Optimizing photovoltaic subretinal implants for high-acuity prosthetic vision
Keywords: Engineering.
Electrical Engineering.

Retinal degeneration diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa, are a leading cause of irreversible vision loss worldwide. They degrade the quality of life and pose a significant burden on the healthcare system. With no cure available, multiple efforts were made with implantable electronics to introduce prosthetic vision by stimulating the remaining visual system. Among these efforts, a technology with photovoltaic subretinal implants has demonstrated the best clinical results so far: formed vision with acuity up to 20/438 in patients blinded by atrophic AMD, closely matching the maximum resolution expected with its 100 µm pixels. However, for a broad acceptance of this technology, prosthetic acuity should significantly exceed the residual vision in AMD patients -- typically no worse than 20/400. Furthermore, acuity above 20/200, the legal limit of blindness in the United States, requires a spatial resolution on the retina of better than 50 µm, which has been shown impossible with the current flat bipolar pixel design, because the local return electrode placed around the stimulating active electrode over-constrains the electric field penetration into the retina. The simple monopolar design, where the local return is removed from each pixel, however, results in excessive crosstalk between pixels, severely compromising contrast and resolution. Various designs have been proposed to overcome the challenges associated with the miniaturization of subretinal pixels: the crosstalk caused by monopolar pixels could be suppressed by transiently turning some active electrodes into returns; pillar-shaped active electrodes of bipolar pixels could reduce the distance between the stimulation sites and the target neurons, hence improving the stimulation strength and the spatial contrast; honeycomb-shaped walls could elevate the return electrode around each pixel to decouple the stimulation strength from the pixel size. Prior to the resource-intensive fabrication and testing of each design, a model should be developed to provide valuable insights and predict the expected performance. This work investigated the design considerations of the photovoltaic subretinal prosthesis and presented a modeling framework to guide the optimization for higher acuity. The modeling framework captured the key features of the system, including the neural response of the bipolar cells to electric fields in the retina, current kinetics at a distributed electrode-electrolyte interface, as well as the photovoltaic circuit dynamics of a very-large-scale electrode array. Using the modeling framework, benchmarks of stimulation strength and contrast were established from prior clinical and animal results. Prosthetic acuity up to the natural resolution limit in rats -- 28 µm -- was then demonstrated with monopolar pixels leveraging the transient returns to suppress the crosstalk. The expected clinical performance of the monopolar pixels with transient returns and the pillar design were also examined using the framework for pixel sizes down to 20 µm, which may enable prosthetic vision with an acuity beyond 20/100. Finally, the computational efficiency of the framework was further improved with sparse and low-rank approximations to enable a design of large implants that can provide a wide field of view with up to 20000 pixels.