Circuit design for a subretinal stimulator
In this project we develop CMOS circuits optimized for the usage in medical implants. The emphasis is on low-power, optimized silicon area, and optimized performance.
As a first example we focus on an active pixel array chip for subretinal implants. In Germany, every year 17,000 people become blind, and about 4000 of those suffer from degenerative retinal diseases, which are called Retinitis pigmentosa (RP) and age-related macular degeneration (AMD). From the different nerve layers in the retinal tissue, only the photoreceptors (rods and cones) of the retina perish.
To restore some visual perception, the retina can be stimulated from either side (epiretinal or subretinal). With a subretinal light sensitive chip, the natural optical system of the eye is still used, allowing the patient to intuitively locate objects. The space beneath the retina is limited. Either just an electrode array can be placed there, or a silicon chip thinned to about 70mm. Experiments from our project partners used a device including a light sensitive CMOS chip and separate direct stimulation electrodes. The device was connected to external power supply by a polyimide-ribbon forming a flexible connection into the eyeball. Feasibility of the wired supply approach for humans was proven, and no perception of the ribbon was reported by the patients.
We work on the design of a next generation subretinal chip. Main concerns are power and stimulation efficiency, and lifetime. Stimulation voltage swing is nearly doubled from +2V to app. ±2V, whereas the power supply is changed from +3VDC to ±2VAC. A digital controller is added to address the pixels sequentially, which reduces the peak supply current. The chip will be connected to a separate supply box using the well proven polyimide-ribbon. The box will be implanted behind the ear and wirelessly supplied from outside like a cochlear implant. To minimize chemical reactions on the periphery, no DC-potential should appear at any terminal of the chip.
A first prototype chip has been published at the ISSCC 2008 in San Francisco.
For the stimulation of the nerve cells, Titanium Nitride electrodes are deposited on top of the chip. These electrodes need to have a large surface to optimize the charge transfer into the nerve tissue.