Electrical stimulation of the nervous system: The principle of safe charge injection with noble metal electrodes

Electrical stimulation of the nervous system is of increasing importance for a variety of prospective prostheses. Many of these involve direct chronic stimulation of the cerebral cortex. The electrochemical reactions which may occur at the Pt or other noble metal|physiological saline interface were discussed. A variety of undesirable reactions may occur, viz, the electrolysis of water, the oxidation of the saline, the dissolution of the metal, and the oxidation-reduction of organic materials. There are two highly desirable methods of injecting charge from a noble metal into tissue: these involve modification of the electrical double layer at the interface, or capacitive coupling via symmetrical surface-layer oxidation-reduction processes. Two principles of tissue stimulation for electrochemically completely safe conditions are proposed: I. Perfect symmetry of the electrochemical processes in the two half-waves of the pulses should be sought. This implies that we do not generate any electrolysis products in solution. One approach to achieve this would appear to involve use of perfectly charge-balanced waveforms of controlled magnitude. 2. The aim should be to inject charge via non-faradaic or surface-faradaic processes, to avoid injecting any possibly toxic materials into the body. The experiments required to specify the amounts of charge which may be injected via these latter processes are described. Considerations affecting alternative choices to the use of Pt for microstimulation electrodes are discussed.