Evaluation of Charge Injection Properties of Thin Film Redox Materials for use as Neural Stimulation Electrodes

Abstract

AbstractThe charge injection limits of a variety of thin film redox materials considered for use as neural stimulation electrodes were evaluated under standardized conditions. Materials tested included the oxides of Ni, Ir, Rh, Ru, and Mn as well as conductive polypyrrole films. Electrodes with geometric areas of, ∼10–4 cm2were tested in bicarbonate buffered saline of pH 7.3. An electrochemically “safe” potential window was determined for each material by cyclic voltammetry. Charge injection capabilities within that window are evaluated with constant current 0.2 msec pulses. Pulse modes include anodic first and cathodic first biphasic pulses and monophasic cathodal pulses applied to electrodes held at an anodic bias. Pulsing from the anodic bias increased the charge limits for all the materials, but the most dramatic improvement was for Rh oxide. The highest charge injection values were obtained with oxides of iridium and rhodium and were of comparable value for the anodic bias mode of pulsing. Where the effect of film thickness was examined, the charge injection limit leveled off at 2–4 mC/cm2geometric as the film thickness increased.