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RESTORING vision with silicon chips designed to be implanted in the eye may
be even harder than anyone thought. The first long-term study of such a chip has
shown that it corrodes within a year.

Various groups are working on devices for restoring limited vision to people
whose light-sensing cells have been damaged by disease. Most are microchips
designed to be implanted under the retina. When they detect light, they directly
stimulate the underlying retinal neurons via an array of microelectrodes.

The chips tested by Martin Stelzle of the Natural and Medical Sciences
Institute in Reutlingen, Germany, and his colleagues are protected by a silicon
oxide coating about 1 micrometre thick. Previous tests in tissue growing in
culture suggested there would be no problems with corrosion.

But when Stelzle tested them in live animals, the results were rather
different. His team implanted nearly 50 chips beneath the retinas of pigs and
rabbits, extracting the chips after up to 28 months. The researchers found that
while the surrounding tissue was not damaged, the longer the chip was in the
eye, the more it deteriorated.

Within six months, the coating began to corrode, exposing the underlying
silicon-based electronics. After a year, the coating had vanished and the chip
was heavily pitted. 鈥淭he chip will function for at least six months, maybe even
longer, but after that the electrical performance will also degrade,鈥� says
Stelzle. Elsewhere in the body, where chips will be exposed to immune attack,
the problems could be even worse.

The team does not know the precise reason for the damage. But Stelzle says
that because of current technology, all chips of this type have a silicon oxide
or silicon nitride coating鈥攁nd initial tests in culture suggest that
silicon nitride is also corroded. New materials will have to be found, he
says.

Stelzle鈥檚 chip hasn鈥檛 been implanted in people. But Optobionics of Wheaton,
Illinois, has implanted similar ones in six people, three in June 2000 and three
in July 2001. Chief information officer David McComb says they are still fine.
鈥淭he implants are stable, in position, and functioning electrically,鈥� he says,
though Optobionics won鈥檛 say if they have restored any vision.

Even if such chips can survive in the body, many experts doubt if it will be
feasible to squeeze enough electrodes onto them to provide decent vision. Other
groups are looking at different methods, such as a coil that wraps round the
optic nerve and stimulates it with a focused electromagnetic field.