Over-pulsing degrades activated iridium oxide films used for intracortical neural stimulation

Microelectrodes using activated iridium oxide (AIROF) charge-injection coatings have been pulsed in cat cortex at levels from near-threshold for neural excitation to the reported in vitro electrochemical charge-injection limits of AIROF. The microelectrodes were subjected to continuous biphasic current pulsing, using an 0.4 V (versus Ag\AgC]) anodic bias with equal cathodal and anodal pulse widths, for periods up to 7 h at a frequency of either 50 Hz or 100 Hz. At charge densities of 3 mC/cm(2), histology revealed iridium-containing deposits in tissue adjacent to the charge-injection sites and scanning electron microscopy of explanted electrodes revealed a thickened and poorly adherent AIROF coating. Microelectrodes pulsed at 2 mC/cm(2) or less remained intact, with no histologic evidence of non-biologic deposits in the tissue. AIROF microelectrodes challenged in vitro under the same pulsing conditions responded similarly, with electrodes pulsed at 3 mC/cm(2) showing evidence of AIROF delamination after only 100 s of pulsing at 100 Hz (10,000 pulses total), while electrodes pulsed at 2 mC/cm(2) for 7 h at 50 Hz (1.3 x 106 pulses total) showed no evidence of damage. In vitro electrochemical potential transient measurements in buffered physiologic saline indicate that polarizing the AIROF beyond the potential window for electrolysis of water (-0.6 to 0.8 V versus AglAgC]) results in the observed degradation. (C) 2004 Elsevier B.V. All rights reserved.