German firm Retina Implant is responsible for the chip that has restored limited sight in experiments with a number of people blinded by retinitis pigmentosa and similar diseases.
Key to the development, is that these diseases destroy only the eye's photoreceptors, and leave its layers of image processing neurons intact.
In the eye, photoreceptors are right at the back.
Bio-electrical signals exit forwards, into transparent layers of neurons and connective nerves that lie on top.
The chip, which is 3mm across and 50µm thick, has a 70µm pitch 40x40 photodiode array, plus a corresponding electrode array.
By sliding this under the neuron layer, between the neurons and the damaged photoreceptors, the implant can both receive images focused by the eye's existing optical system, and inject signals directly into the original neurons.
According to the company, photodiodes of this size cannot produce sufficient power to directly stimulate neurons, so the chip includes signal conditioning, and has to receive power from an external source.
This is provided along a cable that exits through the side of the eyeball and is led to an inductively coupled power receiver under the skin near the subjects ear.
"Assigned to each pixel field are two photocells, one amplifying circuit and a stimulating electrode," said the firm. "The microchip yields a visual field of about 12°, allowing mobility and object recognition in space."
Further experiments are planned where power will be coupled into, or transmitted to, the chip directly.
