Spider silk: its threads are stronger than steel of the same size, it can stretch and absorb water. Some kinds of spider silk can transmit light like optical fibers, or form fibers that can be woven into body armor. Now it's being turned into flexible electrical conductors.
Researchers at Florida State University's National High Magnetic Field Lab have found a way to coat spider silk with a very thin layer of carbon nanotubes, which are also stretchable and flexible but conduct electricity as well as a copper wire. Such a conductor could work as a sensitive pressure detector or be a component in electronics designed to be implanted in the body.
The Florida State team have already built a heartbeat sensor as proof-of-concept, but postdoctoral researcher Eden Steven, the lead author of the study, published in this week's Nature, told DNews the coated silk could just as well work as an artificial muscle. "Earlier studies on lifting strength showed it is some fifty times stronger than muscle," he said.
Other research teams have tried mixing spider silk with metals to get electrical conductivity, but the silks don't stretch as far as pure silk. Coating the silk in metal also cuts off moisture, so it can't shrink or expand the way it normally does. Steven said he wanted a material that was porous enough to let water in, so he decided to try carbon nanotubes.
To make the conductive threads, the group collected threads from the webs of golden silk spiders Nephila clavipes, which are common orb weavers in the American south. The chief advantage of these spiders is that the webs are often large -- three feet across -- making it easy to gather the dragline silk, which is what anchors the web in place.
Next, they mixed the threads with dry nanotubes, which stuck to the threads, but not strongly. Adding drops of water to the bundles of the carbon-covered silk encouraged adhesion and then they pressed the wet threads between two Teflon sheets.
Single fibers pulled from the bundle were covered with the nanotubes. "It was really quite surprising," Steven said. "The way we coat the fiber, we only use water and pressure."
In stretch tests, the researchers found that the coated silk could stretch to 50 percent beyond its original length without losing much electrical conductivity. "This was also quite surprising; we expected it to be brittle," Steven said. At the same time, the threads also retained spider silk's ability to expand when exposed to moisture.
There was more. When they attached a battery to the coated silk and ran current through it, the silk heated up, driving out the moisture. That made the fiber shrink by a small amount -- about 1 percent. But the "pull" on anything attached to it was incredibly strong. While other experiments have shown the ability of spider silk to expand and contract after absorbing water, this is the first time anyone has made it happen in response to electric current, Steven said. That link to electricity opens up applications such as artificial muscles.
To make the heartbeat sensor, the researchers attached the silk strands to a battery and a device that measures electrical resistance. They taped the silk threads to a person's wrist. The threads stretched a small amount in response to the person's pulse, altering their resistance with every heartbeat.
Other scientists have used spider silk in experiments with medical devices because it naturally biodegradable, and experiments have explored its use as a kind of fiber-optic cable to transmit data. But the electrical conductivity makes it that much more useful, noted John A. Rogers, a materials scientist at the University of Illinois at Urbana. Rogers was not involved in this study, but he has explored the use of the silk in dissolving electronics.
"In that sense, the research is very much complementary to our own," he said, as it expands the number of ways to use the silk.
Manufacturing more of these fibers depends the silk supply, and spiders can only make so much at a time. But making the coating won't require any exotic manufacturing, and that's a huge plus. "Sometimes the simple things are best," Steven said.