Carbon nanotubes – incredibly strong, electrically conductive, hollow molecules of carbon about a nanometer in diameter – have for more than a decade been prized by materials scientists. They’ve added them to batteries to increase their surface area and are developing light-emitting nanotubes for telecommunications.
Now University of Texas researchers have demonstrated that mats of single-walled carbon nanotubes can communicate electrical signals to neurons, suggesting that the tubes could be used as an electrical interface between neural prosthetics – devices used to replace damaged or missing nerves – and the body. This is good news for those hoping to use nanotubes to stimulate or replace nerve cells in the eye, brain, and spinal cord.
The Texas researchers grew rat neurons on thick mats of carbon nanotubes seeded on flexible plastic sheets. Instead of treating the mats like a foreign surface, neurons take well to the nanotubes, says Todd Pappas, director of sensory and molecular neuroengineering at the University of Texas Medical Branch, who led the research. The nanotubes absorb an important neural protein and form a roughly textured carpet on which nerves grow readily. When Pappas and colleagues at Rice University sent an electrical charge across the sheet, the neurons responded with an electrical signal of their own, called an action potential, indicating that they got the message.
[For images of nanotubes and neurons click here.]
An example of a neural prosthetic in use is the cochlear implant, which uses electrodes that respond to sound and send electrical signals directly into the brains of people with severe hearing loss. Likewise, neuroscientists are developing retinal prosthetics they hope will restore vision in the blind. The electrical interface in neural prosthetics usually consists of metal electrodes or silicon coated with metal, says Pappas.
If they’re proved safe for use in the body, carbon nanotubes may have advantages over traditional electrodes. Long-term implants can cause inflammation and scarring, because the body treats them like foreign material. In addition to carbon nanotubes’ advantages of strength, flexibility, and conductivity, their surfaces can be covered with molecules that look friendly to cells.
Pappas says researchers would like nanotubes to mimic the kind of support neighboring cells offer one another, although they are “not yet sure what cells want.” Scientists might try attaching molecules that encourage growth and stability, for example. “Surface modifiers need to be chosen so that the cell considers the nanotubes part of its natural [environment],” says Nicholas Kotov, an associate professor of chemical engineering at the University of Michigan.
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.