Can a Self-powered Retinal Prosthesis Support 100,000 Pixels in the Macula?

Abstract

Abstract: : Purpose: Several factors limit the resolution of an electronic retinal prosthesis: (1) Distance between electrodes and target cells determines the threshold voltage, current and power per electrode, as well as minimal separation between neighboring electrodes to avoid cross-talk. (2) Heating of the retina by electric current limits the density of an electrode array. (3) Electrolysis and erosion of electrodes limits current density and pulse duration. (4) Ambient light intensity within the eye limits electrode size and density in a self-powered prosthetic. Successful design of a high-resolution autonomous prosthesis will require optimization of all these parameters. Methods: We calculate threshold values of voltage, current, power and packing density required for cell stimulation as a function of electrode size and distance between electrodes and the target cells. We estimate heating of tissue using models of passive heat conductance and active heat transport by blood flow. We calculate the maximal number of electrodes that can be powered by photovoltaic cells using ambient light on retina. We propose two new chip designs that can provide more efficient stimulation. Results: With an external power supply, pixel density is limited primarily by heating of retina, and the proximity of electrodes to cells is a critical determinant of the current required for cell stimulation. When target cells are separated from electrodes by 50 um, only 13 pixels/mm² are possible without heating the tissue more than 1^oC. When cells are in contact with electrodes, the density can be increased to 10,000 pixels/mm². However, ambient light intensity at the retina is sufficient for continuous activation of only one photovoltaic cell per 3 mm² at a +/-50 mV depolarization level. We describe two new designs for prosthetic chips that are capable of activating up to 100,000 pixels using only ambient light in the eye. The first system uses an auxiliary photovoltaic collector over peripheral areas of retina or in the anterior chamber to gather power for electrodes in the macula. The second system is based on electrostatic deflectable membranes activated by photodiodes, which can serve as an interface with neural retina using the tactile sensitivity of cells. Conclusions: It is theoretically possible to design self-powered photosensitive implants that would support sufficient amount of macular activation sites to provide good visual acuity using only ambient light in the eye.

Keywords: retina • visual acuity • retinal culture