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S. F. Cogan, J. Ehrlich, T. D. Plante, D. B. Shire, M. Gingerich, J. F. Rizzo; Sputtered Iridium Oxide Films (SIROFS) for Retinal Stimulation Electrodes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):660.
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© ARVO (1962-2015); The Authors (2016-present)
As part the Boston Retinal Implant Project effort to develop a sub-retinal prosthesis to restore vision to the blind, thin films of sputtered iridium oxide (SIROF) have been evaluated as low impedance, high charge-injection capacity coatings for electrodes in retinal prostheses.
SIROFs were deposited by DC reactive sputtering over gold metallization on flexible multielectrode arrays fabricated on thin (~10 µm) polyimide substrates. SIROF thickness and electrode areas of 200-1000 nm and 1960-125600 µm2, respectively, were investigated. SIROFs were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronoamperometric measurements of currents required to sustain anodic bias. The charge-injection capacities of the SIROFs were evaluated in an inorganic interstitial fluid model (model-ISF) in response to charge-balanced, cathodal-first current pulses. Charge injection capacities were measured as a function of cathodal pulse width (0.2 - 1 ms) and potential bias in the interpulse period (0.0 to 0.7 V vs. Ag|AgCl).
Adherent SIROF could be deposited with cathodic charge storage capacities of at least 85 mC/cm2 (50 mV/s CV). The SIROF exhibited a potential independent impedance between -0.4 and 0.6 V vs. Ag|AgCl for the entire frequency range (0.05 - 105 Hz) investigated. Unlike activated iridium oxide (AIROF), the SIROF open-circuit potential exhibits a near-Nernstein response to pH, suggesting that H+ and OH- are the principal counterions involved in SIROF redox processes. Depending on the pulse parameters and electrode area, charge-injection capacities ranged from 1-5 mC/cm2, comparable with AIROF pulsed under similar conditions. Arrays, with 8 or 15 electrodes, 400 µm in diameter and coated with 300 nm thick SIROF, were subjected to long-term pulsing in model-ISF at 37oC. The electrodes were pulsed at a charge-injection density of 0.76 mC/cm2 at 16 Hz for periods up to 400 days without SIROF delamination or loss of charge-injection capacity.
In this in vitro study, SIROFs exhibited high charge injection capacities and stability over extended periods of pulsing. The sputter deposition process is well-suited to thin-film, multi-electrode arrays. SIROF is thus a candidate coating for planar electrodes, particularly microelectrodes (area < 10000 µm2), that might be used in retinal prostheses.
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