Characteristics of the Metal-Tissue Interface of Stimulation Electrodes

The success of chronic stimulation with implanted metal electrodes depends in part on an understanding of the electrochemical mechanisms controlling charge transfer across the metal-tissue interface. Time-and frequency-domain impedance behavior of metal electrodes are briefly reviewed, and a model of the physical structure of the interface and an equivalent circuit reflecting electrochemical mechanisms are outlined. Some theories dealing with the observed changes in electrode resistance and capacitance under various experimental conditions are described. Charge-transfer mechanisms at high stimulation intensities, including surface gas adsorption and desorption, are considered. The relation of electrode and tissue damage to faradaic and capacitive stimulus current components, and the relevance of this to stimulus waveforms and the underlying electrochemistry are discussed.