Spider silk darkened with a coating of carbon nanotubes can tell if your heart just skipped a beat.

Spider silk darkened with a coating of carbon nanotubes can tell if your heart just skipped a beat.

Following a few simple steps, researchers have made a silk-nanotube hybrid that is tough, flexible and electrically conductive. The material might find uses in a range of bendy medical sensors.

Long known as one of nature's toughest and most flexible materials, spider silk is not naturally conductive. Scientists have previously married metals such as gold with spider silk, but those hybrids didn't allow the silk to stretch as much as usual.

To create a conductive but less rigid silk, Eden Steven at Florida State University in Tallahassee collected bundles of silk from a species of golden orb-weaver spider. He polarised a powder of carbon nanotubes so that the tubes would stick to the naturally charged silk, then mixed the materials with a few drops of water and pressed them between two sheets of Teflon.

Wrap, shrink

When the material dried out, the silk was coated with a thin layer of nanotubes. This composite is three times tougher than spider silk alone. As the silk naturally expands and contracts when exposed to different humidity levels, the new, flexible hybrid can be easily manipulated to create good electrical contact for wiring. "We simply wind the coated fibre around the contact area and, by controlling the humidity, we can let it shrink. The wire grips the contact area without having to use a conducting paste or solder."

The carbon-silk combination is also sensitive enough to detect the electrical signals from a heart pulse.

Commercially available pulse-detectors are often made of rigid materials. By contrast, the silk-based version can be wrapped around irregularly shaped objects, such as wrists or fingers, without losing sensitivity.

Kitchen simplicity

"These results open new opportunities in moulding and shaping actuators or sensors, where you could potentially think about different geometries or forms," says bioengineer Kimberly Hamad-Schifferli of the Massachusetts Institute of Technology. There are other methods of combining carbon nanotubes with biological materials, she adds, but they usually require expensive equipment and chemicals, and the end result is not mouldable.

"What's really astonishing is that the method of incorporation of the carbon nanotubes is incredibly simple," she says. "It looks like something you could do in your kitchen at home."

Scaling up production may be a challenge, though, as it is hard to farm spider silk in large amounts. But there has been recent progress making synthetic silk, Steven says, which could pave the way for large-scale production.