gms | German Medical Science

Purpose: In patients blinded by atrophic age-related macular degeneration, subretinal photovoltaic implant (PRIMA) provides prosthetic vision with acuity up to 20/420, closely matching its 100 µm pixel size. Testing the next-generation implants with smaller pixels in rats with retinal degeneration is challenging because they don’t exhibit the subretinal debris layer observed in diseased human eyes, which impacts the distance between the electrodes and the target neurons in the inner nuclear layer (INL). This study aims at replicating the debris layer by a scaffold of a similar thickness constructed above the pixels.

Methods: Polymer scaffolds of 20–45 µm in thickness were designed to be porous to minimize the light scattering and to allow electric field penetration into the retina, while preventing migration of the inner retinal cells into the openings. Scaffolds with holes of 4, 7, 10 and 12 µm in width were fabricated using two-photon lithography (Nanoscribe). Devices were implanted beneath the degenerate rat retina (RCS), and the retinal migration was monitored by OCT imaging. Stimulation thresholds and grating acuity were measured in-vivo by visually evoked potentials (VEP). After 4–6 weeks, cellular migration was also assessed using histology and confocal microscopy.

Results: Printed scaffolds did not impede the NIR illumination of the photovoltaic pixels. Structures of up to 45 µm in height could withstand the implantation procedure and were stable in-vivo for up to 6 weeks. Openings of up to 7 µm in width prevented the cell somas from migrating into the scaffolds. Histology demonstrated that voids in the scaffold mesh were filled with organic material.

Conclusions: The scaffolds kept the INL cells at comparable distances from the electrodes as observed in humans. Organic material filling the voids in the scaffold helps match the electrical impedance of the retina rather than saline.