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E. Greenbaum, C.A. Sanders, D. Zhou; Dynamic Interactions at the Retinal Prosthesis Electrode Interface . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3200.
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© ARVO (1962-2015); The Authors (2016-present)
Visual prosthesis microelectrode arrays, implanted in close proximity to the retina, bypass disease–damaged photoreceptors, and transfer electrical charge across the electrode/vitreous interface to functioning neural cells. Physiological considerations of safe stimulation restrict the maximum charge density that may be applied at the electrode surface. The maximum safe charge per phase depends upon electrode geometry and effective surface area, factors that alter electrode capacitance and charge density. Recommended limits for safe charge injection for Pt electrodes implanted in neural tissue are set below the threshold of water electrolysis at pulse configurations that are least likely to produce harmful byproducts of faradaic ionic transformations in the vitreous. Pulses least damaging to cortical neural tissue are balanced, biphasic and equal, resulting in zero net charge injection at the interface.
We have modeled the electrode/vitreous interface and have studied the dynamic interactions of Pt electrodes with electrolyte solutions under conditions approximating the physiological parameters of temperature, electrode configuration and local electrolyte environment found in the human eye. Platinum electrodes were charged with currents from a stimulus generator. The pulse patterns were mono or biphasic with variable amplitude and 1 msec duration. Pulse configuration and charge density were correlated with thresholds for electrolysis (hydrogen evolution) and free chlorine formation. In addition, scanning electron microscopic images (SEM) were taken of Pt wire and Pt gray (higher surface area) (Zhou, US Patent, 2003) electrodes before and after biphasic stimulation to test the effect of effective versus geometric electrode surface area on in situ electrode dissolution.
Results of phasic pulse stimulation tests showed that hydrogen was formed in the vitreous at a charge density of 0.2 mC/cm2 with a monophasic pulse. With a charge balanced biphasic pulse, hydrogen was not evolved until the charge density exceeded 1 mC/cm2. SEM imaging of Pt electrodes energized with biphasic pulses at a charge density of 1 mC/cm2 showed appreciable solubilization of Pt wire electrodes but no deterioration of Pt gray surfaced electrodes.
In choosing waveform configurations for stimulation of retinal prosthesis electrode arrays, current reversal and charge balance are the determinants of long–term operational safety. High surface area Pt microelectrodes are more durable than smooth surfaced electrodes for chronic stimulation of the retina.
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