BECAUSE they are both strong and lightweight, composite materials made from carbon fibres are the darlings of engineers in the aerospace industry. Unfortunately, such materials deteriorate over time. Wind and rain attack the glue that sticks the layers of carbon fibres together. As a consequence, the layers peel away from one another. Many people have tried to solve this problem, without success. A new method aims to do so by stitching the carbon-fibre layers together.

Brian Wardle of the Massachusetts Institute of Technology and his colleagues noted that past attempts to reinforce composite materials, by sewing the layers together as well as gluing them, often ended up damaging the fibres because the metal needles and threads were so thick. Although the sewing did help the composite stay together, its strength was diminished. Dr Wardle reasoned that if a needle and thread could be made thin enough, such damage could be eliminated.

Carbon nanotubes, which are just billionths of a metre across, seemed the perfect needle. Because these tubes are a thousandth of the diameter of carbon fibres, they can slip into the microscopic spaces between them.

Finding a thread for the nanoscopic needle to pull required more thinking. Dr Wardle and his colleagues started exploring the interactions between carbon nanotubes and polymer glues. They discovered that when the nanotubes were just touching the glue, they sucked it up by capillary action, the same physical mechanism by which plants draw water from the soil. Because the nanotubes drew glue into themselves, they could be used as both needles and threads.

The team placed polymer glue between two carbon-fibre layers and heated it. They then placed tens of trillions of nanotubes on each of the carbon-fibre layers such that the tubes could just touch the glue. When the glue was hot and runny, the carbon nanotubes sucked it up through the carbon-fibre layers. The layers were then pressed together and the glue was allowed to cool, before the material was tested for strength and toughness.

Dr Wardle and his colleagues found that the technique would make aerospace parts ten times stronger than at present. The nanotubes themselves are cheap and, as a happy coincidence, having them run perpendicular to the carbon-fibre layers makes the composite over a million times more electrically conductive than it otherwise would be. That means aeroplanes built with such materials would be able to disperse lightning strikes rapidly, avoiding electrical damage during storms as well as being better protected against whatever wind and rain can throw at them.