Fabrication and Characterization of Platinum-Iridium Electrodes with Micro-Structured Surfaces For Neural Stimulation Ap
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Fabrication and Characterization of Platinum-Iridium Electrodes with Micro-Structured Surfaces For Neural Stimulation Applications Sachin S. Thanawala a, Daniel G. Georgiev b, Afzal Khan b, Ronald J. Baird b, Gregory Auner b. a.
Department of Biomedical Engineering.
b.
Department of Electrical and Computer Engineering.
Wayne State University, Detroit, MI-48202.
ABSTRACT Controlled structuring of electrode surfaces on a microscopic scale is expected to decrease the impedance and improve the current injection capabilities of neural stimulation electrodes. We have identified conditions for the fabrication of micro-bumps on platinum-iridium alloy surfaces by means of KrF excimer laser (λ=248nm) irradiation under ambient conditions. A regular array of closely spaced micro-bumps with diameters of about 5µm and heights of about 3µm was generated on the polished face of a platinum-20%iridium wire with a diameter of 75µm. A projection system with a demagnification factor of 9 was used to image a mask with a pattern of circular-holes on the polished face of the wire. Several thousand pulses at a repetition rate of 10Hz and a fluence of 3.0 J/cm2 were applied to produce the micro-bumps. The modified electrode surfaces were studied by optical microscopy and scanning electron microscopy, and the results show the formation of micro-bumps of reproducible shape. Simple two-electrode AC impedance measurements in physiological saline in the frequency range of interest to neural stimulation applications show a considerable decrease in the impedance of micro-structured electrodes with respect to the impedance of a polished electrode. INTRODUCTION Recently there have been increasing interest in developing neural prostheses that employ electrical stimulation of neurons for treatment of a number of diseases and conditions [1,2]. Electrical stimulation of neurons relies on efficient injection of charge into the neural tissue by means of electrodes that are made of highly biocompatible materials. One of the factors influencing such an efficient charge injection is the surface area and the actual surface topography of the electrode. Electrodes must have a large surface area to maintain the stimulus charge densities within their reversible charge injection limits, i.e. to prevent electrochemical electrode degradation that is often toxic [3]. At the same time, the electrodes need to be small enough to be conveniently placed in the central or peripheral nervous system, in the proximity of the neurons that are to be stimulated. In addition, fabrication of electrode surfaces that have features with sizes matching the sizes of the neuron processes (i.e. several microns) can be expected to be beneficial to the electrical stimulation process A most simple technique for neural stimulation and/or recording utilizes implanting of thin insulated wires made of biocompatible metal alloys with excellent electrical and suitable mechanical properties such as Pt-Ir alloys. In this paper, we present a method for controlled
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