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E. Greenbaum, C. A. Sanders, J. D. Weiland, M. S. Humayun; Electrostatic Potential Mapping and Spatial Resolution at the Visual Prosthesis/Vitreous Humor Interface. Invest. Ophthalmol. Vis. Sci. 2007;48(13):648.
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© ARVO (1962-2015); The Authors (2016-present)
Visual prosthesis microelectrode arrays, implanted on the retinas of photoreceptor impaired patients, transfer patterned electrical stimuli across the electrode/vitreous humor interface to the visual neural pathway. To be effective, electrodes must transfer enough charge through the vitreous to exceed neuronal depolarization threshold potentials. However, physiological considerations of safe stimulation restrict the maximum charge density that may be applied to individual electrodes in the array. Decreasing electrode areas and increasing electrode numbers in the newer designs of multipixel prosthesis arrays necessitate investigation of the distribution of interfacial potentials over the active surface of individual and clustered, energized electrodes.
Electric field potentials were mapped above single Pt electrodes and multiple (16 Pt and 60 gold) electrode arrays. Test electrodes were immersed in physiological electrolyte medium. The three-electrode test circuit configuration consisted of a stimulating electrode, a 10 µm recording electrode, and a counter electrode, energized by a stimulus from a pulse generator. The recording electrode was mounted on a xyz translation stage and moved incrementally vertically and horizontally over the stimulating electrode. Pulses were monophasic or symmetrical biphasic, applied at a maximum charge density of 1 mC/cm2. Voltages between the stimulating and counter electrode and between the recording electrode and counter electrode was monitored on a digital oscilloscope.
Potential profiles over multielectrode arrays were obtained by fixing the position of the recording electrode over an electrode in the center of the array while individually stimulating other electrodes in its vicinity. Potentials from active nearest neighbor electrodes propagated a potential over the center electrode half as high as the voltage from the active electrode. Activation of the next concentric layer of closest electrodes dropped the potential over the center electrode to a third. Cross-talk was higher when track lines of distant activated disks ran next to the center recording electrode.
Potential mapping above single prosthesis electrodes is useful for quantifying the charge from a point source and its propagation through a conductive medium. In multielectrode arrays, the spatial arrangement of electrode disks, their size and pitch, and the metal tracks running between the disks determine the potential profile over the surface of individual electrodes.
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