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Y. Terasawa, K. Osawa, M. Ozawa, T. Tokuda, J. Ohta, Y. Tano; Large-Surface-Area Electrodes Based on Bulk Micromachining. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3020.
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We have been developing a visual prosthesis based on suprachoroidal approach, in which current pulses up to 0.5µC/phase must be safely injected via 500-µm-diameter electrodes. This specification exceeds the charge delivery capacity of conventional planar platinum electrodes. The purpose of this study is to develop an electrode which satisfies both limited diameter (0.5mm) and safety of charge injection up to 0.5µC.
Bullet-shaped electrodes (see the figure attached) were cut out with an automatic lathe from a bulk platinum bar. The dimensions of electrodes are 500µm in diameter, 500µm in height respectively. Three-stranded platinum-iridium wires were welded onto bottoms of electrodes. Backsides of each electrode and stranded wires were covered with a 50-µm-thick parylene film. We measured the electrode impedance and charge delivery capacity in phosphate-buffered saline.
Appropriate selection of tools for the lathe allowed us to form a large number of electrodes from bulk materials. Charge delivery capacities of each electrode were 88.8±12.9µC/cm2(mean, SD) for cathodic-first(CF) pulses and 112.9±14.1µC/cm2 for anodic-first(AF) pulses respectively. This corresponds to 0.62±0.091µC AF pulses and 0.78±0.10µC for CF pulses.
We succeeded in injecting current pulses over 0.5µC without exceeding the charge delivery capacity for platinum via 500-µm-diameter electrodes. We are planning chronic stimulation study in vivo to ensure the electrode performance for clinical applications.
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