Researchers say they may soon be able to repair injured and worn-out cartilage with the help of nanotubes. Currently, patients must either go under the knife to mend faulty cartilage (connective tissue that normally pads the ends of bones at joints to keep them from grinding against one another). But scientists say they may one day be able to insert microscopic carbon nanotubes into injured joints—such as knees—encouraging new, stronger cartilage cells to grow in place damaged or thinning ones.

Researchers report in the Journal of Biomedical Materials Research Part A that they successfully grew cartilage around carbon nanotubes in their lab—and are optimistic that one day they will be able to duplicate the feat inside the human body. They may get a step closer in September, when they plan to implant carbon nanotubes in sheep joints to test—for the first time—their technique outside the lab.

Thomas Webster, an associate engineering professor at Brown University, and Brown researcher Dongwoo Khang, along with Grace Park, a research scientist (and one of Webster's former PhD students) at Becton, Dickinson and Company, a Franklin Lakes, N.J.–medical technology firm, say they grew cartilage cells by placing chondrocytes (cartilage-forming cells) and carbon nanotubes together on a polycarbonate urethane surface. As expected, cartilage cells grew around the nanotubes, which are so strong that scientists now use them to reinforce plastic. Researchers say they hastened new cell production by sending electrical surges through the nanotubes, which are also excellent conductors of electricity.

Scientists envision implanting nanotubes through small incisions (in, say, a knee) that a patient's own cartilage cells would colonize. The benefit, Webster says, is that the cartilage would grow more quickly and be stronger than if it was not supported by nanotubes—similar to the way that steel rebar is used to reinforce cement or concrete. He notes that nanotubes would adhere well to existing cartilage. "The patient will have a faster return [than if they used cartilage without nanotubes] to an active lifestyle that they probably have not had in a long time," he says.

Orthopedic surgeons commend Webster's research but say it is too early to tell whether his approach will be successful. Although the idea of creating stronger, better-adhering cartilage sounds good, placing permanent particles such as carbon nanotubes inside the joint may introduce other problems, says Freddie Fu, chairman of the Department of Orthopaedic Surgery at the University of Pittsburgh's School of Medicine and its Medical Center. His major concern: that carbon nanotubes may not be biologically compatible with existing cartilage tissue in the joint.

"Ideally, the architecture of the scaffold should mimic that of the native tissue to be repaired," says Wei Shen, a postdoctoral research associate also in the department.

Scientists for decades have been seeking ways to repair cartilage without resorting to traditional surgery, which typically involves removing damaged cartilage through an incision in the joint while trying to preserve as much of the healthy tissue as possible. One breakthrough was the development in 2002 of gels made of synthetic materials such as polyhydroxyethyl methacrylate, which can be injected into a joint where it solidifies and becomes a cushion with the same shock-absorbing function as cartilage. The problem is, Webster says, the gel pads do not always adhere well to the remaining cartilage in the joint, which means that many patients require follow-up injections. Another concern, Fu says, is that even if the gel sticks it may not last long.

Researchers at the Massachusetts Institute of Technology, Harvard Medical School and the University of Colorado at Boulder are currently trying to develop a gel into which they could place a patient's own cartilage cells that would reproduce once the solution was injected into a target joint. This approach, however, does not use carbon nanotubes, which Webster believes would provide a more durable fix.

Webster has been studying the possibility of growing tissue around nanomaterials since 1998 when he was a graduate student at Rensselaer Polytechnic Institute in Troy, N.Y. "Nanoscale materials are increasing growth in all of these tissue types," he says. "The key is getting the nanomaterials to mimic the roughness of the natural tissue, which creates more surface energy and allows for the absorption of proteins important for the tissue to function."

Webster has come a long way since his original experiments with in vitro bone tissue growth. Over the past decade, he added bladder, cartilage, central nervous system, and vascular tissue growth to his repertoire. The principle is the same in each: Growing cells are more likely to adhere to and thrive on a rough nanotube surface than on smooth bone or fraying cartilage. He is now working with a team of 26 biomedical and tissue engineering researchers at Brown, armed with a $500,000 grant from the National Science and Technology Council's federal National Nanotechnology Initiative, to see how far he can push his ideas.

"The use of nanotechnology in scaffolds to assist with regenerating cartilage is novel," says Constance Chu, director of the University of Pittsburgh Medical Center's Cartilage Restoration Program and an orthopedic surgeon specializing in cartilage regeneration and osteoarthritis, "and would be of high interest if it can eventually improve the functional properties of the regenerated cartilage."