▪ Abstract Retinal prostheses represent the best near-term hope for individuals with incurable, blinding diseases of the outer retina. On the basis of the electrical activation of nerves, prototype retinal prostheses have been tested in blind humans and have demonstrated the capability to elicit the sensation of light and to give test subjects the ability to detect motion. To improve the visual function in implant recipients, a more sophisticated device is required. Simulations suggest that 600–1000 pixels will be required to provide visual function such as face recognition and reading. State-of-the-art implantable stimulator technology cannot produce such a device, which mandates the advancement of the state of the art in areas such as analog microelectronics, wireless power and data transfer, packaging, and stimulating electrodes.

Stimulating electrode materials must be capable of supplying high-density electrical charge to effectively activate neural tissue. Platinum is the most commonly used material for neural stimulation. Two other materials have been considered: iridium oxide and titanium nitride. This study directly compared the electrical characteristics of iridium oxide and titanium nitride by fabricating silicon substrate probes that differed only in the material used to form the electrode. Electrochemical measurements indicated that iridium oxide had lower impedance and a higher charge storage capacity than titanium nitride, suggesting better performance as a stimulating electrode. Direct measurement of the electrode potential in response to a biphasic current pulse confirmed that iridium oxide uses less voltage to transfer the same amount of charge, therefore using less power. The charge injection limit for titanium nitride was 0.87 mC/cm/sup 2/, contradicting other reports estimating that titanium nitride was capable of injecting 22 mC/cm/sup 2/. Iridium oxide charge storage was 4 mC/cm/sup 2/, which is comparable to other published values for iridium oxide. Electrode efficiency will lead to an overall more efficient and effective device.

In this retinal prosthesis project, a rehabilitative device is designed to replace the functionality of defective photoreceptors in patients suffering from retinitis pigmentosa (RP) and age-related macular degeneration (AMD). The device consists of an extraocular and an intraocular unit. The implantable component receives power and a data signal via a telemetric inductive link between the two units. The extraocular unit includes a video camera and video processing board, a telemetry protocol encoder chip, and an RF amplifier and primary coil. The intraocular unit consists of a secondary coil, a rectifier and regulator, a retinal chip with a telemetry protocol decoder, a stimulus signal generator, and an electrode array. This paper focuses on the design, fabrication, and testing of a microchip which serves as the telemetry protocol decoder and stimulus signal generator. It is fabricated by MOSIS with 1.2-mm CMOS technology and was demonstrated to provide the desired biphasic current stimulus pulses for an array of 100 retinal electrodes at video frame rates.