Scientists at the Space Vacuum Epitaxy Centre (SVEC) in Houston are experimenting with thin, photosensitive ceramic films that may one day be implanted in human eyes to restore lost vision.
‘There are some diseases where the sensors in the eye, the rods and cones, have deteriorated but all the wiring is still in place,’ said Dr. Alex Ignatiev, a professor at the University of Houston who directs the SVEC. In such cases, thin-film ceramic sensors could serve as substitutes for bad rods and cones.
Scientists have tried to build artificial rods and cones before using silicon-based photodetectors. But silicon is toxic to the human body and reacts unfavourably with fluids in the eye.
‘We are conducting preliminary tests on the ceramic detectors for biocompatibility, and they appear to be totally stable’ he said. ‘In other words, the detector does not deteriorate and neither does the eye.’
‘These detectors are thin films, grown atom-by-atom and layer-by-layer on a background substrate – a technique called epitaxy,’ added Ignatiev. ‘Well-ordered, ‘epitaxally-grown’ films have the best optical properties.’
The ceramic detectors are said to be much like ultra-thin films found in modern computer chips. ‘We can use our semiconductor expertise and make them in arrays – like chips in a computer factory,’ added Ignatiev. 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 reportedly 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,’ said 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. 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 (UTMS) in Houston will be the surgeon in charge.
‘An incision is made in the white portion of the eye and the retina is elevated by injecting fluid underneath,’ explained 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.’