A bionic eye that can restore sight to the blind should be commercially available within two years, scientists behind the revolutionary technology announced today.

The sophisticated Argus II artificial retina has been cleared by American regulators to begin trials on between 50 and 75 people suffering from two of the most common causes of blindness, paving the way for millions more to benefit from similar implants in the future.

If the research progresses well, a device could be on the market early in 2009 at a likely cost of about £15,000, according to Mark Humayun, Professor of Ophthalmology at the Doheny Eye Institute, a part of the University of Southern California.

An early version of the prosthetic retina has already been fitted to six patients with retinitis pigmentosa, a degenerative and incurable eye condition that affects one in 3,500 people. All have recovered the ability to detect light and motion, and even to make out large letters and to distinguish between objects such as a cup, a knife and a plate.

The second generation device that is now starting trials should provide even better vision, as it contains 60 light-sensitive electrodes compared to just 16 in the previous model.

Further improvements are expected to follow within five to seven years, with a 1000-electrode implant that will enable previously blind people to recognise faces, Professor Humayun said.

“The ultimate aim to allow people recognise faces, and to allow the completely blind to get around on their own,” he told the American Association for the Advancement of Science conference in San Francisco.

“The first phase began in 2002, and the results were not what we expected: we thought they would only see light and dark, but they have done far better than that.

“They can differentiate between a cup, a plate and a knife. They can see motion. They can avoid stumbling around into large objects. That is just with 16 electrodes, and we’re now going up to 60. The models suggest 1,000 will be enough for face recognition, and we hope to get there in five to seven years.”

The bionic eye consists of three elements. First, a miniature camera worn in a pair of dark glasses, which transmits images to a radio received implanted next to the patient’s eye.

This then sends a signal on to a tiny chip of silicon and platinum, about 4mm square, that sits on the retina. The chip’s electrodes stimulate the ganglion cells that transmit visual information to the optic nerve and onwards to the brain, which can then construct a visual image.

“A plate is seen as a saucer of light, and a knife as a runway of light,” Professor Humayun said. “It works by building up images like a dot matrix printer, or pixels on a computer screen.” The implant is suitable for people who are blind because they have lost the photoreceptor cells known as rods and cones that respond to light - the electrodes effectively provide artificial replacements. This includes those with macular degeneration -- the commonest cause of blindness, which affects up to 15 per cent of over-75s - who are also eligible for the new trial.

The technology cannot restore sight to patients who are blind because of severe optic nerve damage, such as that caused by glaucoma, or because of a stroke in the visual cortex.

Professor Humayun said it would also work better for people who have been able to see as older children or adults, than for those who have been blind since birth or shortly afterwards.

It generally takes patients a month or two to get used to the Argus device, before their brains learn to interpret the images. While the operation to install it originally took seven hours, it now takes 90 minutes.

In the first phase of the trials, patients were able to use the implant only in the laboratory, though for the past year they have also been allowed to try it at home. “Perhaps what we’re most excited about in this next study is we will be able to test the new device with patients at their homes, churches, schools and similar locations,” Professor Humayun said.

Patients fitted with the artificial retina “see” in real time, though their field of vision is very small. This should be improved by the larger network of electrodes in the new version.

They have also reported seeing vibrant colours, though these are thought to be artefacts generated by the brain and not a reflection of the outside world, as the electrodes are not designed to pick up colour. In the longer term, it may be possible to tune electrodes to respond to light of different wavelengths, allowing genuine colour vision.

The device has been developed in partnership with a company called Second Sight, and Professor Humayun said it would probably cost around £15,000. “These devices do cost a lot, and a fair benchmark would be a cochlear implant, which costs around $30,000,” he said.

The trials will be conducted at five centres in the US, on patients over 50. The US Food and Drug Administration has insisted on older subjects as they have less to lose if the experiments go wrong. Thousands of blind people have already volunteered.

The number of people in the UK estimated to suffer from age-related macular degeneration is estimated at 400,000.

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