Spider silk, used by spiders to spin their webs, has many unique properties. It is very flexible depending on humidity and is tougher than Kevlar, a polymer used in bullet-proof vests. Now, researchers have found a way to retain these properties while turning the silk into an electrical conductor by reinforcing it with carbon nanotubes.

The researchers who have developed this technology envision applications in artificial muscles, sensors and as enhanced actuators in prosthetics for humans and robots. They used the silk of a species of the golden silk orb-weaver ( Nephila clavipes), which is commonly found in warmer regions of the Americas.

The silk is a protein composed of amino acids that are arranged into blocks and coils where the blocks are connected to each other by the coils. The blocks are mainly responsible for the strength of the silk while the coils, for the elasticity.

The procedure

To reinforce the silk and make it a good conductor, the team mixed functionalised carbon nanotubes with silk from N. clavipes, added a few drops of water, applied pressure and shear, and then air-dried them. The water ensured the nanotubes dispersed uniformly, which is important since they impart electrical conductivity.

“Since the carbon nanotubes coating is very thin, it’s also porous, allowing moisture to be absorbed or released by the silk fibre,” said Eden Steven, one of the researchers and first author of their paper published in Nature Communications on September 10. Steven is from the National High Magnetic Field Laboratory, Florida State University.

He observed that the nanotube-silk fibre was able to stretch and absorb water as the humidity changed.

Thus, the resistance of the fibre changed, allowing it to be correlated with humidity.

Based on this, it could be used in humidity and strain sensing, as well as actuators in prosthetics: artificial muscles made of this silk could be made to extend or contract by raising or lowering the humidity.

“The carbon nanotubes coating allows the biomimetic muscle to be driven by application of electrical currents,” Steven said.

These potential applications are aided by the fact that spider silk has 50 times more lifting power than typical biological muscle, according to previous studies. It is also naturally very tough, flexible and versatile, and eco-friendly.

The tensile strength of the silk derived from N. clavipes is higher than steel’s by a factor of six. It is also known that the silk is as elastic as nylon and much lighter at the same time. The nanotubes, for their share, are as good conductors of electricity as copper wires are.

In fact, “the electrical resistance is also strain dependent,” Steven added. “The fibre can be stretched by as much as 50 per cent and it still retains its electrical continuity.” He hopes that this combination of nanomaterials and biomaterials could be leveraged more in the future, and an interdisciplinary approach inspired by nature taken to solving humankind’s problems.