A prosthesis is provided comprising a substrate having a distal end and a proximal end; and a plurality of electrodes located at or adjacent the distal end of the substrate. The distal end of the substrate is configured for insertion, via an incision, between first and second tissue layers, such as the sclera and choroid in the eye. The prosthesis tapers in thickness towards the distal end and has a substantially curved profile at least on one surface so that the prosthesis can be inserted into position without needing a guide and without causing damage to the tissue or the prosthesis. The prosthesis can include an electrode interface unit located at or adjacent the proximal end of the substrate which locates between the tissue layers. An anchor portion can be provided that extends from the substrate into the incision.