Subretinal implantation of semiconductor-based photodiodes: Progress and challenges

Retinal diseases that result in photoreceptor degeneration may spare the in ner retinal layers. This review concerns a prosthetic approach to restoring visual function through the use of a semiconductor-based microphotodiode a rray implant, designed to be placed under the neural retina in the subretin al space. The fundamental idea is that current generated by the device in r esponse to light stimulation will alter the membrane potential of overlying neurons and thereby activate the visual system. Initial acute studies indi cated that the implant will function in the subretinal space in the absence of an external power supply. More recent and ongoing studies involve chron ic subretinal implantations in normal animals. Post-operative studies have demonstrated that implant function will persist for many months. These chro nic studies have also assessed the biocompatibility of the implant. Photore ceptors are lost directly overlying the implant, due to the blockade of cho roidal circulation to the outer retina by the solid disk device. In compari son, the inner retina maintains its characteristic lamellar structure. Away from the implant site, the retina retains normal anatomy and function. Fut ure studies are needed to determine whether the implant can establish a fun ctional connection to the inner retina and to determine the quality of this connection.