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J. Ehrlich, S. Cogan, P. Troyk, T. Plante, D. Bradley, M. Bak, R. Erickson, D. McCreery, E. Schmidt, V. Towle; Care and Feeding of AIROF Microelectrodes for Visual Prostheses . Invest. Ophthalmol. Vis. Sci. 2004;45(13):4208.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate how the nature of the stimulus driving waveform affects the electrochemical integrity of activated–iridium–oxide film (AIROF) microstimulating electrodes being proposed for use in visual prostheses. Although commonly used, the symmetric biphasic constant–current waveform introduces a limitation in the use of highly–beneficial anodic biasing, used to increase the cathodal charge–injection capacity of AIROF, because the charge balancing anodal phase may polarize the electrode beyond a safe positive voltage limit. Methods: AIROF electrodes, with a nominal surface area of 2,000 ± 300 µm2, were subjected to cathodal–first, biphasic, constant–current pulsing as a function of anodic bias. Charge–injection limits were calculated from the electrode voltage waveforms by correcting for the access resistance contribution. The charge–injection limit was defined as the maximum cathodic charge that resulted in the corrected waveforms exceeding either a cathodic limit of –0.6 V, or an anodic limit of +0.8 V vs. Ag|AgCl. Current waveforms using symmetric, with equal (1:1) cathodic and anodic pulse widths, and asymmetric pulse width ratios of 1:2, 1:4, and 1:8 were investigated. Charge–injection limits for each waveform were determined at bias levels from 0.0 V to 0.7 V in 0.1 V increments. Results: The optimum waveform for maximizing charge–injection used a +0.6 V bias and 1:8 (0.4 ms to 3.2 ms) cathodic–to–anodic pulse–width ratio, with the maximum charge being 3.3 mC/cm2. For the 1:1 waveshape the maximum safe charge–injection limit was 1.8mC/cm2 at +0.5V bias at which the anodic limit was reached. The near–doubling of the charge capacity for the 1:8 waveform would be significant for some visual prostheses currently under development. The use of an essentially monophasic waveform and anodic bias, which is the limiting case of the asymmetric waveform, should provide an additional increase in charge–injection capacity but with some complications in terms of the design of driving circuitry. In any case, maintaining long–term electrochemical stability for AIROF electrodes, may require the use of suitable reference electrode. Conclusions: AIROF electrodes offer higher charge–injection capacity than conventional noble metal electrodes. However, more sophisticated electronic driving conditions may be necessary for implanted long–term stability.
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