Research, led by scientists at the Hansen Experimental Physics Laboratory, Stanford University, has demonstrated clinically relevant retinal responses from experimental animal models following implantation of photovoltaic arrays (doi:10.1038/nm.3851). The arrays were shown to illicit retinal responses with a spatial resolution of 64 ± 11 mm, a response estimated to be equivalent to approximately half of the normal visual acuity in healthy animals. A surgical procedure, combined with the wireless and modular array technology, may have significant application to restoring useful vision in patients suffering a range of retinal degenerative disorders.
Retinal prostheses that aim to stimulate the surviving neurons of the inner retina in patients are not new. Two basic approaches are currently under development – epiretinal and subretinal devices. Epiretinal approaches (such as the Argus II device, Second Sight Inc.) generally aim to stimulate retinal ganglion cells (RGCs) while subretinal devices (such as the Alpha IMS, Retinal Implant) aim to stimulate inner retinal neurons, such as bipolar cells. According to the Stanford team both these approaches have various shortcomings, which they believe can be overcome by a new device in which “photovoltaic subretinal pixels convert pulsed light into electric current, thereby enabling a completely wireless implant”. Multiple small (1-2mm) arrays are placed underneath the retina allowing a significant visual field to be tiled without the need for an external power source. A single module of the photovoltaic prosthesis, manufactured on silicon-on-insulator wafers, is composed of 70-μm-wide pixels separated by 5-μm trenches arranged in a 1-mm-wide hexagonal pattern.
According to the research group, the cortical activation threshold with the 70-μm pixel arrays was 0.55 mW/mm2 with 10-ms pulses—a threshold four times lower than previously reported and two orders of magnitude below the ocular safety limit for the 880nm to 915nm wavelength range. In addition, the research group reported that at the highest settings used, even over the course of several hours, responsiveness did not appear to decrease or show any signs of tissue damage, suggesting the safety of the system. Nevertheless, before any human studies can be designed, significant testing in larger animals will be required for safety and efficacy. In terms of resolution and acuity, the authors of the study concluded that, “our photovoltaic pixels provide much tighter confinement of the electric field compared to the monopolar electrodes in other implants. This helps reduce cross-talk between neighboring electrodes, thereby increasing contrast and spatial resolution of the stimulation”.