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Carbon nanotube fibres seen interacting with E. coli bacteria for over 15 days. ((Rensselaer Polytechnic Institute/ Ganesan and Raja) )

A super-strong nanomaterial with potential industrial and medical applications inhibits growth in mammalian cells while sustaining the growth of bacteria, according to two studies.

Researchers at Rensselaer Polytechnic Institute in Troy, N.Y., also found the size, type and concentration of carbon nanotubes changed their interaction with living cells.

The authors suggest the seemingly contradictory results suggest the environmental and health impacts of the nanomaterial are more complex than first thought and demand further study.

The first study, published in the journal Toxicology Letters, found that the material inhibited the growth of heart muscles in rats.The effect was seen with large clusters of nanotubes and even more so when the nanotubes were finely dispersed.

The second study found the material had an opposite effect on bacteria, actually sustaining growth of samples of Escherichia coli (E. coli).

Both studies were presented Sunday at the 233rd American Chemical Society (ACS) National Meeting in Chicago.

"It appears that in order to see a real environmental impact of nanomaterials, a significantly long duration study similar to ours is needed to get further insight into the physiology of biological interactions in general, and bacterial interactions in particular," said Pavan Raja, a doctoral candidate in chemical and biological engineering who worked on both research teams, in a statement.

Carbon nanotubes are cylindrical carbon molecules, thousands of times narrower than the width of a human hair. They can be several millimetres in length.

Their conductivity and strength have made them the subject of research for potential industrial applications, including use in electronics, computer chips and optics.

Carbon nanotubes are also considered an ideal mechanism for drug delivery, but for reasons still unclear the material is toxic to mammalian cells. Past research has focused on combining the material with synthetic polymers to create a non-toxic material.

A study last year by the University of London's School of Pharmacy also showed water-soluble carbon nanotubes would flush out of mice without being retained in the organs, raising thehopes the material could be used safely.

But the potential environmental impact of the material is still unknown. Lab experiments at Georgia Institute of Technology published last year found multi-walled carbon nanotubes can remain suspended with water containing organic materials for more than a month, raising questions about their impact if they are spilled into the environment.

Jaehong Kim, an assistant professor in the Georgia Tech School of Civil and Environmental Engineering, told the Georgia Tech Research News in December a new emphasis must be placed on how these materials are stored.

"We have to continue to study nanomaterial transport in the lab using various [natural organic matter] sources to try to better understand their potential interaction in the natural environment," he said.