Development of the Electronics and Electrodes for a Safe Direct Current Stimulator
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Currently available commercial neuroprostheses are limited in their functionality by the necessity of biphasic pulsatile stimulation. Direct current (DC) has the capability both excite and inhibit neural activity; however, direct current cannot be applied directly to neurons due to the charge injection thresholds of electrode-electrolyte interfaces that when violated result in harmful, corrosive faradaic reactions. We are developing a Safe Direct Current Stimulator (SDCS) that safely applies ionic direct current (iDC). The design of the SDCS uses a series of eight valves in conjunction with four alternating current electrodes to rectify ionic current in microfluidic channels. The rectified iDC can then be directly applied to neural tissue without the issues associated with conventional direct current. Although previous work has shown a functional proof of concept, the next generation of the device must be miniaturized to be viable as an implantable device. I designed and assembled the electronics of the device on a 23-mm circular printed circuit board (PCB). I then designed tests to confirm proper operation of the device. Analysis of the outputs confirmed the ability to cycle through the different valve and electrode configurations and provide a constant current level that is robust to changes in impedance. The alternating current electrodes must be designed to ensure the rate of faradaic reactions are kept to a minimum. I fabricated electrodes with varying lengths of PtIr wire. I then designed and conducted preliminary experiments to verify their stability. These voltage waveforms were then analyzed and parameters related to the rate of faradaic reactions were extracted. The results of the experiments suggest that electrodes fabricated from 30 cm of PtIr wire not only exhibited the lowest rate of faradaic reactions, but also did not experience significant changes over the course of 24 hours of constant stimulation. Although this work outlines significant steps made towards the development of the SDCS device, aspects of the device such the accompanying microfluidic layer and ionic electrodes must be developed before a functional device can be assembled and tested. Upon completion, the device will then be tested for efficacy and longevity.