An <i>in vitro </i>Analysis of Metal Electrodes for Use in the Neural Environment

Abstract

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 bi-phasic 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.

Effects of Electrical Stimulation of Brain

“…The apparent discrepancy in the findings of White and Gross [32] as contrasted with those of Johnson and Hench [13] is probably due to their techniques for evaluating electrode dissolution. Both groups em ployed current densities of 100 mA/cm2.…”

“…Despite exten sive pericapillary and frequent cytoplasmic damage to cerebral capillary endothelium, HRP or red blood cells were not observed in extracellular parenchymatous spaces and endothelial tight junctions were remarkably intact. In general, topically observed extravasation of Evans blue dye at electrode sites increased proportionately to the intensity of the stimulus, however it should be emphasized that these observations are limited es sentially to leptomeninges [13,24].…”

“…They stated that iridium, rhodium, platinum and palladium have lifetimes of the order of decades. Johnson and Hench [13] at the Univer sity of Florida have shown that platinum electrodes with a surface area of 1.0 mm2 will undergo corrosion when pulsed for extended periods of time (1,000-2,400 h with a charge per phase (Q) of 0.5 //C and a charge dens ity (QD) of 50 /iC/cm2. Their recent investigations suggest that ceramic electrodes, e.g.…”

Long term Cochlear Implant electrode improvement for stimulation and sensing neuronal activity

“…Noble metals mentioned above including gold (Au), palladium (Pd) and rhodium (Rh) are generally preferred for electrical stimulation [19]. However, even the working of these noble metals for electrical stimulation under saline environmental conditions cause corrosion.…”

Adverse Effects of Electrical Energy Applied to the Nervous System

“…They have shown that platinum and rhodium are satisfactory materials from the electrochemical standpoint if the charge density does not exceed 175 /¿C/real cm2 in each half wave of the stimulating pulse. However, Johnson and H ench [22,23] have noted that platinum electrodes subjected to 0.1 A/cm2 for 2,300 h in either fetal calf serum or artificial CSF will undergo corrosion. White and Gross [50] continuously pulsed a large number of electrode materials in physio logical saline using varying current amplitudes with a frequency of 50 pulses/sec for periods of time up to 9 months.…”