Retinal prostheses: current challenges and future outlook.

Blindness from retinal diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa (RP), usually causes a significant decline in quality of life for affected patients. Currently there is no cure for these conditions. However, over the last decade, several groups have been developing retinal prostheses which hopefully will provide some degree of improved visual function to these patients. Several such devices are now in clinical trials. Unfortunately, the possibility of electrode or tissue damage limits excitation schemes to those that may be employed with electrodes that have relatively low charge densities. Further, the excitation thresholds that have been required to achieve vision to date, in general, are relatively high. This may result in part from poor apposition between neurons and the stimulating electrodes and is confounded by the effects of the photoreceptor loss, which initiates other pathology in the surviving retinal tissue. The combination of these and other factors imposes a restriction on the pixel density that can be used for devices that actively deliver electrical stimulation to the retina. The resultant use of devices with relatively low pixel densities presumably will limit the degree of visual resolution that can be obtained with these devices. Further increases in pixel density, and therefore increased visual acuity, will necessitate either improved electrode-tissue biocompatibility or lower stimulation thresholds. To meet this challenge, innovations in materials and devices have been proposed. Here, we review the types of retinal prostheses investigated, the extent of their current biocompatibility and future improvements designed to surmount these limitations.