Development and evaluation of thin-film flexible microelectrode arrays for retinal stimulation and recording

We have described the development of a flexible microelectrode array with potential applications in the large scale recording of neural signals and in focal electrical stimulation for use as a prosthetic implant in degenerative retinal diseases. The array under test consisted of 61 platinum electrodes of 5 µm diameter with 60 µm spacing connected by 8 µm wide gold tracks encased in a flexible polyimide substrate of 15 µm thickness from which recordings were taken from 16 electrodes. The device was tested on an exposed frog eyecup preparation which is characterized by small retinal ganglion cells of similar dimensions to those present in the human retina. The responses of these cells evoked by photic stimulation consisted of trains of action potentials of high signal-to noise ratio at each of the recording sites. Delivery of cathodal constant voltage pulses and constant current pulses to specific electrodes in the array led to the generation of action potentials in adjacent electrodes, implying that retinal ganglion cells in the proximity had been stimulated. Since prolonged stimulation with supra-threshold voltages impaired neither electrode structure nor retinal function, these results provide a sound basis for scaling up the number of array electrodes to deliver focal electrical pulses to the retina, as would be required by a viable epiretinal prosthesis.