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Micro and Nano Systems

2009;():1-6. doi:10.1115/IMECE2009-10213.

This paper presents an analytical and experimental investigation of energy loss mechanisms in surface-micromachined resonators. The numerical models of anchor loss and thermoelastic damping are created in ANSYS/Multiphysics, according to a separation-and-transfer method and a thermal-energy method, respectively. Surface-micromachined resonators are fabricated using the SUMMiT V MEMS foundry process and the measured Quality (Q) factors from these resonators are compared with the created numerical models, showing good agreement. The measured highest Q value is 35,088 at a resonant frequency of 1 l MHz. Thermoelastic damping is found to be the dominant loss in these surface-micromachined resonators.

Commentary by Dr. Valentin Fuster
2009;():7-17. doi:10.1115/IMECE2009-10757.

The objective of this research work is to investigate the displacement control of smart beams of different boundary conditions using photostrictive optical actuators. The authors have developed a computational method useful for design of systems incorporating thin film photostrictive actuators. The element has been implemented in an in-house finite element code. A finite element for static analysis of photostrictive thin films has already been developed and verified with analytical analysis approach of another author. Also the effect of different parameters such as actuator thickness, incident light intensity and convective heat transfer coefficient in the actuation of beam using the thin film photostrictive actuators has been investigated by the authors. In this current work, derived finite element for static analysis of photostrictive thin films has been used to investigate the application of photostrictive actuators for optimum displacement control of beam structure of various boundary conditions. Studies are performed on the effects of various actuator location and length on photoactuation. Photostrictive materials are ferrodielectric ceramics that have a photostrictive effect. The photostrictive effect arises from a superposition of the photovoltaic effect and the converse piezoelectric effect. Photostrictive materials are (Pb, La)(Zr, Ti) O3 ceramics doped with WO3 , called PLZT, exhibit large photostriction under uniform illumination of high-energy light. Photostrictive actuators can directly convert photonic energy to mechanical motion. Photostrictive materials can produce strain as a result of irradiation from high-intensity light. Neither electric lead wires nor electric circuits are required. Thus, photostrictive actuators are relatively immune from electrical interference. They have potential use in numerous MEMS devices where actuation of microbeams is a common phenomenon.

Commentary by Dr. Valentin Fuster
2009;():19-24. doi:10.1115/IMECE2009-10776.

Finite element thermal analysis and comparison with experiments of microscale laser joining of biocompatible materials, polyimide (PI) and nanoscale coating of titanium (Ti) on glass (Gl), is vital for the long-term application of bio-implants and important for the applications of nanoscale solid coatings. In this study, a comprehensive three dimensional (3D) transient simulation for thermal analysis of transmission laser micro-joining of dissimilar materials has been performed by using the finite element (FE) code ABAQUS, along with a moving Gaussian laser heat source. The laser beam (wavelength of 1100 nm and diameter of 0.2 mm), moving at an optimized velocity (100 mm/min), passes through the transparent PI, gets absorbed by the absorbing Ti, and eventually melts the PI to form the bond. The laser bonded joint area is 6.5 mm long on three different Ti coating thicknesses of 400, 200 and 50 nms on Gl surface. Non-uniform mixed meshes have been used and optimized to formulate the 3D FE model and ensure very refined meshing around the bond area. During the microscale laser heating finite element modeling shows widths of PI surface experiencing temperatures above the glass transition temperature are similar to the widths of bonds observed in experiments for coating thicknesses of 400 and 200 nms of Ti on Gl. However, for the case of 50 nm coating bond width using finite element analysis cannot produce and is lower than the bond width observed experimentally.

Commentary by Dr. Valentin Fuster
2009;():25-31. doi:10.1115/IMECE2009-11531.

Recently, we have shown that in-service repair of stiction failed MEMS devices is possible with structural vibrations. In order to further understand this phenomenon and better predict, theoretically, the onset of repair we have constructed an apparatus to determine the Mode I, II, and III interfacial adhesion energies of MEMS devices failed on a substrate. Though our method is general, we are specifically focused on devices created using the SUMMiT V process. An apparatus has been constructed that has 8 degrees-of-freedom between the MEMS device, the surface on which the device is failed, and a scanning interferometric microscope. Deflection profiles of stiction failed MEMS (micro-cantilevered beams 1500 microns long, 30 microns wide, and 2.6 microns thick) have their deflection profiles measured with nanometer resolution by the scanning interferometric microscope. Then non-linear elastic models are used in order to determine the interfacial adhesion energy between the failed micro-cantilevers and the surface. In this work we report the interfacial energies from Mode I and Mixed Mode I and II type failures. We also show further experimental results of repair of stiction failed MEMS and corresponding modeling results that use data from the Mode I and II experiments.

Commentary by Dr. Valentin Fuster
2009;():33-38. doi:10.1115/IMECE2009-11617.

One of the outstanding issues in the analysis of contacting surfaces regards properly accounting for the multi-scale nature of the surface topography. Many treatments of surface contact, particularly those that derive from the well known Greenwood Williamson (GW) model, proceed from the basis that one can determine a representative curvature of the surface peaks (or summits) by means of computing the appropriate numerical derivative of the sampled profile data. However, as has been demonstrated over the last 20 years or so, numerical derivatives of surface data are sensitive to the lateral spacing between sampled surface points. In view of the potential ambiguity associated with such a determination of peak curvature, a number of models have been put forth that account for the fact that topographical features exist at many scales of observation. In the present work, predictions of a recently developed spectral-based model of elastic (rough) surface contact are compared to those of a deterministic numerical computation in which the equations of elasticity are solved at every nodal point of a 3D surface.

Topics: Computation
Commentary by Dr. Valentin Fuster
2009;():39-44. doi:10.1115/IMECE2009-11649.

In this paper, we presented is a four-terminal piezoresistive sensor commonly referred to as a van der Pauw (VDP) structure for its application to MEMS pressure sensing. In a recent study, our team has determined the relation between the biaxial stress state and the piezoresistive response of a VDP structure by combining the VDP resistance equations with the equations governing silicon piezoresistivity and has proposed a new piezoresistive pressure sensor. It was observed that the sensitivity of the VDP sensor is over three times higher than the conventional filament type Wheatstone bridge resistor. To check our theoretical findings, we fabricated several (100) silicon diaphragms with both the VDP sensors and filament resistor sensors on the same wafer so both the sensor elements have same doping concentration. The diaphragms were subjected to known pressures, and the pressure sensitivities of both types of sensors were measured using an in-house built calibration setup. It was found that the VDP devices had a linear response to pressure as expected, and were more sensitive than the resistor sensors. Also, the VDP sensors provided a number of additional advantages, such as its size independent sensitivity and simple fabrication steps due to its simple geometry.

Commentary by Dr. Valentin Fuster
2009;():45-51. doi:10.1115/IMECE2009-11718.

The actuator performance of high temperature shape memory alloys (HTSMAs) can be significantly influenced by viscoplastic mechanisms that appear in metals under high temperature conditions. In this work we investigate experimentally and theoretically the coexistence of phase transformation and viscoplastic behavior in HTSMAs. The experimental study shows that decrease in the thermal cycling rate causes decrease in the actuation strain, as additional irrecoverable strain is generated due to creep mechanisms. The interaction between viscoplasticity and transformation is studied further using TEM. In order to simulate the material response, we propose a constitutive model based on an existing shape memory alloy model, which aims to capture the simultaneous phase transformation and viscoplastic behavior of a HTSMA. After calibration, the proposed model is used in order to identify the impact of the loading rate in the material’s response.

Commentary by Dr. Valentin Fuster
2009;():53-56. doi:10.1115/IMECE2009-11739.

We present experimental results characterizing the changes in electrical transport of single disordered carbon nanowires (diameter 150–250 nm) to the changes in microstructure within the nanowires induced by synthesis temperature. The material system studied is a nanoporous, semiconducting disordered carbon nanowire obtained from the pyrolysis of a polymeric precursor (polyfurfuryl alcohol). Unlike the other allotropes of carbon such as diamond, graphite (graphenes) and fullerenes (CNT, buckyballs), disordered carbons lack crystalline order and hence can exhibit a range of electronic properties, dependent on the degree of disorder and the local microstructure. Such disordered carbon nanowires are therefore materials whose electronic properties can be engineered to specifications if we understand the structure-property correlations. Using dark DC conductivity tests, measurements were performed from 300K to 450K. The charge transport behavior in the nanowires is found to follow an activation-energy based conduction at high temperatures. The conductivity for nanowires synthesized from 600°C to 2000°C is calculated and is linked to changes in the microstructure using data obtained from SEM, TEM and Raman spectroscopy. The electrical properties of the nanowire are shown to be linked intrinsically to the microstructure and the degree of disorder, which in turn can be controlled to a great extent just by controlling the pyrolysis temperature. This ability to tune the electrical property, specifically conductivity, and map it to the structural changes within the disordered material makes it a candidate material for use in active/passive electronic components, and as versatile transducers for sensors.

Topics: Carbon , Nanowires
Commentary by Dr. Valentin Fuster
2009;():57-62. doi:10.1115/IMECE2009-11917.

This paper investigates the influence of indium segregation on the strain fields and electronic structures of self-assembled InAs/GaAs quantum dot structures with and without an In0.15 Ga0.85 As interlayer. We propose a new out-of-plane mismatch strain to interpret an experimental phenomenon. The new mismatch strain simulation successfully analyzes the strain fields and energy levels of InAs quantum dots (QDs). Numerical results reveal that indium segregation would improve the penetration behaviour of the z-axis strain component and the relaxation of the hydrostatic strain. The transition energy of samples A and B without In segregation are 0.991 and 1.028 eV, respectively. The energy difference of two samples agrees well with the previous experimental results. The transition energy of samples A and B may be consistent with presented experiment data at R = 0.84–0.85. In our calculations, indium segregation not only made the transition energy increase significantly as segregation efficiency increased, but also the confinement position of the electron and the heavy-hole shifted toward the top of the QD. Similar phenomena can also be observed for other segregation efficiencies.

Topics: Quantum dots
Commentary by Dr. Valentin Fuster
2009;():63-69. doi:10.1115/IMECE2009-12379.

This paper deals with the problem of static instability of nano switches under the effect of Casimir force and electrostatic actuation. The nonlinear fringing field effect has been accounted for in the model. Using a Galerkin decomposition method and considering only one mode, the nonlinear boundary value problem describing the static behavior of nano-switch, is reduced to a nonlinear boundary value ordinary differential equation which is solved using the homotopy perturbation method (HPM). In order to ensure the precision of the results, the number of included terms in the perturbation expansion has been investigated. Results have been compared with numerical results and also with previously published analytical results. It was observed that HPM modifies the overestimation of N/MEMS instability limits reported in the literature and can be used as an effective and accurate design tool in the analysis of N/MEMS.

Commentary by Dr. Valentin Fuster
2009;():71-75. doi:10.1115/IMECE2009-12539.

Crystalline films grown epitaxially on a substrate consisting of a different crystalline material are of considerable interest in optoelectronic devices and the semiconductor industry. The film and substrate have in general different lattice parameters. This lattice mismatch affects the quality of interfaces and can lead to very high densities of misfit dislocations. Here we study the evolution of these misfit dislocations in a single crystal thin film. In particular, we consider the motion of a dislocation gliding on its slip plane within the film and its interaction with multiple obstacles and sources. Our results show the effect of obstacles such as precipitates and other dislocations on the evolution of a threading dislocation in a metallic thin film. We also show that the material becomes harder as the film thickness decreases in excellent agreement with experiments.

Topics: Thin films
Commentary by Dr. Valentin Fuster
2009;():77-83. doi:10.1115/IMECE2009-13083.

Information on material properties of structural thin films for MEMS fabrication is very limited. The small information available in the literature suggests that the Young’s modulus of structural thin films such as polysilicon can change up to 30% with heavy doping at room temperature. Accurate knowledge of these variations is critical for proper design as well as operation of MEMS devices, especially for applications that require them to be exposed to harsh environmental conditions. In this paper, devices for the on-chip characterization of the Young’s modulus of polysilicon as a function of the doping concentration conditions are presented. Analytical modeling has been performed to predict the change in the devices’ pull-in voltage as a function of doping concentration. The devices were fabricated using the PolyMUMPs process on two different polysilicon layers on the same chip separated by a layer of oxide. The top layer devices are heavily doped while the bottom layer devices are left lightly doped. The lightly doped devices serve as a reference, allowing some account for fabrication uncertainties in order to ensure consistent results. Devices for measuring in-plane stresses, out-of-plane stress gradients and specially designed resistor structures that account for the effect of contact resistance have also been fabricated to monitor these quantities while testing. The devices will be tested using a customized vacuum chamber to study the effect of phosphorus concentration on these structures.

Commentary by Dr. Valentin Fuster
2009;():85-87. doi:10.1115/IMECE2009-13228.

The mechanical characteristics of the epoxy matrix found in filler reinforced polymer composites is important for determining strength and performance. Locally, property variations in regions surrounding fillers can influence the overall macroscopic composite response to loading. We investigate local nanomechanical stiffness of reinforced epoxy composites by using atomic force acoustic microscopy. The effects of tip shape on the contact mechanics at the epoxy interface are found to influence the reported results significantly and will be discussed in context of different tip models. The results have direct correlation to the effect of near-filler interphase regions and the long-term influence of environmental conditions on the polymer composites.

Commentary by Dr. Valentin Fuster
2009;():89-100. doi:10.1115/IMECE2009-12366.

This paper presents a theoretical and experimental investigation of the response of electrostatically actuated parallel-plate resonators when subjected to mechanical shock. Resonators are commonly employed in resonant sensors, where they are operated at low pressure for enhanced sensitivity making their response to external disturbances such as shock a critical issue. A single-degree-of-freedom system is used to model a resonator, which is electrostatically driven by a DC load superimposed to an AC harmonic load. Simulation results are demonstrated in a series of shock spectra that help indicate the combined influence of shock, DC, and AC loads. The effect of the shock duration coinciding with the AC harmonic frequency is investigated. It is concluded that accounting for the electrostatic forces, especially the AC load, is crucial when addressing the reliability and performance of resonators against shock. It is found that for specific shock and AC excitation conditions, the resonator may experience early dynamic pull-in instability. Experimental work has been conducted on a capacitive sensor to verify the obtained theoretical results. The sensor is mounted on top of a small shaker and then both are placed inside a vacuum chamber. Acceleration pulses were applied on the sensor while powered by DC and AC load. The response of the device was monitored using a laser-Doppler vibrometer. The experimental results were compared to the theoretical results and were found to be in good agreement.

Commentary by Dr. Valentin Fuster
2009;():101-104. doi:10.1115/IMECE2009-12932.

In this paper, we present the results of the temperature measurements performed on topology optimized polysilicon microgrippers using Raman spectroscopy. The results reveal that the temperature profile along the actuators is in correspondence with the finite element simulation results presented in [1] except an offset of ∼250 °C due to chip heating. In order to predict this behavior, we included a section of the carrier chip into the finite element model. We also fabricated new devices with wider electrodes to reduce the overall Joule heating. Both finite element simulations and experimental results show that the devices with a wider electrodes design lead to a temperature drop of ∼50 °C as compared to the devices with the previous electrode design.

Commentary by Dr. Valentin Fuster
2009;():105-112. doi:10.1115/IMECE2009-10157.

This paper presents a model to analyze contact phenomenon in microsystems, actuated by ramp voltages, which has applications in frequency sweeping. First-order shear deformation theory is used to model dynamical system using finite element method, while finite difference method is applied to model squeeze film damping. The model is validated by static pull-in results. The presented hybrid FEM-FDM model is utilized to compute values of contact time and dynamic behavior. Considering this model, effects of different geometrical and mechanical parameters on contact time are studied. The influence of imposing the additional reverse voltage on dynamic characteristics of the system is also investigated. It is shown that magnitude and position of applying the reverse voltage is very important in preventing pull-in instability.

Commentary by Dr. Valentin Fuster
2009;():113-118. doi:10.1115/IMECE2009-10158.

In this study, dynamic pull-in instability and snap-through buckling of initially curved microbeams are investigated. The microbeams are actuated by suddenly applied electrostatic force. A finite element model is developed to discretize the governing equations and Newmark time discretization is employed to solve the discretized equations. The static pull-in behavior is investigated to validate the model. The results of the finite element model are compared with finite difference solutions and their convergence is examined. In addition, the influence of different parameters on dynamic pull-in instability and snap-through buckling is explored.

Topics: Microbeams
Commentary by Dr. Valentin Fuster
2009;():119-125. doi:10.1115/IMECE2009-10201.

We present and discuss a variance-reduced stochastic particle simulation method for solving the relaxation-time model of the Boltzmann transport equation. The variance reduction, achieved by simulating only the deviation from equilibrium, results in a significant computational efficiency advantage compared to traditional stochastic particle methods in the limit of small deviation from equilibrium. More specifically, the proposed method can efficiently simulate arbitrarily small deviations from equilibrium at a computational cost that is independent of the deviation from equilibrium, which is in sharp contrast to traditional particle methods. The proposed method is developed and validated in the context of dilute gases; despite this, it is expected to directly extend to all fields (carriers) for which the relaxation-time approximation is applicable.

Commentary by Dr. Valentin Fuster
2009;():127-133. doi:10.1115/IMECE2009-10525.

Recent years has witnessed a large increase in the use of vibrating Micro-Electro-Mechanical-Systems (MEMS) especially in the expanding wireless telecommunication industry. In particular, the use of microresonators to generate or filter signals has facilitated a reduction in the size of many popular cell phones. Advances in microfabrication have increased the ability to create complex MEMS devices. Finite Element Analysis (FEA) has widely been used in the design of these devices. To obtain accurate simulations of complex MEMS devices, a dense FEA mesh is required resulting in computationally demanding simulation models. Arnoldi Model Order Reduction has been investigated and implemented to improve the computational efficiency of MEMS simulations. Using ANSYS, a popular FEA program, a micro resonator model was created. With Arnoldi, a Krylov subspace was extracted from the model and the model was projected onto the subspace reducing the model size. A harmonic simulation over normal operating frequencies was performed on the reduced model and compared with a simulation of the original model. It was found that the computational time was drastically reduced through the use of Arnoldi while achieving similar accuracy as compared to the original model.

Commentary by Dr. Valentin Fuster
2009;():135-142. doi:10.1115/IMECE2009-10837.

This paper presents an O(N) algorithm and its preliminary computer simulation results for virtual prototyping of molecular systems with a simple chain structure. The algorithm is based on proper integration between an internal coordinate method (ICM) and a multibody molecular model. ICM method makes the use of recursive relations possible between two adjacent subsets within a molecular system. The multibody molecular model takes the benefits of freezing degrees of freedom of some lightly excited high frequency bonds. Because these high frequency bonds would force the use of very small integration step sizes, which severely limits the time scales for virtual prototyping of dynamics of molecular conformation over long periods of time. Thus a new multiscale model and efficient algorithm is produced to increase computational efficiency for virtual prototyping of dynamical behaviors of molecular confirmation. This paper will be initially directed towards introduction of the new model and algorithm. Then attention will be turned to the implementation of the algorithm at macro scale, which can be used to demonstrate the validity of the procedure and algorithm. Final focus will be turned to the implementation of the algorithm to a simple molecular chain at micro scale. The algorithm gives an O(N) computational performance for formation/solution of equations of motion for a molecular chain system.

Commentary by Dr. Valentin Fuster
2009;():143-150. doi:10.1115/IMECE2009-10856.

The Knudsen Pump (or Knudsen Compressor) is an unconventional micro-scale gas pump driven by the rarefied gas phenomenon of thermal creep, which is commonly induced by applying a temperature gradient along the wall of thermal creep channels. Previous experimental and simulation results have demonstrated satisfactory performances for Knudsen Pumps using the “linear wall temperature” heating concept. Employing a different heating mechanism, the present work used an isolated heating element placed in front of but not in direct contact with the thermal creep channel. The thermal creep flow was then induced by this isolated heating element instead of the direct temperature gradient along the thermal creep channel wall. Using the DSMC (Direct Simulation Monte Carlo) simulation technique, cases with various heaters’ sizes and operating pressures were studied here to investigate the limitation of thermal creep flows induced by an isolated heater. The maximum pressure ratio in the simulation domain was found to be varied with the heater sizes. This preliminary study of the “isolated heater” heating mechanism is proven to be viable for driving the thermal creep flows and be used in Knudsen Pumps.

Commentary by Dr. Valentin Fuster
2009;():151-156. doi:10.1115/IMECE2009-11253.

In this paper, the size-dependence of the elastic behavior of silicon nanofilms terminated by (100) surfaces is studied by means of molecular dynamics with the modified embedded atom method (MEAM). The results indicate that the (100) surfaces undergo 2×1 reconstruction, which significantly influences the mechanical properties of ultra-thin films. The simulations are carried out at room temperature and structural relaxation is performed. The effective Young’s modulus, in extensional mode, is determined for different thicknesses. The surface energy, surface stress and surface elasticity of layers near the surfaces (non-bulk layers) in the thin silicon films are obtained. The surface properties of nanofilms of a few layers are shown to deviate from thicker films, suggesting a size-dependence of surface parameters and, especially, surface energy. Finally, the results of a recently developed semi-continuum approach are compared with the molecular dynamics results. Below 3 nm, there is a difference between the effective Young’s modulus, calculated by the semi-continuum approach and that provided by MD, suggesting that the continuum approach can no longer provide accurate results.

Commentary by Dr. Valentin Fuster
2009;():157-163. doi:10.1115/IMECE2009-11282.

A computational, physics-based, bulk thermal conductivity model of a neat carbon nanotube network at room temperature is developed using classical finite element techniques. The model is based on experimentally available stochastic distributions of length, diameter, chirality, and orientation, and uses theoretical results for thermal contact resistance from the literature and molecular dynamics simulations for the stochastic nature of tube separation distance. Understanding the thermal transport properties of carbon nanotube networks at various operating temperatures is crucial for the industrial acceptance of these materials in aerospace and electrical applications. Mechanisms of thermal transport are discussed including; thermal conductivity along the tube and inter-contact resistance between the tubes, where the later is considered the dominating factor. The effect of variations of several of the aforementioned stochastic factors influencing the bulk conductivity is investigated, and results demonstrate that changes in the nanotube length play a significant role in improving the bulk conductivity of the network. In addition, a brief study into localized power flow is presented and lends insight into a possible cause of premature network failure.

Commentary by Dr. Valentin Fuster
2009;():165-172. doi:10.1115/IMECE2009-12186.

This paper presents an extended Kantorovich approach to investigate the vibrational behavior of electrically actuated rectangular microplates. The model accounts for the electric force of the excitation and for the applied in plane loads. Starting from a one term Galerkin approximation and following the extended Kantorovich procedure, the partial differential equation governing the microplate vibration, is discretized to two ordinary differential equation with constant coefficients. These equations are then solved analytically and iteratively with a rapid convergence procedure for finding microplate natural frequencies and modeshapes. Results in some specific cases are validated against other theoretical results reported in the literature. It is shown that rapid convergence, high precision and independency of initial guess function make the EKM an effective and accurate design tool for design optimization.

Topics: Microplates
Commentary by Dr. Valentin Fuster
2009;():173-177. doi:10.1115/IMECE2009-12846.

Recently, the study, analysis and prototyping of biologically inspired adhesives pads have been subject of growing interest. These synthetic adhesives consist of rafts of tiny protruding fibers. The adhesion performance of these micro-engineered products is highly dependent on the geometrical and mechanical properties of microfibers and the surface they adhering to. Small fluctuations in these parameters can drastically change their adhesion performance. In this investigation, a more comprehensive mathematical model of a single micro-fiber with adhesion capability in contact with an uneven surface has been developed. To simulate realistic conditions, this analytical model could be extended to an array of micro-fibers. Using Monte Carlo techniques it was possible to study the behavior of an array of these micro-fibers under several degrees of uncertainty. The results deduced by this novel modeling approach are in good agreement with experimental measurements of adhesion performance in synthetic adhesive pads available in literature.

Commentary by Dr. Valentin Fuster
2009;():179-185. doi:10.1115/IMECE2009-10155.

This paper presents a novel design of poly-silicon corrugated diaphragm which can be used for micro-machined microphone. The microphone is built on a corrugated poly-silicon diaphragm with eight-leg supports. In order to reduce the residual stress, the diaphragm is designed with a corrugated ring. A theoretical calculation is presented to show the result of the new design; the FEM simulation results have shown that the eight-leg support diaphragm is the best compared to other two types. The eight-leg-support diaphragm has more deformation than the all-edge-support diaphragm. The fabrication process is also presented in order to confirm that the new design is possibly built by the new technology.

Commentary by Dr. Valentin Fuster
2009;():187-192. doi:10.1115/IMECE2009-10342.

Two key challenges to portable gas chromatography are reducing preconcentrator power consumption and accurate temperature control of adsorbent. This paper presents the results of thermal modeling performed to optimize a microfabricated preconcentrator based on a silicon microhotplate and utilizing Metal Organic Framework (MOF) adsorbents. From this modeling, two design changes are presented that reduce the power consumption by 1.5 W and reduce temperature variation across the microhotplate by 50%.

Commentary by Dr. Valentin Fuster
2009;():193-194. doi:10.1115/IMECE2009-10383.

Rapid creation of devices with microscale features is a vital step in the commercialization of a wide variety of technologies, such as microfluidics, fuel cells and self-healing materials. The current standard for creating many of these microstructured devices utilizes the inexpensive, flexible material poly-dimethylsiloxane (PDMS) to replicate microstructured molds. This process is inexpensive and fast for small batches of devices, but lacks scalability and the ability to produce large surface-area materials. The novel fabrication process presented in this paper uses a cylindrical mold with microscale surface patterns to cure liquid PDMS prepolymer into continuous microstructured films. Results show that this process can create continuous sheets of micropatterned devices at a rate of 3.94 in2 /sec (100 mm2 /sec), almost an order of magnitude faster than soft lithography, while still retaining submicron patterning accuracy.

Commentary by Dr. Valentin Fuster
2009;():195-201. doi:10.1115/IMECE2009-10562.

Typically, a scanning probe microscope (SPM) moves the scanning tip along a zig-zag trajectory. For a given scanning mechanism, the time needed to image an area depends mainly on the number of samples and the size of the image. The imaging speed is further compromised by drifts associated with the substrate and the piezo scanner. It is therefore desirable to improve the imaging speed with limited impact to the effective resolution of the resulting image. In this paper, an adaptive sampling algorithm based on the fractal compression theory of iterated transformations is proposed to address the tradeoff between scanning time and resolution. Instead of scanning a substrate pixel by pixel in an ordered way, the proposed adaptive algorithm starts with a coarse mode scan, measuring only the registered positions within blocks of determined size. Registered positions are defined to constitute the four corners of a specified block. A unique feature of the proposed algorithm is that if the differences among the four registered positions are greater than a preset threshold, the scan process will automatically switch from a coarse mode scan to a zoom-in mode scan. The original block is then divided into two subblocks with HV partitioning method. In the zoom-in mode scan, the microscope uses a finer resolution to scan one of two subblocks. Each sub-block is then treated as another specified block, and the process of coarse mode scanning is repeated until the desired image is synthesized. The main characteristic of this approach is based on the assumption that image redundancy can be efficiently exploited on a block-wise basis. This procedure is particularly relevant to SPMs designed around closed-loop feedback scanners which are gaining in popularity because of their ability to eliminate drift and hysterisis. By applying an adaptive sampling scheme, we have demonstrated that the number of the required samples can be significantly reduced and the scanning process can be accelerated with minimal impact to the image quality.

Commentary by Dr. Valentin Fuster
2009;():203-204. doi:10.1115/IMECE2009-10948.

Input-shaping is an open-loop control technique for dynamic control of electrostatic MEMS. In MEMS applications, open-loop control is attractive as it computes a priori the required system input to achieve desired dynamic behavior without using feedback. In this work, a 3-D computational electromechanical analysis is performed to preshape the voltage commands applied to electrostatically actuate a torsional micromirror to a desired tilt angle with minimal residual oscillations. The effect of higher vibration modes on the controlled response is also investigated. We show that, depending on the design of the micromirros, the first bending mode of the micromirror structures can have significant effect on the dynamic behavior of the system, which is difficult to suppress by using the step-voltage open-loop control. We employ a numerical optimization procedure to shape the input voltage from the real time dynamic response of the mirror structures. The optimization procedure results in a periodic nonlinear input voltage design that can effectively suppress the bending mode effect.

Topics: Mirrors
Commentary by Dr. Valentin Fuster
2009;():205-214. doi:10.1115/IMECE2009-10953.

This research regards to a two-dimensional lateral pushing nanomanipulation using Atomic Force Microscope (AFM). Yet a reliable control of the AFM tip position during the AFM-based manipulation process is a chief issue since the tip can jump over the target nanoparticle and then the process can fail. However, a detailed Modeling and understanding of the interaction forces on the AFM tip is important for prosperous manipulation control and a nanometer resolution tip positioning. In the proposed model, Lund-Grenoble (LuGre) dynamic friction model is used as friction force on the contact surface between the nanoparticle and the substrate. This model leads to a stick-slip behavior of the nanoparticle that is so similar to the experimental behavior in nanoscale. Derjaguin interaction force is applied between the AFM tip and the nanoparticle which considers both attractive and repulsive interactions. The AFM is modeled by lumped-parameters model. A controller is designed based on the proposed dynamic model in order for positioning the AFM tip during a desired nanomanipulation task. Optimal sliding mode approach is used to design the controller. In this approach sliding surface which is used in the sliding mode approach is selected optimally based on the linear quadratic (LQ) method.

Commentary by Dr. Valentin Fuster
2009;():215-223. doi:10.1115/IMECE2009-11050.

A rigid body moving with fluid in a narrow tube is expected to be developed for future engineering applications such as a capsule endoscopy, and it is also applied to some parts of industry. This paper deals with the flow characteristics around a single rigid body with a hole in its center and transient motion of the body when the body is influenced by pressure force from upstream. The model considered the width of the gap between the body and the wall is smaller than a diameter of a tube so that the force on the body can be numerically and analytically estimated as a viscous friction force. It was assumed that the flow is axisymmetric, laminar and taken to be Newtonian and incompressible. It was obtained that, with the hole in its center, the terminal velocity of the body becomes smaller than the average velocity at the inlet. Moreover, because there is a stagnation on the body, the pressure increases behind the body.

Commentary by Dr. Valentin Fuster
2009;():225-234. doi:10.1115/IMECE2009-11071.

Application of atomic force microscope (AFM) as a manipulator for pushing-based positioning of nanoparticles has been of considerable interest during recent years. Nevertheless comprehensive researches has been done on modeling and the dynamics analysis of nanoparticle behavior during the positioning process. The development of dynamics modeling of nanoparticle is crucial to have an accurate manipulation. In this paper, a comprehensive model of pushing based manipulation of a nanoparticle by AFM probe is presented. The proposed nanomanipulation model consists of all effective phenomena in nanoscale. Nanoscale interaction forces, elastic deformation in contact areas and friction forces in tip/particle/substrate system are considered. These effects are utilized to derive governing dynamics of the lumped model of AFM and nanoparticle during the manipulation process. The utilized friction models are a modified Coulomb approach and Lund-Grenoble (LuGre) model. The former is a combination of both normal force and contact surface area. The latter is dependent on the velocity of the nanoparticle and leads to stick-slip behavior of the nanoparticle. Finally, the compatibility and effectiveness of the two proposed models are simulated and compared.

Commentary by Dr. Valentin Fuster
2009;():235-242. doi:10.1115/IMECE2009-11076.

Key mechanical requirements for advanced Hard Disk Drive (HDD) recording heads are a minimal flying height and a perfect track following. By applying Micro Electro-mechanical Micro Systems (MEMS) technology, a Slider with an Integrated Microactuator (SLIM) enabling both in a cost competitive way was created. This paper describes the fabrication process for the system’s electromagnetic microatuator and emphasizes the technology enhancements achieved after a process redesign.

Commentary by Dr. Valentin Fuster
2009;():243-245. doi:10.1115/IMECE2009-11281.

Fabrication of controlled geometry solid-state nanopores (SSNs) using a transmission electron microscope (TEM) is described. Controlling nanopore geometry allows for optimization of the shape and structure of the pore based on the need for specific applications. By manipulating TEM parameters such as current, relative stage settings, and dwell time it is possible to manufacture controlled geometry SSNs in silicon nitride (Si3 N4 ) membranes. Nanopores with circular, elliptical, and triangle-like cross-sectional areas were fabricated.

Commentary by Dr. Valentin Fuster
2009;():247-256. doi:10.1115/IMECE2009-11586.

This paper investigates the design optimization of an electrostatically actuated microcantilever resonator that operates in air. The nonlinear effects of electrostatic actuation and air damping make the structural dynamics modeling more complex. There is a need for an efficient way to simulate the system behavior so that the design can be more readily optimized. This paper describes an efficient analytical approach for determining the optimum design for a microcantilever resonant mass sensor. One simple case is described. The sensor design is a square plate that is coated with a functional polymer and attached to the substrate with folded leg springs. The plate has a square hole in the middle to reduce the effect of squeeze film damping. With the analytical approach, the optimum hole size for maximum sensitivity is found.

Commentary by Dr. Valentin Fuster
2009;():257-262. doi:10.1115/IMECE2009-11629.

Testing methods and apparatus for studying capillary self-assembly processes are presented. This system permits the control of key self-assembly process variables so that relationships between process rates and yields and the process variables can be tested. Part arrival energies and angles are controlled by dropping through a fluid at terminal velocity onto fixed substrate binding sites. Using this system, the assembly probability at the low energy limit is shown to match a simple area fraction relationship.

Commentary by Dr. Valentin Fuster
2009;():263-269. doi:10.1115/IMECE2009-11639.

Metallic large area mold inserts (LAMIs) are essential for the replication of polymer microfluidic devices. Successful molding of micro- or nanoscale features over large areas is dependent on improving the dimensional control of the mold inserts, particularly those fabricated by electrodeposition using the LIGA or UV-LIGA processes. A systematic approach to controlling the internal stress of the nickel deposits, which was essential for predicting the final flatness of the LAMIs prior to electroplating, was carried out. The internal stress of the nickel deposits from a nickel sulfamate solution was estimated using a bent strip stress measurement method after maintaining electroplating chemicals and conditions and reducing contamination. Over-electroplating of the nickel LAMIs was performed on SU-8 electroplating molds on 150 mm diameter Si wafers. Detailed characterization of the nickel LAMIs to determine the relationship between the overall flatness of the LAMIs and the internal stress identified a suitable process window in terms of the current densities (10–20 mA/cm2 ) and the internal stress (−8.3 ∼ −3.0 MPa) for the high quality nickel LAMIs with an overall flatness of 100 μm.

Topics: Nickel , Stress
Commentary by Dr. Valentin Fuster
2009;():271-277. doi:10.1115/IMECE2009-11772.

To address the challenges in developing miniature directional microphones, a novel micro-fabricated directional microphone inspired by the superacute ears of the parasitoid fly Ormia ocharacea is presented in this paper. It consists of two clamped circular silicon diaphragms structurally coupled by an oxide/nitride composite bridge. The separation between the diaphragm centers is 1.25 mm, about the same size as the fly ear. A finite element model is developed to achieve a better understanding of the microphone device and guide the optimal design of the miniature microphones. Using a low coherence fiber optic interferometer detection system, the experiment shows that the directional sensitivity of this device is equivalent to a conventional microphone pair that is 9 times larger. Validating the feasibility to replicate the fly ear in a man-made structure, this work is expected to significantly impact many different fronts that require miniature sensors for sound source localization.

Topics: Ear , Modeling , Microphones
Commentary by Dr. Valentin Fuster
2009;():279-282. doi:10.1115/IMECE2009-11938.

Poly(ether ether ketone) (PEEK) is an aromatic, very high temperature semi-crystalline polymer which exhibits a technologically useful combination of mechanical and chemical properties. In this study carbon nanofibers (CNFs) were used to prepare nanocomposites from PEEK using a polymer crystallization technique at intermediate temperatures. The solution processing technique was used to uniformly disperse the CNFs in the polymer solution and to prepare the nanocomposite samples with different loading of CNFs. Microstructural characterization shows dispersion at very low loading of CNFs, but agglomerates were formed at higher loading. Thermal analysis was used as a means to understand the effect of CNFs on the physical properties of the PEEK nanocomposites.

Commentary by Dr. Valentin Fuster
2009;():283-288. doi:10.1115/IMECE2009-11977.

Dielectrophoresis is the process where nonuniform electric field causes the translational motion of uncharged, polarized particles. Recently dielectrophoresis has become the most widely used process in the field of nanomanufacturing as the translational motion of a carbon nanotube caused by the dielectrophoresis assembles it in the spacing between the electrodes. This process enables engineers to replace the traditional metal (copper or aluminum) wire with carbon nanotubes (CNTs) in the miniature electronic devices. Various process models have been developed and parametric studies have been carried out to understand the process better and to improve the quality and reliability of assembly of CNTs. We consolidate the scattered knowledge of the dielectrophoresis process and represent the integrated knowledge in the form of a complex network by connecting the process parameters based on the relationships they shares. We find that the bipartite relationships exist between some of the process parameters and we represent them in the form of bipartite graphs. We also represent these graphs as incidence matrices and verify whether each graph fulfills the condition of being bipartite. We apply the shortest path algorithm to find an even length path between the any two process parameters which turns out to be an efficient method to estimate the unknown state variables of the process and to access the real time state of the assembly process quickly and efficiently. One can also use bipartite graph to identify the non-contributing variables and eliminate the over constraint situation by applying Gauss elimination method.

Commentary by Dr. Valentin Fuster
2009;():289-293. doi:10.1115/IMECE2009-12045.

Recently, our research group has proposed a MEMS-based solid state corrosion sensor, which is based on embedding metal particle into elastomeric polymers to form a composite-based sensing material. The chemical and dimensional properties of the metal particles and polymer matrix will provide the tailorability in sensor sensitivity, selectivity, time response, and operating life-span. However, the oxidization of metallic particles prior to embedding is adverse for electrical transduction of such sensor. This paper will be based on the investigation of chemical etching protocols used to remove the oxide coating from metal particles without adversely alter the particle itself. The etching process must also be compatible with common MEMS fabrication processes and not limited by the wide range of particle sizes used (30nm–100um). More specifically, metal particles such as Titanium, Aluminum, Nickel, and Stainless Steel are currently being used and investigated.

Commentary by Dr. Valentin Fuster
2009;():295-300. doi:10.1115/IMECE2009-12070.

A novel method of thermoplastic fusion bonding (TPFB), or thermal bonding, for polymer fluidic devices was demonstrated. A pressure cooker was used in a simple sealing and packaging process with precise control of the critical parameters. Polymer devices were enclosed in a vacuum-sealed polymer container. This produced an even pressure distribution and a precise temperature boundary condition over the whole surface of the device. Deformation indicators were integrated on the devices to provide a rapid means of checking deformation and pressure distribution with the naked eye. Temperature, pressure, and time are the fundamental parameters of TPFB. The temperature and pressure are dominated by the material and contact area of the device. The temperature and pressure can be manipulated by controlling the water vapor pressure. The boiling solution guarantees an accurate, constant temperature boundary condition. Time can be eliminated as a variable by choosing a sufficient time to achieve good bonding, since there was no apparent damage to the microstructures after one hour. This new method of TPFB was demonstrated for sealing and packaging a PMMA (polymethylmethacrylate) microfluidic device. Good results were obtained using the vacuum sealed polymer container in the pressure cooker. This method is also suitable for scaling up for mass production.

Topics: Pressure , Bonding , Polymers
Commentary by Dr. Valentin Fuster
2009;():301-302. doi:10.1115/IMECE2009-12371.

The polypyrrole (Ppy) is widely known as a conducting polymer, and many types of microactuators have been demonstrated based on volume change induced by oxidation-reduction reaction of polymer in liquid or dry environments. However, the thermomechanical property of Ppy is not well known. Here we report the measurement of Young’s modulus of polypyrrole nanowires and coefficient of thermal expansion (CTE) by Lateral Force Microscopy. Young’s modulus was measured via tip-deflection of a cantilevered Ppy nanowire. By measuring the applied force and deflection, the Young’s modulus was determined to be 2.3 ± 0.7 GPa. A thin metallic (Cu) film was deposited on only one side of the Ppy nanowire to form bimorph nanoactuators. The CTE was determined to be 12 × 10−6 /K.

Topics: Nanowires
Commentary by Dr. Valentin Fuster
2009;():303-308. doi:10.1115/IMECE2009-12433.

This paper presents an investigation into the response of a clamped-clamped microbeam to mechanical shock under the effect of squeeze-film damping (SQFD). In this work, we solve simultaneously the nonlinear Reynolds equation, to model squeeze-film damping, coupled with a nonlinear Euler-Bernoulli beam equation. A Galerkin-based reduced-order model and a finite-deference method (FDM) are utilized for the solid domain and for the fluid domain, respectively. Several results showing the effect of gas pressure on the response of the microbeams are shown. Comparison with the results of a multi-physics nonlinear finite-element model is presented. The results indicate that squeeze-film damping has more significant effect on the response of microstructures in the dynamic shock loads compared to the quasi-static shock loads.

Commentary by Dr. Valentin Fuster
2009;():309-313. doi:10.1115/IMECE2009-12460.

This paper presents methodologies for rapid prototyping of complex and functional polymeric MEMS structures using lithography steps only. In order to overcome stiffness and strength limitations of the polymeric structures, reinforcing nano-materials were added to enhance stiffness. A challenge in processing polymeric MEMS devices is achieving the release of the MEMS structure from the substrate. This paper presents several process sequences and photoresist combinations that produce freestanding polymeric MEMS structures. Negative photoresist (SU8) and positive photoresist (AZ4620) materials were used in spin coat, bake, exposure and selective development steps. This paper describes the experiments used for determining the optimal process parameters and the release performance. In addition, the paper also illustrates exposure control techniques that reduce process-induced defects such as heat–induced bubbles, degassing and overexposure.

Commentary by Dr. Valentin Fuster
2009;():315-320. doi:10.1115/IMECE2009-12548.

Piezoelectric actuators (PEAs) are frequently used in a wide variety of micromanipulation systems. However their accuracy is limited due to hysteresis nonlinearity. Also investigation of the fundamental properties of the piezoceramics depicts that external mechanical loads cause inclination in hysteresis loop which can deteriorate tracking performance furthermore. A novel modeling and control approach is proposed in this paper, for precision trajectory tracking control of piezoelectric actuators under dynamic load condition. First the hysteresis nonlinear function based on Bouc-Wen hysteresis model is approximated by a Taylor series expansion. Then an adaptive trajectory tracking control is proposed based on the backstepping method using the developed mathematical model. The asymptotical stability in displacement tracking and robustness to the dynamic load disturbance can be provided using the proposed control approach. Experimental results are illustrated to verify the efficiency of the proposed method for practical applications.

Commentary by Dr. Valentin Fuster
2009;():321-327. doi:10.1115/IMECE2009-12676.

In this paper, a passive mechanism is proposed for regulating the axial forces due to thermal stresses induced in MEMS resonant sensors. It is shown that by using this mechanism, one can control the axial force or thermal stresses to be compressive or tensile, or zero. An analytical model and a finite element model are developed for this study. It is shown that the analytical model is a powerful tool to predict the overall response and/or optimize the design, while the finite element model can be used for fine tuning and obtaining more accurate results.

Commentary by Dr. Valentin Fuster
2009;():329-334. doi:10.1115/IMECE2009-10500.

A novel MEMS-based boiler is fabricated and tested. The device is designed to operate from low-temperature heat sources using capillary action channels. The channels supply working fluid to the heated boiler surface, eliminating the need for traditional working fluid pumps. Two basic types of construction are evaluated. First, a more traditional silicon-based device is constructed and tested. Fabrication of the silicon boiler utilizes standard micro-fabrication practices. Second, a copper-based unit is fabricated and tested. Fabrication of the copper boiler focusses on low-cost techniques performed outside the scope of traditional micro-fab procedures. Results of these tests show the promise of non-traditional metals in low-temperature MEMS-based applications. The effectiveness of the copper boilers is shown to be 60% greater than their silicon counter parts. The copper-based prototypes exhibited a maximum evaporation rate for working fluid pumped across the boiling surface of 4.21 mg/sec.

Commentary by Dr. Valentin Fuster
2009;():335-339. doi:10.1115/IMECE2009-10987.

The operation of a MEMS-based micro heat engine at resonant and sub-resonant conditions is presented. Both model and experiments are used to investigate resonant and sub-resonant operation of the engine. In this work, we look at the pressure-volume diagrams of an engine operated at resonance and sub-resonance. Model predictions of the PV diagram are in favorable agreement with measured data. The results show that resonant operation is beneficial. At resonance, the pressure and volume in the engine cavity are decoupled and more mechanical work is observed. The PV diagram describes an elliptical shape. However, for an off-resonant operation the pressure and volume become more coupled and less mechanical work is observed. The PV diagram is described by a sigmoidal shape.

Commentary by Dr. Valentin Fuster
2009;():341-342. doi:10.1115/IMECE2009-11562.

We propose a convenient and easy method to harvest electric potential from plants based on streaming energy. Streaming potential and streaming current are well known phenomenon in the field of microfluidics. Plants also possess micron sized negatively charged xylem and phloem conduits where ionic sap fluid move by virtue of plant pressure. We predict that the movement of ionic sap could cause a streaming potential difference between the upper and lower portions of the stem. Two 400 μm thick Ag/AgCl electrodes probes were implanted (one at near the root and the other near the shoot) and sealed with PDMS resin. A multichannel multimeter was employed to continuously monitor and record the potential difference across the two probes. Thus a clear understanding of the streaming potential in plant system can be obtained.

Commentary by Dr. Valentin Fuster
2009;():343-352. doi:10.1115/IMECE2009-11636.

Wireless sensor networks (WSN) are a promising technology for ubiquitous, active monitoring in residential, industrial and medical applications. These nodes combine a radio transceiver, microcontroller and sensors into a low power package. A current bottleneck for widespread adoption of WSN’s is the power supplies. While the power demands can be somewhat alleviated through novel electronics, any primary battery will have a finite lifetime. Energy harvesting, from ambient vibration, light, and heat sources, offers an opportunity to significantly extend the lifetime of the nodes and possibly provide perpetual power. Thermal energy is an ideal source for WSNs due to the availability of low-grade ambient waste heat sources. Thermoelectric devices convert temperature gradients into DC electric power in compact form factors. Efficient device designs require hundreds of high-aspect ratio semi-conductor microelements fabricated electrically in series and thermally in parallel. This design requirement presents problems for standard microfabrication techniques due to thickness limitations of standard semiconductor processes. We present a new method of contact dispenser printing, specifically developed to additively create microscale generators. Initial materials performance results show promising results and are further detailed in this work.

Commentary by Dr. Valentin Fuster
2009;():353-359. doi:10.1115/IMECE2009-11799.

Phase change materials (PCMs) store thermal energy through a phase transition. Extending the use of PCMs to transient thermal management of electronic handheld devices requires storage capacity beyond 195 kJ/m3 , typical limit of a paraffin. Nano-structures can potentially increase the material choice for PCMs. Melting behavior and heat of fusion are empirically known to be altered in nano-structures. In this paper, we present thermodynamic modeling to show that confinement can lead to an enhancement in the heat of fusion of soft matter. We present material concepts that can serve as such enhanced PCMs. We also discuss modifying existing calorimetry techniques to measure the heat capacity of nanometer scale thin films of soft matter. Design and sensitivity details of nano-calorimeter are presented to analyze phase change phenomenon in ultra-thin polymer films.

Commentary by Dr. Valentin Fuster
2009;():361-367. doi:10.1115/IMECE2009-12281.

The paper describes the realization of the α-prototype of a portable power device consisting of an electrical generator with a power output of about 300 W driven by a small gas turbine set. The device is so small that it can be properly defined an ultra micro device, capable of supplying electric power in stand alone conditions and for prolonged periods of time (up to 24 hours continuously). In practice the device can be used as a convenient substitute (or replacement) for all current battery storage systems and is significantly smaller, lighter and most likely more reliable than the few existing internal combustion engines of comparable power output. The particular nomenclature is UMGTG-UDR1 (Ultra-Micro Gas Turbine Generator). The final configuration of the prototype (for which a patent is pending) is described in the paper as well, together with some of the results of the final operational tests.

Commentary by Dr. Valentin Fuster
2009;():369-376. doi:10.1115/IMECE2009-12405.

The German Artificial Sphincter System (GASS) project aims at the development of an implantable sphincter prosthesis driven by a micropump. During the last few years the feasibility of the concept has been proven. At present our team’s effort is focused on the compliance to safety regulations and on a very low power consumption of the system as a whole. Therefore a low-voltage multilayer piezoactuator has been developed to reduce the driving voltage of the micropump from approximately 300 Vpp to 40 Vpp. Doing so, the driving voltage is within the limits set by the regulations for active implants. The operation of the micropump at lower voltages, achieved using multilayer piezoactuators, has already resulted in a much better power efficiency. Nevertheless, in order to further reduce power consumption, we have also developed an innovative driving technique that we are going to describe and compare to other driving systems. A direct switching circuit has been developed where the buffer capacitor of the step-up converter has been replaced by the equivalent capacitance of the actuator itself. This avoids the switching of the buffer capacitor to the actuator, which would result in a very low efficiency. Usually, a piezoactuator needs a bipolar voltage drive to achieve maximum displacement. In our concept, the voltage inversion across the actuator is done using an h-bridge circuit, allowing the employment of one step-up converter only. The charge stored in the actuator is then partially recovered by means of a step-down converter which stores back the energy at the battery voltage level. The power consumption measurements of our concept are compared to a conventional driving output stage and also with inductive charge recovery circuits. In particular, the main advantage, compared to the latter systems, consists in the small inductors needed for the power converter. Other charge recovery techniques require very big inductors in order to have a significant power reduction with the capacitive loads we use in our application. With our design we will be able to achieve approximately 55% reduction in power consumption compared to the simplest conventional driver and 15% reduction compared to a charge recovery driver.

Commentary by Dr. Valentin Fuster
2009;():377-385. doi:10.1115/IMECE2009-12873.

Vibration based energy harvesting has wide potential applications in areas such as wireless sensors networks and ultra low power devices. While there have been various technologies through which vibration energy has been harvested, there is a considerable need to improve the power density of such devices. Recently, efforts have been made in developing MEMS scale devices as they would have increased power density and also provide ease of integration with wireless sensors and low power electronic devices. The aim of this paper is to present the generic and specific design considerations for vibration energy harvesting at the MEMS scale for electrostatic, electromagnetic and piezoelectric techniques. The effect of external load such as load resistance employed for peak power output on the total damping in the system is discussed. The typical MEMS scale vibrating structures such as cantilever beam, fixed-fixed beam and membrane are also presented.

Commentary by Dr. Valentin Fuster
2009;():387-390. doi:10.1115/IMECE2009-13027.

Dye sensitized solar cells (DSSCs) are promising photovoltaic devices as they offer advantages such as low cost and easy for fabrication et al. The key part of the original DSSC is a sintered film of nanoparticles which has a large surface area for the absorption of dyes. It has been reported that boundaries of nanoparticles diminish the efficiency of charge transport in the nanoparticle network, and lead to charge–carrier recombination. The one dimensional morphology of the nanofiber is believed to improve electron transport efficiency without sacrificing the high specific surface area for the adsorption of dyes. In this paper, TiO2 nanofibers are used to replace TiO2 nanoparticles in the DSSC. The film of nanofibers was synthesized by electrospinning process and collected on the transparent conductive glass substrate. The precursor used for the electrospinning of the nanofiber consists of titanium (IV) isopropoxide, acetate acid, ethanol and polyvinylpyrrolidone(PVP). After the electrospinning process, nanofibers were pretreated at 120°C for 2 hours and annealed at 500°C in atmosphere for another 2 hours. Then DSSC with the film of TiO2 nanofibers were assembled and characterized through electrical measurements. Open circuit voltage of 0.7V and short circuit current densities of 0.45mA/cm2 were achieved.

Commentary by Dr. Valentin Fuster
2009;():391-397. doi:10.1115/IMECE2009-10186.

We present a Brownian Dynamics model of dsDNA-molecule separation using periodic nanofilter arrays. Particular attention is paid to the model’s ability to capture relevant experimental results. The effect of various device parameters on molecule selectivity is investigated. Moreover, our model is used for validating the theoretical prediction of Li et al. [Anal. Bioanal. Chem., 394 , 427–435, 2009] who proposed a separation process featuring an asymmetric device and an electric field of alternating polarity. Good agreement is found between our simulation results and the predictions of the theoretical model of Li et al.

Commentary by Dr. Valentin Fuster
2009;():399-404. doi:10.1115/IMECE2009-10287.

In layered crystals of the AIIIBIIICVI2 family the presence of wide temperature ranges is indicated, in which on the primitive translation of the lattice of the initial phase superlattices with periods of 5–15 nm are superimposed. At that, the neutron-diffraction patterns show the presence of superstructural reflections: both reflections that are multiple of the initial translation of the lattice and incommensurable superstructural ones. Our research showed that layered crystals of the TlInS2 family are crystallized with the formation of both incommensurable and commensurate (of the initial translation) superlattices and could be used for the generation of Terahertz radiation.

Topics: Crystals
Commentary by Dr. Valentin Fuster
2009;():405-413. doi:10.1115/IMECE2009-10336.

This paper discusses the design, analysis, fabrication and characterization of a MEMS device for nano-manufacturing and nano-metrology applications. The device includes an active cantilever as its manipulator that is integrated with a high-bandwidth two degree-of-freedom translational (XY) micro positioning stage. The cantilever is actuated electrostatically through a separate electrode that is fabricated underneath the cantilever. Torsion bars that connect the cantilever to the rest of the structure provide the required compliance for cantilever’s out-of-plane rotation. The active cantilever is carried by a micro-positioning stage, which enables high-bandwidth scanning to allow manipulation in three dimensions. The design of the MEMS (Micro-Electro-Mechanical Systems) stage is based on a parallel kinematic mechanism (PKM). The PKM design decouples the motion in the X and Y directions and restricts rotations in the XY plane while allowing for an increased motion range with linear kinematics in the operating region (or workspace). The truss-like structure of the PKM also results in increased stiffness and reduced mass of the stage. The integrated cantilever device is fabricated on a Silicon-On-Insulator (SOI) wafer using surface micromachining and deep reactive ion etching (DRIE) processes. The actuation electrode of the cantilever is fabricated on the handle layer, while the cantilever and XY stage are at the device layer of the SOI wafer. Two sets of electrostatic linear comb drives are used to actuate the stage mechanism in X and Y directions. The cantilever provides an out-of-plane motion of 7 microns at 4.5V, while the XY stage provides a motion range of 24 microns in each direction at the driving voltage of 180V. The resonant frequency of the XY stage under ambient conditions is 2090 Hz. A high quality factor (∼210) is achieved from this parallel kinematics XY stage. The fabricated stages will be adapted as chip-scale manufacturing and metrology devices for nanomanufacturing and nano-metrology applications.

Topics: Cantilevers
Commentary by Dr. Valentin Fuster
2009;():415-416. doi:10.1115/IMECE2009-10454.

This paper reports on the design, fabrication, and testing of a multiple-beam tuning-fork gyroscope featuring high Quality factors (Q). A multiple-beam tuning-fork structure is designed to achieve high Qs in its drive mode and sense mode. The gyroscope is fabricated on a 30μm-thick SOI wafer using a one-mask fabrication process. The measured Qs of the fabricated gyroscope are 162,060 in the drive-mode and 85,168 in the sense mode at an operation frequency of 16.8kHz. Under a frequency split of 6Hz, the prototype device demonstrates a rate sensitivity of 0.02mV/°/sec.

Topics: Q-factor
Commentary by Dr. Valentin Fuster
2009;():417-418. doi:10.1115/IMECE2009-10646.

In this work, we demonstrate the use of a voltage-applied Atomic Force Microscopy (VAFM) local anodic oxidation nanolithography process to precisely fabricate small (<20 nm) structures from graphene and carbon nanotube material. These graphitic materials have exceptional electrical properties which give them a niche in emerging nanoelectronics applications requiring quantum structures. While several methods for nanoscale patterning of these materials exist, the VAFM nanolithography technique has lately been shown to address the fabrication issues of graphitic nanodevices on the order of tens of nanometers [1]. If the tip is raised sufficiently from the substrate, in high atmospheric humidity, a water meniscus forms between the two (Fig 1). Application of an appropriate electric field between the tip and substrate dissociates the H2O molecules into H+ and OH-. The H+ ions rush towards the negatively charged tip and the OH-ions gather near the positively substrate. The oxygen reacts with the carbon in the graphitic material to form volatile or nonvolatile carbon oxides depending on the voltage applied. This oxidation, coupled with the x-y scanning capability of the AFM allows for thin structure patterning ability. Depending on such process parameters as applied voltage, pulse width, tip dimensions, contact force, and humidity, the oxidation of the graphitic material into carbon oxides enables the formation of insulating trenches or bumps to make any structure or morphology conceivable [2]. This technique can also be performed in the ambient environment, eliminating several fabrication steps, such as the poly(methyl methacrylate) (PMMA) processing required in conventional electron-beam lithography process. We have used the VAFM technique in preliminary studies to cut few layer graphene and “draw” insulating patterns on highly ordered pyrolyzed graphite (HOPG). A negative bias of 10V applied to the AFM tip with no feedback in a high humidity atmosphere created 0.5 nm deep trenches spaced 27 nm apart. Preliminary experiments have also been conducted on 50 nm diameter multi-walled carbon nanotubes. A negative bias of 5V to the AFM tip pulsed for 100 ms segmented the multi-walled nanotube at selected points. Single wall carbon nanotubes were grown using chemical vapor deposition. Graphene was mechanically exfoliated and prepared using methods described elsewhere [3] on 300 nm SiO2 on Si substrate. The samples were connected electrically to ground and placed in an AFM system (Pacific Nanotechnology NANO-I) with environmental control. The samples were imaged in contact mode with an electrically conductive sharp AFM tip after which humidity was raised to 40–45%. Once the humidity was sufficiently raised, the tip was raised from the desired location on either the Carbon nanotubes or graphene/graphite and feedback was turned off. Patterns were drawn by the tip in this configuration with applied tip voltage running anywhere from −5V to −10V. See Figs. 2 and 3 for results on graphene and carbon nanotubes. Currently, a parametric study on AFM lithography on graphene and carbon nanotubes is underway. By varying voltage, humidity, tip speed, dwell time, and tip-substrate distance, we will determine the optimal conditions required to accomplish precise patterning of graphene and controlled segmentation of carbon nanotubes. In conclusion, we have demonstrated a voltage-applied technique utilizing an atomic force microscope tip to pattern nanoscale features on graphitic materials. A systematic study on oxidation parameters is forthcoming.

Commentary by Dr. Valentin Fuster
2009;():419-423. doi:10.1115/IMECE2009-10699.

The feasibility of using room temperature ionic liquids (RTILs) as the electrowetting liquid for capillary force microgrippers was studied. The non-volatility and thermal stability of ionic liquids make them suitable for droplet based microgripping application in high temperature and vacuum environments. Electrowetting on co-planar electrodes was utilized to dynamically change the contact angle of a 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF6 ) liquid bridge to control the capillary lifting forces. The lifting force generated by the liquid bridge was experimentally characterized. The maximum capillary force was 146μN. The dynamic response of the BmimPF6 liquid bridge was also characterized.

Topics: Force , Temperature
Commentary by Dr. Valentin Fuster
2009;():425-426. doi:10.1115/IMECE2009-10707.

In this paper a new theoretical method is developed for finding the amount and position of forces and or mass acting on a piezoelectric microcantilever sensor. In this proposed method, the mechanical parameter of microcantilever and accordingly the response of microcantilever to the forces will be changed by exerting the compression force within microcantilever, via applying different voltage to piezoelectric layers. In addition to some theoretical computations, these results generate an inverse problem to deduce the forces and their positions.

Topics: Sensors
Commentary by Dr. Valentin Fuster
2009;():427-432. doi:10.1115/IMECE2009-10812.

Conventional capacitive MEMS gyroscopes require close matching between the resonant frequencies of drive mode and sense mode. However, the uncertainties in the microfabrication process impair the robustness of the gyroscopes and often lead to unpredictable device performance. This paper analyzes a 4 degree-of-freedom (DOF) non-resonant gyroscope which is less vulnerable to the fabrication perturbations. Unlike the conventional resonant gyroscope which has only one resonant frequency for drive and sense modes, the 4-DOF gyroscope includes two resonant frequencies for each mode. The non-resonant gyroscope design aims to reduce resonance frequency matching, namely to minimize the effect of the inevitable fabrication uncertainties as well as to increase the bandwidth with less sacrifice to the sensitivity. The device performance is analyzed and optimized by the behavior model approach in CoventorWare which significantly accelerates the simulation compared to the traditional finite element method. The optimized non-resonant gyroscope with higher fabrication tolerance as well as enhanced device performance is proven to be an effective design and can be used in a wide range of applications.

Commentary by Dr. Valentin Fuster
2009;():433-438. doi:10.1115/IMECE2009-10895.

This paper reports on a new Lloyd-mirror interference lithography system whose mirror angle is variable with respect to the substrate. We demonstrate that this tunable Lloyd-mirror interferometer significantly increases the nanopatterning area with greater controllability compared to a conventional fixed-angle system for pattern periodicity over 500 nm. The new configuration capable of the independent control of mirror angle will enhance the advantages of Lloyd-mirror interferometer for large-area nanopatterning with more flexibility.

Commentary by Dr. Valentin Fuster
2009;():439-443. doi:10.1115/IMECE2009-10904.

In this article, a rapid, sensitive, and disposable microfluidic immunosensor is presented for point-of-care testing and clinical diagnostics. For the first time, a heterogeneous immunoassay without blocking process is achieved by using protein A functionalized polydimethylsiloxane microchannels. C-reactive protein (CRP), a biomarker for inflammation and cardiovascular disease risk assessment, is selected as a model analyte to demonstrate the sensitivity of this blocking-free microfluidic heterogeneous immunoassay. A four parameter logistic function is used to model and assess the data. The limit of detection obtained is 0.36 μg/mL, which is lower than the cut-off value for clinical diagnosis. The overall assay is completed in 23 min. The assay procedure can be modified for detection of other disease biomarkers or virulent pathogens by using different capture and detection antibodies.

Commentary by Dr. Valentin Fuster
2009;():445-448. doi:10.1115/IMECE2009-10938.

A new capacitive type of tilt sensor using metallic ball is proposed primarily to get over the contact problem in measurement of tilt angle. Its structure and fabrication process are simpler than other previous sensors. Capacitive sensing type has many advantages such as simplicity, non-contact measurement of angle, long-throw linear displacement, and sub-micron plate spacing comparing to other types. The dimension of this prototype sensor is 20mm × 20mm and the diameter of the polystyrene tube is 5mm and the thickness of the tube is 0.15mm. The test result shows the linear relationship between tilt angles and capacitances.

Topics: Sensors
Commentary by Dr. Valentin Fuster
2009;():449-453. doi:10.1115/IMECE2009-10992.

In this paper, an one-layer valveless micropump which can work as a micromixer is numerically studied. The micropump consists of a pump chamber, a neck channel, an outlet channel which is perpendicular with the two opposite inlet channels. A side of the pump chamber is enclosed by a vibrating diaphragm which deforms volume of the pump chamber with time, then excites the flow from the inlet to the outlet of the pump. The attained flow rate at the outlet shows the advance of the present micropump in comparison with conventional nozzle-diffuser micropumps. The mixing performance of the devices is studied by Poincare map technique. It is observed that the mixing is enhanced with an inclined rib in the neck channel.

Topics: Micropumps
Commentary by Dr. Valentin Fuster
2009;():455-459. doi:10.1115/IMECE2009-11094.

Hot embossing is an effective technology for replicating micro-scale features in polymeric materials, but large-scale adoption of this method is hindered by high capital costs and longer cycle times relative to other technologies. This paper details a hot embossing machine design strategy motivated by maximum production speed with minimal capital cost. Innovative design aspects include the choice of new ceramic substrate heaters for electrical heating, design of a moveable heat sink to minimize heat load during the heating cycle, and the careful design of the thermal elements to minimize the heating and cooling cycle times. The hot embossing equipment fabricated from this design has a capital cost estimated to be an order of magnitude less than currently available options. The minimum cycle time is two minutes, and microstructures are replicated within a maximum area of 25mm by 75mm. The hot embossing machine has been tested to characterize the process variability. Runs of polymethylmethacrylate (PMMA) parts manufactured using this equipment are measured to have submicron variation under a variety of processing conditions.

Commentary by Dr. Valentin Fuster
2009;():461-462. doi:10.1115/IMECE2009-11121.

This paper determined the optimal periodic mechanical stimulation of live bone cells from the intracellular calcium oscillation induced by shear stress. The shear stress-induced intracellular calcium responses of cells on a micro-cell chip were measured to study the mechanotransduction of bone cells. From the measured static and dynamic characteristics of the internal cellular signaling in cells, the optimum duration of the mechanical stimulation is determined.

Commentary by Dr. Valentin Fuster
2009;():463-467. doi:10.1115/IMECE2009-11469.

Mixing has become a challenge in micro-fluidic systems because of the low Reynolds number in micro-channels. The method which is implemented in this paper is to use freely-swimming bacteria to enhance the mixing process. Accordingly, the Serratia marcescens bacteria were used for this matter. The mixing performance of the system is quantified by measuring the diffusion rate of Rhodamine B in a particular section of a channel connected to a chamber with varying Rhodamine B concentration. The concentration of Rhodamine B was measured using the Laser Induced Fluorescence (LIF) technique. The channel is in the form of a pipe and is closed on the extending side. In this paper, it is demonstrated that the corresponding diffusion coefficient can be augmented by bacterial participation and that this augmentation can be continued for several hours, depending on the environmental conditions. Additionally, it is shown that the mixing process reacts in response to modifications to the chemical environment of the system, which in turn affect the metabolic activity of the bacteria. Also, a 30 mM glucose buffer was used to show the impact of food on the performance of the bacterial system. It is thus shown that the existence of glucose increases the mixing ability of bacteria.

Commentary by Dr. Valentin Fuster
2009;():469-470. doi:10.1115/IMECE2009-11628.

Thermoelectric coolers (TECs) are solid state cooling devices that produce a temperature difference under an applied voltage. Thermo electric coolers are made by assembling P and N type Bismuth Telluride elements in series. Previous work has shown that microscale components can achieve higher performance in many applications than macroscale devices. [1, 2]. However, current assembly techniques cannot assemble and produce the smaller devices effectively. This paper will look at a water-based method to compare to prior solder-based assemblies.

Commentary by Dr. Valentin Fuster
2009;():471-474. doi:10.1115/IMECE2009-11760.

A high-pressure microvalve technology based on the integration of discrete elastomeric elements into rigid thermoplastic chips is described. The low-dead-volume valves employ deformable polydimethylsiloxane (PDMS) plugs actuated using a threaded stainless steel needle, allowing exceptionally high pressure resistance to be achieved. The simple fabrication process is made possible through the use of poly(ethylene glycol) (PEG) as a removable blocking material to avoid contamination of PDMS within the flow channel while yielding a smooth contact surface with the PDMS valve surface. Burst pressure tests reveal that the valves can withstand over 24MPa without leakage.

Commentary by Dr. Valentin Fuster
2009;():475-479. doi:10.1115/IMECE2009-11833.

We describe new transfer method of carbon nanotube (CNT) film onto the poly-dimethysiloxane (PDMS) based on the poor adhesion between Si wafer and Au layer. To combine the CNT film with the polymer-MEMS field, it is required to transfer CNT film onto the polymer substrates. CNT film was fabricated by vacuum filtration method and was transferred onto the Au-deposited Si wafer. Using photolithography process, CNT film was patterned and PDMS is pouring and curing on the wafer. After peeling off the PDMS, patterned CNT film was transferred which was embedded into the PDMS. The possibility of embedded CNT film in the micro system was demonstrated in the application of electro-thermal actuator.

Commentary by Dr. Valentin Fuster
2009;():481-485. doi:10.1115/IMECE2009-11934.

The ability to produce three-dimensional micro- and nanoscale features at low cost is desirable for many applications such as microfluidic devices, micro and nanomechanical systems, photonic crystals and diffractive optics. For example, micro and nanostructures patterned on the sidewalls of microfluidic devices allow better control over the wetting behavior of fluids flowing through the microchannel. In this study we report on a simple and effective process that allows direct integration of microstructures into a microfluidic device via a modified molding process. The key for the process is to use a thin poly(dimethylsiloxane) layer having microgratings as an intermediate stamp which was placed between a brass mold insert with microfluidic features and a PMMA sheet, which was followed by hot embossing. Using this method, we have demonstrated the formation of micropatterns on non-planar surfaces and at the sidewalls of microfluidic devices, as confirmed using scanning electron microscopy. The designed process will fill the gap in current micro- and nanofabrication technologies in that most of the technologies allow for patterning only on planar substrates.

Commentary by Dr. Valentin Fuster
2009;():487-494. doi:10.1115/IMECE2009-12503.

In this study, static deflection and Instability of double-clamped nanobeams actuated by electrostatic field and intermolecular force, are investigated. The model accounts for the electric force nonlinearity of the excitation and for the fringing field effect. Effects of mid-plane stretching and axial loading are considered. Galerkin’s decomposition method is utilized to convert the nonlinear differential equation of motion to a nonlinear algebraic equation which is solved using the homotopy perturbation method. The effect of the design parameters such as axial load and mid-plane stretching on the static responses and pull-in instability is discussed. Results are in good agreement with presented in the literature.

Commentary by Dr. Valentin Fuster
2009;():495-501. doi:10.1115/IMECE2009-12554.

Atomic force microscopes (AFM) are widely used for feature detection and scanning surface topography of different materials. Contrast of topography images is significantly influenced by the sensitivity of AFM micro cantilever which means enhancement of sensitivity leads to increase of topography images resolution So, in the last years numerous scientists interested in studying the effects of different parameters such as geometric one on the sensitivity of AFM micro cantilevers. V-shape micro cantilever types of AFMs probe are widely used to scan various types of surfaces. In V-shape micro cantilevers, there are many geometric and design parameters which influence the flexural sensitivity of the micro beam, noticeably. In this paper evaluation of optimum geometric parameters and optimum cantilever slope is considered as a significant purpose in order to obtain maximum flexural sensitivity by using genetic algorithm optimization method. In the calculations, the normal and lateral interaction forces between AFM tip and sample surface is considered and modeled by linear springs which represent the contact stiffness of the sample surface. Also, a relation for flexural sensitivity of AFM cantilever as a function of geometric parameters and cantilever slope is derived which is used in optimization step by employing a genetic algorithm program. Using genetic algorithm method, the optimum geometric parameters and cantilever slope are calculated which maximize the flexural sensitivity of the first mode of a V-shape cantilever for various values of normal contact stiffness. These optimum parameters versus normal contact stiffness are presented in some result figures. The results show that for any contact stiffness, there are a cantilever slope and a set of geometrical parameters which provide the maximum sensitivity for AFM probe. Adopting these parameters for the design of V-shape micro cantilever according to the sample contact stiffness, maximum flexural sensitivity can be obtained, so that high contrast images are reachable.

Commentary by Dr. Valentin Fuster
2009;():503-512. doi:10.1115/IMECE2009-12765.

Magnetic micro-electro-mechanical-systems (MEMS) present new class of micro-scale devices that incorporate magnetic materials as sensing or active elements. It exploits the properties of magnetic materials by incorporating them in conventional microfabricated systems. Although their application for microactuation purposes has been limited, the prospects of remote control and large displacements renders them useful, and even unavoidable in certain circumstances. Recognizing the fact that poor electromagnetic flux in micro domain happens to be the most stringent limitation, measures to improve the magnetic field generated by an electromagnetic coil are studied using a microactuator that incorporates a coil and a hard magnetic film deposited on a flexure membrane. This paper describes the design of the microactuator, analysis and optimization that maximizes the deflection. This study also presents an overview of magnetic microactuators covering the scaling effects, materials and processes used in their fabrication and critical review of their limitations.

Commentary by Dr. Valentin Fuster
2009;():513-518. doi:10.1115/IMECE2009-12790.

A capacitive temperature sensor with separate thermal actuation and capacitive readout is introduced. A bi-layer plate with fixed-free boundary condition is used for the thermal actuation to change the gap between two parallel electrodes used for capacitance measurement. Different coefficients of thermal expansion (CTE) of the two layers in actuator cause out-of-plane deformations in the plate when the temperature changes. The proposed design has the capability to control the response of the sensor by increasing its sensitivity in a given temperature range. To obtain the desired characteristic C-T curve, the design utilizes asymmetric geometries. Different design parameters such as the size of the bi-layer plate and the sense electrodes are considered as design variables. ANSYS® FEM simulations are used to extract the C-T responses of different geometries. The results of the FEM simulations show that for a given fabrication process and material properties, the design can be modified to provide the highest sensitivity and linearity in the C-T response for a given temperature range. This temperature sensor can be used for remote and on-chip temperature measurement or temperature compensation.

Commentary by Dr. Valentin Fuster
2009;():519-525. doi:10.1115/IMECE2009-12895.

The efficiency of hybrid solar cell depends mainly on the exciton dissociation efficiency and charge mobility. The exciton dissociation efficiency can be improved by increasing the interfacial area between the nanoparticles and polymer. Charge mobility can be improved by proper distribution of nanoparticles in polymer to form better permitting path of each material. Both these parameters are strongly dependant on better distribution of nanoparticles in the polymer. The approach used in this research is the application of star dispersant to the photo active layer, specifically designed for conducting polymers. This dispersant will modify the arms of conducting polymer to have a high compatibility with nanoparticles and provide better distribution. The patterning of these polymers is achieved by wet etching process. Finally, Indium is used as a contact between P3HT and ITO to measure voltage and current characteristics. A number of specimens are prepared with and without the introduction of star dispersant. Absorption spectrum analysis and Photoluminescence (PL) measurements are performed to characterize the optical properties of active layer. Parametric study involving influence of the nano-composite film morphology with and without star dispersant for Photoluminescence measurement and I-V characteristics of hybrid nano-polymer solar cells have been studied. Structural characterization revealed that with the application of the dispersant, better mixing of the nanoparticles and the polymer can be achieved. This will in turn increase the interface area and improve exciton dissociation.

Topics: Polymers , Solar cells
Commentary by Dr. Valentin Fuster
2009;():527-533. doi:10.1115/IMECE2009-12930.

Development of new sensors with precise and innovative measurement mechanisms is a key in underpinning the competitiveness of industries like automotive, aviation, and power generation. Due to progresses made in micromachining technologies, fabrication of such sensors for multifunctional applications and their integration with readout circuits is easily achievable. In this paper a new multifunctional sensor for the simultaneous measurement of pressure and temperature is proposed and modeled. It uses membranes and beams as active bodies and capacitance measurement as readout system. The sensor can be fabricated with available CMOS-compatible foundry processes. The results of the finite element simulations are presented for pressures up to 1 MPa and temperature changes up to 250 °C.

Commentary by Dr. Valentin Fuster
2009;():535-544. doi:10.1115/IMECE2009-12980.

We have developed and demonstrated a technique for optical excitation of mechanical resonance that does not require coherent, monochromatic, or time-varying light. Previous methods for optically exciting mechanical motion in microscale devices required monochromatic, coherent light or time varying light. This technology could allow sunlight (or other ambient light source) to drive a MEMS device. It could also be used to convert sunlight to mechanical energy and subsequently to electrical energy through piezoelectric or capacitive techniques, essentially a micromechanical analog to the photovoltaic cell. We have demonstrated this method of optical excitation of a MEMS cantilever using simple cantilever beam structures fabricated using Sandia National Laboratories’ SUMMiT V™ process. The bimorph structure was created with polysilicon and aluminum. The minimum power to induce resonance was 3.5–4 mW of optical power incident on the cantilever under a vacuum of less than 1 mTorr. Resonance was observed at 45.6 kHz (slightly less than the 48.5 kHz predicted by FEA).

Topics: Resonance , Wavelength
Commentary by Dr. Valentin Fuster
2009;():545-552. doi:10.1115/IMECE2009-13346.

Although high aspect ratio micro and nanoscale polymer features have been replicated in a range of polymers using injection molding, researchers have also used tooling inserts with a range of sizes, aspect ratios, and tooling materials. In this work, microscale features with molded in polymethylmethacrylates using three types of tooling with similar features. The tooling materials included silicon wafers with an antistiction coating, gold-coated nickel inserts, and a metal-polymer hybrid tooling. Tooling was evaluated based on the ease of melt filling and part ejection; the replication quality as characterized using optical profilometry, confocal microscopy, and scanning electron microscopy; and the damage to the tooling after repeated use. With lower aspect ratio features, the tooling type did not significantly affect replication, but for higher aspect ratio features the hybrid tooling provided far better replication than the silicon tooling. This difference was attributed to retardation of heat transfer in the features of the hybrid tooling. All three tooling materials exhibited polymer-free surfaces after injection molding.

Commentary by Dr. Valentin Fuster
2009;():553-557. doi:10.1115/IMECE2009-10253.

The interface between intersecting microfluidic multicomponent flow is investigated experimentally. Three microchannel configurations are studied. Each configuration has a main channel and an intersecting daughter channel. In two configurations, the channel cross sections are equal and square with the intersection either at 90 or 45 degrees. In the third configuration, the intersection is at 90 degrees, the cross sections are square and the daughter cross section is smaller than the main cross section. In the configurations with equal channel cross sections, microsphere solutions of 2, 4 and 7% spheres (by weight) are compared to each other as well as all water flows. Flow visualization is achieved using confocal fluorescence microscopy. A three-dimensional rendering of the location and shape of the interface is examined for a Reynolds number of approximately one. The presence of microspheres does not appear to strongly influence the location of the flow interface. For flows with equal cross section, the interface downstream of the junction is reasonably planer (two dimensional). Strong three-dimensional effects are shown for flows with unequal cross section.

Commentary by Dr. Valentin Fuster
2009;():559-566. doi:10.1115/IMECE2009-10354.

Numerical modeling of fluid-structure interaction problems are challenging in the field of computational fluid dynamics because of the complex geometries involved and freely moving boundaries. Flapping of an inextensible filament in a uniform fluid flow is such a problem which mimics the swimming of energy harvesting eel fish. Recently, immersed boundary method has found much attention in simulating fluid-structure interaction problems due to its easiness in grid generation and memory and CPU savings. In the present work, we employed an improved version of immersed boundary method proposed by Shin et al. [1] which combines the feedback forcing scheme of the virtual boundary method with Peskin’s regularized delta function approach. A FORTRAN code is developed for the simulation of flexible filament flapping in a uniform fluid flow. The code is validated for the bench mark problem of two-dimensional flow over a circular cylinder. A single filament hanging under gravitational force is simulated using the developed code which is analogous to a rope pendulum and the results are compared with available analytical results. The results are found to be in good agreement. Finally, the interaction of the flapping filament in the uniform fluid flow is studied for different flow and structure parameters. The production of a series of vortex procession obtained in the case of flapping of filament is in good agreement with the previous available experimental and numerical results.

Topics: Fluid dynamics
Commentary by Dr. Valentin Fuster
2009;():567-570. doi:10.1115/IMECE2009-10422.

Based on diffusion mechanism, a program-controlled cyclic particle extraction on an integrated PDMS microfluidic platform is presented. The platform comprises on-chip pneumatic peristaltic pumps and valves connected to a PC-based relay board, hence allowing programmable manipulations. The main concept is cycling a sample stream along with an extraction stream multiple times to enhance the particle separation. A sample solution containing 3-μm and 0.5-μm particles was utilized to demonstrate the process. The average flow rate was 4.75 mm/s and the extraction channel length was 84.3 mm. The relatively accumulative concentration for the 0.5-μm particle was 7.52% at the 1st cycle and became 37.99% at the 10th cycle. The result shows an expected improvement of particle concentration after the cyclic extraction. Higher efficiency can be achieved with more cycles.

Commentary by Dr. Valentin Fuster
2009;():571-576. doi:10.1115/IMECE2009-10564.

Miniature robots should be precisely controlled because of a small workspace and size of their shapes. Small error of control could lead to failure of tasks such as an assembly. Tracking is one of the most important techniques because control of a small scale robot is hard to accomplish without object’s motion information. In this paper, we compare the feature based and the region based tracking methods with microbiorobot. Invariant features can be extracted using Scale Invariant Feature Transfrom (SIFT) algorithm because microbiorobot is a rigid body unlike a cell. We clearly showed that the feature based tracking method track exact positions of the objects than region based tracking method when objects are close contacted or overlapped. Also, the feature based tracking method allows tracking of objects even though partial object disappears or illumination is changed.

Commentary by Dr. Valentin Fuster
2009;():577-582. doi:10.1115/IMECE2009-10565.

Drug delivery systems have had a profound impact on several branches of medicine. Engineers and researchers alike have labored to create a controlled drug delivery device capable of regulated dosage release and a specific cell targeting mechanism. The growing field of biomimicry has inspired several of these drug systems, though success has been limited. The flagellated low Reynolds number propulsion system of Salmonella typhimurium has inspired this specific delivery complex. In this system, the helical flagellar filaments of S. typhimurium are isolated from the bacteria’s cell body and are bound to functionalized paramagnetic microspheres. As a magnetic field is applied to this device, the microsphere rotates, inducing rotation of the helical flagella. This motion creates a locomotive force and drives the device in a predestined direction.

Commentary by Dr. Valentin Fuster
2009;():583-588. doi:10.1115/IMECE2009-10582.

Electrophoresis is the motion of dispersed particles relative to a fluid under the influence of an electric field. Presently this phenomenon of electrokinetics is widely used in biotechnology for the separation of proteins, sequencing of polypeptide chains etc. The separation efficiency of these biomolecules is affected by their aggregation. Thus it is important to study the interaction forces between the molecules. In this study we calculate the electrophoretic motion of a pair of colloidal particles under axial electric field. The hydrodynamic and electric double layer (EDL) interaction forces are calculated numerically. The EDL interaction force is calculated from electric field distribution around the particle using Maxwell stress tensor and the hydrodynamic force is calculated from the flow field obtained from the solution of Stokes equations. The continuous forcing approach of immersed boundary method is used to obtain flow field around the moving particles. The EDL distribution around the particles is obtained by solving Poisson-Nernst-Planck (PNP) equations on a hybrid grid system. The EDL interaction force calculated from numerical solution is compared with the one obtained from surface element integration (SEI) method.

Commentary by Dr. Valentin Fuster
2009;():589-597. doi:10.1115/IMECE2009-10634.

Since the inception of microfluidics, the electric force has been exploited as one of the leading mechanisms for driving and controlling the movement of the operating fluid (electrohydrodynamics) and the charged suspensions (electrokinetics). Electric force has an intrinsic advantage in miniaturized devices. Because the electrodes are placed cross a small distance, from sub-millimeter to a few microns, a very high electric field is rather easy to obtain. The electric force can be highly localized with its strength rapidly decaying away from the peak. This makes the electric force an ideal candidate for spatial precision control. The geometry and placement of the electrodes can be used to design electric fields of varying distributions, which can be readily realized by MEMS fabrication methods. In this paper we examine several electrically driven liquid handling operations. We discuss the theoretical treatment and related numerical methods. Modeling and simulations are used to unveil the associated electrohydrodynamic phenomena. The modeling based investigation is interwoven with examples of microfluidic devices to illustrate the applications. This paper focuses on detailed physical simulations of component-level operations. Since the components must be integrated to form a functional system in order to provide desired services, system-level complexities in both architecture and execution also need to be addressed. Compared to the state of the art of computer-aided design for microelectronics, the modeling aid for microfluidics systems design and integration is far less mature and presents a significant challenge, thus an opportunity for the microfluidics research community.

Commentary by Dr. Valentin Fuster
2009;():599-604. doi:10.1115/IMECE2009-10678.

We studied the effect of textured hydrophobic surfaces on drag reduction in Newtonian laminar flow through a rectangular channel. Fine fabrication was given to the test wall surfaces so that the groove pattern and the groove area ratio may be changed methodically, and their surfaces are coated with PTFE. Drag reduction was estimated by pressure loss measurement in 0.5×5 mm and 12×12 mm channels. Visualization experiment was carried out to reveal a mechanism of drag reduction from the form of air-water interface standpoint. A series of experiments showed that the air-water contact area ratio and the air layer thickness influence the drag reduction, and the maximum drag reduction ratio is 15.6%.

Commentary by Dr. Valentin Fuster
2009;():605-611. doi:10.1115/IMECE2009-10765.

Myoglobin is one of the important cardiac markers, whose concentration increases from 90 pg/ml to over 5000 pg/ml in the blood serum of heart attack patients. Separation and detection of myoglobin play a vital role in deciding the cardiac arrest in advance, which is the challenging part of ongoing research. In the present study, one of the electrokinetic approach i.e., dielectrophoresis (DEP) is chosen to manipulate the myoglobin molecule in aqueous solution. A generalized theoretical expression is developed for the dielectrophoretic force acting on an arbitrary shape of the particle. Dielectric myoglobin model is developed by approximating the shape of the molecule as sphere, oblate and prolate spheroids. Mathematical model for simulating dielectrophoretic behavior of a myoglobin molecule in a microchannel is developed. The microchannel consists of parallel array of electrodes at the bottom wall. Finite element based approach is considered to solve the problem. The variation in the Clausius-Mossotti factor with respect to the applied electric field frequency is observed for aqueous solution of myoglobin. The crossover frequency is obtained as 30 MHz for given properties, for all the shapes of molecule. Shifting of crossover frequency with conductivity of medium is observed. The simulation results indicate that, the electric field and DEP forces are maximum at the edges of the electrodes and minimum elsewhere. The results also indicate that, DEP force exponentially decayed along the height of the channel.

Commentary by Dr. Valentin Fuster
2009;():613-617. doi:10.1115/IMECE2009-10772.

Preconcentration of cardiac proteins was demonstrated using isotachophoresis (ITP) on a polymethyl methacrylate (PMMA) microchip. PMMA microchip was formed using solvent imprinting and temperature-assisted solvent bonding. ITP experiments were performed on two types of microchip: one containing straight microchannel and the other one with 10X step reducing microchannel. In ITP experiments, Hydrochloric Acid (HCl) was used as leading electrolyte, while Aminocaproic Acid (EACA) was used as terminating electrolyte. Three fluorescent proteins, cTnI Labeled w/ Pacific Blue, Green Fluorescent Protein (GFP) and R-Phycoerythrin (PE), were allowed to separate and concentrate in presence of a constant electric field. Microchip ITP experiments show that the sample proteins were concentrated and stacked into adjacent zones. The final concentration of protein zones were calculated from the microchannel dimensions and initial volume of proteins. In straight microchannel, the concentration factors for PE, GFP, and cTnI proteins were 80, 40, and 30, respectively. The concentration factors were 10 fold higher in the step reducing microchannel.

Commentary by Dr. Valentin Fuster
2009;():619-625. doi:10.1115/IMECE2009-10773.

In this paper, we experimentally studied the evaporative behavior of the nanofluid droplets (fluid containing metal nanoparticles) on nanoporous superhydrophobic surfaces. Uniformly dispersed in water, gold chloride (AuCl3 ) nanoparticles of varying sizes (10–250 nm) and concentrations (0.001–0.1% wt) were tested as nanofluids. Porous anodized aluminum oxide (AAO) with a pore size of 250 nm was tested as a nanoporous superhydrophobic surface, coated by a self assembled monolayer (SAM). During the evaporation in a room temperature and pressure, the evaporation kinetics (e.g., contact angle, contact diameter, and volume) of the nanofluid droplets was measured over time by using a goniometer. In the beginning, the initial droplet contact angles were significantly affected by the nanoparticle sizes and concentrations such that as the concentration increased, the initial contact angle decreased, which was more pronounced at larger particle sizes. During evaporation, despite the different particle sizes and concentrations, there were two distinct stages shown, especially for the change of contact angles, i.e., gradual decrease in the beginning, followed by rapid decrease in the end. No remarkable wetting transition from de-wetting (Cassie) to wetting (Wenzel) state was shown during the evaporation. Evaporation rate was influenced by nanoparticles such that it was significantly mitigated with the nanofluid droplet of the highest concentration (0.1% wt). The scanning electron microscope (SEM) images show that the ring-like dry-out pattern forms after the evaporation of nanofluids with lower concentrations (0.001%, 0.01% wt), whereas the one with higher concentrations (0.1%wt) forms a uniformly distributed pattern. These results demonstrate that nanoparticle sizes and concentrations make significant effects on interfacial phenomena in droplet evaporation on nanostructured surfaces, which will impact many engineering applications and system designs based on droplets such as microfluidics and heat transfer.

Commentary by Dr. Valentin Fuster
2009;():627-631. doi:10.1115/IMECE2009-10778.

This paper presents a mathematical model for pH gradient ITP in a microfluidic system. The mathematical model is based on mass conservation, charge conservation and electroneutrality condition in the system. A finite volume based numerical model is developed to simulate pH dependent isotachophoresis (ITP) in microfluidic devices. Numerical results of pH dependent ITP are obtained for straight and dog-leg microchannels. For both channels, five ionic components are used to simulate the model ITP system. The ITP results obtained from dog-leg microchannel capture the band broadening and band dispersion observed in T-channel junction. However, no such dispersion is noticed for ITP in the straight microchannel.

Commentary by Dr. Valentin Fuster
2009;():633-641. doi:10.1115/IMECE2009-10926.

Ultrasound based on-line cleaning for hollow fiber (HF) membrane filtration of synthetic wastewater was studied. An ultrasonic transducer was submerged into a filtration system in order to get an efficient cleaning of HF membranes in fouling conditions. An ultrafiltration (UF) HF membrane with the pore size at 10,000 NMWC is employed to purify waste water. The focus of this study is on the effects of temperature, ultrasonic frequency, ultrasonic power intensity and caviation micro-bubbles as well as the transmembrane pressure (TMP) performance. Experimental evidence reveals that the permeate flux increased with the application of ultrasound after fouling by sullage solution for one hour. The micro-bubble size measured by laser PDA system shows a decreased tendency with the increase of ultrasonic frequencies, and larger micro-bubbles have greater contribution to the increase of permeate flux. Results futher shows that the permeate flux measured with lower ultrasonic frequency or higher power intensity maintained higher value in general as feeding sullage water and maintain a higher risk to extend membrane pore size. In addition, the rise of the temperature around filtration system has less impact on permeate flow rate in online ultrasound system when the temperature of feed solution maintained constant.

Commentary by Dr. Valentin Fuster
2009;():643-648. doi:10.1115/IMECE2009-10927.

Joint effect of traveling wave dielectrophoresis and AC electroosmotic fluid flow is used to sort bacteria from other particles and increase the bacteria output concentration in a microfluidic device. The device consists of a thin and long rectangular channel with two interdigitated electrode arrays, one at the bottom and one at the top of the channel, that are used to generate a nonuniform electric field. A four-phase signal at high frequency superposed on a low frequency signal is applied. At the end of the channel, the fluid is collected in two outputs: the bacteria are collected on one side and fluid without bacteria is collected on the other side. We have previously demonstrated a method to optimize cell separation using multiple frequency dielectrophoresis. The device presented here illustrates a novel use of multiple frequencies that permits the combined use of traveling wave dielectrophoresis and AC electroosmotic fluid flow.

Commentary by Dr. Valentin Fuster
2009;():649-653. doi:10.1115/IMECE2009-10967.

A device based on a magnetic Coulter counting principle to detect metal particles in lubrication oil is presented. The device detects the passage of ferrous and non ferrous particles by monitoring inductance change in a coil. First, the sensing principle is demonstrated at the mesoscale using a solenoid. Next, a microscale device is developed using a planar coil. The device is tested using iron and aluminum particles ranging from 100μm to 500μm. The testing results show the device is capable of detecting and distinguishing ferrous and non-ferrous metal particles in lubrication oil. The design concept demonstrated here can be extended to a microfluidic device for online monitoring of ferrous and non-ferrous wear debris particles.

Topics: Wear , Lubrication
Commentary by Dr. Valentin Fuster
2009;():655-661. doi:10.1115/IMECE2009-10986.

In present study, stretching dynamics of electrically tethered λ-DNA (48.5kbp) in SiO2 nanochannels has been investigated. At high electrical fields (above 20kV/m), elongations of electrically tethered DNA molecules were observed. At high E-fields, DNAs were tethered in nanochannels and were spontaneously elongated along the nanochannels up to about 90 percent of its contour length. With E-field turned off, the measured relaxation time was about 10 sec from stretching with 20kV/m. In current study, observed behaviors of DNA molecules in nanochannels were explained by field-induced dielectrophoretic DNA trap due to the particular cross-sectional geometry of nanochannels. Also the elongation ratio between 20kV/m and 60kV/m cases and the effect of E-field distribution in the transverse plane on field-induced dielectrophoretic tethering force are discussed based on “worm-like chain” model. The FEM simulation was done to verify induced dielectrophoretic tethering force into the nanohorn.

Topics: DNA
Commentary by Dr. Valentin Fuster
2009;():663-667. doi:10.1115/IMECE2009-11008.

An efficient cooling system consisting of a plate, on which copper nanorods (nanorods of size ∼100nm) are integrated to copper thin film (which is deposited on Silicon substrate), a heater, an Aluminum base, and a pool was developed. Heat is transferred with high efficiency to the liquid within the pool above the base through the plate by boiling heat transfer. Near the boiling temperature of the fluid, vapor bubbles started to form with the existence of wall superheat. Phase change took place near the nanostructured plate, where the bubbles emerged from. Bubble formation and bubble motion inside the pool created an effective heat transfer from the plate surface to the pool. Nucleate boiling took place on the surface of the nanostructured plate helping the heat removal from the system to the liquid above. The heat transfer from nanostructured plate was studied using the experimental setup. The temperatures were recorded from the readings of thermocouples, which were successfully integrated to the system. The surface temperature at boiling inception was 102.1°C without the nanostructured plate while the surface temperature was successfully decreased to near 100°C with the existence of the nanostructured plate. In this study, it was proved that this device could have the potential to be an extremely useful device for small and excessive heat generating devices such as MEMS or Micro-processors. This device does not require any external energy to assist heat removal which is a great advantage compared to its counterparts.

Commentary by Dr. Valentin Fuster
2009;():669-675. doi:10.1115/IMECE2009-11214.

In microfluidic systems external forces are frequently applied to fluids or colloidal suspensions in order to accomplish or enhance mass transport tasks. The complexities of microscale geometries and material properties, however, can cause discrepancies between theoretical predictions and the actual values of the applied force. Therefore a calibration experiment is necessary to validate the actual magnitude of the applied force. One method of such in vivo calibration is through observations of tracer particle motions using particle tracking velocimetry (PTV). In microfluidic applications, the tracer particles of choice are typically submicron in diameter and therefore undergo significant Brownian motion. Further complicating the matter is the presence of the solid channel boundaries whose presence can lead to hindered Brownian motion and position-dependent hydrodynamic drag. In this paper we present a Langevin simulation study of the effects of normal and hindered Brownian motions, and the time between image acquisitions on the accuracy of external force measurements based on PTV. It is found that the relative strength between the random forces that cause Brownian motion and the applied external force plays a critical role in measurement accuracy. We also found that hindered Brownian motion and the associated sampling trajectory biases contribute additional force measurement inaccuracies when PTV is conducted in the vicinity of a solid boundary.

Commentary by Dr. Valentin Fuster
2009;():677-685. doi:10.1115/IMECE2009-11278.

In recent years, microfluidic devices that generate micron sized droplets/bubbles have found widespread applications in drug delivery, microanalysis, tumor destruction, as ultrasound agents and in chemical reactions at the micron level. In the current work, simulations results are being presented for a T-junction device for the formation of micron-sized droplets using the lattice Boltzmann method. Flow regimes obtained as a consequence of two immiscible fluids interacting at a T-junction are presented for a range of Capillary numbers and different flow rates of the continuous and dispersed phases. Through lattice Boltzmann based simulations, regime maps are presented that distinguish parallel flows from droplet flows. It is shown that as the Capillary number increases, the transition zone which separates parallel and droplet flows shrinks, and is influenced by the viscosity ratio as well.

Commentary by Dr. Valentin Fuster
2009;():687-688. doi:10.1115/IMECE2009-11312.

We propose a hydrodynamic focusing based particle filtration method. Three microchannels associated for the filtration were networked with an island structure to control a boundary to limit particle sizes. The boundary was easily controlled by adjusting lengths of the three microchannels. Using hydrodynamic focusing in the microfluidic network, we have successfully designed and tested the device to filtrate particles of 10-μm and 20-μm. By adjusting the ratios of hydraulic resistances (or outlet channel lengths) using the built-in valves, the boundary distance was controlled. The proposed method could continuously sort particles by size. Thus, the proposed sorting method can be applicable for many fields for biology and biomedical engineering.

Commentary by Dr. Valentin Fuster
2009;():689-694. doi:10.1115/IMECE2009-11327.

We are examining techniques for manipulation of microfabricated elements using arrays of bacteria as microactuators. Flagellated Serratia marcescens bacteria are attached to microstructures using a blotting technique that creates a bacterial monolayer carpet. These bacterial carpets naturally self-coordinate to propel the microstructures. We refer to these constructs as microbiorobots (MBR). Generally, the motion pattern of the MBRs is largely rotational in nature, and the center of mass deviates no more than several hundred microns from its original position. However, the angular velocity and orientation of the MBRs may be controlled using ultraviolet light stimulus, and the translational position may be adjusted using electrokinetic stimulus. Here, we demonstrate precision positional adjustment of a microbiorobotic transporter that is used to engage and transport cube-shaped particles 10 μm on each side.

Commentary by Dr. Valentin Fuster
2009;():695-696. doi:10.1115/IMECE2009-11410.

Freestanding bilayer lipid membranes provide an exceptional platform for measurements of lipid/protein interactions and ion translocation events at the single molecule level. For drug screening applications, large arrays of individual bilayer supports are required. However, an effective method for generating, stabilizing, and monitoring arrays of lipid bilayers remains elusive. Here we investigate a novel approach towards the facile generation of bilayer arrays for high throughput screening. The approach takes advantage of fundamental microfluidic capabilities by combining an emulsion generator with droplet-interfaced membrane formation, allowing for fully-automated production of membrane arrays whose density is, in principle, unlimited.

Commentary by Dr. Valentin Fuster
2009;():697-698. doi:10.1115/IMECE2009-11432.

A key requirement for the effective study of interactions between analytes and ion channels is the ability to dynamically vary analyte type and concentration to a membrane-bound ion channel within a planar phospholipid membrane (PPM). Here an open well microfluidic PPM apparatus supporting dynamic perfusion is presented. The plastic chip supports the manual formation of bilayer membranes that are resistant to pressure disturbances during perfusion with stability on the order of several hours. Using a chamber volume of 20 μL and a flow rate of 0.5 μL/min, the system enables rapid perfusion without breaking the membrane. The perfusion capability is demonstrated through gramicidin ion channel measurements.

Topics: Membranes
Commentary by Dr. Valentin Fuster
2009;():699-703. doi:10.1115/IMECE2009-11549.

This paper explores flow in complex nano-sized channels by use of molecular dynamics. Due to the small nature of these channels and to better capture wall effects, non-equilibrium molecular dynamics simulations were performed. Straight, constricted and sawtooth channels were studied. The function used for modeling the particle interactions is the Lennard-Jones 6–12 potential. Stochastic boundary conditions are used in conjunction with periodic boundary conditions in a 3D domain. Computational enhancements including cell subdivision and neighbor listing provide increased efficiency. The channels were homogeneous in the depth dimension and the results were averaged in the depth direction in order to improve averages. Velocity profiles at several locations were computed and are presented in the paper. The eventual goal of this research is to study the effects of time-dependent inflow and pressure drops so as to understand the flow in nano channels in the human bone.

Commentary by Dr. Valentin Fuster
2009;():705-711. doi:10.1115/IMECE2009-11579.

This paper presents an experimental study of flow evaporation in non-uniform microchannels, demonstrating the ability to provide a stable flow of evaporated fluid for energy conversion and chip cooling applications. Two mechanisms are proposed to stabilize the internal flow evaporation. The first mechanism is to establish a temperature gradient along the channel to separate the room temperature inlet fluid from the steam exit flow. The second mechanism is to change the direction of the surface tension forces acting on the meniscus to fix its position along the channel. To achieve this, shaped channels are formed of contractions and expansions with varied wall angles. The device consists of a silicon wafer with through-etched complex microchannels, that is anodically bonded to a glass wafer on each side. Inlet and exit holes for the fluid are machined in the glass wafers. Water is forced through the chip while it is heated on the exit side of the three layer chip. The qualitative nature of the two-phase flow along the shaped channels is observed through the glass cover wafer, for different flow rates and wall temperatures. The temperature gradient achieved with different thickness of channel walls shows agreement with the modeling results. Also, the benefit of having multiple expansions in the channels was demonstrated. By using these two mechanisms the onset of water evaporation was fixed along the channel. This will lead to the development of adequate two-phase flow micro heat exchangers.

Commentary by Dr. Valentin Fuster
2009;():713-717. doi:10.1115/IMECE2009-11618.

The problem of passive micromixing has been analyzed. Laminar flow in microchannels implies that transport processes including mass transfer are dominated by diffusion; inertial effects and have little to no effect on mixing in small scale conduits. Study of the diffusion theory shows that the velocity at the interface of fluids intended to be mixed is a determining factor. Higher velocity at the diffusion interface increases the mass flux across. By means of unique sidewall design, the laminar velocity profile can be passively altered such that the maximum of the profile coincides with the transversely progressing diffusion fronts repeatedly through the mixing channel. Based on the idea of increasing diffusion interfacial velocity, a passive mixing channel has been designed using numerical simulation tools, fabricated and characterized.

Commentary by Dr. Valentin Fuster
2009;():719-723. doi:10.1115/IMECE2009-11700.

The interest in micropower generation using the high energy density provided by hydrocarbon fuels as a portable power and heat source has stimulated research on combustion in microdevices. As the length scale of a combustion channel is decreased, the surface area-to-volume ratio increases approximately inversely with the critical dimension. The resulting high surface heat loss is a limiting factor to the size of a microcombustor. However when using arrays of micro-combustors, some of the surface heat loss in a channel becomes heat source for its neighbors. Combustion of methane/air mixture in an array of channels is studied as a function of gas velocity and distance between channels and is compared to the case of a single channel. Arrays of channels are shown to have self-sustained combustion when no such combustion is possible in a single channel.

Commentary by Dr. Valentin Fuster
2009;():725-731. doi:10.1115/IMECE2009-11705.

Low pressure driven ultrafiltraion (UF) processes has been applied in various industries due to its economical and easy operated benefits. Hollow fiber membrane is one of the most used membrane configuration in industry, membrane fouling is the major challenge for widely usage. Most of the investigation of UF was carried out by experiments to determine the effect of different operating conditions on permeate flux. However, experiments provide limited insight information on the membrane performance. In addition, the prediction of permeate flux under different operating conditions is necessary for experimental design and optimization. The purpose of the present study is to develop a numerical model to simulate the UF process and investigate the UF mechanism. A numerical model was developed using commercial CFD package (FLUENT). The effects of various operating conditions on permeate flux were determined by experiments and simulations, the comparison of the experimental and CFD results shows good agreements. Controlling membrane fouling will maintain a high productivity. The simulations were carried out to investigate the efficiency of removing accumulated particles on membrane surface by installing spacer filaments in membrane channels. The results suggested that the zigzag type spacer has d/h = 0.5 and l/h = 5 is more economical and efficient in reducing fouling.

Commentary by Dr. Valentin Fuster
2009;():733-738. doi:10.1115/IMECE2009-11796.

E. coli O157:H7 strains represent the most important group of food-borne pathogens. PCR-amplified intimin gene of pathogenic E. coli O157:H7 was detected heterogeneously via a microfluidic chip that consists of streptavidin-coated nanoliter chambers. Biotinylated primers and digoxigenin labeled deoxyuridine triphosphate (dUTP) were incorporated into the amplified intimin (eaeA) gene by an off-chip PCR thermal cycler. The amplified products were injected into the chip where they were immobilized via streptavidin-biotin interaction. Detection of the products using alkaline phosphatase (AP) conjugated anti-digoxigenin was performed with an epi-fluorescent microscope. This assay was capable of detecting 0.06 ng/μL biotin-digoxigenin-dsDNA conjugate distinctly, which is a hundred fold more sensitive than the traditional detection by agarose gel.

Commentary by Dr. Valentin Fuster
2009;():739-740. doi:10.1115/IMECE2009-11844.

Ionic current through highly confined nanochannels receives significant attention over the past decade because of the novel phenomena from the overlapped electric double layers. In addition, nanoscale Coulter-type single molecule sensors detect the modulation of ionic current through individual nanopores to sense the translocation of single molecules or nanoparticles. The baseline ionic current and its noise determine the sensitivity of the Coulter-type single molecule sensors. All published research on ionic current through nanochannels focuses on the effects of overlapped electric double layers without paying much attention to the ionic transport through nanopores for high concentration electrolytes with non-overlapped double layer. Therefore, the ionic current data for high concentration electrolytes through nanochannels are limited and not systematic.

Topics: Nanopores
Commentary by Dr. Valentin Fuster
2009;():741-748. doi:10.1115/IMECE2009-11875.

A two dimensional numerical simulation is conducted to investigate the flow and heat transfer characteristics of single phase liquid laminar flow through rough microchannels. The wall roughness is simulated in a series of cases with rectangular, triangular and trapezoidal elements, respectively. Shape factor and peak position have been used to analyze the influence of roughness elements on centerline velocity distribution, pressure drop and Nusselt number. It is found that the shape factor has a significant effect on the centerline velocity distribution, pressure drop and Nusselt number. It is also found that, for a given shape factor, the effect of peak position on pressure drop is strongly than centerline velocity distribution and heat transfer. In addition, for all considered roughness element shapes, the rectangular element displays a poor heat transfer and large pressure drop.

Commentary by Dr. Valentin Fuster
2009;():749-750. doi:10.1115/IMECE2009-11876.

Focusing particles into a tight stream is usually a necessary step prior to counting, detecting and sorting them in microfluidic devices such as flow cytometers [1] and continuous-flow separators [2]. The diverse approaches to particle focusing may be classified as active or passive ones in nature. Active focusing utilizes external force field(s) to manipulate particles other than the force to pump the particle solution. This category covers the conventional sheath flow focusing method and the various sheathless focusing methods where a transverse acoustic, AC dielectrophoretic, or magnetic force must be externally imposed. Passive focusing exploits the geometric topology-induced internal force(s) to alter the particle motion. So far dielectrophoretic, hydrodynamic and inertial forces have been demonstrated to focus particles [3]. Here, we introduce a novel passive particle focusing technique in electrokinetic flow through curved microchannels. This focusing stems from the cross-stream dielectrophoretic motion induced by channel curvatures [4], and is demonstrated in a serpentine and a spiral microchannel.

Commentary by Dr. Valentin Fuster
2009;():751-752. doi:10.1115/IMECE2009-11885.

Particle (both biological and synthetic) separation is important for a wide range of applications in industry, biology, and medicine. In microfluidic devices particles have been separated based on either extrinsic labels (e.g., fluorescence- and magnetic-activated sorting) or intrinsic properties (e.g., size, charge, density, etc.). The latter may take place in a batchwise or continuous-flow process. The batch-process separation typically includes filtration, chromatography, and electrophoresis. In the continuous-flow separation, an external force field (e.g., acoustic, electrical, magnetic, and optical, etc.) acts on particles at an angle to the flow direction and deflects them to different flow paths [1]. Here we introduce a continuous particle separation technique in electrokinetic flow through curved microchannels. This separation results from the cross-stream dielectrophoretic motion induced by channel curvatures [2]. It eliminates the use of in-channel micro-electrodes or micro-obstacles that are required in present dielectrophoresis-based particle separation techniques [3].

Commentary by Dr. Valentin Fuster
2009;():753-757. doi:10.1115/IMECE2009-11895.

Integration of nanohole array based sensing in microfluidic environments has been focus of several recent works. However, nanohole array based sensing performed to date has involved dead-ended holes that fail to harness the potential benefits provided by flow-through operation. Potential benefits include enhanced transport of reactants via nanoconfinement and solution sieving. Here, we describe computer based analysis related with nanofluidic transport and solid mechanics under a flow-through sensing scheme. We also demonstrate experimental nanofluidic flow-through transport and its application to real-time monitoring of self-assembled monolayer creation and biomarker detection.

Topics: Flow (Dynamics)
Commentary by Dr. Valentin Fuster
2009;():759-760. doi:10.1115/IMECE2009-11903.

Cell lysis is a necessary step in the analysis of intracellular contents. It has been recently demonstrated in microfluidic devices using four methods: chemical lysis, mechanical lysis, thermal lysis, and electrical lysis [1]. The locally high electric fields needed for electrical lysis have been achieved using micro-electrodes and micro-constrictions for pulsed and continuous DC electric fields, respectively. However, since the two determining factors of electrical lysis are field strength and exposure time, opposing pressure-driven flow must often be used in pure DC lysis to reduce the velocity of the cells and to ensure the cells spend sufficient time in the high electric field region [1,2]. Using DC-biased AC fields can easily fulfill these requirements as only the DC component contributes to cell electrokinetic transport. Prior to lysis, cell concentration can be increased by trapping using dielectrophoresis (DEP), which may occur with either DC or DC-biased AC electric fields [3,4]. This operation is useful in cases where the cell supply is limited or when the cell concentration is too low in general. In this work, red blood cells are used to demonstrate the smooth switching between electrical lysing and trapping in a microchannel constriction. The transition between lysis and trapping is realized by tuning the DC component in a DC-biased AC electric field.

Topics: Microchannels
Commentary by Dr. Valentin Fuster
2009;():761-766. doi:10.1115/IMECE2009-12017.

Electrokinetic flow in microchannels in the vicinity of charged membranes or nanochannels gives rise to a polarization phenomena that can be used to control analyte. Utilizing this concept this paper focuses on a novel method to trap and pre-concentrate an analyte surrounded by a radial planar Nafion membrane. Numerical simulations are conducted to demonstrate the exclusion enrichment phenomena and the relative effect of the convective component on concentration and electric fields is demonstrated. Preliminary experimental results demonstrate the promise of the concept.

Commentary by Dr. Valentin Fuster
2009;():767-771. doi:10.1115/IMECE2009-12340.

A microfluidic device was fabricated via UV lithography technique to separate nonmagnetic fluoresbrite carboxy microspheres (∼4.5 μm) from the ferrofluids made of magnetic nanoparticles (∼10 nm). A mixture of microspheres and ferrofluid was injected to lithographically developed Y shape micro channels, and then by applying the external magnet field, the fluoresbrite carboxy microspheres and ferrofluids were clearly separated into different channels because of the magnetic force acting on those nonmagnetic particles. During the fabrication, a number of different parameters, such as UV exposure times, UV power level and photoresist thickness were tested to optimize for our needs. In addition, in the magnetic field testing, different pumping speeds, and particle concentrations associated with the various distances between the magnet and the microfluidic system were studied for an efficient separation.

Commentary by Dr. Valentin Fuster
2009;():773-778. doi:10.1115/IMECE2009-12464.

Bioluminescence detection is often achieved by using luciferase as an enzyme. When it is implemented in a microfluidic device, the enzyme must be properly mixed with luciferase assay reagents (LAR) to achieve enzymatic reactions. Two microfluidic reactors are investigated in this work for bioluminescence detection. The reactors were fabricated in poly(methylmethacrylate), PMMA, by hot embossing using a mold master with the reactor layouts made by high-precision micromilling. Reactor I device contains staggered herringbone mixers. Reactor II device has the same layout except that the mixers were replaced with smooth channels. We found that the mixing efficiency in Reactor I was 17.8 times higher than Reactor II. Theoretical analysis of the experimental results indicated that the required channel length of mixing was linearly proportional to the flow rate. A calibration curve for luciferase was obtained for both reactors. The limit of detection in Reactor I was determined to be 0.14 μg/mL of luciferase.

Topics: Microfluidics
Commentary by Dr. Valentin Fuster
2009;():779-784. doi:10.1115/IMECE2009-12691.

Recent trends in micro and nano fabrication techniques have opened a new era for microfluidic based immunosensing devices. In immunosensing microfluidic device, the buffer solution transports the different biomolecules and cells. The interaction between the cell and surface of the microchannel takes place during this transport. In the present study, the effect of interaction between the cell and the immobilized biomolecule on the cell transport is analyzed theoretically. A single cell transport is studied with the interaction between the cell surface and the microchannel wall. The type of immobilized biomolecule on the surface and the surface properties of the cell decide the interaction force between cell and biomolecule. In the present analysis, the interaction force between the cell and modified microchannel is considered as a bond force between ligand and receptor. The bond force is equated as an additional rolling friction to investigate the effect of bond force on the cell transport behavior. The coefficient of rolling friction is determined through non-dimensional analysis. The non-dimensional governing equation is solved to investigate the effect of different operation parameters on cell velocity. The cell velocity experiences a resistance while attaining the maximum velocity. This resistance depends on different operating parameters and forces acting on the cell. It is observed that, higher cell density delays the attainment of maximum cell velocity. It is also observed that, the value of maximum cell velocity is function of Reynolds number and bond length. Finally, it is demonstrated that, the bond density and contact area have no effect on the cell velocity behavior beyond the maximum bond density.

Commentary by Dr. Valentin Fuster
2009;():785-789. doi:10.1115/IMECE2009-12891.

Direct current dielectrophoretic (DC-DEP) effects on the electrophoretic motion of charged polystyrene particles through an L-shaped microchannel were experimentally and numerically studied. In addition to the electrostatic and hydrodynamic forces, particles experience a negative DC-DEP force arising from the interaction between the dielectric particle and the induced spatially non-uniform electric field occurring around the corner of the L-shape microchannel. The latter force causes a cross-stream DEP motion so that the particle trajectory is shifted towards the outer corner of the turn. A two-dimensional (2D) Lagrangian particle tracking model taking into account the induced DC-DEP effect was used to predict the particle trajectory shift through the L-shaped channel, which achieves quantitative agreement with the experimental data.

Commentary by Dr. Valentin Fuster
2009;():791-792. doi:10.1115/IMECE2009-10915.

This paper presents three-dimensional micro flow measurement system “OCTIV”, which is based on Optical Coherence Tomography. This was applied to solid-liquid two-phase flow in the microchannel. Consequently, OCTIV has an attractive two-component and three-dimensional velocimetry for micro flow.

Commentary by Dr. Valentin Fuster
2009;():793-794. doi:10.1115/IMECE2009-11866.

2-Color Optical Coherence Dosigraphy, which could visualize micro-scale drug infiltration, was applied to lipidrich plaques of rabbit with drug administration. Calculated distributions of absorption coefficient had good coincidence with drug infiltration into tissue. Therefore, 2C-OCD has a promising modality for micro-visualizing assay of drug delivery system.

Commentary by Dr. Valentin Fuster
2009;():795-798. doi:10.1115/IMECE2009-12137.

Here we focus on recent advances in understanding the deformation and fracture behavior of collagen, Nature’s most abundant protein material and the basis for many biological composites including bone, dentin or cornea. We show that it is due to the basis of the collagen structure that leads to its high strength and ability to sustain large deformation, as relevant to its physiological role in tissues such as bone and muscle. Experiment has shown that collagen isolated from different sources of tissues universally displays a design that consists of tropocollagen molecules with lengths of approximately 300 nanometers. Using a combination of theoretical analyses and multi-scale modeling, we have discovered that the characteristic structure and characteristic dimensions of the collagen nanostructure is the key to the ability to take advantage of the nanoscale properties of individual tropocollagen molecules at larger scales, leading to a tough material at the micro- and mesoscale. This is achieved by arranging tropocollagen molecules into a staggered assembly at a specific optimal molecular length scale. During bone formation, nanoscale mineral particles precipitate at highly specific locations in the collagen structure. These mineralized collagen fibrils are highly conserved, nanostructural primary building blocks of bone. By direct molecular simulation of the bone’s nanostructure, we show that it is due to the characteristic nanostructure of mineralized collagen fibrils that leads to its high strength and ability to sustain large deformation, as relevant to its physiological role, creating a strong and tough material. We present a thorough analysis of the molecular mechanisms of protein and mineral phases in deformation, and report discovery of a new fibrillar toughening mechanism that has major implications on the fracture mechanics of bone. Our studies of collagen and bone illustrate how hierarchical multi-scale modeling linking quantum chemistry with continuum fracture mechanics approaches can be used to develop predictive models of hierarchical protein materials. We conclude with a discussion of the significance of hierarchical multi-scale structures for the material properties and illustrate how these structures enable one to overcome some of the limitations of conventional materials design, combining disparate material properties such as strength and robustness.

Topics: Bone
Commentary by Dr. Valentin Fuster
2009;():799-802. doi:10.1115/IMECE2009-10537.

Typically, cancer diagnosis relies on morphological examination of surgically removed tissue samples. However, diagnosis based on morphological examination is difficult, not accurate and often requires large amounts of biological materials. Thus, additional more accurate markers are needed to further increase the diagnostic accuracy of cancer cells. Despite having similar morphological features, the cancer and normal cell populations show significantly different mechanical properties. The mechanical properties of cancer cells that have been identified as important factors for the diagnosis of cancer are the cell stiffness or elasticity and cell adhesion. We present an acoustic resonant platform that is used in liquid environments and it is able to monitor the attachment of normal and cancer cell lines. The biosensor is based on a MEMS sheer horizontal surface acoustic wave (SH-SAW) piezoelectric resonator. We consider the SH-SAW piezoelectric resonator because it is better suited for liquid sensing applications due to the minimal damping of the acoustic wave. The miniature size of this biosensor allows us to perform single cell electrical measurements which will provide information on the progression of cell adhesion, cell growth and viscoelasticity changes of normal and cancer cells. A commercial quartz crystal microbalance (QCM) is initially used to study the cell attachment process and correlate the relationship between the electrical measurements and the mechanical properties of cells. The commercial QCM could be used as a functional biosensor utilizing living cells as biological signal transduction elements.

Commentary by Dr. Valentin Fuster
2009;():803-804. doi:10.1115/IMECE2009-10815.

High quality surface patterns of macromolecules are a key component of many microfluidic and microstructured devices. The technique of microcontact printing uses an elastomeric stamp to selectively transfer molecules to a pretreated substrate, and it has emerged as one of the most ubiquitous and versatile ways of creating micropattnered surfaces. This technique is especially well suited for transferring a single chemical pattern, but many chemically and biologically relevant surfaces require multiple complementary molecular patterns. The research demonstrated here utilizes a high-precision passive alignment system to generate these patterns using sequential microcontact printing steps. The technique relies on mechanical alignment and does not require optical alignment of each stamp; the resulting data shows a placement variation of less than 5 μm.

Topics: Printing
Commentary by Dr. Valentin Fuster
2009;():805-809. doi:10.1115/IMECE2009-11230.

This study presents a stable and flexible method for fabricating a free-standing polymer membrane with perforated micro- and nanopores using an imprint lithography combined with a pressed self-perfection method and a sacrificial layer technique. For the fabrication, micropores were initially patterned on a double resist layer: the upper SU-8 resist layer as an active membrane layer and the lower life-off resist used as a sacrificial layer. The membrane with micropores was then pressed with a flat quartz wafer to reduce pore size down to sub-micrometer. Finally, a free-standing SU-8 membrane with perforated micro- and nanopores was successfully lifted-off from the substrate by dissolving the sacrificial layer.

Commentary by Dr. Valentin Fuster
2009;():811-814. doi:10.1115/IMECE2009-12575.

We present a novel immobilization technique via physical adsorption of biomolecules onto