Pillar electrodes reduce in-vivo stimulation thresholds for subretinal prosthesis

Abstract

Purpose :
Smaller pixels in subretinal prosthesis have higher stimulation thresholds due to limited penetration depth of electric field into the inner nuclear layer (INL). This can be remedied by placing active electrodes on pillar structures, which penetrate into the middle of the INL. Here we evaluate the effects of such implants in-vivo.

Methods :
Photovoltaic arrays with (a) 55 and 40µm pixels having flat geometry (F55 and F40), and (b) pixels with their active electrodes on 10µm tall pillars (Pil55 and Pil40), were implanted subretinally in rats with retinal degeneration (Royal College of Surgeons, RCS). Visually evoked potentials (VEP) were recorded via screw electrodes placed onto the skull above visual cortices on both hemispheres. Collimated NIR laser light (915nm) was projected onto the implant. VEPs were measured using full-field flashes at 2Hz with varying irradiances and pulse durations.

Results : With pillars on 55µm pixels, the irradiance threshold was reduced by nearly a factor of 2 (from 1.0±0.27 to 0.55±0.15 mW/mm²), and the duration threshold decreased by a more than a factor of 3 (from 0.29±0.11 to 0.08±0.02 ms). With 40µm pixels, signals were much weaker, and the effect of the pillars was less pronounced: irradiance threshold decreased by 28% (from 1.8±0.58 to 1.3±0.3 mW/mm²) and duration threshold by 16% (from 0.83±0.17 to 0.7±0.12 ms). VEP waveforms with pillar electrodes were more complex than with flat pixels: they included components with larger latency, each with its own irradiance and duration thresholds.

Conclusions : Lower stimulation thresholds enable wider dynamic range of stimulation, which should provide larger number of resolvable levels of grey - a feature crucial for complex visual tasks, such as face recognition. For image modulation by pulse duration using DMD projector on video goggles, 55mm pixels with pillars may offer 3 times wider dynamic range due to lower duration threshold. However, with 40mm pixels, signals remained quite low despite the pillars, and therefore other geometries should be considered for such small pixels.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.