By Jonathan Fildes
Science and technology reporter, BBC News, San Francisco
A bionic eye implant that could help restore the sight of millions of blind people could be available to patients within two years.
A receiver under the eye surface passes the signals back to the chip
US researchers have been given the go-ahead to implant the prototype device in 50 to 75 patients.
The Argus II system uses a spectacle-mounted camera to feed visual information to electrodes in the eye.
Patients who tested less-advanced versions of the retinal implant were able to see light, shapes and movement.
"What we are trying to do is take real-time images from a camera and convert them into tiny electrical pulses that would jump-start the otherwise blind eye and allow patients to see," said Professor Mark Humayun, from the University of Southern California.
BIONIC EYE TECHNOLOGY
1: Camera on glasses views image
2: Signals are sent to hand-held device
3: Processed information is sent back to glasses and wirelessly transmitted to receiver under surface of eye
4: Receiver sends information to electrodes in retinal implant
5: Electrodes stimulate retina to send information to brain
Retinal implants are able to partially restore the vision of people with particular forms of blindness caused by diseases such as macular degeneration or retinitis pigmentosa.
About 1.5 million people worldwide have retinitis pigmentosa, and one in 10 people over the age of 55 have age-related macular degeneration.
Both diseases cause the retinal cells which process light at the back of the eye to gradually die.
The new devices work by implanting an array of tiny electrodes into the back of the retina.
A camera is used to capture pictures, and a processing unit, about the size of a small handheld computer and worn on a belt, converts the visual information into electrical signals.
These are then sent back to the glasses and wirelessly on to a receiver just under the surface of the front of the eye, which in turn feeds them to the electrodes at the rear.
The whole process happens in real time.
First-generation, low-resolution devices have already been fitted to six patients.
"The longest device has been in for five years," said Professor Humayun.
"It's amazing, even with 16 pixels, or electrodes, how much our first six subjects have been able to do."
Terry Byland, 58, from California was fitted with an implant in 2004 after going blind with retinitis pigmentosa in 1993.
"At the beginning, it was like seeing assembled dots - now it's much more than that," he said.
"When I am walking along the street I can avoid low-hanging branches - I can see the edges of the branches."
Mr Byland is also able to make out other shapes.
"I can't recognise faces, but I can see them like a dark shadow," he said.
The new implant has a higher resolution than the earlier devices, with 60 electrodes.
It is also a lot smaller, about one square millimetre, which reduces the amount of surgery that needs to be done to implant the device.
The technology has now been given the go-ahead by the US Food and Drug Administration to be used in an exploratory patient trial.
This will take place at five centres across America over two years, with 50-75 patients aged over 50.
If successful, the device could be commercialised soon after, costing around $30,000 (£15,000). Other devices could then be developed with higher resolution or a wider field of view, said Professor Humayun.
Future work includes studying the effects the implants have on the brain.
"We are actually studying what happens to the visual cortex over time," said Professor Humayun.
The research was presented at the American Association for the Advancement of Science (AAAS) annual meeting in San Francisco, US.