separation of DnA and Proteins

Microfabricated regular sieving structures hold great promise as an alternative to gels to improve biomolecule separation speed and resolution. In contrast to the disordered gel porous network, these regular structures also provide well-defined environments ideal for study of molecular dynamics in confining spaces. How-ever, previous regular sieving structures have been limited for sep-aration of long DNA molecules, and separation of smaller, physi-ologically-relevant macromolecules, such as proteins, still remains as a challenge. Here we report a microfabricated anisotropic siev-ing structure consisting of a two-dimensional periodic nanoflu-idic filter array (an Anisotropic Nanofilter Array, or ANA). The designed structural anisotropy in the ANA causes differently-sized molecules to follow different trajectories, leading to efficient sepa-ration. Continuous-flow Ogston sieving-based separation of short DNA and proteins as well as entropic trapping-based separation of long DNA were achieved, thus demonstrating the potential of the ANA as a generic sieving structure for an integrated biomol-ecule sample preparation and analysis system.

Cell Stimulation, Lysis, and Separation in Microdevices

Quantitative data on the dynamics of cell signaling induced by different stimuli requires large sets of self-consistent and dynamic measures of protein activities, concentrations, and states of modi-fication. A typical process flow in these experiments starts with the addition of stimuli to cells (cytokines or growth factors) under controlled conditions of concentration, time, and temperature, followed at various intervals by cell lysis and the preparation of extracts. Microfluidic systems offer the potential to do laborious assays in a reproducible and automated fashion [1].Figure 1 shows quantification of the stimulation of a T-cell line with antibodies performed in a micro-fluidic device with integrat-ed cell lysis. The device is capable of resolving the very fast kinet-ics of the cell pathways, with protein activation levels changing 4-fold in less than 15 seconds [2]. The quantification of the lysate is currently performed off-chip using electrophoretic separation. To effectively extract meaningful data from cellular preparations, many current biological assays require similar labor-intensive sample purification steps.Micro-electrophoretic separators have several important advan-tages over their conventional counterparts, including shorter separation times, enhanced heat transfer, and the potential to be integrated into other devices on-chip. However, the high voltages required for these separations prohibit using metal electrodes in-side the microfluidic channel. A PDMS isoelectric focusing device with polyacrylamide gel walls [3] has been developed to perform rapid separations by using electric fields orthogonal to fluid flow. This device and its variants have been shown to focus organelles, low-molecular-weight dyes, proteins, and protein complexes (Fig-ure 2a) in seconds. Simulations have driven the development of improved device configurations, such as tandem IEF stages (Fig-ure 2b).

Microreactors for Synthesis of Quantums Dots

We have fabricated gas-liquid, segmented-flow reactors with multiple temperature zones for the synthesis and the overcoating of quantum dots (QDs). In contrast to single-phase flow reactors, the segmented flow approach enables rapid mixing and narrow residence time distribution, factors which strongly influence the ultimate QD size distribution. The silicon-glass reactors accom-modate a 1-m-long reaction channel (hydraulic diameter ≈ 400 µm) and swallow side channels for multiple additional injections of precursors inside the main channel (Figure 1). Pressure-drop channels were added in order to avoid backflow into the side chan-nels. Two temperature zones are maintained, a heated region (> 260 °C) and a cooled quenching region (< 70 °C). Measurements of the flow distribution (Figure 2a) show that this side manifold de-sign results in very uniform distribution even at very low nominal flow rates. As a model system, monodispersed CdSe and CdSe/ZnS QDs were prepared using this reactor. For the preparation of CdSe QDs, cadmium and selenium precursor solutions were de-livered separately in the cooled region and were thereafter mixed in the heated region. An inert gas stream is introduced further downstream to form a segmented gas-liquid flow, thereby rapidly mixing the precursors and initiating the reaction, as was shown in a previous work [1]. In the case of the synthesis of CdSe/ZnS QDs, CdSe cores are introduced directly inside the main channel, while Zn and S precursors are added through the side swallow channels, allowing the overcoating. The reaction is stopped when the fluids enter the cooled outlet region of the device. When we vary the process parameters (temperature, precursors flow rates). the size of the cores material can be tuned without sacrificing the monodispersity. In addition, the overcoating of CdSe cores allows shifting the absorbance spectrum (5 nm), due to the presence of the ZnS layering (Figure 2b).

Microfluidic Synthesis and Surface Engineering of Colloidal nanoparticles

There has been considerable research interest over the last de-cade in fabricating core-shell materials with tailored optical and surface properties. For example, core-shell particles of silica and titania have drawn attention due their potential for trapping light at specific frequencies. This optical property depends on the for-mation of nanolayers on nano- or micro-cores. To obtain useful particles, these layers need to be uniform and even. These lay-ered particles also need to be distinct and monodispersed. While nanolayer formation is successful in batch reactions, nonunifor-mity, agglomeration, and secondary nucleation often occur. We have developed microfluidic routes for synthesis and surface-coat-ing of colloidal silica and titania particles. The chief advantages of a microfluidic platform are precise con-trol over reactant addition and mixing and continuous operation. Microfluidic chemical reactors for the synthesis and overcoating of colloidal particles are shown in Figures 1a and 1b, respectively [1-2]. Figure 2a is an SEM micrograph of silica particles synthe-sized in a microreactor (Figure 1a) operated in segmented gas-liq-uid flow mode. Figure 2b shows a silica nanoparticle coated with a thick shell of titania. We have also fabricated integrated devices combining synthesis and overcoating to enable continuous multi-step synthesis of core-shell particles.

Organic Synthesis in Microreactor Systems

Enhanced heat and mass transfer, reduced reaction volume, and the ability to run several experiments in parallel render mi-croreactors powerful instruments for scanning and optimizing chemical reaction conditions. Furthermore, the high mechanical strength and thermal conductivity of silicon microreactors permit the exploration of organic syntheses at higher temperatures and pressures than can be achieved with conventional bench-scale equipment. An example of these benefits is demonstrated in the aminocarbonylation reaction study [1]. Traditionally, these reac-tions are performed at atmospheric conditions and with tempera-tures at or below the boiling point of the solvent (toluene, 110ºC). However, in silicon microreactors (Figure 1), it is possible to reach pressures exceeding 100 bar [2] and temperatures above 800ºC [3]. Exploration of the aminocarbonylation reaction offers in-formation that can be useful for the optimizing selectivity of the synthesis; higher CO pressures enhance α-ketoamide formation and increased temperatures favor amide formation.Once the chemical reaction is complete, it is desirable to separate the toxic gas from the liquid phase. Although negligible on the macro-scale, surface forces play a dominant role in microfluidics. Creating a capillary-based system (Figure 2) [4] makes it possible to take advantage of these forces. The liquid phase wets the cap-illaries and prevents the gas from penetrating the capillary ma-trix through the proper adjustments of pressure drops across the separator. Similarly, this concept can be applied to heterogeneous reactions that involve two immiscible liquids. Due to this micro-technology, microreactor systems can be assembled for multi-step synthesis and separation that could not easily be achieved in tra-ditional laboratory environments. As a result, high throughput experiments can be performed and entire chemical processes can be optimized efficiently with microreactor systems.

Autothermal catalytic Micromembrane Devices for Portable High-purity Hydrogen Generation

The high efficiency and energy density of miniaturized fuel cells provide an attractive alternative to batteries in the portable-power-generation market for consumer and military electronic devices [1-3]. The best fuel cell efficiency is typically achieved with hydrogen, but safety and reliability issues remain with cur-rent storage options. Consequently, there is continued interest in reforming of liquid fuels to hydrogen. The process typically in-volves high-temperature reforming of fuel to hydrogen combined with a low-temperature PEM fuel cell, which implies significant thermal loss. Owing to its high hydrogen content (66%) and ease of storage and handling, methanol is an attractive fuel. However, partial oxidation of methanol also generates CO, which can poi-son the fuel cell catalyst [1]. Previously [4] we have successfully demonstrated hydrogen pu-rification using thin (~200 nm) Pd-Ag membranes using electri-cal heating. Further, integration of these devices with LaNiCoO3 catalyst allowed methanol reforming at 475oC with 47% fuel con-version [5]. In the current work, we fabricate a novel autother-mal reformer for hydrogen generation and purification using bulk micromachining techniques. This device combines the reforming unit with a catalyst loaded microreactor for combustion of hy-drogen not recovered through the Pd- Ag membrane, generated CO, and unreacted methanol. The energy from the combustion heats the reformer to the operating temperature (~450C). High thermal conductivity of silicon ensures efficient heat transfer from combustor to reformer. In the first phase, Pd-Ag membrane stability post-fabrication was tested; results indicated a pin-hole- and crack-free layer. Further, we successfully demonstrate high-pressure operation (up to 1.6 atm) of the device for enhanced hydrogen flux. The microburner has also been characterized with hydrogen oxidation over platinum catalyst. Work on reforming methanol for hydrogen generation and characterization of ther-mal responses is currently under progress.

Thermal Management in Devices for Portable Hydrogen Generation

The development of portable-power systems employing hydro-gen-driven solid oxide fuel cells continues to garner significant interest among applied science researchers. The technology can be applied in fields ranging from the automobile to personal elec-tronics industries. This work focuses on developing microreaction technology that minimizes thermal losses during the conversion of fuels – such as light-end hydrocarbons, their alcohols, and am-monia – to hydrogen. Critical issues in realizing high-efficiency devices capable of operating at high temperatures have been addressed: specifically, thermal management, the integration of materials with different thermophysical properties, and the devel-opment of improved packaging and fabrication techniques.A new fabrication scheme for a thermally insulated, high-temper-ature, suspended-tube microreactor has been developed. The new design improves upon a monolithic design proposed by Arana et al. [1]. In the new modular design (Figure 1), a high-temperature reaction zone is connected to a low-temperature (~50°C) package via the brazing of pre-fabricated, thin-walled glass tubes. The design also replaces traditional deep reactive ion-etching (DRIE) with wet potassium hydroxide (KOH) etching, an economical and time-saving alternative. A brazing formulation that effectively ac-commodates the difference in thermal expansion between the silicon reactor and the glass tubes has been developed. Autother-mal combustion of hydrogen, propane (Figure 2), and butane has been demonstrated in ambient atmosphere and in a vacuum.

Microfluidic Systems for the Study of Vascular networks

Mechanical forces are important regulators of cell biology in health and disease. Cells in the vascular system are subjected to fluid shear stress, cyclic stretch, and differential pressure [1-3],[1-3],, and at the same time they receive multiple biochemical cues. All these factor into the integrated response of the tissue. A micro-fluidic bioreactor has been constructed to facilitate studies into the roles of both biophysical and biochemical factors on capil-lary morphogenesis. The device is made of PDMS, cured on The device is made of PDMS, cured on The device is made of PDMS, cured onThe device is made of PDMS, cured on an SU-8 patterned wafer. Then a scaffold material, collagen, is induced into a specific region of devices that was designed to keep its shape and properties. Cells are seeded via one flowCells are seeded via one flow channel on the surface of the scaffold and then subjected tothe scaffold and then subjected to scaffold and then subjected to and then subjected toand then subjected to controlled mechanical factors like surface shear and trans-endo-mechanical factors like surface shear and trans-endo-surface shear and trans-endo-thelial pressure, or biochemical angiogenic factors, inducing theor biochemical angiogenic factors, inducing theinducing the formation of vascular sprouts that extend across the scaffold to a second flow channel. With the bioreactor, cells on the scaffold. With the bioreactor, cells on the scaffoldcells on the scaffold form a confluent monolayer and generate sprouts. They show. They show different responses and interactions with the scaffold, followingfollowing the angiogenic factors, fluidic factors, surface characteristics and scaffold properties. Experiments are now under way to find theExperiments are now under way to find thefind the relations between cell responses and controlled factors. The de- The de- The de-The de-veloped system is the first system that can control biochemical andis the first system that can control biochemical andthat can control biochemical andcan control biochemical andcontrol biochemical and biochemical and mechanical factors together, and it can be used for comparing thecan be used for comparing thecomparing the effects of angiogenic factors under controlled environment withenvironment with with enhanced view. It can also be applied to study the process of an-to study the process of an-giogenesis that entails the growth of vascular sprouts emanating from one endothelial surface and connecting with the other.

Patterning and Processing of Thermosensitive Hydrogels for

Hydrogels have been an active area of research for a variety of applications due to their ability to retain large volumes of water within their polymer gel networks. Stimuli-responsive hydrogels provide the added advantage of the ability to control the water retention by means of external stimuli. For example, N-isopro-pylacrylamide (NIPAAm) is a thermosensitive hydrogel that ex-hibits a Lower Critical Saturation Temperature (LCST) around 32°C, above which the gel becomes hydrophobic and expels the water molecules, resulting in a drastic swelling/shrinking ratio. The goal of this project is to utilize this pseudo-binary transition in the fields of microfluidics and drug delivery.By imbedding magnetic nanoparticles into the gel networks, the Hamad-Schifferli group [1] could control the temperature of the gels by inducing eddy currents by means of an oscillating magnet-ic field. We are developing the concept further into micro-scale devices that can be monolithically integrated into many micro-fluidic systems. We have demonstrated the ability to photopattern the hydrogels and have shown control of the swelling behavior by controlling the amount of cross-linking in the network. This al-lowed for the creation of hydrogel valves for microfluidic devices. Unlike pressure controlled valves, these valves do not require any physical interconnects to macro-scale devices. This advantage could prove extremely useful in the commercialization of micro-fluidic analysis systems where users might not have equipment such as syringe pumps or air compressors available. In addition to valves, applications of the swelling behavior to micropumps are also being examined.

A Large-strain, Arrayable Piezoelectric Microcellular Actuator by Folding Assembly

A low-power, piezoelectric, contracting cellular MEMS actuator has been developed that demonstrates a peak strain of 3% under a 10 V stimulus. Since the motion of the end effecter is linear and in-plane, the actuator can be arrayed in series to amplify the total stroke or in parallel to amplify the total force, as needed. Loca-tion of the piezoelectric member through the structural center of stiffness reduces the potential for parasitic out of plane bending present in previous designs [1].Cellular actuators arrays can be assembled into a larger array of actuators. We demonstrated that sets of cellular microactuators can be assembled out of plane by folding them over thin gold hinges. To our knowledge, this study is the first effort in this field. The gold hinges serve dually as mechanical assembly guides and electrical interconnects. Long chains of devices may be assem-bled by rolling out of plane. Figure 2 shows a smaller collection, assembled by folding three actuator triplets onto one another. Actuation of the collection is contingent on the manufacturing of functional thin-film PZT.

Thermal Ink Jet Printing of Lead Zirconate Titanate Thin Films

The ferromagnetic and piezoelectric properties of ceramic lead zirconate titanate (PZT) thin films have made PZT an appealing choice for micro-sensors and actuators. Significant work has been done integrating PZT with standard MEMS processes, including the development of PZT sol-gels for spin coating [1-2]. Crack-ing is often a problem with PZT spin coating due to the brittle nature of the films coupled with the thermal strain experienced during annealing. This propensity for cracking limits the overall thickness deposited and the size out of plane features over which PZT can be reliably coated. Furthermore, spin coating requires a large volume of the expensive PZT precursor solution. We pro-pose thermal ink-jet printing of a modified PZT sol-gel as a new method of depositing PZT films for MEMS applications. Pre-liminary work has shown ink jetting to be a reliable method for depositing PZT films of the correct thickness for MEMS applica-tions and that annealed films can crystallize into the piezoelectric perovskite phase using the same thermal process developed for spin-coated PZT (see Figure 1) [3]. The goal of this research is to develop a deposition process that will enable reliable manufactur-ing of high-quality PZT films with greater deposition flexibility and lower material costs than spin coating. Thermal ink jetting technology supports a wide range of ink viscosities and solid particle contents. The ink composition can therefore be adjusted to control both the contact angle of solution with the substrate (1000Å Pt/ 200Å Ti) and the as-deposited film thickness. This flexibility allows for the deposition of films with thickness and uniformity that are acceptable for the fabrication of piezoelectric devices (see Figure 2). Multiple layers can be depos-ited to attain the thickness as needed. Currently, annealed films have been prepared as thick as 0.5 µm, corresponding to an as deposited thickness of approximately 1 µm. This is comparable to the current limit of standard spin-coated PZT sol-gel processed; printing of thicker films is under investigation.

Piezoelectric Micro-power-penerator: MEMS Energy-harvesting Device for Self-powered Wireless corrosion-monitoring System

A novel thin-film, lead zirconate titanate Pb(Zr,Ti)O3 (PZT), energy-harvesting MEMS device is being developed for autono-mous wireless monitoring systems. It is designed to harvest en-ergy from parasitic vibrational energy sources and convert it to electrical energy via the piezoelectric effect. We envision that harvesting parasitic energy from the vortex-induced vibration of the oil pipelines will deploy a massive number of microsensors along the hundreds of miles of pipeline in very cold and remote areas. The proposed system consists of a corrosion sensor, a ra-dio transceiver, a microcontroller, a power management module, and a piezoelectric micro power generator (PMPG) to supply the needed power of the system without replacing batteries. The new pie-shaped design for the harvester (about a size of a nickel) has a radical departure from previous design concepts. This energy harvester design can be regarded as revolutionary as the first self-rectifying piezoelectric power generator. The new design avoids the high Q resonance, which is also a big change from previous designs. This will enable more robust power gen-eration even if the frequency spectrum of the source vibration varies unexpectedly. Furthermore, the beam shape is optimized to achieve uniform allowable strain throughout the PZT layer. Cur-rently, the first prototype, which is shown schematically, is being fabricated at MTL.

Lateral-line-inspired MEMS-array Pressure Sensing for Passive Underwater navigation

A novel sensing technology for unmanned undersea vehicles (UUVs) is under development. The project is inspired by the lateral line sensory organ in fish, which enable some species to form three-dimensional maps of their surroundings [1-2]. The canal subsystem 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 objects [2]. It also aids schooling [5]. Similarly, by measuring pressure variations on a vehicle surface, an engineered dense pressure sensor array allows the identifica-tion and location of obstacles for navigation (Figure 1). We are demonstrating proof-of-concept by fabricating 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 (Fig-ure 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 measured. At this stage, the individual MEMS pressure sensors are being constructed and tested.In parallel to the construction of a sensor array, techniques are being developed to interpret the signals from a dense pressure ar-ray by detecting and characterizing wake structures such as vorti-ces and building a library of pressure distributions corresponding to basic flow obstructions. In order to develop these algorithms, experiments are being performed on coarse arrays of commercial pressure sensors

Fabrication of a Fully-integrated Multiwatt µTurboGenerator

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, andsensors, robotics, and, robotics, and other applications. Building upon the results presented in [1], the current research is aimed at fabricating a fully-integrated, multiwatt micro turbogenerator on silicon that can produce 10 micro turbogenerator on silicon that can produce 10 W DC output power (Figure 1). One of the main challenges in-in-volves the seamless integration between silicon and the magnetic the seamless integration between silicon and the magnetic components required to generate power. The generator requires a NiFe soft magnetic back iron and laminated stator for flux redirection as well as NdFeB permanent magnet pieces to serve as flux sources (Figure 2). In addition, copper windings must be fabricated above the laminated stator to couple to the alternating flux in order to extract electrical power from the machine.Great strides have been made in the past year to quantify the requirements on the magnet pieces that will go into the rotor housing. Manufacturing accuracy of the pieces is critical because variations in the magnet geometries create an overall rotor imbalance, which can cause the rotor to crash during transcritical operation. A procedure in which the gaps around in which the gaps around the gaps around the magnet pieces are filled with solder and then polished back using chemical-mechanical planarization has been developed; this process can reduce the effective imbalance of the rotor by anan order of magnitude.The assembly and packaging procedure for the turbogenerator is also critical because the embedded permanent magnets cannot withstand temperatures much above 150 oC. This temperature restriction rules out the use of fusion bonding for the final die-level assembly after rotor insertion. Based on results presented by Choe, et al. [2], an eutectic In-Sn bonding scheme that requires only 140 oC has been researched. In this scheme, Cr/Au is de-posited on one bonding surface and Cr/Sn/In/Au is deposited on the other surface; both depositions are done using an e-beam evaporator without breaking vacuum. By painting no-clean flux on both surfaces and compressing the dies together on a hot plate, we form the bond.

A Portable Power Source Based on MEMS and carbon nanotubes

There is a growing need for small, lightweight, reliable, highly efficient and fully rechargeable portable power sources. The focus of this project is the design and modeling of a system in which energy is stored in the elastic deformation of carbon nanotube (CNT)-based springs. The CNTs are coupled to a MEMS electric generator. When the CNT deformation is released, the stored energy actuates the generator, which then converts the energy into electricity. The MEMS generator may be operated in reverse, as a motor, in order to wind the CNT springs and recharge the system. Alternatively, the stored elastic energy may be used to supply a mechanical load directly. This project is motivated by recent research into the mechanical properties of CNTs. The CNTs have a high stiffness, low defect density, and a consequently high yield strain that enables them to store elastic energy with significantly greater energy density than typical spring materials such as high-carbon steel. Models suggest that CNTs can be reversibly stretched by up to 15% [1]; lower strains of up to 6% have been demonstrated experimen-tally to date [2-3].This type of system offers several important potential advantages. First, due to CNTs’ high strength, high flexibility, and low defect density, they can store energy at very high energy density. Con-sidering just the CNT-based spring itself, the energy density of an array of CNTs stretched to a reversible 15% strain is about 1500 W-hr/kg, about ten times the energy density of Li-ion batteries. The energy density of the final system will be lower because of the finite conversion efficiency of the generator and the weight of both the supporting structure and the generator hardware. In addition, because energy storage in the CNT system is based on stretching chemical bonds rather than breaking and reforming chemical bonds as in batteries, the CNT-MEMS generator sys-tem has the potential to operate at higher power densities, un-der harsher conditions, to deeper discharge levels, and through a greater number of charge-discharge cycles than a chemical bat-tery.The system architecture consists of a CNT-based energy storage element, an energy release rate mechanism, and a MEMS gen-erator. This project is examining and modeling different varia-tions on this system architecture that incorporate different modes of deformation of the CNT-based energy storage element, vari-ous types of generators, different types of coupling between the storage element and the generator, and different size scales for the various components. One conceptual example is illustrated below, in which the axial relaxation of an axially-stretched CNT-based storage element is converted to rotational motion of a wheel. The wheel is coupled to a piezoelectric generator through a mechanism that regulates the rate of energy release, much as in a mechanical watch.

A MEMS Steam Generator

Previous work [1] has shown that MEMS technology has signifi-cant potential to create more compact, higher- performing hard-ware for chemical oxygen iodine lasers (COIL). In COILs, the laser medium is a flowing gas that must be pumped through the system at high mass flow rates to ensure proper system opera-tion. As a result, compact pumps with high pumping rates are a key element of the COIL system. One promising component of a MEMS COIL system would be a compact MEMS pump system in which the pump action is provided in part by micro steam ejectors and the micro steam generators that supply their driving fluid. This work describes the design and modeling of a microscale hydrogen peroxide (H2O2)-based steam generator to supply such a MEMS pump system. Hydrogen peroxide is a read-ily available, inexpensive, nontoxic, and environmentally friendly fluid that may be catalytically decomposed to form steam. Steam generation by the catalytic decomposition of H2O2 also finds oth-er important applications in the MEMS field beyond pumping, particularly in the area of thrust generation. Compared to their macroscale counterparts, MEMS H2O2-based steam generators offer better performance, notably improved mixing, and higher uniformity due to the absence of moving parts [2-3].A complete MEMS steam generator consists of a peroxide res-ervoir, an injector, a reactor, and a converging-diverging nozzle to accelerate the exiting flow, as shown in Figure 1. Initial work focuses on the design of the reactor and nozzle. Liquid H2O2 in aqueous solution is injected into the reactor, where it decom-poses into steam and oxygen gas upon contact with the catalyst. A continuous supply of homogeneous liquid catalyst is used, as it avoids the aging problem typically exhibited by heterogeneous catalysts [4]. The gaseous products of the reaction are then accel-erated to supersonic velocities through the converging-diverging nozzle. The work to date indicates that a MEMS steam generator designed to minimize heat transfer to the environment can pro-vide complete, compact, uniform decomposition of peroxide into steam suitable to drive a MEMS pumping system.

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 typi-cally employ large hardware and use reactants relatively ineffi-ciently. 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 performance. A complete microCOIL system includes microchemical reactors, microscale supersonic nozzles, and micropumps. System models incorporating all of these elements predict significant performance advantages in the microCOIL approach [1].Initial work is focused on the design, microfabrication, and demonstration of a chip-scale singlet oxygen generator (SOG), a microchemical 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; sur-face-to-volume ratio increases 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 cool-ing channels for efficient heat removal [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.

An Integrated Microelectronic Device for Label-free nucleic Acid Amplification and Detection

While there have been extensive advances in miniaturized poly-merase chain reaction (PCR) systems, progress on integrated mi-crofabricated readout mechanisms has been rather limited, and most systems rely on off-chip optical detection modules to mea-sure the final product. Existing optical detection platforms typi-cally include CCD cameras, photodiodes, and photomultiplier tubes. While such hardware has adequate sensitivity for detecting PCR products in sample volumes significantly lower than that of bench-top systems, most are difficult to miniaturize and integrate into a compact analytical system. For example, some portable systems incorporating external LEDs and photodetectors can weigh between 1 kg and 4 kg each. To address these limitations, several groups have successfully embedded photodetectors within integrated PCR platforms. However, these devices still rely on external excitation sources.To address this limitation, we have developed an integrated mi-croelectronic device for amplification and label-free detection of nucleic acids (Figure 1) [1]. Amplification by PCR is achieved with on-chip metal resistive heaters, temperature sensors, and microfluidic valves. We demonstrate a rapid thermocycling with rates of up to 50°C/s and a PCR product yield equivalent to that of a bench-top system. Amplicons within the PCR product are detected by their intrinsic charge with a silicon field-effect sensor. Similar to existing optical approaches with intercalators such as SYBR Green, our sensing approach can directly detect standard double-stranded PCR products while in contrast our sensor oc-cupies a micron-scale footprint, dissipates only nano-watt power during operation, and does not require labeling reagents. By com-bining amplification and detection on the same device, we show that the presence or absence of a particular DNA sequence can be determined by converting the analog surface potential output of the field-effect sensor to a simple digital true/false readout.

Monitoring of Heparin and its Low Molecular Weight Analogs by Silicon Field Effect

Heparin is a highly sulfated glycosaminoglycan that is used as an important clinical anticoagulant. Monitoring and control of the heparin level in a patient’s blood during and after surgery is essen-tial, but current clinical methods are limited to indirect and off-line assays. We have developed a silicon field-effect sensor for di-rect detection of heparin by its intrinsic negative charge [1]. The sensor consists of a simple microfabricated electrolyte-insulator-silicon (EIS) structure encapsulated within microfluidic channels (Figure 1). As heparin-specific surface probes, we used the clinical heparin antagonist protamine or the physiological partner an-tithrombin III. The dose-response curves in 10% PBS revealed a detection limit of 0.001 U/ml, which is orders of magnitude lower than clinically relevant concentrations. We also detected heparin-based drugs, such as the low-molecular-weight heparin enoxaparin (Lovenox®) and the synthetic pentasaccharide hepa-rin analog fondaparinux (Arixtra®) (Figure 2), which cannot be monitored by the existing near-patient clinical methods. We de-monstrated the specificity of the antithrombin III functionalized sensor for the physiologically active pentasaccharide sequence. As a validation, we showed correlation of our measurements to those from a colorimetric assay for heparin-mediated anti-Xa activity. These results demonstrate that silicon field-effect sensors could be used in the clinic for routine monitoring and maintenance of therapeutic levels of heparin and heparin-based drugs and in the laboratory for quantitation of total amount and specific epitopes of heparin and other glycosaminoglycans.

Weighing of Biomolecules, Single cells and Single nanoparticles in Fluid

Nanomechanical resonators enable the measurement of mass with extraordinary sensitivity. Previously, samples as light as 7 zepto-grams (1 zg = 10-21 g) have been weighed in vacuum, and proton-level resolution seems to be within reach. Resolving small mass changes requires the resonator to be light and to ring at a very pure tone—that is, with a high quality factor. In solution, viscosity severely degrades both of these characteristics, thus preventing many applications in nanotechnology and the life sciences where fluid is required. Although the resonant structure can be designed to minimize viscous loss, resolution is still substantially degraded when compared to measurements made in air or vacuum. An entirely different approach eliminates viscous damping by placing the solution inside a hollow resonator that is surrounded by vac-uum (Figure 1). We have recently demonstrated that suspended microchannel resonators can weigh single nanoparticles (Figures 2), single bacterial cells. and sub-monolayers of adsorbed proteins in water with sub-femtogram resolution (1 Hz bandwidth). Cen-tral to these results is our observation that viscous loss due to the fluid is negligible compared to the intrinsic damping of our sili-con crystal resonator. The combination of the low resonator mass (100 ng) and high quality factor (15,000) enables an improvement in mass resolution of six orders of magnitude over a high-end commercial quartz crystal microbalance [1]. This gives access to intriguing applications, such as mass-based flow cytometry, the direct detection of pathogens, or the non-optical sizing and mass density measurement of colloidal particles.

Integrated System for cancer Biomarker Detection

There is evidence to suggest that the next generation of cancer-screening tests may employ not just one, but a small panel of less than ten biomarkers that together add statistical power to the de-tection of specific cancers. While immunoassays such as ELISA are well established for detection of antigen-based biomarkers, the fidelity of the assay is governed by the disassociation constant, Kd, of the antibody-antigen complex. If the antigen concentra-tion is significantly below Kd, then the binding kinetics are slow and readout precision of the antigen-antibody complex can be degraded by noise.We propose a general approach for improving the performance of ligand-receptor assays. The approach is based on a nano-fluidic device that controllably concentrates a dilute sample and an ultra-sensitive suspended microchannel resonant mass sensor that detects specific biomarkers within the concentrate. Since the amplification (or gain) of the concentrator is adjustable, the dynamic range and detection limit of the immunoassay can be governed by the properties of the concentrator and not Kd. Since the integrated concentration/detection system is batch-fabricated by conventional foundry-level processing techniques, the cost per device could potentially be less than ten dollars.Over the past year, we have fabricated the first generation of in-tegrated systems (Figure 1). The devices appear to be functional based on initial visual inspections. We are currently validating the performance of the system by using quantum dots for a calibration assay. We are also in the process of validating the performance of the concentrator and mass sensor (as individual components) with prostate-specific antigen so that we can make comparisons to existing methods in terms of sensitivity and selectivity.

Passive Microwave Transponders for Passive, Real-time, and High-sample-rate Localization

Passive surface acoustic wave (SAW) transponders have been used for RFID applications because of their zero-power and long-range (100m) capabilities. This work presents current research into utilizing SAW transponders for localizing objects. The SAW transponders have many advantages over existing localization solutions, including small size (mm x mm), zero-power, high-ac-curacy, longer range (100m), and kilohertz update rates. Unhin-dered localization of objects is desirable for many applications, from human computer interaction to product tracking to security. The SAW transponders offer improvements to existing solutions for asset tracking, location of lost articles, ubiquitous computing, tracking of people with special needs or prisoners, workers in hazardous situations, human machine interfaces, virtual training environments, security, location of short-range mobile sensors, and biomedical research. The goal of this project is to prototype this tracking system and evaluate the feasibility of a commercial system.Multiple RADAR measurement stations use phase-encoded chirps to selectively track individual SAW transponders by tri-angulation of range and/or angle measurements. Update rates on the order of 10kHz with accuracies better than 10cm3 are conceivable. A 300-nm deep-uv contact-mask lithography nano-fabrication process to create these SAW devices is under develop-ment. Figure 1 shows the block diagram of the electronic test setup that is being used to characterize devices. Figure 2 shows a micrograph of a few of our fabricated devices. Recent results from this investigation will be presented, including the character-ization of our first devices.

Macroscopic Interfaces to Parallel Integrated Bioreactor Arrays

Macroscopic fluidic interfaces are important for improving the usability of microfluidic devices. For example, in our previously developed parallel integrated bioreactor arrays [1], two needle punctures were required to fill each fluidic reservoir, one for fluid injection using a syringe and another needle to vent the air dis-placed by the injected fluid. While suitable for internal labora-tory use, such an inconvenient fluid injection procedure impedes the adoption of this new bioreactor technology. We have developed a fluid injection port that automatically vents the displaced air and is compatible with standard laboratory pi-pette tips. The principle of operation is shown in Figure 1. On opposite sides of each fluid reservoir (Figure 1a), there is a fluid injection channel and a vent channel. Both of these channels ter-minate in vias that connect the channels to the seat of the pipette interface nipple. The pipette-interface-nipple (Figure 1e) is an elastomer structure, which, when compressed by its housing (Fig-ure 1c), seals closed both vias such that fluid can neither escape nor enter the fluid reservoir. When a pipette tip is inserted into the interface-nipple, it is deformed and allows air to escape from the vent port (Figure 1f). Meanwhile, the inserted pipette tip seals to the via connected to the fluid injection channel and fluid can be injected into the reservoir. The particular bow-tie shape of the pipette-interface-nipple was chosen such that when it is inserted into a rectangular housing, sufficient compressive force would seal the central slit closed while also allowing space for the nipple to expand upon insertion of the pipette tip. Fabricated devices exhibited good operating characteristics and provided a seal against a greater-than-10psi back pressure.

MIT-OSU-HP Focus center on non-lithographic Technologies for MEMS and nEMS

This newly formed center is part of an overall set of centers on MEMS/NEMS fundamentals supported by DARPA. The MIT-OSU-HP Focus Center aims to develop new methods for fabrica-tion of MEMS and NEMS that do not use conventional litho-graphic methods. 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 demonstration 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 spec-trum from active materials, photonic and electronic materials, to mechanical 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.

A Micromachined Printhead for the Evaporative Printing of Organic Materials at Ambient Pressure

Organic optoelectronic devices are promising for many commer-cial applications if methods for fabricating them on large-area, low-cost substrates become available. Our project investigates the use of MEMS in the direct patterning of materials needed for such devices. By depositing the materials directly from the gas phase, without the liquid phase coming in contact with the sub-strate, we aim at avoiding the limitations due to inkjet printing of such materials.We developed a MEMS-enabled technique for evaporative print-ing of organic materials. This technique does not require a vac-uum ambient, has a fast printing rate (1 kHz), and can be scaled up to an array of individually addressable nozzles. The MEMS printhead comports a microporous layer with integrated heaters for local evaporation of the materials. Figure 1 shows the micro-fabricated device: an array of 2 micron pores and an integrated thin film platinum heater sit in the center of a silicon membrane. The material to be printed is delivered to the porous region in liquid or gas phase and deposits inside the pores (see Figure 1, top left). The integrated heater then heats up the porous area (see Fig-ure 2, top) and the material is re-evaporated from the pores onto the substrate. The main limitation of this printhead is the failure of the thin-film platinum heater at temperatures above 800°C (see Figure 2 bottom).This printhead was used, together with inkjet technology for the delivery of material to the pores, to print molecular organic semi-conductors (see other abstract in this volume). Our technique enables printing of organic optoelectronics over large areas and can be used to print on a variety of substrates, does not require a vacuum ambient, and thus could enable low-cost printing of optoelectronics.

Surface Micromachining Processes using non-lithographic Technologies

Conventional MEMS fabrication relies heavily on planar lithog-raphy and IC technology. While these techniques are well-suited for relatively flat devices such as the semiconductors, they are drastically limited in the design and fabrication of three-dimen-sional devices such as MEMS. From a commercial viewpoint, the semiconductor paradigm is also a poor fit for MEMS because the lower volume and demands make it more difficult 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 and decreased costs.Our project investigates severing MEMS fabrication from the semiconductor paradigm via non-lithographic technologies. We have previously shown how MEMS can be used for the direct patterning of small molecular organics [1]. Using similar con-cepts, we intend to show that surface micromachining can also be achieved.The first stage of the project is to directly pattern a structural layer over a spacer and successfully release a cantilever. We have successfully patterned metal silver over various spacer materials, including polyethylene glycol (PEG), polyvinyl acetate (PVA), and UNITY™ sacrificial polymer, and we are currently working on the release process. This technique will ultimately be used to con-struct simple structures, such as cantilevers and bridges, to test the structural material’s mechanical properties. The next stage of this project will consist of using this process to fabricate cantile-vers and integrate them with other non-lithographic techniques to fabricate an accelerometer. Subsequent stages will consist of creating a library of non-lithographic processes so that entire MEMS devices can be fabricated without the use lithography.

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 antennae as airflow sensors has been suggested [1], and recent discoveries of our collaborators reveal that mechanosensory in-put from the antennae of flying moths serves a similar role to that of the hind wings of two-winged insects, detecting Coriolis forces and thereby mediating 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 M. Sexta by attachment of microsystems seems plausible. The goal of our project is to design and fabricate micromechani-cal actuators, which will be mounted onto the moth antennae (Figure 1). Our collaborators will study the flight control mecha-nism by mechanical stimulation. Our first step is to fabricate “dummy” silicon rings for our biolo-gist collaborators for implant experiments. The diameters along the antenna vary from tip to base, being thickest in the middle. As a result, in order to prevent the ring’s being thrown off, the mounting of the silicon ring onto the base cannot be as simple as pushing it from the tip with a large inner hole. On the other hand, the sizes of the antennae vary from moth to moth. Two-piece construction was designed and fabricated to be like a “zip strip” to meet the mounting requirements (Figure 2). Future work will focus on refining the design and fabrication of the mounting kit and integrating actuators into it. To generate adequate displace-ment, strain amplification will be needed, such as reported by Conway, et al. [3].

MEMS Micro-vacuum Pump for Portable Gas Analyzers

There are many advantages to miniaturizing systems for chemi-cal and biological analysis. Recent interest in this area has led to the creation of several research programs, including a Micro Gas Analyzer (MGA) project at MIT. The goal of this project is to develop an inexpensive, portable, real-time, and low-power approach for detecting 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 en-abling 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.There has been a great deal of research done over the past 30 years in the area of micro pumping devices [1, 2]. We are cur-rently developing a displacement micro-vacuum pump that uses a piezoelectrically driven pumping chamber and a pair of piezoelectrically driven active-valves; the design is conceptually similar to the MEMS pump reported by Li et al. [3]. We have constructed an accurate compressible 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 fabricated three generations of the MEMS vacuum pump over the past year and are currently working on the fourth. A schematic of the pump is shown in Figure 1. For ease in testing we have initially fabricated only Layers 1-3 and have constructed a testing platform that, 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 three 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 third generation pump test data is shown in Figure 2. Figure 2a shows the pressure versus flow rate characteristics of the pump; note that the data compares very well with models. Figure 2b shows the output flow rate versus actuation characteristics of the pump. Notice that the flow goes to zero each time the piston is actuated upwards (red bar). All three pistons demonstrated similar perfor-mance illustrating a pump with fully functioning pistons and teth-ers. Next, we hope to characterize the pumping characteristics of this and the upcoming fourth-generation pumps.

Microfabricated Electrodes for Solid Oxide Fuel cells

The solid oxide fuel cell (SOFC) is an energy-conversion device that produces electricity directly through the electrochemical re-action of hydrogen (H2) and oxygen (O2). The SOFC also allows utilization of hydrocarbons, such as methane (CH4), via internal reforming or direct electrochemical oxidation, giving these sys-tems the flexibility of using a variety of commercially available fuels. The high energy-density of hydrocarbon fuels makes the SOFC attractive for large-scale stationary systems and as a re-placement to batteries for powering portable electronic devices. Standard SOFCs operate in the temperature range of 800°C-1000°C and have an open circuit voltage of approximately 1-volt. The most common SOFC materials are yttria-stabilized zirconia (YSZ) for the electrolyte, strontium-doped lanthanum manganite (LSM) for the cathode and nickel (Ni)-YSZ for the anode. Al-though elevated temperature operation allows the use of non-noble metal catalysts and excellent high-grade heat exhaust to be used for additional power generating cycles, there are numer-ous advantages to lowering the SOFC operating temperature to around 600°C. Benefits such as lower thermal stress, reduced cell degradation, utilization of metallic components, and shorter startup times are a few. However, at these lower temperatures the poor electrochemical activity of the electrodes, in particular the LSM cathode, leads to unacceptable voltage losses that lower the efficiency and performance of the SOFC. Oxygen reduction mechanisms on the perovskite material LaxSr1-xMnO3-d (LSM) has been widely studied; however, no final con-clusion on the molecular level mechanisms for oxygen reduction has been made. To probe the oxygen reduction reaction, we fabricate electrodes with precise geometries (50-200 µm) using thin-film deposition techniques (sputtering and laser ablation) and subsequent photolithography to investigate the fundamen-tal electrode mechanisms and rate-determining reactions. The electrochemical impedance spectroscopy (EIS) response of a La1-xSrxMnO3-d (LSM) microelectrode on 8YSZ is then analyzed as a function of geometry and temperature using a microprobe sta-tion equipped with a high temperature stage, as Figure 1 shows. Our preliminary EIS results1 shown in Figure 2a show at least four distinct reaction processes for oxygen reduction on LSM/8YSZ: (i) ion transport in 8YSZ with average activation energy (Ea) of 1.16±0.02eV, (ii) surface diffusion on LSM with Ea rang-ing from 1.34±0.05eV to 1.65±0.03eV, (iii) at least one surface chemical process on LSM with Ea ranging from 1.71±0.02eV to 1.88±0.02eV and an average capacitance 3.4x10-4 F/cm2, and (iv) a mixed bulk/TPB charge transfer process with Ea ranging from 2.42±0.02eV to 3.05±0.03eV and an average capacitance of 3.2x10-3 F/cm2. The overall oxygen reduction process is il-lustrated in Figure 2b, with the rate-limiting reaction for ORR found to be from mixed bulk/TPB charge transfer processes be-low 700°C and shifts to surface chemical reactions above 700°C.

A MEMS-relay for Power Applications

Contact travel and heat dissipation are important requirements of electrical power switching devices such as MEMS-relays and MEMS-switches. Whereas low-power MEMS-based RF switches have been vigorously studied, few studies have been reported on high-power MEMS-relays. This paper presents a MEMS-relay for power applications. The device is capable of make-break switching; has large contact travel, on the order of 10’s of µm; and has low contact resistance, on the order of 120 mΩ. Testing has demonstrated current carrying capacity on the order of sev-eral amperes and hot-switching of inductive loads, on the order of 10mH, without performance degradation.The MEMS-relay, shown in Figure 1a, is bulk micromachined in (100) silicon and bonded to a glass substrate. Anisotropic etching is used to fabricate the oblique and parallel (111) contact surfaces, having nanometer-scale surface roughness [1]. Figure 1b shows a cross section of the open fabricated contacts. An offset between the wafer-top and the wafer-bottom KOH masks produces the contact geometry shown. The silicon contact metal surfaces are created by evaporation and electroplating with a conductive film, shown in Figure 1c. A thermal oxide layer provides insulation between the actuators and the contacts. Deep reactive ion-etch-ing (DRIE) is used to pattern a parallelogram-flexure compliant mechanism and a pair of rolling-point “zipper” electrostatic ac-tuators [2]. Nested masks are used to pattern both wafer-through etches. Figure 2 illustrates the process used to fabricate the de-vice.

A Silicon-etched, Electrical-contact Tester

We are developing a bulk micromachined contact tester to inves-tigate the electro-tribological performance of micro- and nano- structured planar electrical contacts [1]. The test device features parallel, planar, nanometer-scale surface roughness contacts etched in silicon coated with thin conductive films. Contacts used in microsystems, probes and interconnects are subject to heat dis-sipation and to electro-mechanical tribological effects. With an understanding of how nanoscale surface and subsurface material structure affect electrical contact resistance and mechanical con-tact wear, a deterministic manufacturing process could be devel-oped to design electrical contacts from crystalline plane surfaces as potential high performance contacts for MEMS devices and related applications. The microfabricated contact tester, shown in Figure 1 and in Figure 2, consists of a pair of parallel planar contact surfaces with nanometer roughness patterned onto two (100) Si substrates. Anisotropic etching is used on one of the substrates to create a membrane that serves as a compliant mechanism for the con-tact tester. A thin conductive film, i.e., Au, is patterned onto the contacts in a Kelvin configuration. The two-piece tester archi-tecture allows for inspection of the contacts before, during, or after testing without destruction of the test device. In one em-bodiment of the tester, a quasi-kinematic coupling enables the alignment between the substrates while providing the initial gap between the contacts. Similar quasi-kinematic designs fabricated in silicon substrates have reported repeatability on the order of 1 micrometer [2]. In a second embodiment of the MEMS-tester a patterned oxide film is used to provide the initial space between the contacts. The tester will be loaded using a commercial na-noindenter to bring the surfaces into contact as contact resistance is measured as a function of the force.

Microfluidic Studies of Biological cell Deformability and Rheology

It is well known that many hereditary or infectious diseases, as well as certain types of cancer, produce alterations in mechanical properties of human cells. In certain diseases, such as malaria, infected cells exhibit reduced deformability and increased cy-toadherance. Such changes alter the circulatory response of red blood cells (RBCs) and limit physiological responses to such dis-eases, such as splenic clearance of parasitized RBCs, and restrict circulation of RBCs in the microvasculature. In other diseases, such as pancreatic cancer under certain conditions, cancerous cells may exhibit enhanced deformability, which may contribute to an increased probability of metastasis.Several projects in the Suresh research group aim to utilize micro-fabricated structures to experimentally evaluate the circulatory re-sponse of diseased human cells. In these studies, microfabricated channels of polydimethylsiloxane (PDMS) are used in conjunc-tion with a fluidic system and a high-speed camera to quantify the biorheological behavior of cells under different conditions. In the case of diseases involving RBCs, cells are made to pass through a narrow (~3 micrometer square) channel under a known pressure differential. During this process, shown in Figure 1, the average velocity and characteristic entrance and exit and shape recovery times are indicative of the overall biorheological response of the cell during microcirculation. Current results indicate large dif-ferences in rheological behavior of cells of different ages and in-cubation times. In addition, initial results from Plasmodium fal-ciparum parasitized RBCs indicate a possible effect of exported proteins from the malaria-inducing parasite to the surface of the RBC membrane on the rheological behavior of RBCs. Similar experiments are also being conducted on pancreatic cancer cells (Panc-1), as seen in Figure 2. In these cells, the biorheology of cells is assessed in environments similar to those experienced dur-ing metastasis. Future work with these systems will aim to further identify particular proteins or biochemical interactions that affect the biorheolgical and circulatory behavior of diseased cells with the aim of developing enhanced understanding of the mecha-nisms of disease progression and possible avenues for treatment.

Phase change Materials for Actuation in MEMS

Phase change materials (Sb and Te alloys) are used for optical data storage in commercial phase change memories, such as rewrit-able compact discs (CD±RW) and rewritable digital video disks (DVD±RW, DVD-RAM) [1]. Recently, they have also shown high potential for the development of phase change random ac-cess memories (PC-RAMs or PRAMs), which might replace flash memories in the future [2]. In this project, thin films of phase change materials are systematically analyzed with regard to their transformation behavior under laser-induced amorphization and crystallization. The goal of this project is to gain a better under-standing of the relationship among the laser parameters, the ma-terial-specific transformation kinetics, and the involved volume changes (and associated mechanical stresses) over a wide range of alloy compositions.The approach to pursuing this goal is to use microfabricated SiN cantilevers as substrates for thin film deposition: The SiN cantile-vers are manufactured by chemical vapor deposition of low-stress SiN on Si wafers, patterning the SiN film using optical lithogra-phy and revealing the cantilevers using dry etching and wet etch-ing. Thin films of phase change materials are subsequently sput-ter-deposited on these SiN cantilevers and are locally switched by laser heating from the amorphous to the crystalline phase (and vice versa). The associated stresses induce a cantilever bending, which is measured by optical microscopy and non-contact in-terferometry as a function of laser annealing parameters, laser quench rate and alloy composition (Figures 1 and 2). Additionally, amorphous films are hot-stage crystallized, which allows the study of the kinetics associated with the crystallization process as well as the force associated with the cantilever bending.The results of this project will help to increase the number of write-erase cycles and the data transfer rate in phase change memories and may lead to other applications of phase change materials in MEMS actuation.

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

There is growing interest in the microfabrication of electrodes for solid oxide fuel cells (SOFCs) in microionic devices [1]. Recently, we reported the fabrication of Pt/(Zr,Y)O2 (YSZ) nanocomposite electrodes by reactive magnetron co-sputtering [2]. Use of X-ray diffraction and X-ray photoelectron spectroscopy (XPS) charac-terization show these composites to be a two-phase system with no change of oxidation state from the constituent compounds. Electrical characterization via impedance spectroscopy demon-strated promising electrochemical properties at low temperatures; an area-specific resistance of 500 Ω cm2 was achieved at 400ºC. To test whether microfabricated thin-film electrolytes may suffer from degradation due to grain boundaries acting as short-circuit-ing diffusion pathways, sputtered NiO diffusion source films were in-diffused along grain boundaries into nanocrystalline CeO2 thin films grown by pulsed laser deposition (PLD), at tempera-tures from 700-800oC. The diffusion profiles were measured by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) at the Institute for Physical Chemistry at RWTH Aachen Univer-sity, Germany. These SIMS spectra, shown in Figure 2, point to a single diffusion mechanism, believed to be grain boundary diffusion, at these relatively low temperatures. Further work to systematically determine the unique opportunities and challenges associated with microstructured SOFCs is currently underway.

Nanowire- and Microsphere-templated Gas Sensors

Novel materials synthesis techniques were used to fabricate nano-structured and macroporous semiconducting metal oxide (SMO) films exhibiting exceptionally high sensitivity to reducing and oxi-dizing gases, as compared to conventionally prepared specimens. Increased sensitivity resulted from an elevated surface area and reduced specimen cross section. Several processing routes were pursued including electronspinning of semiconducting metal ox-ide (SMO) nanowires into a highly porous mat structure and mi-crosphere templating followed by pulsed laser deposition (PLD) of macroporous SMO material onto the microsphere templates.The TiO2/poly(vinyl acetate) composite nanofiber mats were electrospun onto interdigitated Pt electrode arrays, producing a mesh of 200-500 nm sheaths filled with ~10 nm thick single-crys-tal anatase fibrils. Testing in the presence of NO2 gas at 300°C demonstrated a minimum detection limit (MDL) of below 1 ppb1. Chemical and physical synthesis routes were combined to prepare macroporous CaCu3Ti4O12 and TiO2 thin films by PLD onto PMMA microsphere-templated substrates. Stable quasi-or-dered hollow hemispheres with diameter and wall thicknesses of 800 nm and 100 nm, respectively, were obtained (Figure 1). Cur-rent-voltage and impedance spectroscopy measurements point to the crucial role played by grain boundary barriers in controlling the electrical properties of these films. The macroporous CaCu-3Ti4O12 films exhibited a much superior H2 gas sensitivity (55ppm MDL) to non-templated films2 (Figure 2), while macroporous TiO2 films exhibit excellent NOx sensitivity. Studies are continu-ing to more carefully correlate sensor response with SMO micro-structure, morphology, and chemistry.

BioMEMS for control of the Stem cell Microenvironment

The stem cell microenvironment is influenced by several factors including cell-media, cell-cell, and cell-matrix interactions. Al-though conventional cell-culture techniques have been success-ful, they offer poor control of the cellular microenvironment. To enhance traditional techniques, we have designed a microscale system to perform parallel cell culture on a chip while controlling the microenvironment in novel ways. To control cell-matrix and cell-cell interactions, we use cell pat-terning. We have developed a simple cell-patterning technique (Figure 1 upper) that can pattern single cells onto arbitrary sub-strates [1]. Using this technique, we patterned clusters of mouse embryonic stem cells (mESCs) with different numbers of cells in each cluster (Figure 1 lower). We have also developed methods for single-cell patterning using dielectrophoresis (DEP), which uses non-uniform AC electric fields to position cells on or be-tween electrodes [2].To control cell-media interactions, we have developed a microflu-idic device for culturing adherent cells over a logarithmic range of flow rates [3]. The device (Figure 2, left) controls flow rates via a network of geometrically-set fluidic resistances connected to a syringe-pump drive. We use microfluidic perfusion to explore the effects of continuous flow on the soluble microenvironment. We cultured mESCs in standard serum-containing media across a 2000× range of flow rates. On day 1, colony areas were roughly constant along the axis of perfusion, implying negligible nutrient depletion. However, by day 3, we observed a significant decrease in colony size along the axis of perfusion at mid-range flow rates (Figure 2, right). At higher flow rates, colonies were uniformly large along the axis of perfusion, implying that nutrient depletion was not significant above certain flow rates. This microfabricated system will serve as an enabling technology that can be used to control the cellular microenvironment in pre-cise and unique ways, allowing us to perform novel cell biology experiments at the microscale.

Microfabricated Devices for Sorting cells Using complex Phenotypes

This research involves the development of numerous microfab-ricated sorting cytometer architectures for genetic screening of complex phenotypes in biological cells. Our various approaches combine the ability to observe and isolate individual mutant cells within surveyed populations. In this work we merge benefits of both microscopy and flow-assisted cell sorting (FACS) to offer unique capabilities in a single platform. Biologists will leverage these new affordances to isolate cells on the basis of observed dynamic and/or intracellular responses, enabling novel avenues for population screening. Our most recent approach to image-based sorting, which comple-ments our earlier work, utilizes a microfabricated array of PDMS microwell structures positioned in the floor of a microfluidic flow chamber (Figure 1) [1-2]. These microwells capture and hold cells in place for microscopy-based imaging, and can be optimized to trap single cells. After inspecting the array using microscopy to determine cells of interest, we apply radiation pressure from an infrared (IR) laser diode to levitate target cells out of the wells and into a flow stream. Released cells can be collected downstream for further analysis. The interconnect-free architecture scales eas-ily; we have presently implemented trap arrays containing more than 10,000 sites.Manipulating live cells, irrespective of the technique, will certain-ly have some effect on cellular behavior and physiology. It is im-perative that we understand the effects of our sorting techniques (both optical and electrical) on cellular physiology over a range of operating conditions for two main reasons: (1) to determine whether there are any gross effects (such as viability and changes in proliferation), and (2) to determine whether there are more subtle effects that alter complex phenotypes of interest. To this end we are designing a microfabricated device to perform electri-cal and optical “dose responses” to determine optimal regions of operation and using fluorescence-based stress reporter cell lines as sensors of physiological state (Figure 2).Figure 1: Microwell-based optical sorting. (A) Schematic sort based on fluorescence localization (nucleus vs. cytoplasm). Laser levitates target cells into the flow stream for downstream collection. (B) Section of well array. We remove a membrane-stained cell from a population of purely nuclear-stained cells. We used an argon laser here; we now use IR-beams to mitigate cell-damage concerns.p Figure 2: Sensing physiological state. (A) Green fluorescent protein (GFP) based stress reporter cell line which shows significant increase in fluorescence intensity, compared to control (B) after a 30-minute heat shock at 44ºC and a 14-hour recovery at 37ºC. Such a live cell sensor will allow us to perform fluorescence-based cell health assays on thousands of cells. Scale bars 20 µm.

Combined Microfluidic/Dielectrophoretic Microorganism Concentrators

This project focuses on the development of microfabricated mi-crofluidic/dielectrophoretic devices capable of concentrating micron-size particles from complex liquids. The concentrated particles of interest, such as pathogenic bacteria and spores, can then be delivered in small aliquots to the appropriate sensor for identification. Our micro-concentrator exploits the phenomenon of dielectrophoresis (DEP)–the force on polarizable particles in spatially non-uniform electric field [1]–to trap particles from the flow stream in order to subsequently concentrate them by release into a smaller volume of liquid. We create the non-uniform elec-tric field using interdigitated electrodes (IDE) at the bottom of the flow channel (Figure 1). To maximize the exposure of particles to the DEP field, we em-ploy a passive microfluidic mixer to circulate the liquid (Figure 1). One question that arises is how to determine the optimal mixer geometry for circulating the liquid, which may differ from the ide-al geometry for mixing two liquids. To answer this question we developed modeling tools and an experimental methodology to quantitatively predict the trapping behavior of particles in these systems. As Figure 2 shows, our modeling is able to predict the efficiency of different mixer configurations, without any fitting parameters. Among the four mixers tested (herringbone mixer (HM) slanted groove mixer (SGM), staggered herringbone mixer (SHM), and smooth channel (SMOOTH)), the HM and SHM perform similarly. This result is unexpected, as the HM is known to be a poor mixer of two liquids, while here we show that it is fine for circulating one liquid [2].

DEP cell-patterning for controlling cellular Organization

The ability to place cells at specific locations on a substrate is a useful tool to study and engineer interactions between cells [1], perform image-based cell selection [2], and create cell-based bio-sensors [3]. The ability to pattern with single-cell resolution is necessary in order to perform studies of single-cell physiology in which these cells are interacting with other cells. We have pre-viously created nDEP-based traps that were used to hold single micron-size beads at chosen locations on a substrate [4]. We have recently extended this work by modifying the design to allow us to manipulate and pattern single cells. We accomplished this modification by adding interdigitated electrodes to minimize non-specific cell adhesion and determining operating parameters that minimized heating and electric field exposure. The resulting structures are termed nDEP microwells to reflect that fact that they present an electrical microwell to incoming cells, allowing only cell-substrate attachment inside the DEP trap. With these nDEP microwells we have been able to place non-adherent cells and pattern adherent cells (Figure 1). Additionally, we have dem-onstrated that our cell-patterning technique does not affect gross cell phenotype as measured by morphology and proliferation. Fi-nally, we have developed a method that combines pressure-driven and convective flows to manipulate cells in two dimensions (Fig-ure 2).

Iso-dielectric Cell Separation

Increased throughput in the techniques used to engineer new metabolic pathways in unicellular organisms demands similarly high throughput tools for measuring the effects of these pathways on phenotype. For example, the metabolic engineer is often faced with the challenge of selecting the one genomic perturbation that produces a desired result out of tens of thousands of possibilities [1]. We propose a separation method – iso-dielectric separation, or IDS – which separates microorganisms continuously based on their intrinsic dielectric properties [2-3]. Because IDS is an equilibrium method, sorting cells according to their unique equi-librium positions in an energy landscape, it offers enhanced speci-ficity over other label-free separation methods [4]. This technol-ogy would enable high throughput screening of cells based upon electrically distinguishable phenotypes. Iso-dielectric separation uses dielectrophoresis (DEP) and media with spatially varying conductivity to create the energy landscape in which cells are separated according to their effective conduc-tivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis, and is thus applicable to uncharged particles, such as cells [5]. The IDS leverages many of the advantages of microfluidics and equilibrium gradient separation methods to create a device that is continuous-flow, capable of parallel separations of multiple (>2) subpopulations from a heterogeneous background, and label-free. We demon-strate the simultaneous separation of three types of polystyrene beads based upon surface conductance as well as sorting non-vi-able from viable cells of the budding yeast Saccharomyces cerevisiae (Figure 2). Current efforts are focused on the separation of Esch-erichia coli based upon the amount of the intracellular polymer poly(hydroxybutyrate) each cell contains.

MEMS Vibration Harvesting for Wireless Sensors

The recent development of “low power” (10’s-100’s of µW) sens-ing and data transmission devices, as well as protocols with which to connect them efficiently into large, dispersed networks of indi-vidual 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 structural health monitoring, par-ticularly for aircraft.This SHM application and the power levels required favor the piezoelectric harvesting of ambient vibration energy. Current work focuses on harvesting this energy with MEMS resonant structures of various geometries. Coupled electromechanical models for uniform beam structures have been developed to pre-dict the electrical and mechanical performance obtainable from ambient vibration sources. The optimized models have been vali-dated by comparison to prior published results [2] and verified by comparison to tests on a macro-scale device [3]. A non-opti-mized, uni-morph beam prototype (Figure 1) has been designed and modeled [4-5]. Dual optimal frequencies with equal peak powers and unequal voltages and currents are characteristic of the response of such coupled devices when operated at optimal load resistances (Figure 2). Design tools to allow device optimi-zation for a given vibration environment have been developed for both geometries. Future work will focus on fabrication and testing of optimized uni-morph and proof-of-concept bi-morph prototype beams. System integration and development, including modeling the power electronics, will be included.

Design, Fabrication, and Testing of Multilayered Microfabricated Solid Oxide Fuel cells (SOFcs)

Microfabricated solid oxide fuel cells were investigated for por-table power applications requiring high energy densities [1]. The thickness of the electrolyte, the travel length of oxygen ions, was reduced down to ~150nm. The tri-layers (yttria-stabilized zir-conia (YSZ) as an electrolyte and platinum-YSZ cermet as cath-ode/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 (25-600°C) [2], respectively, require careful thermome-chanically stable design of µSOFCs.First, material properties of the ultra-thin YSZ were character-ized experimentally and found to be significantly different than those of bulk YSZ [3]. Second, based on the obtained proper-ties, maximum stresses in the plates at 625ºC were analyzed us-ing non-linear von Karman plate theory [4]. The stresses showed three regions with sidelength variation: an un-buckled regime, a 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-~180µm sidelength and total ~450nm 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-struc-tured ultra-thin electrolyte, anode, and cathode layers, additional studies are needed to improve specimens and test setup and to assess µSOFCs’ long-term operational stability.

A Compact Flash X-Ray Source Based Upon Silicon Field Emitter Arrays

X-rays are used for non-destructive imaging in virtually every industry today. They enable doctors to make diagnostic decisions to quality assurance for electronics and industrial components. The machines that do these tasks are large, bulky, and occasionally slow due to the design of the X-ray system. Every X-ray imaging system is generally limited to up to 2 X-ray sources due to the cost but primarily the size. The majority of commercial X-ray sources are based upon thermionic emission or a heated filament similar to that of an incandescent light bulb. Because of the high temperature of the thermionic source, it is difficult to shrink the size of the X-ray tube. Field emission is a solution that has been touted for decades, but it has always had reliability problems. We resolve these problems by demonstrating that a high-performance and potentially compact flash X-ray source can be realized. The current X-ray setup, completed in collaboration with Massachusetts General Hospital (MGH) and shown in Figure 1, has been shown to be reliable enough to take hundreds of images for computed tomography to reconstruct a 3D image. These X-rays were taken in pulsed mode, where short bursts of a few hundred nanoseconds are used to turn the field emitter on, the first demonstration of its kind.

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

Magnetrons are a highly efficient (>90%), high-power 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 the emission of resonant microwave radiation. Current state-of-the-art magnetrons use a heated metal filament to thermionically emit electrons into vacuum continuously and are not addressable. This work seeks to replace the heated metal filament as a source of electrons with Si field emitter arrays to improve efficiency and increase power, especially when several sources are combined. Si field emitter arrays, schematically shown in Figure 1, are devices that are normally off and are capable of high current densities plus spatial and temporal addressing. These arrays consist of many sharp Si tips sitting on long Si nanowires that limit the current of the electron emission. Electrons from the Si tips tunnel into a vacuum as a result of the high electric field of the applied bias on the polysilicon gate. Pulsing the electric field applied on the gate can turn the arrays on and off. The proposed use of Si field emitter arrays in a magnetron will allow injection locking and hence phase control of magnetrons. Phase-controlled magnetrons have multiple applications in areas where high-power microwave sources are desired. Currently, Si field emitter arrays have been designed for the magnetron; our collaborators at Boise State University are testing them.

Acoustically-active Surface for Automobile Interiors Based on Piezoelectric Dome Arrays

The surfaces of automobile interiors can be rendered acoustically active by mounting on them thin, wide-ar-ea membranes with arrays of small acoustic transduc-ers. Each small, individually addressable transducer functions as a speaker or a microphone, and an en-tire pixelated acoustic membrane enables directional sound generation and sensing. The frequency response of the wide-area acoustically-active surface is deter-mined by those of the small isolated acoustic transduc-ers, which thereby yields better tunability of the band-width through designing pixel dimensions. As a result, the acoustically-active surface can work either in the audio frequency range for noise cancellation, personal entertainment, and communication with the vehicle, or - in the ultrasonic frequency range - for gesture de-tection, alertness monitoring, etc., which collectively improve the comfort and safety of the automobiles.This project seeks to develop and demonstrate a thin, wide-area acoustic “wallpaper” based on an array of dome-shaped piezoelectric transducers, which exhib-its outstanding performance and is deemed the most suitable option for miniaturization and scalable fabri-cation. Dependencies of device performance for both speaker and microphone applications on the material properties, dome dimensions, and back cavity struc-ture have been studied through theoretical modeling and numerical simulation. For speaker applications, a 12-μm thick, 8-inch wafer-size acoustic wallpaper con-sisting of an array of PVDF domes that are sub-1-mm in diameter is capable of generating over 60 dBSPL a half meter away with a 1-kHz, 10-V driving voltage, which can be further enhanced by scaling up the area and reducing the thickness of the membrane. On the other hand, reducing the radius of PVDF domes will lead to an extended bandwidth into the ultrasound range at a small cost of sound pressure level in the audio frequen-cy range. We have developed a scalable process to fabri-cate such acoustic wallpapers. A 1×1 cm2 sample (Figure 1) has been fabricated for demonstration and will be scaled up to a 10×10 cm2 wallpaper to explore prospec-tive applications of acoustically active surfaces.

The Scanning Anode Field Emission Microscope: A Tool for Mapping Emission Characteristics of Field Emitter Array Devices and Structures

We developed a scanning anode field emission micro-scope (SAFEM) for characterizing field emission array (FEA) devices and structures (see Figure 1). The SAFEM is designed to accurately and precisely scan and posi-tion a probing anode tip over emitter tips of an FEA and acquire maps of emitter tip current IE(x,y) and an-ode-to-emitter voltage VAE(x,y,) from which the map of spatial variation of the field factor β(x,y) and the distri-bution f(β) are extracted. The scanning and positioning movement is achieved by using two positioning stages. The first stage holds the device so a sample can move in the xyz direction with a travel range of 26 mm and resolution of 6 nm. The second stage holds the probing anode and can also move in the xyz direction with a travel range of 5 mm and a resolution of 0.03 nm. Its probing and imaging of emission currents from FEA devices and structures enable use of the SAFEM to ob-serve the aging, i.e., temporal evolution of FEA devices in vacuum and gas ambient, and to measure accurately the turn-on and operating voltage of FEA devices. The SAFEM is operational and has been used to obtain cur-rent maps of various FEA devices, as in Figure 2. Images obtained by the SAFEM have been used to optimize the fabrication of FEAs for high-current and long-lifetime operations. The resolution of the SAFEM is determined by scan step size, relative sizes of emitter tip radii (rE), an-ode tip radius (rA), and emitter array pitch (TP): rE << rA << TP . To obtain a well-resolved current map of FEA tips, the SAFEM is operated in the pulsed mode. The scanning motion, voltage (or current) sourcing, and current (or voltage) measuring functions of the SAFEM can be independently operated in either a pulsed or continuous mode. In the completely pulsed mode, the scanning anode moves a discrete scan step and enters a wait state long enough for the movement response to reach steady state. Once the steady state is reached, the source measure unit turns on the anode voltage and waits for the emission current response to also reach a steady value before the command to measure the cur-rent is issued. The duty cycles of the pulsing operation are critical to obtaining a well resolved current map. Operation in the pulsed mode significantly reduces the described noise current from the FEA substrate, nearby edges, nearby tips, and stage movements compared to other modes of operation.

Highly Uniform Silicon Field-Emitter Arrays

Cold cathodes based on silicon field-emitter arrays (FEAs) have shown promise in a variety of applications requiring high-current-density electron sources. However, FEAs face a number of challenges that have prevented them from achieving widespread use in commercial and military applications. One problem limiting the reliability of FEAs is emitter tip burnout due to Joule heating. The current fabrication process for FEAs results in a non-uniform distribution of emitter tip radii. At a fixed voltage, emitters with a small radius emit a higher current while emitters with a large radius emit a lower current. Therefore, emitters with a small radius reach their thermal limit due to Joule heating at lower voltages and consequently burn out. Previous solutions to mitigating tip burnout have focused on limiting the emitter current with resistors, transistors, or nanowires in order to obtain a more uniform emission current.In this project, we focused on increasing the uniformity of emitter tip radii as a means to reduce tip burnout. Figure 1 shows a typical distribution of emitter tip radii for FEAs. The non-uniform distribution of emitter tip radii first forms during the photolithography step that defines the array of “dots” that become the etching mask for the silicon tips. In our FEA fabrication process, we used a trilevel resist process that nearly eliminated the light wave reflected at the photoresist/silicon interface and hence improved the uniformity of the dot diameter. Furthermore, we integrated the emitter tips with silicon nanowires to improve their reliability. Figure 2 shows a diagram of the fabricated structure. Our fabrication process resulted in FEAs with a more uniform emission current and a potentially longer lifetime.

CMOS Opto-nanofluidics

CMOS (complementary metal-oxide-semiconductor) foundries offers designers access to nanometer scale patterns, a suite of readout and interface circuits, and, more importantly, the capability to mass-manufacture their designs. These features of microelectronic CMOS foundries have been extensively utilized for photonic applications. This abstract introduces their application to opto-nanofluidics. In MicroTAS 2017, we first reported the process of fabricating nanofluidic channels inside CMOS chips by defining the channels using the polysilicon gate layer and releasing the channels by sacrificial etching. Since then, we developed a packaging approach to accommodate mm-sized dies, which are the norm in multi-project wafer runs. The CMOS opto-nanofluidic chip in Figure 1 was fabricated in a 65-nm SOI CMOS process (10LP+) with integrated photodetectors.The packaging approach employs a low-cost epoxy material to extend the die area and a back-side machining step to planarize and thin the wafer down for subsequent lithography and etch steps. The I-V curves in Figure 2 indicate that the photodetectors are fully operational after the substrate extension and nanochannel release. Future chips will include an integrated amplifier with 0.6 μV/√Hz simulated input referred noise for improved sensitivity by lock-in detection. We plan to apply this CMOS opto-nanofluidic platform for single-molecule manipulation and sensing applications.

Increasing the Yield of Atmospheric Pressure Microsputtering for Fabrication of Agile Electronics

Additive manufacturing (AM) promises new, flexible production; however, while AM excels at creating structural parts, it cannot make functional objects well, e.g. multi-material structures such as electronic components and circuits. Sputtering, which removes material from a target atom-by-atom by using a plasma, is used in IC fabrication finely layered, multi-material fabrication. By miniaturizing the dimensions of the plasma reactor down to sub-millimeter scale, the sputterer can operate at atmospheric pressure, obviating the need for a vacuum. However, at atmospheric pressure, collisions with gas molecules scatter most of the sputtered material, preventing it from reaching the substrate. We develop plasma microsputterer technology that allows for high-resolution, high-quality deposition of arbitrary patterns, without any templates, pre-, or post-processing; recent results with a gold target include creating imprints with electrical conductivity within an order of magnitude to that of bulk metal. We explore two methods to minimize sputtered ma-terial scattering and to increase the deposition rate (yield). The first method minimizes the gap between the sputtering target and the substrate (Fig. 1): the sputtering target is placed 150 µm above the substrate. Dielectric barriers confine the plasma, forcing the plasma to connect the target wire and anode without damaging the substrate. This approach yields 0.2 nm/s (40 pg/s)—twice previous results. However, significant substrate heating occurs, which is incompatible with temperature-sensitive substrates. The second method harnesses convection to drive the sputtered material towards the substrate (Fig. 2). We surround the microsputter target (100 µm diameter) with a strong jet of air (100 m/s, 0.5 mm thick coaxial flow) to force air molecules to transport the sputtered material. This method greatly increases the yield (1 nm/s, 20 ng/s)— 30% of the sputtered material reaches the substrate. Current work focuses on further increasing the deposition rate by increasing the rate at which atoms are sputtered.

Silicon MEMS Compatible Bipropellant Micro Rocket Engine Using Steam Injector

Rocket engines miniaturized and fabricated using MEMS or other techniques have been an active area of research for two decades. At these scales, miniaturized steam injectors like those used in Victorian-era steam locomotives are viable as a pumping mechanism and offer an alternative to pressure feed and high-speed turbo-pumps. Storing propellants at low pressure reduces tank mass, and this improves the vehicle empty-to-gross mass ratio; if one propellant is responsible for most of the propellant mass (e.g., oxidizer), injecting it while leaving the others solid or pressure-fed can still achieve much of the potential gain. Previously, the principal investigator and his group built and tested ultraminiature-machined micro jet injectors that pumped ethanol and also explored liquid and, more recently, hybrid engine designs. Recent work has focused on designing and implementing a whole-engine test article that simultaneously integrates a steam injector, boiler, decomposition chamber, fuel injector and thrust chamber, that is practical to build, and that is compatible with MEMS fabrication. An axisymmetric engineering mockup in brass was built to demonstrate the feasibility of the design concept (see Figure 1). Configurations that combine electrically-driven pumps with steam injectors by, for example, using electric pumps to pump fuel or coolant and a steam injector motivated by boiled coolant to pump oxidizer are also being explored. These would allow pressurized tanks to be avoided altogether while still being compatible with miniaturization via MEMS.

Gated Silicon Field Ionization Arrays for Compact Neutron Sources

Neutron radiation is widely used in various applications, ranging from the analysis of the composition and structure of materials and cancer therapy to neutron imaging for security. However, most applications require a large neutron flux that is often achieved only in large infrastructures such as nuclear reactors and accelerators. Neutrons are generated by ionizing deuterium (D2) to produce deuterium ions (D+) that can be accelerated towards a target loaded with either D or tritium (T). The reaction generates neutrons and isotopes of He, with the D-T reaction producing the higher neutron yield. Classic ion sources require extremely high positive electric fields, on the order of 108 volts per centimeter (10 V/nm). Such a field is achievable only in the vicinity of sharp electrodes under a large bias; consequently, ion sources for neutron generation are bulky. This work explores, as an alternative, highly scalable and compact Si field ionization arrays (FIAs) with a unique device architecture that uses self-aligned gates and a high-aspect-ratio (~40:1) silicon nanowire current limiter to regulate electron flow to each field emitter tip in the array (Figure 1). The tip radius has a log-normal distribution with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm in diameter and is within 200 nm of the tip. Field factors, β, > 1 × 106 cm-1 can be achieved with these Si FIAs, implying that gate-emitter voltages of 250-300 V (if not less) can produce D+ based on the tip field of 25-30 V/nm. In this work, our devices achieve an ionization current of up to 5 nA at ~140 V for D2 at pressures of 10 mTorr. Gases such as He and Ar can also be ionized at voltages (<100 V) with these compact Si FIAs (Figure 2).

Silicon Field Emitter Arrays (FEAs) with Focusing Gate and Integrated Nanowire Current Limiter

The advent of microfabrication has enabled scalable and high-density Si field emitter arrays (FEAs). These are advantageous due to compatibility with complementary metal-oxide semiconductor (CMOS) processes, the maturity of the technology, and the ease in fabricating sharp tips using oxidation. The use of a current limiter is necessary to avoid burn-out of the sharper tips. Active methods using integrated MOS field-effect transistors and passive methods using a nano-pillar (~200-nm wide, 8-µm tall) in conjunction with the tip have been demonstrated. Si FEAs with single gates reported in our previous works have current densities of >100 A/cm2 and operate with lifetimes of over 100 hours. The need for another gate (Figure 1) becomes essential to control the focal spot size of the electron beam as electrons leaving the tip have an emission angle of  12.5. The focus electrode provides a radial electric field that reduces the lateral velocity of stray electrons and narrows the cone angle of the beam reaching the anode. Varying the voltage on the focus gate reduces the focal spot size or achieves an electron beam modulator for radio frequency applications. In this work, we fabricate dense (1-μm pitch) double-gated Si with an integrated nanowire current limiter (Figure 2). The apertures are ~350 nm and ~550 nm for the extractor and focus gates, respectively, with a 350-nm-thick oxide insulator separating the two gates. Electrical characterization of the fabricated devices shows that the focus-to-gate ratio (VFE/VGE) can be used to control the anode current (Figure 2). When the focus voltage exceeds the gate voltage, the field superposition increases the extracted current, and vice versa. These devices can potentially find applications as high-current focused electron sources in flat panel displays, nano-focused X-ray generation, and microwave tubes.

Electron Transparent Anodes for Field Emission Cathodes in Poor Vacuum

Nanoscale Vacuum Channel Transistors (NVCTs) using field emission sources could potentially have superior performance compared to solid state devices of similar channel length. This is due to ballistic transport of electrons, shorter transit time and higher breakdown voltage in vacuum. Furthermore, there is no opportunity for ionization or avalanche carrier multiplication imbuing NVCTs with very high Johnson figure of merit (~1014 V/s). However, field emitters need ultra-high vacuum (UHV) for reliable operation as the field emission process is sensitive to barrier height variations induced by adsorption/desorption of gas molecules. Small changes in the barrier height cause exponential variations in current. Poor vacuum also leads to generation of energetic ions that bombard the emitters, altering the work function and degrading electrical performance. To overcome the UHV requirement, graphene can be used to nano-encapsulate the field emitter in UHV or a gas (e.g. He) with high ionization energy. Separation of the electron tunneling region from the electron acceleration region enables emission of electrons in UHV and electron transport in poor vacuum, if not atmospheric conditions. For mechanical strength, a multi-layer graphene structure that is transparent to electrons while being impervious to gas molecules/ions is necessary. In this work experimentally characterize the electron transparency of graphene membranes using arrays of gated Si field emitters with 1 µm pitch (Figure 1) that exhibit transistor-like characteristics. Using an energized multi-layer graphene/grid structure (Figure 2) in combination with emitter arrays, we measured extremely high electron yield perhaps due to secondary emission from electrons impinging on the graphene layer. Adopting this architecture for NVCTs will allow the realization of empty state electronics capable of functioning at higher frequencies (THz regime) higher power and harsher conditions (high radiation and high temperature) compared to solid state electronics.

GaN Vertical Nanowires with Self-Aligned Gates for Field Emission Applications

Field emitters (FE), or namely vacuum transistors, are promising for harsh-environments and high-frequency electronics thanks to their radiation hardness and scattering-free electron transport. However, the stability and operating voltage still need improvement to enable circuit applications. To overcome these issues, III-Nitrides are excellent candidates due to their strong bonding energies and tunable electron affinities. Though the material properties of III-Nitrides are promising, so far, there are few works demonstrating sub-100 V turn on as most III-N FEs are still two-terminal structures.In this work, we demonstrate a novel GaN nanowire (NW) FEs based on self-aligned gates to reduce the gate-emitter turn-on voltage (VGE, ON) below 30 V. The GaN on Si wafer was grown by Enkris Semiconductor, Inc. Thanks to a new GaN processing technology, we successfully fabricate GaN NWs with width of 60 nm and aspect-ratio of 5 (Figure 1 (a)). The gate stack is then conformally deposited. We then finish the device fabrication by dry etching to open FEs’ tips (Figure 1 (b)). We measure the transfer characteristics with a suspended 0.5-mm-diameter tungsten ball biased at +500 V as an anode (Fig. 2(a)). Device turns on at 27 V. This device demonstrates the lowest turn-on voltage among GaN field emitters in literature, as well as excellent current density (Fig. 2 (b)) and shows great potential for integrated circuit applications.

3D-Printed, Miniature, Multi-material, Valve-less, Magnetically Actuated Liquid Pumps

Miniaturized pumps can be used to supply precise flow rates of liquid in compact systems. Numerous micro-fabricated positive displacement pumps for liquids with chamber volumes that are cycled using valves have been proposed. Pumps made via standard (i.e., cleanroom) micro-fabrication typically cannot deliver large flow rates without integrating hydraulic amplifi-cation or operating at high frequency due to their small pump chambers.3D-Printing has recently been explored as a pro-cessing arena for microsystems; in particular, research-ers have reported 3D printed pumps for liquids and gas-es with performance on par with or better than coun-terparts made with standard microfabrication. Building on earlier work on printed magnetically actuated liquid pumps, we 3D-printed multi-material, magnetically driven, valve-less miniature liquid pumps. We used the fused filament fabrication (FFF) method: a thermoplastic filament is extruded from a hot nozzle to create, layer by layer, a solid object. The body of the pump is printed in Nylon 12, while the actuation mag-net is printed in Nylon 12 containing NdFeB micro-par-ticles. The devices are driven by a non-contact rotating magnet and employ valve-less diffusers to greatly sim-plify operation. Our low-cost, leak-tight, miniature devices are microfabricated using 150- and 225-µm layers with a multi-step, multi-material printing process (Figure 1) that monolithically creates all key features with <13-µm in-plane misalignment. Each pump has a frame, a 225-µm-thick membrane connected to a piston with an embedded magnet, a chamber, two diffusers, and two fluidic connectors (Figure 2). Fabrication of the pump requires under 75 minutes and costs less than $3.89. Fi-nite element analysis of the actuator predicts a maxi-mum stress of 15.7 MPa @ 100 μm deflection, i.e., below the fatigue limit of Nylon 12 for infinite life (i.e., 19 MPa). Water flow rate up to 1.68 ml/min at an actuation fre-quency of 204 Hz was measured.

Measurement of the Condensation Coefficient of Water Using an Ultrathin, Nanoporous Membrane

In applications ranging from electronics cooling and power generation cycles to distillation, liquid-vapor phase change phenomena play a critical role. At their fundamental (kinetic) limits, evaporation and con-densation are dictated by the resistance to molecules crossing the liquid-vapor interface, which is quantified by the condensation coefficient. Despite its fundamen-tal importance and widespread use in heat transfer models such as the Schrage equation, the condensation coefficient of water has been difficult to characterize, with experimental results and theoretical calculations spanning three orders of magnitude. Experimental measurement has been challenging because three con-ditions must be satisfied: sensitivity to the condensa-tion coefficient is high, temperature of the liquid-vapor interface is precisely yet noninvasively measured, and the concentration of contaminants at the liquid-vapor interface is low. To achieve a precise measurement of the condensation coefficient of water, we have fabricat-ed an ultrathin (~200 nm), nanoporous (~150 nm diame-ter), hydrophobic membrane for forward-osmosis (FO) driven transport (Figure 1). Due to the ultrathin, low-as-pect ratio dimensions of the membrane, we achieve high sensitivity to the condensation coefficient and avoid undesired contaminant buildup at the interface because the membrane is freestanding. Since transport is driven by osmotic pressure, the system can be main-tained at isothermal conditions such that the tempera-ture can be precisely measured in the bulk water with-out interfering with the liquid-vapor interface. These experimental measurements of the condensation co-efficient of water are crucial for modeling liquid-vapor phase change in nanoscale systems and advanced ther-mal management devices.

In-plane Gated Field Emission Electron Sources via Multi-material Extrusion

Field emission is the quantum tunneling of electrons to vacuum due to local high electrostatic fields; such high fields can be generated at a moderate voltage using nanosharp, high-aspect-ratio tips. Compared to thermionic counterparts, field emission cathodes consume less energy, respond faster, and can operate in poorer vacuum, making them attractive in compact applications such as nanosatellite electric propulsion, portable mass spectrometry, and handheld X-ray generation. A wide variety of materials has been explored as field emitters; the research in field emission electron sources has focused on carbon nanotubes (CNTs) due to their nanosized tip diameter, high aspect-ratio, high electrical conductivity, and excellent chemical stability. However, most manufacturing methods for CNT field emission electron sources have associated large cost, long processing time, need of static masks for defining in specific locations the nanostructured emitting material and/or the electrode(s), and large gate interception (or the need for advanced assembly methods to attain high transmission). In this project, we are developing low-cost field emission cathodes via multi-material extrusion. The devices are flat plates with two concentric imprints (Figure 1): an imprint made of CNTs (emitting electrode), symmetrically surrounded on both sides by an imprint made of Ag microparticles (extractor gate). Unlike the great majority of field emission cathodes reported that have an out-of-plane gate electrode, our devices have an in-plane gate that significantly reduces the cost and manufacturing complexity of the device and also facilitates high gate transmission. Our devices can emit electrons in vacuum with as little as 62 V applied between the CNT imprint and the Ag imprint and achieve over 97% gate transmission (Fig. 2). Current work focuses on increasing the imprint density to attain larger current density emission and on developing ballasting structures for attaining large and uniform array emission.

Additively Manufactured, Miniature Electrohydrodynamic Gas Pumps

A corona discharge is a self-sustained physical phenomenon induced around the sharper electrode of a diode due to sharply nonuniform electric fields within the interelectrode space. Ion propagation across such a space is accompanied by collisions with neutral particles, resulting in bulk fluid movement known as ionic wind. In contrast to traditional counterparts, ionic wind pumps have no moving parts, respond faster, and produce significantly less noise, drawing great interest in applications such as air propulsion and electronics cooling. Currently, ionic wind pump technology is far from practical in applications that require large flow velocity, flow rate, and power efficiency; another concern is the stability of the pump, given that ion accumulation in the interelectrode space can cause an electric short during sustained operation. Researchers have proposed using active electrodes with a plurality of field enhancers arranged in parallel (multiplexing) to maximize throughput; however, the reported multi-needle devices are serially assembled, and their performance is inferior to that of single-needle counterparts. This project uses metal additive manufacturing and electropolishing to create miniature, multi-needle ionic wind pumps. Our devices are needle-ring corona diodes composed of a monolithic inkjet binder-printed active electrode (Figure 1), made in stainless steel 316L, with a plurality of sharp, conical needles and a thin plate copper counter-electrode, with electro-chemically etched apertures aligned to the needle array. Five-needle ionic wind pumps eject air at 2.9 m/s and at a volumetric flow rate of 343 cm3/s, three times larger than the flow rate of a single-tip device with comparable efficiency (Figure 2). Current work systematically studies the relevant parameters to optimize the design of the electrohydrodynamic pump.

3D-Printed Silver Catalytic Microreactors for Efficient Decomposition of Hydrogen Peroxide

Microreactors increase the surface-to-volume ratio of their reactants and by-products, resulting in faster, more efficient reactions and better heat transfer than in their non-miniaturized counterparts, leading to higher throughput per unit of reactor active volume and to better selectivity in the species produced by the reactor. The great majority of microreactors are made of polydimethylsiloxane (PDMS)—a material that cannot operate at elevated pressures or temperatures. Other reported microreactors are made in silicon, ceramics, or metals; although these materials are compatible with high-pressure and high-temperature operation, they have associated a very high production cost because they are made in a semiconductor cleanroom or with specialized, low-throughput tooling, e.g., electro discharge machining. Hydrogen peroxide (H202), a water-soluble oxidant, spontaneously decomposes in the presence of heat or a catalyst. Applications of a H2O2 catalytic reactor include monopropellant rocket propulsion, steam generators, and pumping; miniaturized versions of such catalytic reactors are of great interest to PowerMEMS. Here, we developed a novel additive manufacturing technique based on silver clay extrusion to create high-pressure compatible and high-temperature compatible, monolithic microfluidics; silver is also a very efficient and effective catalyst for the decomposition of H2O2. Our microreactors are composed of a water-tight microchannel connected to the exterior via two fluidic ports (Figure 1). The experimental performance of the microreactor as a catalytic decomposer of H2O2 matches well our reduced-order modeling estimates (Figure 2), attaining a decomposition efficiency of 87% for a flow rate of 5 μL/min of H2O2 with an initial concentration of 30% w/w. Current research focuses on exploring other applications, e.g., heat exchangers.

Management of Brine Effluent from the Desalination Plant

While large-scale desalination has been a mainstay in a country with a severe water shortage for many decades, management of high concentration brine effluent (> 50,000 TDS) has posed technological, economic, and environmental challenges. There are two research directions to treat the brine effluent effectively: (1) reduce the total volume of effluent onshore and (2) discharge the effluent offshore to minimize environmental impact. Interestingly, the production of effluent is tons of liters, but both studies implement a diffusion process of molecule in the effluent, which appears on a microscopic scale. Here, two technologies are briefly introduced: ion concentration polarization (ICP) to reduce effluent volume and offshore discharge of effluent using plunging liquid jets to minimize environmental impact.ICP process is a novel electrochemical desalination technology, which emerged within the last decade as a viable option for effluent treatment (Figure 1). ICP employs only a cation exchange membrane (CEM) to utilize a higher diffusivity of chloride (t^(β-)), which is the majority ion in the effluent. Compared with conventional electrochemical desalination such as ED (electrodialysis), it is more energy-efficient, less susceptible to various fouling, and can be implemented with a much smaller footprint. Our group has developed and matured the technology over the years to realize the first-ever lab-scale ICP desalination prototype (~0.1L/min), demonstrating its technical and economic feasibility, and secured several key intellectual properties on this technology. For the offshore discharge of brine, we are investigating the use of plunging liquid jets (Figure 2) through laboratory experiments. Similar to the widely used offshore discharge outfalls such as submerged or surface jets, plunging jets also utilize the high momentum and negative buoyancy of brine to induce mixing with the surrounding ocean water and reduce the concentration of contaminants such as salt, anti-fouling agents, and anti-scalants that, in turn, reduce the environmental impact. However, unlike these outfalls, plunging jets also introduce air into the water column which, when dissolved, can reduce the environmental impact associated with the creation of hypoxic (low dissolved oxygen) zones.

Reduced-order Modeling of Oil Transport in Internal Combustion Engines Based on Autoencoder

Reducing emissions of internal combustion engines is the major focus in the modern automotive industry. Lubrication oil leakage from the piston ring pack is critical to oil consumption and emission. The oil transport mechanism is not well understood due to the computational complexity of the oil motion and ring pack dynamics and experimental difficulty. This raises our interest to build a reduced-order model predicting the oil movement inside the ring pack with acceptable accuracy and efficiency.In this work, we proposed a neural-network-based method to perform model order-reduction (MOR) on the computational fluid dynamics (CFD). First, we use a variational autoencoder (VAE) to address the reduce basis of the fluid field and encode the original space into the reduced space. Second, we apply a recurrent neural network (RNN) to learn the dynamics of the reduced space. To guarantee the stability of system dynamics, certain physics-based conservation law and stability regularization are included in the loss function. This method can reduce the fluid dynamics model calculation time by orders of magnitude with acceptable accuracy for analysis. With the reduced-order oil transport model coupled with the piston ring dynamics model, we can quantitatively analyze the mechanisms for oil leakage and inspire design optimization in automotive industry. The methodology developed in the work is not limited to fluids. The same procedures are applicable to other physical system modeling. Further, the methodology can interpret the neural network behavior from the perspective of model order-reduction.

A Four-terminal Nanoelectromechanical Switch Based on Compressible Self-assembled Molecules

Nanoelectromechanical (NEM) switches are under investigation as complements to, or substitutes for, CMOS switches owing to their intrinsic quasi-zero static leakage, large ON-OFF conductance ratio, and high robustness in harsh environments. For most NEM contact switches, a trade-off between high actuation voltage and the risk of stiction failure seems inevita-ble due to the strong van der Waals attraction between contacts at the nanoscale. This attraction leads to un-favorable dynamic power consumption and decreased reliability. Through this research, we have developed a novel tunneling NEM switch, termed a “squitch”, based on a metal-molecule-metal junction whose tunneling gap can be modulated by compressing the molecule lay-er with electrostatic force created by a voltage applied between the metal electrodes. In contrast to conven-tional NEM contact switches, direct contact of squitch electrodes in the ON state is avoided by assembling a molecular spacer between the electrodes; the molecu-lar spacer acts to hold the squitch together and helps reduce hysteresis and the possibility of stiction failure.A multi-terminal squitch has been demonstrated using a chemically-synthesized Au nanorod as a floating top electrode, and bottom Au electrodes patterned with electron beam lithography. With the help of a peeling technique that we have developed, Au electrodes are created with sub-nanometer roughness. The electrodes include two actuation gates recessed by several nanometers via a graphene sacrificial layer. By choosing molecules with appropriate chain lengths, we are able to define nanometer-wide electrode-to-nanorod gaps, which can be subsequently adjusted by a bias voltage applied between the gate electrodes. With a proper bias voltage, we can exponentially modulate the conduction current through a small variation of the gating voltage. Our squitch has been experimentally demonstrated to exhibit low actuation voltage and hysteresis, which supports its prospects in ultra-low power logic applications.

Silicon Field Ionization Arrays Operating > 200 V for Deuterium Ionizers

Devices that can field-ionize gas molecules at low bias voltages are essential for many applications such as ion mobility spectrometry and highly selective porta-ble gas sensing. Field ionization consists of a valence electron of a gas atom or molecule tunneling through a potential barrier, commonly into a vacant energy state of the conduction band of a metal at the anode. Clas-sic ion sources require extremely high positive electric fields, of the order of 108 volts per centimeter. Such fields are only achievable in the vicinity of very sharp electrodes under a large bias. Ion sources based on mi-crowave plasma generation have demonstrated high currents and high current densities. Yet, they are bulky and require large magnetic fields. Alternatively, single or arrays of gated tip structures have been used as field ionizers, but they emit low currents (~10 nA). Early tip burn-out due to non-uniform tip distribution and low voltage breakdown are the two main causes of such low currents. In this work, Si field ionization arrays (FIAs) with a unique device architecture that uses a high-aspect-ratio (~50:1) silicon nanowire current limiter to regulate electron flow to each field emitter tip in the array is proposed. The nanowires are 10 μm in height, 1 µm apart and 100-200 nm in diameter. A dielectric matrix of (Si3N4/SiO2) supports a poly-Si gate while a 3 μm thick dielectric holds the contacts. Current densities >100 A/cm2 and lifetime > 100 hours have already been reported. The tip radius has a log-normal distribution varying from 2 to 8 nm with a mean of 5 nm and a standard deviation of 1.5 nm, while the gate aperture is ~350 nm. Field factors, β, > 1 × 106 cm-1 can be achieved with these devices implying that voltages of 250-300 V (if not less) can produce D+ ions based on the tip field of 25-30 V/nm. Completed chips on a package are shown in Figure 1 together with a schematic of the test set-up for field ionization.Breakdown at the mesa edge at voltages ~70 V was the reported by Guerrera, et al. However this has now been overcome by etching a vertical sidewall profile (Figure 2) with a combination of both SF6 and C4F8 flowing simultaneously (Figure 2). I-V characterization in air demonstrates breakdown occurs within the active region (Figure 2) possibly due to the narrow gate apertures and the short oxide thickness from the tip to the poly-Si gate. Initial results show that further etching this oxide to expose the nanowire increases the oxide separation to the gate, which in turns increases the breakdown voltage (Figure 2), thus enabling the Si FIAs to be operated at voltages exceeding 200 V.

Highly Uniform Silicon Field Emitter Arrays

Cold cathodes based on silicon field emitter arrays (FEAs) have shown promising potential in a variety of applications requiring high current density electron sources. However, FEAs face a number of challenges that have prevented them from achieving widespread use in commercial and military applications. One problem limiting the reliability of FEAs is emitter tip burnout due to Joule heating. The current fabrication process for FEAs results in a non-uniform distribution of emitter tip radii. At a fixed voltage, emitters with a small radius emit a higher current while emitters with a large radius emit a lower current. Therefore, emitters with a small radius reach their thermal limit due to Joule heating at lower voltages and consequently burnout. Previous solutions to mitigating tip burnout have focused on limiting the emitter current with resistors, transistors, or nanowires in order to obtain more uniform emission current.In this project, we focus on increasing the uniformity of emitter tip radii as a means to reduce tip burnout. Figure 1 shows a typical distribution of emitter tip radii for FEAs. The non-uniform distribution of emitter tip radii first forms during the photolithography step that defines the array of “dots” which become the etching mask for the silicon tips. In our FEA fabrication process, we use a tri-level resist process that nearly eliminates the light wave reflected at the photoresist/silicon interface, and hence improves the uniformity of the dot diameter. Furthermore, we integrate the emitter tips with silicon nanowires to improve their reliability. Figure 2 shows a diagram of the fabricated structure. We expect our fabrication process to result in FEAs with more uniform emission current and potentially longer lifetime.

High Current Density Silicon Field Emitter Arrays (FEAs) with Integrated Extractor and Focus Gates

Cold electron sources have been identified as alter-natives to thermionic emitters due to their lower op-erating temperature, instant response to the applied electric field, and their exponential current-voltage characteristics. With the advent of microfabrication, the generation of high electric fields around sharp emitters to tunnel electrons was made possible. Scal-able and high-density field emitter arrays (FEAs) based on Si are advantageous due to compatibility with CMOS processes, maturity of technology, and the ease to fabricate sharp tips using oxidation. The use of a cur-rent limiter is necessary to avoid burning of the sharp-er tips; active method using an integrated MOSFET, or passive methods using a nanopillar (~200 nm wide, 10 µm tall) in conjunction with the tip has been demon-strated. Si FEAs reported by Guerrera, et al., exhibited high current densities exceeding 100 A/cm2 and having lifetimes of over 100 hours. The need for another gate (Figure 1) becomes essential to control the focal spot size of the beam as the tips become blunt with time and as a consequence, the turn-on voltage also increases (Figure 2). With the focus electrode, stray electrons extracted by the gate closest to the tip will be captured and only electrons emitted within a certain cone angle will reach the anode, thus achieving a narrower focal spot size compared to a single gated Si FEA. The voltage on the focus gate can be varied with time to maintain a fixed focal spot size or even as an electron control switch. Indeed having a high positive focus voltage pulls all the extracted electrons and can be used to prevent electrons reaching the anode. This also offers the opportunity for fast switching of these Si FEAs, which has been a limitation thus far of these devices. In this work we are optimizing the process steps to fabricate Si FEAs with the two integrated gates and current limiter, to characterize the effects of the focus gate on electron emission. These devices will find applications in flat panel displays, nanofocused X-ray sources, microwave tubes, and triodes.

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, schematically shown in Figure 1, are devices that are normally off and are capable of high current densities plus spatial and temporal addressing. These arrays consist of many sharp tips made of silicon sitting on long silicon nanowires that limit the current of the electron emission. Electrons from the silicon tip tun-nel into a vacuum as a result of the high electric field of the applied 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 magnetrons have multiple applications in areas where high- power microwave sources are desired. Currently, Si field emitter arrays have been designed for the mag-netron and are undergoing testing with collaborators at Boise State University.

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 consumption. This substantial reduction in the capital cost will drastically increase the availability of semi-conductor fabrication technology and enable experi-mentation, prototyping, and small-scale production to occur locally and economically. Our research in the last few years has been primarily focused on developing and characterizing tools for the 1” Fab. In previous years, we demonstrated a deep reactive ion etching (DRIE) tool and a corresponding modular vacuum tool architecture, and we are now working to develop a reactive magnetron sputtering tool and an inductively coupled plasma-based PECVD tool (ICP-CVD) for depositing a wide variety of materials. The reactive magnetron sputtering tools operates using a 2” target and a direct sputtering configuration and is fully integrable with the modular tool architecture of the 1” Fab. We have demonstrated the functionality of the tool with the depositions of copper, aluminum, and via reactive sputtering, aluminum nitride. The system has been characterized using a response surface methodology and consistent, uniform depositions with <6% variation across the wafer have been shown. The ICP-CVD tool has also been built within the modular tool architecture and is being tested with depositions of SiO2, SiNx, and a-Si. The use of an ICP source allows depositions to occur at temperatures as low as 25°C, with low hydrogen incorporation, and quality approaching that of LPCVD depositions. Film stress and index of refraction are also controllable.

Is the Surface Wickability the Single Descriptor of Critical Heat Flux during Pool Boiling?

Enhancement and estimation of critical heat flux (CHF) are two of the most important research areas of pool boiling. It is well-known that microstructured surfaces can extend the limit of CHF up to ~250% higher than that of a flat surface. The mechanism for this enhance-ment has generally been accepted as the wickability of structured surfaces originating from liquid propaga-tion within the surface structures driven by capillary pressure. We investigated the applicability of this the-ory based on the accumulated data of previous studies and our experimental data. We first calculated capil-lary pressure and permeability of structured surfaces to characterize liquid propagation rate analytically. We then performed pool boiling experiments on silicon mi-cropillar surfaces to measure CHF values. We found that there is no distinct relationship between the CHF and wickability contrary to a general notion. Our results suggest that although liquid wicking has been found to be important, the parameter wickability defined by previous works alone is not sufficient to describe CHF. In addition to the wickability, we propose that there may be other important parameters that also change along with the surface structures, e.g., the diameter of vapor columns and bubble departure size, among others, which need further investigation.

Carbon Nanotubes Based-field Emitters by 3-D Printing

In field emission, electrons are ejected from a solid sur-face via quantum tunneling due to the presence of a high local electrostatic field. Compared to state-of-the-art thermionic electron sources, field emission cathodes consume significantly less power, are faster to switch, and could operate at higher pressure. Field emission cathodes have a wide range of applications such as X-ray sources, flat-panel displays, and electron microscopy. Several materials, e.g., Si, ZnO, and graphene, have been explored as field emission sources; however, carbon nanotubes (CNTs) are very promising to implement field emission cathodes due to their high aspect ratio, high electrical conductivity, excellent mechanical, and chemical stability, and high current emission density. Reported approaches for fabricating CNT field emitters include screen printing and direct growth of nanostructures (e.g., plasma-enhanced chemical vapor deposition) where a static stencil, i.e., mask, is involved to produce patterned structures in specific locations. These masks increase the time and cost needed to iterate the pattern, affecting the prototype optimization of the cathode. Ink direct writing (IDW), i.e., the creation of imprints by extrusion of liquid suspensions through a small nozzle, has emerged as an attractive maskless patterning technique that can accommodate a great variety of materials to create freeform imprints at low-cost (Figure 1). An imprint with CNTs protruding from the surface of the imprint (Figure 2), strongly adhered to the substrate can achieve stably high-current emission when an electric field is applied. We are currently working on the design and optimization of the formulation of a CNT-based ink, to eventually demonstrate low-cost field emission sources.

Electron Impact Gas Ionizer with 3-D Printed Housing and NEMS Si Field Emission Cathode for Compact Mass Spectrometry

Mass spectrometry is widely used to quantitatively determine the composition of samples. However, the bulky size and high-power consumption of conven-tional mass spectrometry instruments limit their por-tability and deployability. One of the key components of a mass spectrometer (MS) is the ionizer. State-of-the-art electron impact gas ionizers use a stream of electrons produced by a thermionic cathode to create ions by fragmentation. Field emission cathodes, based on quantum tunneling of electrons triggered by high electrostatic fields, are a better alternative for portable mass spectrometry of gases compared to mainstream thermionic cathodes because they consume signifi-cantly less power, are faster to switch, and could oper-ate at higher pressure.In this project, we are developing a compact electron impact gas ionizer based on a cleanroom-microfabricated cathode and a 3-D printed ionization housing (Figure 1). The cathode is an array of nano-sharp silicon field emitters with proximal, self-aligned extractor gate, while the ionization housing is composed of an ionization region surrounded by an ionization cage, an anode electrode, a repeller electrode, and a dielectric structure that holds together the electrodes. To produce ions (i) a high enough bias voltage is applied between the extractor gate and the silicon tips, shooting electrons into the ionization region, (ii) the anode electrode attracts the emitted electrons, forcing them to interact with the neutral gas molecules within the ionization region, (iii) the bias voltage of the ionization cage maximizes the ionization yield of the interaction between the electrons and the neutral gas molecules, and (iv) the repeller electrode pushes ions out of the ionization cage. Figure 2 shows an assembled ionizer. Current work is focused on characterization of the field emission cathode and gas ionizer at various conditions.

Printed Piezoelectric Thin Films via Electrohydrodynamic Deposition

Piezoelectric components have found applications in a variety of fields including energy harvesting, biological and chemical sensing, and telecommunications. The creation of piezoelectric thin films has made possible the implementation of exciting devices that operate at higher frequency (a consequence of the reduction of the thickness of the piezoelectric material) including highly sensitive gravimetric biosensors and acousto-fluidic actuators. However, traditional manufactur-ing methods for piezoelectrics require a high vacuum, show low deposition rates, involve expensive and com-plex equipment, and require additional microfabrica-tion processes to achieve the required geometries via patterning and lithography.Electrohydrodynamic deposition harnesses the electrospray phenomenon to create ultrathin imprints from liquid feedstock (Figure 1). When the electrospray emitter operates in the cone-jet mode, stable jetting of the liquid feedstock allows for the direct writing of structures, thus, eliminating the need for steps for material removal, e.g., mask transfer and etching (Figure 2). In addition, electrohydrodynamic deposition can operate at room temperature without the need for a vacuum and can be scaled-up via electrospray emitter multiplexing.This project aims to produce piezoelectric thin films suitable for acoustic resonators and actuators via electrospray jetting of nanoparticle-doped liquid feedstock. Initial work revolved around the optimization of the deposition parameters and formulation of the liquid feedstock for the reduction of the printed line’s width and thickness, elimination of the “coffee ring” effect, and analysis of the crystallographic orientation of the films. Current work focuses on improving the film’s homogeneity, increasing the crystal orientation towards a highly oriented film, and its piezoelectric characterization and application as a sensor.

Gravitationally-driven Wicking Condensation

Vapor condensation is routinely used as an effective means of transferring heat or separating fluids. Film-wise condensation, where the condensate completely covers the condenser surface, is prevalent in typical in-dustrial-scale systems. Dropwise condensation, where the condensate forms discrete liquid droplets, can im-prove heat transfer performance by an order of mag-nitude compared to filmwise condensation; however, current state-of-the-art dropwise technology relies on functional hydrophobic coatings, which are often not robust and therefore undesirable in industrial condi-tions. Furthermore, low surface tension condensates, like hydrocarbons, pose a unique challenge since coat-ings used to shed water often do not repel these fluids. We demonstrated a method to enhance condensation heat transfer using gravitationally-driven flow through a porous metal wick, which takes advantage of the condensate’s affinity to wet the surface and also eliminates the need for condensate-phobic coatings. The condensate-filled wick has a lower thermal resistance than the fluid film observed during filmwise condensation, resulting in an improved heat transfer coefficient of up to an order of magnitude and comparable to that observed during dropwise condensation. The improved heat transfer realized by this design presents the opportunity for significant energy savings in natural gas processing, thermal management, heating and cooling, and power generation.

Boron Arsenide Crystals with High Thermal Conductivity and Carrier Mobility

Overheating presents a major challenge in modern electronics industry due to the increasingly higher power density. High temperatures not only limit de-vice performance, but also greatly reduce reliability and lifetime. To effectively dissipate heat from an elec-tronic chip, materials with high thermal conductivity (k) are crucial. Common electronic materials such as copper and silicon exhibit a room temperature (RT) k of 401 Wm-1K-1 and 148 Wm-1K-1, respectively. In compar-ison, diamond holds the current k record of about 2000 Wm-1K-1 at RT. However, natural diamond is scarce, and synthetic diamond still suffers from slow growth, low quality, and high cost. In addition, significant thermal stresses can arise from the large mismatch in the co-efficient of thermal expansion between diamond and common semiconductors.Recently, first-principles calculations predicted a very high RT k of about 1400 Wm-1K-1 for cubic boron arsenide (BAs), rendering it a close competitor for diamond. Our materials collaborators from the University of Houston and UCLA have grown samples of different sizes and qualities. We carried out thermal transport measurement of these sub-millimeter to millimeter-sized samples using time-domain thermoreflectance (TDTR) (Figure 1), among other methods. In some samples, we have reached thermal conductivity as high as 1200 Wm-1K-1.We have also carried out the first-principles calculation of electron and hole mobility in boron-based III-V materials. We predict that BAs has both high electron (1400 cm2V-1s-1 at RT) and hole (2110 cm2V-1s-1 at RT) mobility (Figure 2). These characteristics, together with the high thermal conductivity, make BAs attractive for microelectronics applications both as device materials and as heat sink materials.

A Simple Fabrication Method for Doubly Reentrant Omniphobic Surfaces via Stress Induced Bending

We developed omniphobic, doubly reentrant surfac-es fabricated with a simple method suitable for use with traditional microfabrication processes. Intrinsic stresses in deposited layers of silicon nitride induced bending of a singly reentrant microstructure, creat-ing the doubly reentrant geometry. Due to the use of standard microfabrication processes, this approach may be extended to a variety of materials and feature sizes, increasing the viability of applying omniphobic doubly reentrant structures for use in areas such as superomniphobicity, anti-corrosion, heat transfer en-hancement, and drag reduction.Figure 1 shows the fabrication process, in which standard photolithography and etches used to create singly reentrant microstructures are adopted. However, due to the stresses in deposited layers of silicon nitride, the singly reentrant structure is bent into a doubly reentrant geometry that renders the surface omniphobic. Figure 2 shows the contact angle of water and FC 40 on the surface. FC 40 has a much lower surface tension than water, which typically makes it difficult to repel. However, due to the double reentrant geometry of this surface, it is repelled.

Micro-engineered Pillar Structures for Pool Boiling Critical Heat Flux Enhancement

Increasing the performance of phase-change heat trans-fer phenomena is key to the development of next-gen-eration electronics, as well as power generation systems and chemical processing components. Surface-engineer-ing techniques could be successfully deployed to achieve this goal. For instance, by engineering micro/nano-scale features, such as pillars, on the boiling surface, it is pos-sible to attain 100% enhancement in pool boiling criti-cal heat flux (CHF). Researchers have been working on several CHF enhancing micro- and nano-structured sur-faces 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 1 cm × 1 cm silicon micropillar surfac-es with different micro-pillar heights have been fabricat-ed using MTL's photolithography and DRIE facilities.The surfaces were characterized using MTL's Scanning Electron Microscope (SEM), as shown in Figure 1a. The surfaces were then characterized by measuring the capillary wicking rate using high-speed imaging and a custom-built capillary tube approach as presented in Figure 1b. The capillary wicking experimental results are presented in Figure 1c is demonstrating the increase in liquid transport capability by increasing the micro-pillar heights.Finally, the performances of such structures were characterized through traditional pool boiling experiments and compared with a flat silicon heater (Figure 1d). The surfaces were tested at atmospheric pressure and saturation temperature using DI water as the working fluid. The results demonstrate the benefits of wicking promoted by these structures in terms of CHF enhancement.

Superhydrophilic and Superhydrophobic Nanostructured Surfaces for Microfluidics and Thermal Management

Nanostructured features can be used to magnify the intrinsic hydro-phobicity or hydrophilicity of a material to create superhydropho-bic and superhydrophilic surfaces [1, 2]. There has been particular interest in these surfaces for a variety of applications including self-cleaning and drag reduction with superhydrophobic surfaces [3-5]. Superhydrophilic surfaces are of interest in anti-fogging and thermal management [6-8]. Past work has demonstrated significant changes in contact angle with minimal hysteresis with the introduc-tion of nanostructured surface features [9]. Current efforts, how-ever, focus on the dynamic robustness and spreading of liquids on such surfaces We have fabricated silicon pillar arrays with cross sections of 500 nm× 500 nm, spacings between pillars of 800 nm, and heights of 5 µm (Figure 1). The pillar arrays are naturally oxidized in air to make them hydrophilic. The interaction of the spreading liquid with the fabricated pillars was studied using diffraction limited microscopy and with an environmental scanning electron microscope (Figure 2). The preliminary data (Figure 2) shows that the liquid-air inter-face is pinned diagonally. Using an energy minimization approach, theory is currently being developed to understand the effect of pil-lar spacing, height, and diameter on spreading dynamics. We have also concurrently coated the silicon pillars with a silane chemistry to create superhydrophobic surfaces. The effect of shape and size of the nanostructures on hydrophobic robustness is currently being investigated.

Design of a Micro-breather for Venting Vapor Slugs in Two-phase Microchannels

Boiling is currently used in a variety of industries as an efficient method of cooling. Boiling, and phase-change in general, are attrac-tive because the latent heat of vaporization can be used to carry and dissipate large heat fluxes. Two-phase microchannels have been of recent interest because they promise compact and efficient solu-tions [1].However, phase-change in microchannels leads to challenges that are not present in macroscale counterparts because the governing forces are different. Surface tension forces become dominant at the microscale whereas buoyancy forces can be neglected. As a result, flow instabilities, large pressure fluctuations, and local liquid dry-out occur in microchannels, which severely limits the overall ther-mal performance.To address these problems, a few solutions have been proposed [2, 3], including the use of porous membranes or hydrophobic ports that allow vapor bubbles to escape from the microchannels as they form. The proposed solutions have drawbacks, including the inabil-ity to sufficiently remove vapor bubbles effectively and eliminate dry-out within the channels.We propose a design for a microscale breathing device that uses the combination of surface chemistry and geometry to separate vapor from a liquid flow. To better understand the physics and governing parameters for the microscale breather, we designed a test device that allows for cross-sectional visualization of a breathing micro-channel (Figure 1). We have conducted various experiments and col-lected image data to help direct our vapor breather design to achieve high vapor removal efficiencies with minimal fabrication effort and control requirements (Figure 2).The successful implementation of a microchannel with an efficient breather will allow for new cooling technologies with higher heat removal capacities that can be effectively used by the semiconduc-tor industry. The breathers also have significant promise as liquid vapor separators for use in micro-fuel cells and other applications that require phase separation at the microscale.

Microfluidic Patterning of P-Selectin for Cell Separation through Rolling

Cell separation based on markers present on the cell surface has ex-tensive biological applications. However, current separation meth-ods involve labeling cells and label removal steps that are often slow and intrusive. Recently, we discovered that it is possible to steer cells interacting transiently with the surface through patterning of recep-tors on the surface [1]. In this paper we report microfluidic pattern-ing of P-selectin receptors to control cell rolling for label-free sepa-ration of cells. We envision a microfluidic device that would perform label-free separation of cells by rolling them on receptor patterned surfaces (Figure 1). The present work is the first step towards real-izing these devices.A microchannel defining the pattern was fabricated in PDMS and reversibly bonded onto a polystyrene substrate. Human P-Selectin was filled inside the microchannel and left overnight for physisorb-tion to complete. Later the PDMS mask was removed, and the surface was washed with PBS and finally incubated in Fetal Bovine Serum to block non-specific interactions. HL-60 myeloid cell suspension was flowed over the surface to verify patterning of P-selectin. We observed that cells interacted selectively with the P-selectin region, showing that the patterning technique was successful (Figure 2). Rolling was clearly observed on the selectin-coated bands and some deflection of cells at the edge was also observed. A few cells were also seen to detach from one band and reattach at another selectin band downstream. This work demonstrates microfluidic patterning of P-selectin that is essential for a device for cell separation based on cell rolling. In the future, these patterns will be incorporated in a smaller microfluidic flow chamber with multiple inlets and outlets for label-free, con-tinuous-flow cell separation.