8.1  Introduction

Retinal implants can help partially restoring vision to patients suffering from degenerative diseases of the retina. Age-related macular degeneration and retinitis pigmentosa by replacing the functionality of no longer working photoreceptors with systematic electrical stimulation to neural cells further down the optical neural path [8, 12].

Clinical trials show that electrical stimulation using epiretinally implanted electrodes causes the appearance of localized white or yellow round phosphenes [12]. These percepts must correspond to excitation of cells in the ganglion cell layer (GCL) or deeper in the retina, as only these cells map to a location under the stimulating electrode; the more superficial nerve fiber layer (NFL) is formed by axons of GCL neurons going towards the optic nerve that belong to ganglion cells away from the stimulating point (Fig. 8.1). Epiretinal electrode arrays having 16 (4 × 4) electrodes have already been successfully implanted in clinical trials, and efforts are ongoing to increase the resolution of the implant; versions with 60 electrodes are currently undergoing the FDA approval process and 240 and more electrodes are being worked on.

Fig. 8.1  Diagram of a transverse cut of retinal model with epiretinal implant close to its surface (not to scale). The retina geometry has been approximated as flat in this model. The epiretinal implant electrically stimulates the ganglion cell layer (GCL) across the nerve fiber layer (NFL) by injecting electrical current using charge-balanced biphasic pulses. The NFL is formed by the axons of cells in the GCL, which curve and bundle together, eventually shaping into the optic nerve, which relays the visual signal to the brain. In all layers but the NFL, the neural pathway is predominantly vertical. In this configuration the 25 electrodes are arranged in a regular 5 × 5 matrix and partially embedded in a dielectric substrate. A three-dimensional view of the implant model is shown in Fig. 8.2