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Stuart F. Cogan, Jinghua Chen, Timothy D. Plante, Marcus D. Gingerich, Joseph F. Rizzo, Douglas Shire; Electrochemical Behavior of Chronically Implanted Sub-retinal Iridium Oxide Stimulation Electrodes. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4964.
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© ARVO (1962-2015); The Authors (2016-present)
The electrochemical properties of sputtered iridium oxide (SIROF) electrodes implanted chronically in mini-pig were characterized as part of the Boston Retinal Implant Project effort to develop a sub-retinal vision prosthesis. The study sought to determine charge-injection and driving voltage characteristics of SIROF electrodes as a function of electrode area. These data were used to evaluate charge-injection limits of SIROF and inform hardware development.
Multielectrode arrays (MEAs) of planar SIROF electrodes having diameters of 50-400 microns, were fabricated on thin polyimide substrates. The MEAs were implanted subretinally in mini-pig using an ab externo approach and a skull-mounted percutaneous connector as an electrical interface. The SIROFs were characterized periodically over 111 days by cyclic voltammetry and by voltage transients measured in response to current pulsing. Similar measurements were taken in vitro in an inorganic interstitial fluid model prior to implantation.
In vivo, 400-micron SIROF electrodes, subjected to 50 Hz, 1 ms current pulses and interpulse bias control for charge-balance, delivered 0.1 mC/phase with electrode polarization well-within safe limits as defined by the potential for water reduction. The 50-micron diameter microelectrodes delivered 100 nC/phase without water reduction. An expected increase in electrode impedance was observed over the course of the study due to tissue response, increasing by a factor of 1.86±0.65 (n=12, averaged over all electrode diameters), and ranging from 4-10 kohm and 28-63 kohm for 400-micron and 50-micron diameter electrodes, respectively. A positive interpulse bias decreased the polarization of the SIROF electrodes, but was not required to deliver pulses at charge levels above reported chronic perceptual thresholds in humans. Cyclic voltammetry and transient measurements at sacrifice suggest that the impedance increase is due to tissue and not changes in the SIROF electrode coatings.
SIROF electrodes delivered stimulus charge densities above reported human perceptual thresholds over a 100 day implantation period. Observed changes and variability in the voltage transient response of the SIROF are probably due to local differences in tissue response.
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