Perforated Membrane as an Interface for Focal Electrical Stimulation of Retina

Abstract

Abstract: : Purpose: Current retinal prostheses use an array of electrodes to stimulate retinal nerve cells at an indeterminate distance. Stimulation threshold voltage and power, as well as cross-talk between neighboring channels, would be greatly reduced if the neural cells could be brought into direct contact with the electrodes. Our goals were: (1) Stimulate controlled migration of neural retina, (2) Demonstrate low threshold neural excitation when cells make contact with electrodes. Methods: We have developed a simple device that consists of a 13 um thick Mylar membrane with an array of perforations (channels) of several microns in diameter in which addressable electrodes can be embedded. Whole retina explants from Sprague-Dawley (P7) rats were harvested and cultured over the membrane. As tissue grew through the channels, samples were fixed with glutaraldehyde for light, scanning, and transmission electron microscopy. Cell stimulation was documented by Fluo-4 fluorescence. Results: We found that tufts of retinal tissue migrated through the channels over a two-day period. The specific layers of retina included in the tuft were dependent on channel diameter. Tuft formation was possible from either the epi-retinal or sub-retinal side of an explant. Electron microscopy revealed that nerve cell processes were present in both the channels and the tufts. The cells inside a 20x20 um channel and those adjacent to the channel were electrically stimulated with 1 ms pulses at a current density of 35 nA/um². Conclusions: We have shown that neuronal retinal tissue can migrate directionally through perforations in a Mylar membrane, and thus grow into direct contact with stimulating electrodes. This proximity may prove critical for reducing the threshold stimulation current and cross-talk of prosthetic electrodes.

Keywords: retinal culture • retina • retinal connections, networks, circuitry