Electrical Characteristics of Chronically Implanted Platinum-Irdium Electrodes

The electrical characteristics of platinum 30 percent-iridium microelectrodes implanted in the cruciate gyrus of cats were monitored during pulsing with charge-balanced, symmetric, cathodicfirst, controlled-current pulse pairs. The amplitude of the pulses spanned the range shown in other studies to be effective as neuronal stimuli, but not to induce histologic or physiologic damage in brain tissue adjacent to the microelectrode tips (10-80 ?, A). Two inflections on the cathodic and anodic charging transients induced by the controlled-current pulses were identified as being due to evolution of molecular hydrogen and the adsorption of oxygen, respectively. Despite the pH dependence of the equilibrium potential of these faradaic reactions, the electrode potentials at the inflections were shown to be independent of stimulus current over most of the useful range of stimulus intensities (20-80/?A, 200-800 ?IC/cm2, and 1-4 A/cm2). Hence, these inflections are valid markers of the true electrode potential, independent of accurate computation of access impedance or a stable reference electrode potential. The implications for monitoring the performance of chronically implanted microelectrodes and for the selection of stimulus parameters is discussed.