BioEd Online

A bionic eye that allows blind people to see has now got a protective coat of diamond that should significantly improve its performance.

The silicon chip retinal implant is being developed by Second Sight, a company based in Sylmar, California, along with a consortium of university researchers. The device needs a hermetic case to prevent it from reacting with fluids in the eye.

"It's as if you're throwing a television into the ocean and expecting it to work," says the company's president, Robert Greenberg. "The approach until now has been to lock it in a big waterproof can, but it's very big and bulky," he explains.

So researchers have developed an ultrananocrystalline diamond (UNCD) film that is guaranteed to be safe, long-lasting, electrically insulating and extremely tough. The coating can also be applied at low temperatures that do not melt the chip's microscopic circuits.

The UNCD film is the first coating to meet all the necessary criteria for the implant, says Xingcheng Xiao, a materials scientist at Argonne National Laboratory, Illinois, who developed the film.

The tiny diamond grains that make up the film are about 5 millionths of a millimetre across. They grow from a mixture of methane, argon and hydrogen passing over the surface of the five-millimetre-square chip at about 400 °C. Xiao and his colleagues have already tested the implants in rabbits' eyes, and saw no adverse reaction after six months. He will present the results on 1 April at the Materials Research Society meeting in San Francisco, California.

Lighter load

A healthy retina contains rod and cone cells that convert light into electrical impulses, which fly to the brain through the optic nerve. But for millions of people with diseases such as retinitis pigmentosa or macular degeneration, these cells do not work properly.

The retinal implant bypasses these diseased cells by electrically stimulating healthy cells that sit beneath them at the back of the eyeball. The patient 'sees' using a pair of glasses carrying a tiny video camera that sends digitized images to the implant using radio waves.

The first human trial of Second Sight's artificial retina has been running since 2002, and it has enabled a formerly blind patient to distinguish between objects such as cups and plates, and even to make out large letters. But with only 16 electrodes, the device does not allow the patient to see a clear picture. For that, thousands of electrodes are needed on the same size of chip, making it even more delicate.

In the first trial, the electronics package was separated from the electrodes and implanted behind the patient's ear because its casing was so bulky, explains Greenberg. The team's goal is to integrate everything into a single unit that fits neatly into the eye. Xiao adds that a UNCD coating could be equally useful for other implantable devices, such as biosensors to monitor a patient's health.

"They've managed to create a diamond film that is of considerably higher quality than other methods have managed," says Doug Shire, a materials engineer at Cornell University in Ithaca, New York, who is part of the Boston Retinal Implant Project. "But the ultimate potential of these devices will only be told in long-term trials," he adds.

Second Sight is planning clinical trials of a 60-electrode device this year, but it may be several more years before the diamond-coated chip is used in humans, says Greenberg.