Researchers aren't sure what makes carbon nanotubes such an effective template for stem cells--or how they help stem cells differentiate. But Webster says a likely answer to both questions is laminin, a glycoprotein in the brain's extracellular matrix that directs the generation of healthy nerve cells. The surface of carbon nanotubes resembles the elongated shape of laminin, and previous research has shown that nanotubes easily attract and adsorb laminin. Laminin, in turn, has a key amino acid sequence that attracts stem cells, stimulating them to turn into neurons. For these reasons, nanotubes may serve as pro-active delivery devices for stem cells.

"Mechanisms that promote neurogenesis and functional recovery are the keys to success in treating stroke," says Cindi Morshead, professor of anatomy at the University of Toronto, who studies stem cell therapies for stroke. Morshead sees Webster's findings as a potentially important step; however, she also cautions that there's a long way to go before such treatments are proven effective. "It is one thing to make new neurons, but it is entirely a different issue to create cells that are functionally integrated into existing networks that can promote brain repair and behavioral recovery," she says.

In future trials, Webster's team will examine behavioral effects over a longer time period, to see if new neurons in diseased areas stay and form long-lasting, functional connections. They will also compare nanotube templates with templates made of silicon and other polymers. Webster suspects that nanotubes will have an advantage over silicon because nanotubes' outer surfaces resemble natural proteins and tissues in the brain, preventing the body from rejecting them and forming scar tissue.

Todd Pappas, director of the Sensory and Molecular Neuroengineering Department at the University of Texas Medical Branch, who studies signaling neurons and nanotubes, says he thinks nanotubes will eventually be incorporated into prosthetic devices. "But we're still in the near term, and we've got to see what these materials do in the long term," he says.