Subretinal monopolar photovoltaic arrays provide pixel size-independent stimulation threshold and 40µm resolution under spatiotemporal modulation

Abstract

Purpose : High visual acuity in prosthetic vision requires effective stimulation by small pixels with little crosstalk. However, due to shallow penetration of the field with planar bipolar pixels, retinal stimulation threshold exceeds the safe charge injection limit with pixels under 40µm. To bypass such limitation, we developed arrays with deep-penetrating field and investigated their stimulation threshold and acuity in vivo.

Methods : The 1.5mm-wide hexagonal arrays of monopolar photovoltaic pixels of 20 or 40µm in size, having a common return electrode at the edge, were produced with and without 25µm tall walls surrounding each pixel. To reduce the crosstalk, the nearby pixels were converted into transient local returns by spatiotemporal modulation of their conductivity. Following subretinal implantation in rats with degenerate retina (RCS), visually evoked potentials (VEP) were recorded in response to pulsed NIR (880 nm) activation of the implants. The full-field stimulation threshold was measured with 10ms pulses at 2Hz, while acuity was measured using gratings alternating at 2Hz, under 4ms stimulation applied either at 64Hz or multiplexed into 4 sub-frames at 256Hz.

Results : The full-field stimulation threshold was 0.057±0.029mW/mm², corresponding to a current density of 0.092mA/mm² on the active electrodes, independent of pixel size and presence of the walls. This is nearly 30 times lower than that with bipolar pixels of 40µm in size. On the day of implantation, VEP amplitude typically exceeds 100µV, but it dropped to tens of µV one week later, and gradually recovered back to the initial level within 8 weeks post-op. Grating acuity matched the pixel size in 40µm arrays.

Conclusions : Photovoltaic prostheses with nearly-vertical electric field in the retina demonstrated stimulation thresholds independent of pixel size, and much lower than those with bipolar pixels. Using optical spatiotemporal modulation of the photodiode conductivity, any subset of pixels can be converted into transient local returns, thereby providing field confinement necessary for high resolution. Presence of the vertical walls did not negatively affect the retinal excitability, paving the way to honeycomb implants with local returns. Resolution of 40µm corresponds to acuity of 20/160, exceeding the threshold of legal blindness in US. Acuity studies with 20µm pixels are in progress.

This is a 2021 ARVO Annual Meeting abstract.