Kotov is working with Pappas to develop retinal implants using nanomaterials. "Single-wall carbon nanotubes can be thin and compatible with the mechanics [muscle contractions] of the eye," says Kotov. Macular degeneration, a condition in which the light-sensing cells in the center of the retina break down, is the most common cause of vision loss in people over 65. To treat the disease, Kotov and Pappas hope to replace the light-sensing nerves with a combination of nanoparticles and carbon nanotubes that can sense light, convert it into an electrical signal, and send a message to the nerves that communicate between the eye and the brain. (These nerves remain intact in patients with macular degeneration.) Kotov says his nanoparticles "can even give color resolution."

But Kotov cautions that research should proceed very slowly to ensure the safety of any nanotube-based prosthetics. Researchers say that while carbon nanotubes appear to be inert and biologically harmless, their effects on the body have not yet been established experimentally (see "Tiny Toxins?").

Thomas Webster, associate professor of materials science and biomedical engineering at Brown University, is conducting some of the first experiments implanting carbon nanotubes in live animals. Webster and researchers at Yonsei University in Seoul injected a solution of carbon nanotubes and stem cells into stroke-damaged areas of rats' brains. "Without carbon nanotubes, the problem is that stem cells don't stay in the hurt area -- they migrate into healthy tissue," says Webster. In his experiments, the nanotubes helped the stem cells stay put, and their rough surfaces and conductivity coaxed the cells to develop into neurons. He says it's too early to find out "what happens to the materials after they do the job," though, or whether the nanotubes will have toxic effects in the long term.