Carbon nanotubes turn on water flow. (Materials Science: From Boston, at a meeting of the Materials Research Society).

Tiny tubes are everywhere in biology, from blood capillaries to channels in cell membranes. Now scientists are finding that synthetic carbon nanotubes have a property that could make them useful for modeling some of the body's narrowest channels.

Using computer simulations, Gerhard Hummer of the National Institutes of Health in Bethesda, Md., and his colleagues have shown that water molecules will quickly enter and flow through a carbon nanotube just 8 nanometers in diameter. A separate set of simulations shows that certain organic molecules also will course through such nanotubes.

In a first step toward understanding how molecules move through natural tubes, the scientist sought fundamental details of how water goes through the nanotubes. For instance, when 1,000 water molecules surround a 13.5-nm-long carbon nanotubes, five of the molecules rapidly enter the tube and form a transient chain. After a few nanoseconds, this chain exits the tube and is replaced by another chain, and so on. The nanotubes conduct water at a rate similar to that of certain channels in the kidneys, says Hummer.

The nanotube simulations are helpful for studying narrow flows in such biological venues as kidneys and cell membranes, says Hummer, who also reported on the water simulations with his colleagues in the Nov. 8 NATURE.

In related work, Hummer's colleague Shekhar Garde of Rensselaer Polytechnic Institute in Troy, N.Y., reported simulations showing that carbon nanotubes also can conduct methane and other organic molecules.

Someday, carbon nanotubes' unusual transport properties might be used in biomedical applications such as highly targeted drug delivery, Hummer speculates. --J.G.