Spider silk is the toughest known material made by organisms, more than 10 times stronger than Kevlar. Now scientists have enhanced it with carbon nanotubes, making it even tougher than normal spider silk and electrically conductive as well, findings that could lead to novel silk-based electronics.
The natural fiber’s strength makes it potentially valuable in a wide range of commercial applications. Researchers are also investigating its medical potential— for instance, they might find use in artificial skin or synthetic tendons.
It can also contract greatly, suggesting it could serve in artificial muscles for robots or prosthetic limbs. For instance, a spider silk fiber only 1 millimeter thick could lift a 5 kilogram weight, suggesting it can carrying out 50 times more work than a comparable amount of biological muscle.
Its great strength and flexibility makes it attractive for use in electronics, but such applications depend on how compatible researchers can make it with electrically conductive materials. That’s why researchers have been combining it with carbon nanotubes, whose electrical conductivity has attracted much interest from industry for decades.
Carbon nanotubes are hollow pipes of carbon just nanometers or billionths of a meter in diameter. Although they are only about the width of a DNA strand, they are roughly 100 times stronger than steel at one-sixth the weight, and their conductive properties for both electricity and heat rival the best metal conductors.
Although spider silk is tough, it becomes soft when exposed to water. The researchers mixed silk fibers they harvested from golden orb-weaver spiders (Nephila clavipes) with a dry powder of carbon nanotubes, wet this blend slightly with a few droplets of water, and rubbed it together.
"We were surprised by how easy it was to coat spider silk fibers with carbon nanotubes," says Eden Steven, a physicist at the National High Magnetic Field Laboratory in Tallahassee, Fla. "We originally tried using conventional coating methods using many chemicals and also high-temperature processing, but later on we found out only water and pressure was enough."
The fibers contracted when the mix dried, and incorporated a uniform coating of nanotubes around themselves.
"This coating is very high quality, very uniform," Steven says.
This hybrid material is not only electrically conductive, but also roughly three times tougher than regular spider silk fibers. Experiments suggest it could perform as well as regular spider silk as an artificial muscle.
"Electrical conductivity was maintained even when the fiber was stretched by 50 percent," Steven says.
The researchers noted the electrical conductivity of their novel fibers could depend on the humidity and the amount of physical strain on the thread. As such, they could also find use in electrical humidity or strain sensors— indeed, the scientists developed a heart pulse sensor with them. Using a gold or a thicker carbon nanotube coating could make its electrical conductivity less vulnerable to humidity.
Scientists are currently attempting to develop synthetic spider silk that Steven said carbon nanotubes may one day get incorporated into. “The versatility of this material means it can be used in many applications,” Steven says.
The scientists detailed their findings Sept. 10 in the journal Nature Communications.
Top Image: Scanning Electron Microscope image of a bent carbon-nanotubes-coated spider silk fiber. The carbon nanotubes coating is intimately connected to the fiber. There were no observable macroscopic cracks. Courtesy High-Performance Materials Institute.