Tuesday, July 18, 2006

Metal coating provides long-life contact lenses

An antibacterial coating could let people keep their contact lenses in for months at a time, its inventor says.

Gas permeable contact lenses are approved for at most 30 days of wear. Even within that time, some lenses can build up a bacterial biofilm, which can obscure vision or cause eye infections.

But this will not happen if lenses are coated with the antibacterial metal selenium, according to Ted Reid of Texas Tech University Health Sciences Center in Lubbock, Texas.

In lab tests on rabbits, Reid found a one molecule thick layer of selenium kept lenses completely free of bacteria for more than two months.

Reid now hopes to use the coating to prevent bacterial growth on other internal devices like heart valves, stents to open blood vessels, or catheters.
Typical lunch

In high quantities, selenium is toxic to people, but Reid's coatings use less than one hundredth the amount of selenium found in a typical lunch. "I've put the contacts into my own eyes and had no damage," he says.

Selenium kills bacteria by catalysing the formation of superoxide radicals, which oxidise their cell walls. In human tissue, the metal pinches electrons from naturally present sulphur and donates them to local oxygen atoms, creating an endless supply of radicals.

These superoxides live only a few nanoseconds - long enough to rupture bacterial cells that might be sticking to the lens, but not long enough to diffuse through the tear layer from the lens coating to the cornea, says Reid. He is now planning to try the lenses in human volunteers to check they cannot damage the cornea.

If all goes well, coated lenses could be available to consumers in two years, he says.
Fighting HIV

When he mixed the combo with the virus in a test tube it lost its ability to infect human cells. Reid thinks superoxides oxidise the cell-recognition proteins on the virus sheath.

This means selenium-coated molecules might have a prophylactic ability, preventing someone who has been exposed to HIV from becoming infected, he says.

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