German and Hungarian researchers have brought sight to nine blind patients with hereditary retinal degeneration, using a subretinally implanted microelectronic chip with 1500 pixels.
The chip size is approximately 3mm x 3mm and is surgically implanted below the fovea (area of sharpest vision in the retina). It provides a diamond-shaped visual field of 15 degrees diagonally across chip corners. It is powered by a subdermal coil behind the ear that is powered from a battery via transdermal inductive transmission.
The core of the implant is a microchip with 1,500 pixels, each 70 x 70 microns. Photocells, an amplifying circuit, and a stimulation electrode are attached to each pixel. The photocells absorb the light entering the eye, transforming it into electrical signals. A tiny power line provides energy from the subdermal coil Sixteen additional electrodes are placed for testing purposes at the tip of the implant. The incoming light intensity controls the amount of current released by each electrode, stimulating the neighboring intact retinal nerve cells electrically. The nerve impulses generated by the retinal cells are processed in the remaining neuronal network of the retina and transmitted via the optic nerve to the visual cortex, creating visual sensations. An unimpaired, regularly functioning optic nerve is required.
“So far, our approach using subretinal electronic implants is the only one that has successfully mediated images in a trial with freely moving blind persons by means of a light sensor array that moves with the eye,” the scientists said. “All the other current approaches require an extraocular camera that does not link image capture to eye movements, which, therefore, does not allow the utilization of microsaccades for refreshing the perceived images.” In most hereditary retinal diseases, such as retinitis pigmentosa, the photoreceptors progressively degenerate, often causing blindness in adult life, and there is no therapy available to treat this disease.