- Nanotubes could be used to deliver drugs (Image: Zhegang Huang)
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"Breathing" nanotubes that contract and expand autonomously could be used to deliver drugs around the body.
Researchers have previously created molecules capable of automatically forming into tiny tubes, but these are just inert pipes. Now Myongsoo Lee of the Seoul National University in South Korea and colleagues have designed nanotubes that can expand and contract.
The tubes are built from layers of six bent molecules that self-assemble into the shape of a hexagon. When placed in water, the interlocking molecules of each hexagon slide closer together when heated and expand again when cooled, so variations in temperature cause the tubes to pulsate. "Our tubule is the first example that undergoes expansion and contraction without changing its tubular structure," says Lee.
Changes in the tube's thickness are due to the central element of the bent molecules, a substance called pyridine – a hydrocarbon with a nitrogen atom attached. The nitrogen atom attracts a cluster of water molecules that force the tube to expand. This attraction is disrupted when the tubes are heated to 60 °C, causing the tube to shrink from a thickness of 11 nanometres to 7 nanometres. Cooling reverses the process.
To test the tubes' new abilities, Lee and colleagues filled them with buckyballs, spherical carbon molecules, so they had something resembling peas in a pod. When the tubes were heated and shrunk, they spat out some of the buckyballs, suggesting they could be used as a nano-pump.
"If you can close off one of the ends you would be able to pump fluid or have a controlled release of the content," agrees Jan van Hest, a chemist at Radboud University Nijmegen in the Netherlands. "Drug delivery would be a possibility" he says, but points out that the biocompatibilty of the nanotubes would have to be investigated first.
It may also be possible to design similar nanotubes that react to changes in pH, rather than temperature, which would make them even more versatile, he says.