Using space technology, scientists have developed extraordinary ceramic photocells that could repair malfunctioning human eyes.

by Steve Price and Dr Tony Phillips

Rods and Cones. Millions of them are in the back of every healthy human eye. They are biological solar cells in the retina that convert light to electrical impulses - impulses that travel along the optic nerve to the brain where images are formed.

Without them, we're blind.

Indeed, many people are blind - or going blind - because of malfunctioning rods and cones. Retinitis pigmentosa and macular degeneration are examples of two such disorders. Retinitis pigmentosa tends to be hereditary and may strike at an early age, while macular degeneration mostly affects the elderly. Together, these diseases afflict millions of people; both occur gradually and can result in total blindness.

"If we could only replace those damaged rods and cones with artificial ones," says Dr. Alex Ignatiev, a professor at the University of Houston, "then a person who is retinally-blind might be able to regain some of their sight."

Years ago such thoughts were merely wishful. But no longer. Scientists at the Space Vacuum Epitaxy Centre (SVEC) in Houston are experimenting with thin, photosensitive ceramic films that respond to light much as rods and cones do. Arrays of such films, they believe, could be implanted in human eyes to restore lost vision.

Image courtesy A. Ignatiev. A schematic diagram of the retina - a light-sensitive layer that covers 65% of the interior surface of the eye. SVEC scientists hope to replace damaged rods and cones in the retina with ceramic microdetector arrays.

"There are some diseases where the sensors in the eye, the rods and cones, have deteriorated but all the wiring is still in place," says Ignatiev, who directs the SVEC. In such cases, thin-film ceramic sensors could serve as substitutes for bad rods and cones. The result would be a "bionic eye."

The Space Vacuum Epitaxy Centre is a NASA sponsored Commercial Space Centre (CSC) at the University of Houston. NASA's Space Product Development (SPD) program, located at the Marshall Space Flight Centre, encourages the commercialisation of space by industry through 17 such projects. At the SVEC, researchers apply knowledge gained from experiments done in space to develop better lasers, photocells, and thin films - technologies with both commercial and human promise.

Scientists at Johns Hopkins University, MIT, and elsewhere have tried to build artificial rods and cones before, notes Ignatiev. Most of those earlier efforts involved silicon-based photodetectors. But silicon is toxic to the human body and reacts unfavourably with fluids in the eye - problems that SVEC's ceramic detectors do not share.

"We are conducting preliminary tests on the ceramic detectors for biocompatibility, and they appear to be totally stable" he says. "In other words, the detector does not deteriorate and [neither does] the eye."

[More] In 1996, during shuttle mission STS-80, astronauts use Columbia's robotic arm to deploy the Space Vacuum Epitaxy Centre's Wake Shield Facility.

"These detectors are thin films, grown atom-by-atom and layer-by-layer on a background substrate - a technique called epitaxy," continues Ignatiev. "Well-ordered, 'epitaxally-grown' films have [the best] optical properties."

Crafting such films is a skill SVEC scientists learned from experiments conducted using the Wake Shield Facility (WSF) - a 12-foot diameter disk-shaped platform launched from the space shuttle. The WSF was designed by SVEC engineers to study epitaxial film growth in the ultra-vacuum of space. "We grew thin oxide films using atomic oxygen in low-Earth orbit as a natural oxidising agent," says Ignatiev. "Those experiments helped us develop the oxide (ceramic) detectors we're using now for the Bionic Eye project."

The ceramic detectors are much like ultra-thin films found in modern computer chips, "so we can use our semiconductor expertise and make them in arrays - like chips in a computer factory," he added. The arrays are stacked in a hexagonal structure mimicking the arrangement of rods and cones they are designed to replace.

The natural layout of the detectors solves another problem that plagued earlier silicon research: blockage of nutrient flow to the eye.

"All of the nutrients feeding the eye flow from the back to the front," says Ignatiev. "If you implant a large, impervious structure [like the silicon detectors] in the eye, nutrients can't flow" and the eye will atrophy. The ceramic detectors are individual, five-micron-size units (the exact size of cones) that allow nutrients to flow around them.

Artificial retinas constructed at SVEC consist of 100,000 tiny ceramic detectors, each 1/20 the size of a human hair. The assemblage is so small that surgeons can't safely handle it. So, the arrays are attached to a polymer film one millimetre by one millimetre in size. A couple of weeks after insertion into an eyeball, the polymer film will simply dissolve leaving only the array behind.

The first human trials of such detectors will begin in 2002. Dr. Charles Garcia of the University of Texas Medical School in Houston will be the surgeon in charge.

These first-generation ceramic thin film microdetectors, each about 30 microns in size, are attached to a polymer carrier, which helps surgeons handle them. The background image shows human cones 5-10 microns in size in a hexagonal array.

"An incision is made in the white portion of the eye and the retina is elevated by injecting fluid underneath," explains Garcia, comparing the space to a blister forming on the skin after a burn. "Within that little blister, we place the artificial retina."

Scientists aren't yet certain how the brain will interpret unfamiliar voltages from the artificial rods and cones. They believe the brain will eventually adapt, although a slow learning process might be necessary - something akin to the way an infant learns shapes and colours for the first time.

"It's a long way from the lab to the clinic," notes Garcia. "Will they work? For how long? And at what level of resolution? We won't know until we implant the receptors in patients. The technology is in its infancy."

Ignatiev has received over 200 requests from patients who learned of the studies from earlier press reports. "I'm extremely excited about this," he says. He cautions that much more research is needed, but "it's very promising."