By Dr David Whitehouse
BBC News Online science editor
Scientists have described their first complete design of an implant that will take the place of light-sensitive cells in the retina of a damaged eye.
Stacey Bent holds a large-scale prototype (Image by Linda A. Cicero)
Current implants use chips that convert light into electrical impulses that are fed to the brain via the optic nerve.
The new device will work differently. It will be placed on a damaged retina and convert light into chemicals that will stimulate nerve cells.
The prototype is being constructed at Stanford University in California.
Dr Stacey Bent of Stanford University calls the device "the holy grail of prostheses". It takes a new approach to replacing a damaged retina, the layer of cells at the back of the eye that detect light and send signals to the brain.
Trauma or disease can damage or destroy retinal cells. Over the past few years implants have been developed to replace them based on electronic chips that turn light into electrical pulses.
But there are difficulties in placing electronics into the eye.
Retinal nerve cells will be stimulated
"The problem with electronic implants is while they are very good it is difficult to make them biocompatible," Dr Bent told BBC News Online.
"What we a trying to do is a different approach from the current attempts using electrodes to stimulate nerve cells in the eye."
The new device works chemically rather than electronically.
"Instead of using electrical stimulation from a chip that converts light into electric impulses, we are using an implant that releases neurotransmitters just as the retina does naturally."
The researchers want light to strike the chip, causing it to release a small amount of neurotransmitter fluid that will stimulate retinal nerve cells.
The implant is to be made of a soft polymer material that will conform to the curvature of the back of the eye.
A key component will be retinal nerve cells that have been persuaded to grow behind the chip so that they can be stimulated effectively.
"We are working on pre-positioning nerve cells in a layer behind the implant," Dr Bent said. This way they will be in a better position to react to the stimulus from the light sensitive cells above them.
Based on work carried out in the past decade, scientists believe they know how to persuade retinal nerve cells to align.
The back of the implant will be coated with proteins designed to attract filaments that grow out of retinal cells.
It is hoped they will connect the implant to the optic nerve, so that signals can be sent to the brain.
Tests are planned of the device in the laboratory. It will be stimulated with light and the nerve cells monitored to see if they respond.
If they are a success, animal tests will follow.