An in vitro analysis of metal electrodes for use in the neural environment.

The results of evaluation of the corrosion response of Pt, Au, Rh, Ir, Pt-10% Ir and Pt-10% Rh electrodes subjected to bipolar current pulses of 0.1 and 1.0 A/geom. cm2 in a simulated neural environment are reported. The criteria for evaluation include the extent and morphology of material removal and corrosion product formation. The chemical changes on the electrode surface and in the test electrolyte are also evaluated. Modified potentiographic methods were developed which monitor interface conditions of the electrodes during chronic, long-term biphasic current passage. Under the test conditions established by the neural model, evaluation of the corrosion response of the candidate electrode materials shows that Rh was moderately damaged at 1.0 A/geom. cm2, while the remaining test electrodes were severely damaged at this current density. While Rh, Ir and Au exhibit corrosion resistance at 0.1 A/geom. cm2, damage to all of the test materials preclude their use in any long-term chronic neural prosthetic implant under the simulated stimulation parameters of the tests. The relative corrosion resistance of the test electrode materials is related to the adherence and coherence of the corrosion product produced on the electrode surface during the first few hundred hours of stimulation. Analysis of the voltage response of the electrodes to current passage provides a means of monitoring the nucleation and growth of the corrosion product.