Gel put springiness back into old lenses

A neat fix for ageing eyes could be tested in humans next year. The treatment, which involves replacing the contents of the lens in the eye with a soft polymer gel, could allow millions of people to throw away their reading glasses.

“At first, we see it being used as an improvement to current cataract surgery,” says Arthur Ho at the University of New South Wales, a key member of the Australian government’s multinational Vision Cooperative Research Centre (Vision CRC) that is working on the technique. “But once it is shown to be safe and effective, we think that more and more younger people who are starting to need reading glasses will adopt it as well.”

The eye’s lens focuses by changing shape. When muscles in the eye relax, the lens is pulled flat to focus on distant objects. When they contract, the lens returns to a fatter shape, bringing closer objects into focus. But as we age, our lenses harden, preventing them reforming into their fatter shape: the lenses of 40-year-old people have only a quarter of their capacity to change shape, or “accommodate”, as they did at birth. After the age of 45, most people need reading glasses, or bifocal glasses.

In the late 1980s, Jean-Marie Parel at the University of Miami showed that replacing the contents of an ageing rhesus monkey’s lens with silicone oil could restore its ability to focus. But silicone oil gradually leaks from the lens capsule. Since then, researchers have been working to develop a polymer that has both the same refractive index as the human crystalline lens and also the right biomechanical properties.

After evaluating more than 30 different polymer formulations created at another Australian research institute, CSIRO Molecular Science in Melbourne, Ho thinks his group has cracked it. Vision CRC is staying tight-lipped about the new formulation while it is being patented. All the team will say is that it is a siloxane-based material, which is cured with UV or visible light after injection to turn it from a liquid to a gel.

Tests on rabbit and monkey eyeballs show the latest formulation has the same average refractive index as a healthy lens and could provide up to 9.5 dioptres of accommodation, enough to allow reading at a distance of only 10 centimetres. So far, the group has conducted tests on only a few human eyes and found the treatment provided between around 6 to 8.5 dioptres of accommodation- more than enough to permit easy reading.

Implanting the gel would be very similar to current cataract surgery, except that the lens is not replaced. Instead, after making a small incision in the cornea, a doctor would cut a tiny hole in the lens capsule and suck out the contents. The gel, which has the consistency of thick oil, is pumped in and a burst of UV or visible light transforms it into a jelly. “This could be a quick, 15-minute procedure,” says Ho.

The gel-injection surgery has been perfected on rabbits by a team led by Parel in Miami. But adult rabbit eye lenses do not accommodate, so the next step is to assess the technique in live rhesus monkeys, which have similar eyes to humans. This work should begin by the end of this year, Ho says, “and in the best-case scenario, we should begin human trials by the end of next year.”

Hugh Taylor, director of the Centre for Eye Research Australia in Melbourne, is enthusiastic. “This technique has enormous potential,” he says. “It will totally change ophthalmic surgery, if we can get it right.” New Scientist issue 9 August 2003.