Orthopaedic implants that monitor the healing process and speed up bone growth may be achievable using carbon nanotubes, say researchers at Brown University in Providence, USA.

Growth spurt: Carbon nanotubes on a titanium implant‘Carbon nanotubes (with a battery in the implant) are able to conduct electricity. So, by measuring the conductivity between two individual carbon nanotubes, we can get the resistance of what is there, which will tell us if it is bone (more conductive), infection (less conductive) or scar tissue (even less conductive),’ explains Tom Webster, Associate Professor of Engineering at Brown University.

Scientists anodised the surface of a titanium implant with hydrofluoric acid and applied an electric current to it, causing pits hundreds of nanometres thick to form on the surface. A cobalt catalyst was applied to the pits and then heated, causing carbon nanotubes to form. Human bone cells added to the implant grew twice as fast as on an untreated titanium surface, possibly due to the electrical stimulation, say researchers. Equally important, the nanotubes also revealed what was growing on the implant.

This information can be transmitted to a handheld device using radio frequencies, much like the technology used in pacemakers.

Currently, X-rays or bone scans are used to monitor implants, but, says Webster, such tests are often not sensitive enough to measure new bone growth. They are also performed infrequently, adds Dr David Farrar, Biomaterials Technology Manager at the Smith & Nephew Research Centre in Heslington, UK.

He says, ‘Doctors might bring a patient back to look at the implant once, but they wouldn’t do a regular series of tests to find out if it is well enough for the patient to start playing golf again. If you could get that feedback in real time, that would be very exciting.’

However, cautions Farrar, it is too soon to get worked up about this technology. ‘So far, the [Brown University] team has only shown that their coating works with bone cells in the lab. The real test will be to see how it performs in the body, which is a much more complicated system.’

Farrar also has concerns about the use of carbon nanotubes. ‘It is early stages in terms of nanotubes and whether they are safe to use in the body. I think when they are immobilised on the surface, as in this case, then perhaps there’s less risk than if they’re allowed to freely float around the body,’ he says. ‘But we do know that very small debris particles can be released from orthopaedic implants and cause problems. So I would be worried about the potential of the nanotubes to break off from the surface.’

Webster notes that in his team’s in vitro tests, the purer carbon nanotubes (containing fewer unreacted catalysts), seemed to be less toxic. He is hoping to carry out animal testing within the next year.

Future intelligent implants could react to medical problems by releasing drugs directly from the implant, adds Webster. This could treat infection, inflammation or address a lack of bone growth.