Abstract

Objective. Retinal prostheses aim to restore sight by electrically stimulating the surviving retinal neurons. In clinical trials of the current retinal implants, prosthetic visual acuity does not exceed 20/550. However, to provide meaningful restoration of central vision in patients blinded by age-related macular degeneration (AMD), prosthetic acuity should be at least 20/200, necessitating a pixel pitch of about 50 µm or lower. With such small pixels, stimulation thresholds are high due to limited penetration of electric field into tissue. Here, we address this challenge with our latest photovoltaic arrays and evaluate their performance in vivo. Approach. We fabricated photovoltaic arrays with 55 and 40 µm pixels (a) in flat geometry, and (b) with active electrodes on 10 µm tall pillars. The arrays were implanted subretinally into rats with degenerate retina. Stimulation thresholds and grating acuity were evaluated using measurements of the visually evoked potentials (VEP). Main results. With 55 µm pixels, we measured grating acuity of 48 ± 11 µm, which matches the linear pixel pitch of the hexagonal array. This geometrically corresponds to a visual acuity of 20/192 in a human eye, matching the threshold of legal blindness in the US (20/200). With pillar electrodes, the irradiance threshold was nearly halved, and duration threshold reduced by more than three-fold, compared to flat pixels. With 40 µm pixels, VEP was too low for reliable measurements of the grating acuity, even with pillar electrodes. Significance. While being helpful for treating a complete loss of sight, current prosthetic technologies are insufficient for addressing the leading cause of untreatable visual impairment—AMD. Subretinal photovoltaic arrays may provide sufficient visual acuity for restoration of central vision in patients blinded by AMD.

Highlights

  • Age-related macular degeneration (AMD) is a leading cause of untreatable vision loss, affecting over 8.7% of the population worldwide [1]

  • Photoreceptors convert light into electrical and chemical signals, which propagate to bipolar cells located in the inner nuclear layer (INL), and to retinal ganglion cells (RGC), which generate trains of action potentials transmitted to the brain via the optic nerve

  • Response to prosthetic stimulation was evaluated by recording visually evoked potentials (VEP) via transcranial electrodes placed above the visual cortex, as described previously [18, 25] and exemplified in figure 3(a)

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Summary

Introduction

Age-related macular degeneration (AMD) is a leading cause of untreatable vision loss, affecting over 8.7% of the population worldwide [1]. Advanced forms of AMD (neovascularization and geographic atrophy) are associated with severe visual impairment, and their prevalence dramatically increases with age: from 1.5% in US population above 40 years to more than 15% in population older than 80 years [2]. Despite losing high-resolution central vision, these patients rarely exhibit visual acuity worse than 20/400 due to preservation of peripheral vision. Prosthetic restoration of sight in such conditions may only be beneficial if acuity reaches 20/200 or better. Gradual loss of photoreceptors leads to visual impairment, while the remaining retinal neurons survive to a large extent [3,4,5]

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