Bhuckory, M. | Search bioRxiv

bioRxiv2022

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

ObjectiveRetinal 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 2x2 mm array of 100m-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. ApproachHere, 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 75m pixels and compared to clinical thresholds with 100 m pixels. Electrical stimulation model calibrated by these clinical and rodent data was used to predict the performance of the implant with smaller pixels. Main ResultsWe found that PRIMA implants with 75m pixels under the maximum safe near-infrared (880nm) illumination of 8 mW/mm2 with 30% duty cycle (10ms pulses at 30Hz) should provide a similar perceptual brightness as with 100 m pixels under 3 mW/mm2 irradiance, used in the current clinical trials. Contrast of the Landolt C pattern scaled down to 75m pixels is also similar under such illumination to that with 100m pixels in clinical settings, increasing the maximum acuity from 20/420 to 20/300. SignificanceComputational model of the photovoltaic subretinal prosthesis defines the minimum pixel size of the PRIMA implants as 75m. Increasing the implant width from 2 to 3 mm and reducing the pixel size from 100 to 75m 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.

bioengineering↗

bioRxiv2021

Electronic "photoreceptors" enable prosthetic vision with acuity matching the natural resolution in rats

Localized stimulation of the inner retinal neurons for high-acuity prosthetic vision requires small pixels and minimal cross-talk from neighboring electrodes. Local return electrodes within each pixel limit crosstalk, but can over-constrain the electric field, thus precluding efficient stimulation with subretinal pixels smaller than 50 m. Here we demonstrate high-resolution prosthetic vision based on a novel design of a photovoltaic array, where field confinement is achieved dynamically, leveraging the adjustable conductivity of the diodes under forward bias to turn the designated pixels into transient returns. We validated computational modeling of the field confinement in such an optically-controlled circuit by ex-vivo and in-vivo measurements. Most importantly, using this strategy, we demonstrated that the grating acuity with 40 m pixels matches the pixel pitch, while with 20 m pixels, it reaches the 28 m limit of the natural visual resolution in rats. This method enables customized field shaping based on individual retinal thickness and distance from the implant, paving the way to prosthetic vision with acuity as high as 20/80 in atrophic macular degeneration.

bioengineering↗