Subretinal implantation and testing of polyimide film electrodes in cats

Background: Progress in the field of microelectronics has led to the development of visual prostheses for the treatment of blinding diseases. One concept under investigation is an electronic subretinal prosthesis to replace the function of lost photoreceptors in degenerative diseases, such as retinitis pigmentosa. Methods: In the subretinal prosthesis design concept, an array of stimulation electrodes is placed in the subretinal space. To test the feasibility of the concept and to determine basic stimulation parameters, wire-bound stimulation devices were used in acute trials for up to 12 h in three eyes in anaesthetised cats. These wire-bound stimulation elements were based on strips of polyimide film. The film strips were introduced through a sclerostomy into the vitreous cavity and via a retinotomy into the subretinal space during a modification of the standard three-port vitrectomy procedure. On entry through the retinotomy, the film was advanced mechanically to the desired position in the area centralis. Perfluorocarbon liquid (PFCL) was used to establish close contact between the electrode array and the outer retina. Stimulation was performed with computer-generated sequences of current waveforms in acute trials immediately after surgical implantation of the stimulation film. Cortical recordings in the primary visual cortex were performed with electrodes placed in locations corresponding to the retinal stimulus site. Results: All three implantations were carried out successfully with the stimulation array implanted beneath the outer retina of the area centralis of the operated eye. The retina was attached over the stimulation array in all cases. No cortical responses were recorded in one of the stimulation sessions. The results from another session revealed clear intracortical responses to subretinal stimulation with polyimide films. Following single-site retina stimulation, the estimates of spatial cortical resolution and temporal resolution were approximately 1 mm and 20-50 ms, respectively. Discussion: Our results indicate that focal subretinal stimulation evokes localised spatio-temporal distribution of cortical responses. These findings offer hope that coarse restoration of vision may be feasible by subretinal electrical stimulation.