Nanotechnology In Retinal Prostheses

Prostheses for vision restoration in visually impaired patients with degenerative disorders like ARMD, retinitis pigmentosa and glaucoma are based on one of three techniques: stimulation of the visual cortex, stimulation of optic nerve, and stimulation of retinal cells (53). The visual prostheses can be broadly classified into cortical and retinal implants, which are further discussed below.

Cortical Implants

These consist of electrodes embedded in the visual cortex. For localized intracortical stimulation with minimal pain, small electrodes and low electrical currents are desired. Nanosize electrodes could potentially be employed in such devices to enhance visual signals. The main advantage of cortical stimulation for vision restoration is that the technique bypasses disease proximal to the primary visual cortex, making it potentially useful for treating diseases of optic nerve and retina.

Retinal Implants

These serve as artificial photoreceptors and consist of an array of microphotodiodes attached to microelectrodes. Depending on the layer of retina receiving the device, retinal prostheses can be classified into epiretinal, when the implant is placed on the surface of the retina between vitreous and inner limiting membrane, and subretinal, when the implant is placed in the subretinal space between outer retina and RPE. Retinal implants are limited in application to disorders in which the visual pathway distal to retina is intact and functional.

During the past decade, tremendous progress has been made towards electronic retinal prostheses; however, it is generally believed that a fully functional long-lasting device is not on the immediate horizon. One of the challenges with retinal prostheses is long-term biocompatibility. For an electronic implant, in addition to ensuring chemical, biophysical and immunological compatibility, the reaction to external electrical stimuli and heating of the tissue need to be considered. To this effect, carbon nanotube (CNT)-based retinal prostheses are being developed to avoid the incompatibility issues with metallic electrodes (54). Carbon nanotubes offer unique mechanical and electrical properties for use in retinal prostheses, such as tunable electrode height to allow fabrication of penetrating microelectrode arrays with controllable stimulation depth, easy penetrability in retina, and good biocompatibility, as suggested by their capability to support retinal ganglion cell growth. Thus, nanotechnology advancement is benefiting the development of smaller, more-efficient, and biocompatible retinal prostheses.

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