Age-related macular degeneration (AMD) is one of the leading causes of blindness in the developed world, with an incidence of 1:500 in patients aged 55–64, and 1:8 in patients over 85 . Retinitis pigmentosa (RP) is an inherited disease blinding about 1 in every 4000 individuals much earlier in life . In both of these conditions the photoreceptor layer degenerates, while the inner retinal neurons survive to a large extent [3–5]. Electrically activating these neurons provides an alternative route for visual information and raises hope for the restoration of sight to the blind.
In a normal retina, photoreceptors convert light into neural signals that are processed by inner retinal neurons, leading to generation of action potentials in the retinal ganglion cells (RGCs). These signals travel to the brain through the optic nerve and serve as the basis for visual perception. Electrical stimulation of the retina with microelectrodes can also produce action potentials in RGCs, creating spatially patterned percepts of light called phosphenes. Indeed, recent clinical trials with retinal prosthetic electrode arrays have restored visual acuity to subjects blinded by retinal degeneration up to 20/1200 using epiretinal placement (facing the ganglion cell side) , and up to 20/550 with subretinal implantation . While this serves as an important proof of concept with clinically useful implications, existing retinal prosthesis designs have a number of shortcomings.
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