The stimulating electrode is a vital component of a neural prosthesis since it is in direct contact with tissue. To ensure an electrode maximizes charge injection while avoiding tissue damage, one must understand the electrochemical characteristics of the electrode material. Two electrode materials were studied in this context: platinum, the most common electrode material for neural stimulation; and conductive diamond, a new material that has promising characteristics for neurological applications. Two electrochemical techniques were used: cyclic voltammetry (CV) and a custom system, the Current Pulse – Capacitor Discharge (CP-CD) Method.
CV of platinum in phosphate-buffered saline (PBS) exhibited electrochemical activity not expected in physiological tissue: platinum reduces or oxidizes the sodium phosphate component of PBS. Diamond did not exhibit such electrochemical activity over its 3-V potential window.
The reduction of O2 to superoxide can be damaging to tissue. At a physiological concentration of O2 in 0.15 M H2SO4, platinum electrodes reduce O2 at potentials negative of approximately +0.6 V vs. Ag/AgCl (its open-circuit potential), and diamond electrodes reduce oxygen negative of -1.2 V.
Contaminant adsorption on an electrode surface can inhibit charge transfer, decreasing its effectiveness at stimulating neural activity. Voltammetry of platinum in the high-organic content Aplysia hemolymph showed significant decreases in current density compared to PBS. CV of diamond in hemolymph indicated some varied electrochemical activity, but no decrease in current densities.
Since CV is a relatively slow technique, the CP-CD method was developed by our lab to study charge transfer at the fast rates of neural stimulation. Preliminary experiments showed only a fraction of the charge transferred during CV can be transferred during fast pulsing. Therefore, historical estimates of charge injection from CV are significantly overestimated.
Overall, the O2 reduction reaction and the organic adsorption issues discovered with platinum may pose limitations in its use for neural stimulation. Diamond’s resistance to O2 reduction and strong anti-fouling properties make it an attractive electrode material for this application. The experiments performed provide a wealth of information on platinum and diamond electrodes, but also provide a protocol for evaluating any electrode or electrolyte in the context of neural stimulation.