Walied A. Moussa

MEMS-Based Power Generation Techniques for Implantable Biosensing Applications

Implantable biosensing is attractive for both medical monitoring and diagnostic applications. It is possible to monitor phenomena such as physical loads on joints or implants, vital signs, or osseointegration in vivo and in real time. Microelectromechanical (MEMS)-based generation techniques can allow for the autonomous operation of implantable biosensors by generating electrical power to replace or supplement existing battery-based power systems. By supplementing existing battery-based power systems for implantable biosensors, the operational lifetime of the sensor is increased. In addition, the potential for a greater amount of available power allows additional components to be added to the biosensing module, such as computational and wireless and components, improving functionality and performance of the biosensor. Photovoltaic, thermovoltaic, micro fuel cell, electrostatic, electromagnetic, and piezoelectric based generation schemes are evaluated in this paper for applicability for implantable biosensing. MEMS-based generation techniques that harvest ambient energy, such as vibration, are much better suited for implantable biosensing applications than fuel-based approaches, producing up to milliwatts of electrical power. High power density MEMS-based approaches, such as piezoelectric and electromagnetic schemes, allow for supplemental and replacement power schemes for biosensing applications to improve device capabilities and performance. In addition, this may allow for the biosensor to be further miniaturized, reducing the need for relatively large batteries with respect to device size. This would cause the implanted biosensor to be less invasive, increasing the quality of care received by the patient.

Top-orthogonal-to-bottom-electrode (TOBE) CMUT arrays for 3-D ultrasound imaging

Two-dimensional ultrasound arrays hold great promise for 3-D imaging; however, wiring of each channel becomes impractical for large arrays or for small-footprint catheter probes for which the number of wires must be limited. Capacitive micromachined ultrasound transducers offer a promising solution for such 2-D array applications, but channel routing is still non-trivial. A top-orthogonal-to-bottom-electrode (TOBE) 2-D CMUT array architecture is presented along with row-column addressing schemes for low-channel-count 3-D ultrasound imaging. An N × N TOBE array is capable of obtaining 3-D images using only 2N channels. An interfacing scheme is presented in which transmit-receive signals are routed along rows while bias voltages are applied along columns, effectively allowing for single-element transmit/receive control. Simulations demonstrated potentially finer resolution and improved side lobe suppression over a previously published row-column-based imaging method. Laser vibrometer testing was done to measure membrane displacement in air and confirmed that single-element air-coupled actuation in transmit mode could be achieved using our proposed interfacing scheme. Acoustic testing was also performed in both transmit and receive modes to characterize the ability of the proposed interfacing scheme to achieve dominant-element transmission and reception in immersion operation. It was seen that membrane displacement in both modes was indeed largely confined to the active area.

A finite element model of a MEMS-based surface acoustic wave hydrogen sensor.

Hydrogen plays a significant role in various industrial applications, but careful handling and continuous monitoring are crucial since it is explosive when mixed with air. Surface Acoustic Wave (SAW) sensors provide desirable characteristics for hydrogen detection due to their small size, low fabrication cost, ease of integration and high sensitivity. In this paper a finite element model of a Surface Acoustic Wave sensor is developed using ANSYS12© and tested for hydrogen detection. The sensor consists of a YZ-lithium niobate substrate with interdigital electrodes (IDT) patterned on the surface. A thin palladium (Pd) film is added on the surface of the sensor due to its high affinity for hydrogen. With increased hydrogen absorption the palladium hydride structure undergoes a phase change due to the formation of the β-phase, which deteriorates the crystal structure. Therefore with increasing hydrogen concentration the stiffness and the density are significantly reduced. The values of the modulus of elasticity and the density at different hydrogen concentrations in palladium are utilized in the finite element model to determine the corresponding SAW sensor response. Results indicate that with increasing the hydrogen concentration the wave velocity decreases and the attenuation of the wave is reduced.

Double-SOI Wafer-Bonded CMUTs With Improved Electrical Safety and Minimal Roughness of Dielectric and Electrode Surfaces

Despite myriad potential advantages over piezoelectric ultrasound transducers, capacitive micromachined ultrasound transducers (CMUTs) have not yet seen widespread commercial implementation. The possible reasons for this may include key issues of the following: (1) long-term device reliability and (2) electrical safety issues associated with relatively high voltage electrodes on device surfaces which could present an electrical safety hazard to patients. A CMUT design presented here may mitigate some of these problems. Dielectric charging is one phenomenon which can lead to unpredictable performance and device failure. Using a previously published 1-D model of dielectric charging, we link minimal dielectric surface roughness with minimal dielectric charging. Previous studies of Fowler-Nordheim tunneling suggest that minimal-surface-roughness electrodes could lead to minimal transdielectric currents (and, hence, slower dielectric charging rates). These principles guided our device architecture, leading us to engineer near atomically smooth electrodes and dielectric surfaces to minimize dielectric charging. To provide maximum electrical safety to future patients, CMUT devices were engineered with the top membrane serving as a ground electrode. While multiple CMUT elements have not been individually addressable in most such designs to our knowledge, we introduce a fabrication method involving two silicon-on-insulator wafers with a step to define individually addressable electrodes. Our devices are modeled using a finite-element package. Measured deflections show excellent agreement with modeled performance. We test for charge effects by studying deflection hysteresis during snapdown and snapback cycles in the limit of long snapdown durations to simulate maximal-dielectric-charging conditions. Devices were also tested in long-term actuation tests and subjected to more than 3 × 1010 cycles without failure.

High-Performance Piezoresistive MEMS Strain Sensor with Low Thermal Sensitivity

This paper presents the experimental evaluation of a new piezoresistive MEMS strain sensor. Geometric characteristics of the sensor silicon carrier have been employed to improve the sensor sensitivity. Surface features or trenches have been introduced in the vicinity of the sensing elements. These features create stress concentration regions (SCRs) and as a result, the strain/stress field was altered. The improved sensing sensitivity compensated for the signal loss. The feasibility of this methodology was proved in a previous work using Finite Element Analysis (FEA). This paper provides the experimental part of the previous study. The experiments covered a temperature range from −50 °C to +50 °C. The MEMS sensors are fabricated using five different doping concentrations. FEA is also utilized to investigate the effect of material properties and layer thickness of the bonding adhesive on the sensor response. The experimental findings are compared to the simulation results to guide selection of bonding adhesive and installation procedure. Finally, FEA was used to analyze the effect of rotational/alignment errors.

On the Feasibility of a New Approach for Developing a Piezoresistive 3D Stress Sensing Rosette

Monitoring of stresses and strains in a structure is important to detect problems during the design or the service cycle of the structure. Piezoresistive sensing rosettes are considered a reliable method for stress and strain monitoring. While few efforts have been focused towards developing a 3D stress/strain sensing rosette, most of the currently developed piezoresistive rosettes extract only in-plane stress/strain components. In this paper, a new approach for building a stress sensing rosette capable of extracting the six stress components and the temperature is presented and its feasibility is verified both analytically and experimentally. The current approach is based on varying the doping concentration of the sensing elements and utilizing the unique behavior of the shear piezoresistive coefficient (π44) in n-Si.

Development and Experimental Evaluation of a Novel Piezoresistive MEMS Strain Sensor

This paper presents the experimental evaluation of a new piezoresistive microelectromechanical systems strain sensor. The sensing chip is highly capable of measuring biaxial state of strain/stress. The sensing elements are p-type piezoresistors on (100) single crystal silicon aligned along [110] and its in-plane transverse. The concept of introducing geometric features to enhance the sensor sensitivity is investigated. The results of experimental evaluation and finite-element analysis (FEA) proved the viability of this concept to improve the sensor sensitivity. The microfabrication process utilizes five doping concentrations to explore the effect of doping level on the sensor performance. The sensor is developed considering applications under varying temperature conditions. Therefore, high doping concentration (more than 1 ×1019 atoms/cm3) is favorable to reduce the sensor thermal drift. As a result, the sensor sensitivity is significantly reduced. Hence, geometric features are introduced in the sensor silicon carrier to compensate for the signal loss through stress concentration effect, which magnified the strain field in the proximity of the sensing elements. In addition, the use of full-bridge configuration reduced the overall temperature coefficient of resistance (TCR). At doping concentration of ~5 ×1019 atoms/cm3, the measured strain sensitivity is 0.035 mV/με for input voltage of 5 volts, which corresponds to an effective gauge factor of ~7 and piezoresistive gauge factor of ~44. The effective gauge factor includes all the signal losses and the effect of bonding adhesive. Design and analysis, prototyping, and experimental evaluation are presented. Finally, guidelines to select the bonding adhesive and packaging scheme are provided.

Nanotechnology's Implications for Select Systems of Renewable Energy

Developments in micro and nanotechnology within the renewable energy industry have the potential to create significant advances for the renewable energy industry. A review of selected renewable energy sectors being influenced by micro and nanotechnology was performed, finding that the most promising areas involve electricity generation, biomass technologies, and hydrogen technologies. Such technologies include: surface microtexturization and nanocrystalline films in photovoltaic and photoelectrochemical cells; nanoscale catalysts and membranes in biomass or thermochemical hydrogen generation; surface utilization of carbon nanotubes in hydrogen storage and fuel cell applications. These advances may increase process yields and efficiencies, and also may lower overall costs.

Simulation of MEMS Piezoelectric Micropump for Biomedical Applications

In this study, we demonstrate the usefulness of Finite Element Analysis (FEA) and simulation techniques in the design of MEMS micropumps. Such pumps provide for the handling of milliliter-scaled fluid volumes desired in many lab-on-a-chip chemical and biomedical applications. This work is focused on a micropump driven by the piezoelectric effect, which in turn invokes the dominant resonance behavior. Because the design of the device is the emphasis of this study, the model was originated in CAD and includes the fme-scale geometric details commonly encountered in a wide variety of micropumps. The model considered in this study is a rectangular micropump with a piezoelectrically actuated diaphragm on its top and two valves on its bottom. The mechanical efficiency of the pump hinges on using resonance to generate sufficient motion of the diaphragm. Mechanical Event Simulation (MES) commercial software from ALGOR was utilized to simulate this motion, and thus provide a method for optimizing the design. The results show that consideration needs to be given to the voltage-driving frequency because of its effect on the pump performance and the stress levels within it.Copyright

Wide-bandwidth piezoelectric energy harvester with polymeric structure

A polymer based energy harvester with wide bandwidth is designed, fabricated and tested in this work. A polymer based structure has a lower resonance frequency compared to a silicon based structure with the same dimensions due to the much lower stiffness of polymeric materials. Therefore, a polymeric energy harvester is more useful for situations with lower ambient vibration frequencies. Aluminum nitride pads are fabricated on an SU-8 membrane to convert mechanical vibration of the membrane to electrical voltage. A new and scalable microfabrication process flow is proposed to properly fabricate piezoelectric layers on SU-8 structures. The nonlinear stiffness due to the stretching strain in the membrane provides a wider harvestable frequency bandwidth than conventional linear oscillators. Wideband energy harvesters are more useful for practical applications due to uncontrollable ambient vibration frequency. The load-deflection equation of the device is calculated using finite element simulation. This equation is then used in an analytical solution to estimate the nonlinear effect of the structure. A bandwidth of ~146 Hz is obtained for the fabricated device and a maximum open circuit voltage of 1.42 V, maximum power of 1.37 µW, and power density of 3.81 µW cm−2 were measured at terminal load of 357.4 kΩ under an excitation acceleration of 4 g. A power output of 10.1 µW and power density of 28.1 µW cm−2 was estimated using a synchronized switch harvesting on interface (SSHI) electrical interface with electrical quality factor of 5. In addition, the lumped element model has been employed to investigate the scaling effect on a polymeric circular diaphragm.