title: Over-pulsing degrades activated iridium oxide films used for intracortical neural stimulation.
abstract: 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 0.4V (versus Ag|AgCl) anodic bias with equal cathodal and anodal pulse widths, for periods up to 7h at a frequency of either 50Hz or 100Hz. At charge densities of 3mC/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 2mC/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 3mC/cm(2) showing evidence of AIROF delamination after only 100s of pulsing at 100Hz (10,000 pulses total), while electrodes pulsed at 2mC/cm(2) for 7h at 50Hz (1.3 x 10(6) 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 0.8V versus Ag|AgCl) results in the observed degradation.
authors.affiliation: EIC Laboratories Inc., 111 Downey St., Norwood, MA 02062, USA. scogan@eiclabs.com
authors.forename: Stuart F
authors.initials: SF
authors.lastname: Cogan
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Guzelian
authors.forename: William F
authors.initials: WF
authors.lastname: Agnew
authors.forename: Ted G H
authors.initials: TG
authors.lastname: Yuen
authors.forename: Douglas B
authors.initials: DB
authors.lastname: McCreery
mesh_terms: D000818:Animals; D002415:Cats; D002540:Cerebral Cortex / Q000502:physiology / Q000528:radiation effects*; D004307:Dose-Response Relationship, Radiation; D004558:Electric Stimulation / Q000379:methods*; D004563:Electrochemistry; D004567:Electrodes, Implanted; D004595:Electroplating; D007495:Iridium / Q000378:metabolism*; D008839:Microelectrodes; D008855:Microscopy, Electron, Scanning / Q000379:methods
chemical_list: C044458:iridium oxide; D007495:Iridium