Pixel size limit of the PRIMA implants: from humans to rodents and back

Objective Retinal prostheses aim at restoring sight in patients with retinal degeneration by electrically stimulating the inner retinal neurons. Clinical trials with patients blinded by atrophic Age-related Macular Degeneration (AMD) using the PRIMA subretinal implant, a 2×2 mm array of 100μm-wide photovoltaic pixels, have demonstrated a prosthetic visual acuity closely matching the pixel size. Further improvement in resolution requires smaller pixels, which necessitates more intense stimulation. Approach Here, we examine the lower limit of the pixel size for PRIMA implants by modeling the electric field, leveraging the clinical benchmarks, as well as using a preclinical animal data to assess the stimulation strength and contrast of various patterns. Visually evoked potentials were measured in RCS rats with photovoltaic implants of 100 and 75μm pixels and compared to clinical thresholds with 100 μm pixels. Electrical stimulation model calibrated by these clinical and rodent data was used to predict the performance of the implant with smaller pixels. Main Results We found that PRIMA implants with 75μm pixels under the maximum safe near-infrared (880nm) illumination of 8 mW/mm 2 with 30% duty cycle (10ms pulses at 30Hz) should provide a similar perceptual brightness as with 100 μm pixels under 3 mW/mm 2 irradiance, used in the current clinical trials. Contrast of the Landolt C pattern scaled down to 75μm pixels is also similar under such illumination to that with 100μm pixels in clinical settings, increasing the maximum acuity from 20/420 to 20/300. Significance Computational model of the photovoltaic subretinal prosthesis defines the minimum pixel size of the PRIMA implants as 75μm. Increasing the implant width from 2 to 3 mm and reducing the pixel size from 100 to 75μm will nearly quadrupole the number of pixels and thereby should significantly improve the visual performance. Smaller pixels of the same bipolar flat geometry would require excessively intense illumination, and therefore a different pixel design should be considered for further improvement in resolution.