Hrishikesh V. Panchawagh

Toward a self-deploying shape memory polymer neuronal electrode

The widespread application of neuronal probes for chronic recording of brain activity and functional stimulation has been slow to develop partially due to long-term biocompatibility problems with existing metallic and ceramic probes and the tissue damage caused during probe insertion. Stiff probes are easily inserted into soft brain tissue but cause astrocytic scars that become insulating sheaths between electrodes and neurons. In this communication, we explore the feasibility of a new approach to the composition and implantation of chronic electrode arrays. We demonstrate that softer polymer-based probes can be inserted into the olfactory bulb of a mouse and that slow insertion of the probes reduces astrocytic scarring. We further present the development of a micromachined shape memory polymer probe, which provides a vehicle to self-deploy an electrode at suitably slow rates and which can provide sufficient force to penetrate the brain. The deployment rate and composition of shape memory polymer probes can be tailored by polymer chemistry and actuator design. We conclude that it is feasible to fabricate shape memory polymer-based electrodes that would slowly self-implant compliant conductors into the brain, and both decrease initial trauma resulting from implantation and enhance long-term biocompatibility for long-term neuronal measurement and stimulation.

Dynamic characterization of MEMS cantilevers in liquid environment using a low-cost optical system

Taking advantage of an off-the-shelf, high-quality but inexpensive optical detector, namely a DVD optical head, a system has been developed for the dynamic characterization of the vertical (out-of-plane) displacement of MEMS structures. The novelty of the proposed setup, besides the proposed field of application, lies in the ability of the system to operate both in air and in a liquid environment. In order to give a representative experimental proof of the functionality of the system, a cantilever array was designed and fabricated via a commercially available multi-user polysilicon surface micromachining process (PolyMUMPs). The frequency response of a tiny silicon cantilever beam (150 µm length, 10 µm width and 1.5 µm thickness) was measured, both in air and in deionized water, and compared against theoretical models of the system. Despite the low-cost signal conditioning system used, it was possible to trace a very clear Bode plot, and to obtain good agreement between experimental data and theoretical predictions. The measurements indicate that this displacement sensor performs very well under many circumstances of interest which require continuous monitoring with large bandwidth. This sensor offers a good low-cost measurement solution for the single-point dynamic characterization of the MEMS devices, particularly when the frequencies involved are high and the working environment is unsuitable for other types of detectors.

Frequency-dependent stability of parallel-plate electrostatic actuators in conductive fluids

We present an electromechanical stability analysis of passivated parallel-plate electrostatic actuators in conductive dielectric media and show that the pull-in instability can be eliminated by tuning the applied frequency below a design-dependent stability limit. A partial instability region is also obtained, where the actuator jumps from the pull-in displacement to another stable position within the gap. The results predict that the stability limit is always greater than the critical actuation frequency, and therefore any device that is feasible to actuate in a conductive fluid can be operated with stability over the full range of motion.

Micromachined Electrical Conductivity Probe for RF Ablation of Tumors

RF ablation is an important technique in cancer treatment. It has been proposed that the effective area treated via RF ablation can be increased by increasing the local electrical conductivity. This is achieved by injection of NaCl solution into the tissue. For an accurate and effective RF ablation treatment using this new method, it is necessary to measure the local electrical conductivity, which varies spatially due to diffusion of sodium chloride. In this paper, we propose a micro probe to measure the local tissue electrical conductivity. The probe consists of two in-plane miniature electrodes separated by a small gap. When the electrodes are in contact with the tissue, the electrical resistance across them can be used to calculate the electrical conductivity. The probe is fabricated by standard photolithography techniques. The substrate material is polyimide and the electrodes are made of gold. A four-electrode probe is used to calibrate the new electrical conductivity micro probe using different concentrations of saline water. The resistance measurements are carried out using an impedance analyzer on different frequencies. The frequency of choice for RF ablation of tumors is 500k Hz and is the one selected for calibration and testing. The micro-probe calibration is then verified by measuring electrical conductivity of a phantom and comparing it with the result measured by the four-electrode probe. Finally, some in vivo tests are performed and the results are compared with literate data.Copyright

A Model for Electrostatic Actuation in Conducting Liquids

This paper presents a generalized model that describes the behavior of micromachined electrostatic actuators in conducting liquids and provides a guideline for designing electrostatic actuators to operate in aqueous electrolytes such as biological media. The model predicts static actuator displacement as a function of device parameters and applied frequency and potential for the typical case of negligible double-layer impedance and dynamic response. Model results are compared to the experimentally measured displacement of electrostatic comb-drive and parallel-plate actuators and exhibit good qualitative agreement with experimental observations. The model is applied to show that the pull-in instability of a parallel-plate actuator is frequency dependent near the critical frequency for actuation and can be eliminated for any actuator design by tuning the applied frequency. In addition, the model is applied to establish a frequency-dependent theoretical upper bound on the voltage that can be applied across passivated electrodes without electrolysis.

Characterization of silicon parallel-plate electrostatic actuator in partially conducting aqueous solution

In this paper we present experimental results on the behavior of a silicon parallel-plate electrostatic actuator operated in a partially conducting aqueous solution. First, an experimental setup based on a laser position sensor is described for dynamic measurement of out-of-plate motion of MEMS structures in liquids. This set up is used to characterize motion of a surface-micromachined, polysilicon piston actuator operated in water using high- frequency drive signals to avoid charge migration that screens actuation potential. Results show that besides actuator design and fluid properties, voltage-displacement characteristics and pull-in phenomenon of parallel-plate actuators also depend on drive frequency.

Design and Characterization of a BioMEMS Device for In-Vitro Mechanical Stimulation of Single Adherent Cells

This paper presents development of a BioMEMS device to mechanically stimulate single adherent cells by means of electrostatic actuation. The main components of the proposed device include a platform for cell placement and an electrostatic comb drive actuator to provide in-plane motion. A high frequency actuation method was used to enable actuation in aqueous solutions. Displacements greater than 5μm were measured when the device was actuated with a 1 MHz square wave signal with 10V peak amplitude in DI water. Additionally, this device was successfully actuated in ionic solutions up to 50mM NaCl aqueous solution using frequencies greater than 30 MHz. Significant electrolysis and corrosion of the polysilicon and metal layers was observed when the devices were actuated in saline solutions with peak voltages greater than 15V, thus indicating that there is a limit on the maximum actuation voltage that can be used. No noticeable actuation was observed in phosphate buffer solution (PBS) or cell culture medium even when frequencies as high as 50 MHz were used due to ion migration. Theoretical calculations suggest that frequencies of the order of 100-500 MHz will be required for actuation in cell culture media. Currently we are in the process of building an experimental set-up to allow use of such high frequencies. Initial results for cell plating experiments on the cell stretcher platform and other considerations for device implementation are discussed in the end.Copyright

Packaging of In-Plane Thermal Microactuators for BioMEMS Applications

This paper addresses two issues related to in-plane, electro-thermal actuators for BioMEMS applications. First, in order to protect the actuator from biological debris and particulates, a packaging technique using a flip-chip bonded polysilicon cap is demonstrated. The encapsulated actuator transmits motion outside the package via a piston, which moves through a small clearance. The second issue addressed is the reduction in efficiency of the thermal actuator in liquids. By coating the packaged actuator with a thin conformal hydrophobic layer via an atomic layer deposition (ALD) process, the liquid is prevented from entering the encapsulation. This avoids direct contact between the actuator and the surrounding liquid thereby improving its efficiency. The unpackaged and packaged actuators were tested in both air and de-ionized water. Although the packaging resulted in a reduction in the performance of the thermal actuator in air, the actuation efficiency in water was significantly improved due to the isolation of the hot arms from the liquid. This packaging technique is also applicable to other MEMS devices and in-plane actuators such as electrostatic comb drives for engineering as well as biological applications.Copyright

Micromachined hot-wire thermal conductivity probe for biomedical applications

A micro thin-film thermal conductivity probe is developed to measure thermal conductivity of biological tissues based on the principle of traditional hot-wire method. The design of this new micro probe consists of a resistive line heating element on a substrate and a RTD based temperature sensor. The transient time response of the heating element depends on the thermal conductivity of the surrounding medium and the substrate. A theoretical analysis of the transient conduction for this configuration where the heater source is sandwiched between two materials (the substrate and the surrounding medium) shows that the composite thermal conductivity calculated from the temperature versus time response is simply the average of the thermal conductivity of the two materials. The experiments conducted to measure thermal conductivity of Crisco and agar gel show a good match with the theoretical and numerical analyses. The technique demonstrates the potential of the microprobe for in vivo measurements of thermal conductivity of biological tissues.