Purchase this article with an account.
KL Roach, L Theogarajan, J Wyatt; An Electrical Model of Electrode Behavior in Retinal Prostheses . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4481.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: To model the electrical behavior of microfabricated electrodes, particularly the nonlinear scaling of the resistance and capacitance and the voltage dependence of charge transfer. A retinal prosthesis requires safe, power-efficient, and effective electrodes. By understanding how various design parameters affect electrode circuit characteristics, it will be possible to minimize power consumption while maintaining high safety levels and effective retinal stimulation. Methods: All measurements were done using either a large platinum return far from the array or a large shared return built into the array. Pulsed-current potentiometry was performed on oxidized and unoxidized iridium electrodes with diameters of 100um and 400um under a range of electrolyte conditions. Cyclic voltammetry was used to measure effective charge storage, verify the capacitance values calculated by pulsed-current potentiometry, and better understand the charge transfer mechanisms. Charge storage and charge transfer were further investigated by recording the voltage decay of a charged, open circuit electrode. Results: The resistive term was very stable under a wide range of pulse amplitudes and durations. With corrections for the series wiring resistance, the resistance scaled linearly with the electrolyte conductivity and inversely with electrode diameter. Capacitance measurements were far less stable, varying considerably with pulse amplitude and duration. Large effective capacitances were seen at high current levels, but the scaling patterns were inconsistent. Electrolyte concentration had a weaker effect, but similarly complex behavior. Charge transfer measurements showed several threshold voltages at which rapid charge decay occurred across the electrode interface. Conclusion: Early tests showed that return selection had no measurable effect on the results. Both returns could be considered large and distant for these purposes. The resistive term matches the simple theoretical model of a hemispherical electrode embedded in an insulating plane and enclosed by a large hemispherical return. Measurements for the capacitance do not fit any of the standard models. More work must be done to effectively model the capacitance and charge transfer terms. Future tests will involve new platinum and iridium arrays with a wider range of electrode diameters and return configurations.
This PDF is available to Subscribers Only