The implants deliver a protein, known as nerve growth factor (NGF), directly to nerve cells in the brain, which stimulates their growth and keeps them from degenerating.

NGF controlls cell regeneration in the rest of the body, but is blocked by the body from entering the brain, so that normally brain cells have no capacity for self-regeneration.

A team from Cornell University, New York, has developed the technology, which has been tested on animals.

Tested on animals

Dr Mark Saltzman and his colleagues have succeeded in releasing NGF via pea-sized pellets implanted in the brains of test animals.

They believe it could be possible to implant them into the area of the human brain vulnerable to Alzheimer's Disease.

NGF has proved useful in the treatment of other diseases: It has been delivered by eye-wash to treat retinal ulcers, and by injection into the bloodstream to treat diabetic neuropathy - a peripheral nerve disease that causes loss of feeling in the extremities.

Better solutions

Harry Cayton, executive director of the Alzheimer's Disease Society, was unimpressed by the US research.

He said: "This does not have very obvious therapeutic possibilities for humans.

"Brain implants are going to be expensive, risky and complicated. We think there are much more useful therapeutic processes being researched in this country which involve sending chemical messages to the brain by taking a pill.