Electromechanical resonators such as quartz crystals, surface acoustic wave (SAW) resonators, and ceramic resonators have become essential components in electronic systems. However, due to their large footprint and difficulty in integrating with CMOS processes, there has been much development in realizing microelectromechanical system (MEMS) resonators that achieve comparable performance yet have smaller footprint and are compatible with CMOS. As with other semiconductor devices, with increasing frequency and with decreasing device size into the submicron scale, variability has started to become a critical issue in MEMS resonators. However, one of the critical challenges is the lack of a characterization method that is accurate but efficient enough to be used for testing the large number of devices necessary to acquire accurate statistical distribution of the parameters of interest. This project proposes an on-chip test circuit that can accurately characterize a large number of resonators for variation analysis and that is general enough that it can be used with a wide range of resonators, not limited to specific frequencies or other properties. The proposed test circuit is based on a transient step response method using a voltage step that can accurately measure the resonant frequencies and the quality factor of devices ^[1] . The circuit employs a sub-sampling method to capture the high-frequency decay signal ^[2] and a simple analog-to-digital converter (ADC) ^[3] allowing complete digital interface, an important feature for test automation. SPICE level simulation combined with a behavioral simulation tool that was developed showed acceptable extraction errors of <1% for RS, <0.1% for Lx, <0.1% for Cx, <100 ppm for fs, and <1% for Qs. A test chip implementing the proposed test circuit has been designed and fabricated in NSC 0.18-um CMOS process.

1. M. Zhang, N. Llaser, H. Mathias, and F. Rodes, “CMOS offset-free circuit for resonator quality factor measurement,” IEEE Electronic Letters, vol. 46, no. 10, p. 706, May 2010.
2. R. Ho, B. Amrutur, K. Mai, B. Wilburn, T. Mori, and M. Horowitz, “Applications of on-chip samplers for test and measurement of integrated circuits,” in Proc. 1998 IEEE Symposium on VLSI Circuits, June, 1998, pp. 138-139.
3. E. Alon, V. Stojanović, and M. A. Horowitz, “Circuits and techniques for high-resolution measurement of on-chip power supply noise,” IEEE Journal of Solid-State Circuits, vol. 40, no. 4, pp. 820-828, Apr. 2005.

Nanostructured solar cells are attracting increasing attention as a promising photovoltaic (PV) technology ^[1] . Generation of free charge carriers in nanostructured PV devices occurs at the electron donor-acceptor interface, analogous to the pn-junction interface in traditional crystalline silicon solar cells. However, recombination at this interface constitutes one of the major charge carrier loss pathways. Thus characterizing and controlling recombination dynamics is critical for informing the design of novel device architectures. Recombination parameters also enable comparisons between different device architectures.

In this work, we employ the transient photovoltage (TPV) technique ^[2] to probe recombination mechanisms under standard operating conditions in three different solar cells, as shown in Figure 1: a poly(3-hexylthiophene) and phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction; a chloroaluminium phthalocyanine and fullerene (ClAlPc:C₆₀) planar mixed heterojunction; and a lead sulfide quantum dot and zinc oxide (QD PbS:ZnO) pn-heterojunction. The normalized TPV data acquired at 0.5-sun illumination intensity are shown in Figure 2a, which compares the recombination lifetimes of charge carriers in these devices. The observed differences in carrier lifetimes may arise from variations in the respective interface morphologies: for example, the slower recombination transients observed in the ClAlPc:C₆₀ device may be attributed to the intrinsic planarity of this particular architecture.  We can also measure the charge carrier lifetime as a function of the light intensity, as shown in Figure 2b; this result confirms that recombination dynamics are faster in P3HT:PCBM and QD PbS:ZnO than in ClAlPc:C₆₀ PV devices.

1. Anonymous, “A sunny outlook,” Nature Photonics, vol. 6, no. 3, p. 129, Mar. 2012.
2. C. G. Shuttle, B. O’Regan, A. M. Ballantyne, J. Nelson, D. D. C. Bradley, J. de Mello, and J. R. Durrant, “Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell,” Applied Physics Letters, vol. 92, p. 3, 2008.

It is desirable to extend the functionality of MEMS to different form factors including large-area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates.

A new method for additive fabrication of thin (125±15-nm-thick) gold membranes on cavity-patterned silicon dioxide substrates using contact-transfer printing is presented for MEMS applications. The deflection of these membranes, suspended over cavities in a silicon dioxide dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. The fabrication process employs a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto the substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactive-ion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm² in area are fabricated, and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28-μm-diameter cavities, as shown in Figure 1. Using the membrane deflection data, Young’s modulus of these gold films is extracted (74±17 GPa), and it is comparable to that of bulk gold. Additionally, a 15 Hz sinusoidally varying voltage of 15 V peak-to-peak amplitude is applied to the MEMS device to demonstrate that the large membrane deflection is a repeatable deflection (Figure 2).

These films can be utilized in microspeakers, pressure sensors, microphones, deformable mirrors, tunable optical cavities, and  large-area arrays of these devices.

1. A. Murarka, C. Packard, F. Yaul, J. Lang, and V. Bulovic, “Micro-contact printed MEMS,” IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), 2011, pp. 292-295.
2. C. Packard, A. Murarka, E. W. Lam, M. A. Schmidt, and V. Bulovic, “Contact-printed microelectromechanical systems,” Advanced Materials, vol. 22, pp. 1840–1844, 2010.
3. A. Murarka, S. Paydavosi, T. L. Andrew, A. I. Wang, J. H. Lang, and V. Bulovic, “Printed MEMS membranes on silicon,” IEEE 25th International Conference on Micro Electro Mechanical Systems (MEMS), 2012, pp. 309-312.

Design and optimization of novel RF Nano-Electro-Mechanical (NEM) resonators such as Resonant Body Transistors (RBT) require modeling across multiple domains, including mechanical (distributed stress and elastic wave models), electrical (semiconductor devices and RF small signal models), and thermal. These domains are all cross-coupled in nonlinear ways and require lengthy finite element multi-physics analyses to solve. Due to the complexity of these structures embedded in the CMOS stack and sensed using active FETs, the day-long time scale of each finite element simulation prevents quick, intuitive parameterization of device design. A reduced model parameterized across all three domains is therefore necessary both for rapid prototyping and for device optimization.

In this work, we are developing an algorithm to automatically generate compact models for NEM resonators. Our compact models are suitable for AC, DC and RF operation of the device and allow the circuit designers to run circuit-level time-domain simulations using any commercial circuit simulator ^{[1] [2]} . The compact models are “parameterized,” so that the circuit designer will be able to instantiate instantaneously models within the circuit simulator for different values of the key device parameters.  Key resonator parameters included in the compact parameterized model are resonant frequency, quality factor, signal strength, isolation, presence of spurious modes, and operating temperature. Values for the model coefficients are calibrated using measurements from NEMS resonator devices. A critically important feature of our models is to guarantee that when circuit designers change arbitrarily values for the device parameters, the compact models will always preserve the physical properties of the original device and will never cause numerical instabilities and convergence issues when connected to other device models and circuits within the circuit simulator ^[3] . Figure 1 shows the layout of a Si-based NEMS-CMOS resonator. Numerical results show a great promise for our technique. We have achieved high quality fit to the measured data, as Figure 2 shows, which offered modeling challenges including the presence of noise and spurious resonant peaks.

1. Z. Mahmood and L. Daniel, “Circuit synthesizable guaranteed passive modeling for multiport structures,” in Proc. of Behavioral Modeling and Simulation Conference (BMAS), Sept. 2010.
2. Z. Mahmood, R. Suaya and L. Daniel, “An efficient framework for passive compact dynamical modeling of multiport linear systems,” in Proc. of Design, Automation and Test in Europe, (DATE), Mar. 2012.
3. Z. Mahmood and L. Daniel, “Guaranteed passive parameterized modeling of multiport passive circuit blocks,” in Proc. of TECHCON, Sept. 2011.

Professor Ed Boyden uses light to precisely control neural activity.  His lab has invented safe, effective ways to deliver light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively ^[1] .  This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health, and his lab has developed animal models of epilepsy and Parkinson’s disease to explore the use of optical control to develop new therapies.

We have recently developed mass-fabricatable multiple light guide microstructures produced using standard microfabrication techniques to deliver light to activate and silence neural target regions along their length as desired ^[2] .  Each probe is a 100- to 150-micron-wide insertable micro-structure with many miniature lightguides running in parallel and delivering light to many points along the axis of insertion.  Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage.  We are currently developing 2-D arrays of such probes so multiple colors of light can be delivered to 3-dimensional patterns in the brain, at the resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics.  Such devices will allow the substrates that causally contribute to neurological and psychiatric disorders to be systematically analyzed via causal neural control tools.  Given recent efforts to test such reagents in nonhuman primates, these devices may also enable a new generation of optical neural control prosthetics, contributing directly to the alleviation of intractable brain disorders.

The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide, and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns ^[2] .  Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe functions as designed ^[2] .  The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable (see Figure 2).  The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule.  This allows for increased modularity and control in initial probe testing.

We are now utilizing transgenic mice, which express optogenetic activators and silencers in cortical pyramidal neurons, to demonstrate optogenetic control of neural circuits in a fashion appropriate for in vivo circuit mapping or brain machine interface prototyping.  Our goal is to explore the degree to which this technology can be used to functionally map neural network connectivity over large, multi-region circuits in the brain, and to subserve a new generation of neural control prosthetics.

1. X. Han and E. S. Boyden, “Multiple-color optical activation, silencing, and desynchronization of neural activity, with single-spike temporal resolution,” PLoS ONE, vol. 2, no. 3, p. e299, Mar. 2007.
2. A. N. Zorzos, E. S. Boyden, and C. G. Fonstad, “A multi-waveguide Implantable probe for light delivery to distributed brain targets,” Applied Optics Letters vol. 35, no. 12, pp. 4133-4135, Dec. 2010.

Sepsis is an adverse systemic inflammatory response caused by microbial infection in blood. In this work, we report a simple microfluidic approach for intrinsic, non-specific removal of both microbes and inflammatory cellular components (platelets and leukocytes) from whole blood, inspired by the in vivo phenomenon of leukocyte margination ^[1] . As blood flows through a narrow microchannel (20 × 20 µm), deformable red blood cells (RBCs) migrate axially to the channel center, resulting in margination of other cell types (bacteria, platelets and leukocytes) towards the channel sides (see Figure 1) ^[2] . With the use of a simple cascaded channel design, the blood samples undergo a 2-stage bacteria removal in a single pass through the device, thereby allowing higher bacterial removal efficiency. As an application for sepsis treatment, we demonstrated separation of Escherichia coli and Saccharomyces cerevisiae spiked into whole blood, achieving high removal efficiencies of ~80% and ~90%, respectively (Figure 2A). Inflammatory cellular components were also depleted by >80% in the filtered blood samples, which could help to modulate the host inflammatory response and potentially serve as a blood-cleansing method for sepsis treatment. The developed technique offers significant advantages including high throughput (~1mL/hr per channel) and label-free separation that allows non-specific removal of any blood-borne pathogens (bacteria and fungi). The continuous processing and collection mode potentially enables the return of filtered blood to the patient directly, similar to a simple and complete dialysis circuit setup. Due to design simplicity, further multiplexing is possible by increasing channel parallelization or device stacking to achieve higher throughput comparable to convectional blood dialysis systems used in clinical settings.

1. H. L. Goldsmith and S. Spain, “Margination of leukocytes in blood flow through small tubes,” Microvascular Research, vol. 27, pp. 204-222, 1984.
2. H. W. Hou, H. Y. Gan, A. A. S. Bhagat, L. D. Li, C. T. Lim, and J. Han, “A microfluidics approach towards high-throughput pathogen removal from blood using margination,” Biomicrofluidics, vol. 6, pp. 024115-13, 2012.

The average diameter of human red blood cells (RBCs) is around 8µm. As RBCs circulate in the body and transport oxygen, they have to deform repeatedly in small blood capillaries. RBC deformability is therefore an important mechanical attribute for efficient oxygen delivery. Several blood related diseases such as malaria, sickle cell anemia, and sepsis are marked with significant alterations in RBC deformability ^{[1] [2] [3]} .

This project studies RBC dynamic deformability using a simple, portable microfluidic device ^[4] . The deformability of individual RBCs can be assessed by the average velocity of RBCs passing through narrow microfluidic channels. The repeated deformations to be experienced by RBCs simulate in vivo blood capillary system. Several blood-related diseases are included in our studies.

1. A. M. Dondorp, E. Pongponratn, and N. J. White, “Reduced microcirculatory flow in severe falciparum malaria: Pathophysiology and electron-microscopic pathology, Acta Tropica vol. 89, pp. 309-317, 2004.
2. O. K. Baskurt, D. Gelmont,  and H. J. Meiselman, “Red blood cell deformability in sepsis,” American Journal of Respiratory and Critical Care Medicine,  vol. 157, pp. 421-427, 1998.
3. S. Chien, “Red cell deformability and its relevance to blood flow,” Ann. Rev. Physiol. vol. 49, pp. 177-192, 1987.
4. H. Bow, IV. Pivkin, M. Diez-Silva, S.J. Goldfless, M. Dao, J.C. Niles, S. Suresh, and J. Han, A microfabricated deformability-based flow cytometer with application to malaria, Lab on a Chip, vol. 11, pp. 1065-1073, 2011.

In the stomach, the biological hydrogel known as mucus protects the stomach wall from the damaging effects of strongly acidic digestive juices inside the stomach lumen. Altered mucus function is linked to gastric diseases including ulcers and cancers. The biophysical mechanisms underlying the barrier are not well understood, due partly to a lack of suitable in vitro tools.

In this work, we developed an in vitro microfluidic system designed to mimic mucus secretion in the stomach (see Figure 1) ^[1] . In our system, mucus components are pumped continuously on-chip into an acidic flow, mimicking in vivo mucus secretion into an acidic stomach lumen. A fluorescent pH indicator added to the samples allows optical tracking of acid diffusion. Our microfluidic system is superior to in vitro macroscale techniques currently used to assay mucus function ^[2] . Advantages of our system include study of barrier function under secretion rather than static conditions, ability to optically measure the pH profile inside the mucus layer, and low sample volume requirement enabling experiments using difficult-to-purify mucus components.

With this system, we demonstrate that continuous secretion of mucin glycoprotein, the dominant protein component of mucus, hinders the diffusion of acid (Figure 2) due to the ability of mucins to directly bind and sequester H⁺ (see ^[1] for more details). We further estimate that the barrier function resulting from direct binding of H⁺ to mucin constitutes a significant portion of the in vivo mucus barrier. This “mucus-secretion-on-a-chip” platform may be used to systematically study the barrier function of each mucus layer component, perform diagnostics of mucus function using small amounts of clinical sample, and test mucus-targeted drugs.

1. D. L. Li, O. Lieleg, S. Jang, K. Ribbeck, and J. Han, “Microfludic in vitro system for quantitative study of stomach mucus barrier function,” Lab on a Chip, 2012, to be published. DOI: 10.1039/C2LC40161D.
2. S. Tanaka, H. H. J. Meiselman, E. Engel, P. H. Guth, O. Furukawa, R. B. Wenby, J. Lee, J. D. Kaunitz, “Regional differences of H+, HCO3-, and CO2 diffusion through native porcine gastroduodenal mucus,” Dig. Dis. Sci., vol. 47, no. 5, pp. 967-973, May 2002.

Although inertial force-induced lateral migration has been extensively studied for almost 50 years and has been utilized in various microchannels to perform size-based separation for cell research, the mechanism of inertial focusing is generally described as the interplay between inertial lift force and dean drag force and lacks information on particle behavior in depth direction, leaving several missing pieces from the physical understanding of inertial focusing parameter space ^{[1] [2] [3]} . Here we present an exploratory study of inertial focusing in planar straight microchannels and spiral microchannels with varying geometry to identify the regimes of particle behavior in response to flow rate and channel dimension. To gather accurate information on the depth direction of a straight channel, we fabricated a pair of straight channels with the same cross-sectional dimensions but different orientations and recorded the focusing positions of particles in the top-down images under the same conditions using these two devices, respectively. Combining the data from these two channels provides unambiguous information on the cross-sectional particle focusing positions. We also developed a polymer-casting technique to fabricate PDMS devices with smooth sidewalls through which one can observe the particle positions at the outermost loop of the planar spiral in the channel depth direction. The data gathered for the same spiral channel but from different directions allowed us to map the distribution of particles in cross-section with a simulated velocity field. With accurate information on particle positions in the cross-sections of straight and spiral channels, we would be able to relate the effect of channel dimension on the force field with the related particle- focusing behavior and identify the key parameters for the optimal design of a size-based separation device targeted at specific size range.

1. S. S. Kuntaegowdanahalli, A. A. Bhagat,, G. Kumar, and I. Papautsky, “Inertial microfluidics for continuous particle separation in spiral microchannels,” Lab. Chip, vol. 9, no. 20, pp. 2973-2980, Oct. 2009.
2. D. Di Carlo, J. F. Edd, K. J. Humphry, H. A. Stone, and M. Toner, “Particle segregation and dynamics in confined flows,” Physical Review Letters, vol. 102, no. 9, p. 094503, Mar. 2009.
3. A. J. Mach, and D. Di Carlo, “Continuous scalable blood filtration device using inertial microfluidics,” Biotechnol. Bioeng, vol. 107, no. 2, pp. 302-311, Oct. 2010.

Although new software techniques enable higher-resolution medical ultrasound imaging, commercial ultrasonic transducer technology has remained largely unchanged for a few decades.  Current transducers are fabricated from bulk PZT using assembly steps that are labor-intensive and limit individual transducers to millimeter-sized features.  With micro-fabrication technology, micro-scale transducers can be easily manufactured at very low cost, but their acoustic power and efficiency may be compromised.  We revisit a piezoelectric micro-machined ultrasonic transducer (PMUT) based on a lead zirconate titanate (PZT) thin film with a view to improve acoustic performance.  Our initial findings show that the inherently high piezoelectric coupling of thin-film PZT produces the deflection necessary for high acoustic pressure applications without significant power requirements or application of a DC bias voltage if the design can be optimized. With its high acoustic pressure output and small size, a PMUT could be used for deep penetration and non-invasive medical imaging, e.g., intracranial monitoring of head injuries.

Our group has derived the equivalent circuit for a bimorph PMUT ^[1] .  This configuration sandwiches a PZT between top and bottom electrodes and actuates it with an applied voltage across the electrodes.  Adding a structural support layer, such as silicon, creating a multimorph device increases the model’s complexity. With separate definition of mechanical and electrical neutral axes, the equivalent circuit derivation extends to include the multimorph design ^[2] .  With this advance, transduction behavior of the PMUT can be more accurately predicted, designs more easily optimized, and results validated with a complete model.  An analytical solution for deflection based on electrode coverage has been derived and the optimum electrode coverage for maximum deflection has been determined.  Based on the modeling results, fabrication of an optimized PMUT design is now underway. Our eventual goal is to incorporate PMUT elements into 1D and 2D arrays with a small form factor to enable high resolution medical imaging.

Figure 1: Cross-sectional view of PMUT element. Design is currently being fabricated.

1. F. Sammoura and S.-G. Kim, “Theoretical modeling and equivalent electric circuit of a bimorph piezoelectric micromachined ultrasonic transducer,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, to be published.
2. F. Sammoura and S.-G. Kim, “Modeling of the neutral axes of a circular piezoelectric micromachined transducer in transmit and receive mode,” Tech. Dig. of Solid-State Sensors and Actuators Workshop, to be published.