Vertical walls surrounding pixels in subretinal space reduce stimulation threshold and improve contrast

Abstract

Purpose : High-resolution visual prostheses require densely packed small pixels, which typically have limited penetration of electric field into tissue and significant cross-talk between neighboring electrodes. We present a novel honeycomb configuration of the stimulating electrode array with vertically separated active and return electrodes, designed to leverage retinal migration for reducing the subretinal stimulation threshold and electrical cross-talk.

Methods : Tissue integration with subretinally implanted honeycomb arrays (silicon, pixel pitch = 40 μm, wall height = 25 μm, implant diameter = 1 mm) in rats with retinal degeneration (Royal College of Surgeons, RCS) was evaluated using OCT, confocal microscopy, and histology. We analyzed the electric field produced by honeycomb arrays with insulating walls, having active electrode at the bottom of each well and return electrode in two configurations: 1) local return on top of the honeycomb, and 2) distant return on the back side of the implant. Stimulation thresholds were assessed using a model of network-mediated retinal stimulation.

Results : On average, 43% (n=3, σ=24%) of the INL cells migrated into the honeycomb cavities with no observable fibrosis. Honeycomb arrays of pixels with local and distant return electrodes exhibit stimulation thresholds (activation of 50% of the cells in the Inner Nuclear Layer, INL) 12 and 17 times lower than that with flat arrays, respectively, while retaining 100% spatial contrast. Local return electrodes on top of the honeycomb allow for extension of the activation zone above the walls to stimulate cells above the cavity, enabling activation of up to 70% (σ=28%) of the INL within safe irradiance limits (< 5mW/mm²), while preserving 100% spatial contrast.

Conclusions : Insulating walls surrounding each pixel in a honeycomb structure align the electric field vertically, matching the orientation of bipolar cells in the retina. This significantly reduces the stimulation threshold, while preserving high contrast between the neighboring pixels. Such structures enable further reduction of the pixel size of subretinal arrays to allow visual prostheses with even higher resolution.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.

 

Inner nuclear layer cells migrated into the subretinally-implanted honeycomb array of 40 μm pitch with 25 μm high walls. Recessed active electrodes and return electrode on top decreases stimulation threshold >10x compared to flat pixels.