Vikram Mukundan

Modeling and Characterization of Electrostatic Comb-drive Actuators in Conducting Liquid Media

Operation of electrostatic actuators in liquid media has various proposed applications, especially in biological environments. The devices are operated by modulating at a frequency higher than the relaxation rate of the ions in solution. We present circuit models based on electric double layer theories to obtain analytical expression for the frequency-dependent force response of electrostatic actuators in ionic media. The model has been compared with experimental measurements of actuation in media of conductivity spanning five orders of magnitude. Further, impedance spectroscopy is used to measure the values of the circuit models, which are compared with the experiments. These measurements also quantify the parasitic impedances in the devices. A conformal layer of Parylene-C is demonstrated as a passivation scheme for the electrodes in corrosive media. The heating effects due to parasitic impedances are also quantified by temperature measurements of devices in fluids.

MEMS Electrostatic Actuation in Conducting Biological Media

We present design and experimental implementation of electrostatic comb-drive actuators in solutions of high conductivity relevant for biological cells. The actuators are operated in the frequency range 1-10 MHz in ionic and biological cell culture media, with ionic strengths up to 150 mmol/L. Typical displacement is 3.5 mum at an applied peak-to-peak signal of 5 V. Two different actuation schemes are presented and tested for performance at high frequency. A differential drive design is demonstrated to overcome the attenuation due to losses in parasitic impedances. The frequency dependence of the electrostatic force has been characterized in media of different ionic strengths. Circuit models for the electric double layer phenomena are used to understand and predict the actuator behavior. The actuator is integrated into a planar force sensing system to measure the stiffness of cells cultured on suspended structures.

Microsystems for Biomechanical Measurements

The use of microtechnology to make biomechanical measurements allows for the study of cellular and subcellular scale mechanical forces. Forces generated by cells are in the few nanoNewton to several microNewton range and can change spatially over subcellular size scales. Transducing forces at such small size and force scales is a challenging task. Methods of microfabrication developed in the integrated circuit industry have allowed researchers to build platforms with cellular and subcellular scale parts with which individual cells can interact. These parts act as transducers of stresses and forces generated by the cell during migration or in the maintenance of physical equilibrium. Due to the size and sensitivity of such devices, quantitative studies of single cell and even single molecule biomechanics have become possible. In this review we focus on two classes of cellular force transducers: silicon-based devices and soft-polymer platforms. We concentrate on the biomechanical discoveries made with these devices and less so on the engineering behind their development because this is covered in great detail elsewhere.

Piezoresistive MEMS Underwater Shear Stress Sensors

We report on the design and performance of underwater piezoresistive floating-element shear stress sensors for direct dynamic measurements. Our design utilizes sidewall-implanted piezoresistors to measure lateral force and infer shear stress, and traditional top-implanted piezoresistors to detect normal forces and pressure transients. A gravity-driven flume was used to test the sensors. FEMLAB simulation and microscale Particle Image Velocimetry experiments were used to characterize the flow disturbance over different gap sizes. The results show no detectable disturbance of the flow over the range of sensor gap sizes evaluated (5-20 µ m).

Microactuator device for integrated measurement of epithelium mechanics

Mechanical forces are among important factors that drive cellular function and organization. We present a microfabricated device with on-chip actuation for mechanical testing of single cells. An integrated immersible electrostatic actuator system is demonstrated that applies calibrated forces to cells. We conduct stretching experiments by directly applying forces to epithelial cells adhered to device surfaces functionalized with collagen. We measure mechanical properties including stiffness, hysteresis and visco-elasticity of adherent cells.

Sidewall epitaxial piezoresistor process for in-plane sensing applications

We report, a novel, selective epitaxial fabrication method to form piezoresistors on the sidewalls of microstructures for in-plane sensing applications. We have fabricated and characterized cantilevered force sensors with lateral piezoresistors. Their sensitivity was found to be 2-7 times more sensitive than most ion-implanted cantilevers we have made with equivalent dopant concentration. In addition, we have characterized noise and electrical characteristics and the effect of trench dimension on deposition rate.

Design, Fabrication, and Characterization of Piezoresistive MEMS Shear Stress Sensors

This paper presents the design, fabrication, and characterization of unique piezoresistive microfabricated shear stress sensors for direct measurements of shear stress underwater. The uniqueness of this design is in its transduction scheme which uses sidewall-implanted piezoresistors to measure lateral force (and shear stress), along with traditional top-implanted piezoresistors to detect normal forces. Aside from the oblique-implant technique, the fabrication process also includes hydrogen anneal step to smooth scalloped silicon sidewalls due to Deep Reactive Ion Etch process, which was shown to reduce 1/f noise level by almost an order of magnitude for the sidewall-implanted piezoresistors. Lateral sensitivity characterization of the sensors was done using a microfabricated silicon cantilever force sensor, while out-of-plane characterization was done using Laser Doppler Vibrometry technique. In-plane sensitivity and out-of-plane crosstalk were characterized, as well as hysteresis and repeatability of the measurements. The sensors are designed to be used underwater for various applications.Copyright

Tools for Studying Biomechanical Interactions in Cells

Cells interact with their environment through forces that are generated and sensed by the cell. Forces generated by cells are in the few nanoNewton to several microNewton range and can change spatially over subcellular size scales. Transducing forces at such size and force scales requires development of platforms that can mechanically interface with cells. We describe several techniques that have been developed to study the role of mechanical forces in cellular processes. The measurement tools include those to measure the forces exerted by the cell on the extracellular environment, internal forces of contraction and the cytoskeletal properties.

Differential Electrode Design for Electrostatic Actuator in Conducting Media

We present the design, modeling and experimental validation of a novel differential actuation scheme to enhance the performance of comb-drive electrostatic actuators in aqueous media. The new design mitigates the losses in parasitic impedance and extends the actuation of the devices into frequency ranges suitable for biological samples, achieving a displacement of 3.5 mum in 100 mM KCl. The frequency dependence of the electrostatic force has been tested by operating the devices in media of different ionic concentrations. Circuit models for the electric double layer phenomena are used to analyze the device behavior.

Modeling and validation of electrostatic actuation in aqueous ionic media

Operation of electrostatic actuators in liquid media has various proposed applications, especially in biological environments. We present circuit models based on electric double layer theories to estimate the frequency-dependent force response of electrostatic actuators in ionic media. This model is particularly useful for predicting the characteristic frequency required to operate the devices. Further, the impedance characteristics of test structures are measured and matched to determine the relevant circuit components. The values obtained from impedance and actuation experiments are compared with the predictions from theory to validate the models.