Strong, versatile little "nanotubes" made out of carbon are considered future stars in nanotechnology research in medicine and industry. Now a study finds that longer threads of the stuff mimic the toxic qualities of asbestos, renewing questions about how carbon nanotubes can be used safely.
Researchers with British institutes and the U.S.-based Project on Emerging Nanotechnologies injected mice with asbestos and with commercial samples of carbon nanotubes of varying sizes. When they examined the lining of the rodents' abdominal cavities, the researchers observed that longer nanotubes behaved like asbestos, provoking inflammation and lesions.
The study was reported Tuesday in Nature Nanotechnology, a scientific journal.
Carbon nanotubes are widely available for sale, but the study's authors and outside experts said they are not certain how extensively the materials have begun to be used in electronic gear, composite metal structures or consumer products. Such uses are expected eventually.
Because of that uncertainty, the researchers hope to pressure companies developing carbon nanotube-based materials to reveal whether they are using longer strands such as the ones that appear to act like asbestos — which was once a wonder material, too, before its cancerous consequences were discovered.
"I think it ups the stakes," said one of the authors, Andrew Maynard of the Project on Emerging Nanotechnologies, a partnership of the Woodrow Wilson International Center for Scholars and the Pew Charitable Trusts. "Up to this point we could talk hypothetically about the risks, but there wasn't enough there to demand action."
Vicki Colvin, a Rice University chemist who directs the National Science Foundation-funded Center for Biological and Environmental Nanotechnology, said carbon nanotubes' potential applications — such as cars that could be 80 percent lighter than today's models, but just as sturdy — are too powerful to ignore. She said the new study drove home the importance of making sure "we know how to handle it." Colvin was not involved in the new research.
The researchers acknowledged their work had limitations and called for more study.
For one thing, they put nanotubes directly into the abdomens of mice and stopped their experiments after a week — before seeing whether the nanotubes went on to induce mesothelioma, the cancer of the organ lining caused by asbestos exposure.
Mesothelioma is slow to develop; it can take 30 to 40 years in humans. But Vincent Castranova, chief of a pathology research unit at the National Institute for Occupational Safety and Health, said signs of the cancer would have been apparent in the mice after a month or two.
Whether that would have happened in a meaningful way is a vital question.
Castranova noted that earlier research in Japan, similar to Tuesday's paper, found that mice injected with carbon nanotubes did develop mesothelioma. But the doses of carbon nanotubes were so high that Castranova questioned the results.
And in research in his labs, in which mice are not injected with nanotubes but breathe it into their lungs — the way people would presumably be exposed — the animals developed inflammation that peaked within seven days of exposure, and returned to normal within one or two months.
"Whether the material is asbestos-like is still a question to be debated," Castranova said. "Having a panic that you have the next asbestos is a little bit premature in my view."
It's also worth noting that the new study did not find an asbestos-like effect with shorter or more tangled strands of carbon nanotubes. That does not mean smaller carbon nanotubes are necessarily safe. It just means that the asbestos-like effects in this experiment did not come from inherent properties of all carbon nanotubes. Rather, those effects came from stacking nanotubes together into a long, thin, asbestos-like fiber, which the body struggles to process.
Carbon nanotubes basically are minuscule, rolled pipes of graphite. They can be as narrow as one nanometer, or one billionth of a meter. (For comparison, a human hair is more than 80,000 nanometers across.)
Because their structure endows them with powerful physical properties, such as strength greater than that of steel, carbon nanotubes are being explored for a wide range of uses in electronics and medicine. Some potential applications involve coating the nanotubes in other substances, which could blunt any toxic effects.
For example, researchers have explored using nanotubes as the mechanism for delivering tiny, cancer-killing smart bombs to tumors. Stanford University scientists involved in such work found that coated, short carbon nanotubes — unlike the ones at issue in the asbestos study — were safely digested by mice after being injected into their bloodstreams.
Maynard said the combination of that research with his group's new study "shows there are no simple answers here. What type of materials you're using and what you're using them for makes a big difference."