Here are two things you might not expect to see together: spider silk coated in carbon nanotubes. This hybrid material is stretchy, super strong and can shrink and grow with humidity, making it potentially very useful for sensors and flexible electronics – and scientists made it by rubbing it between their fingers.
This nanotube-coated spider silk, described in the journal Nature Communications, could be useful for devices such as heart monitors inside the body and might even act like synthetic muscle.
Spider silk is well known as a biological Holy Grail of fabrics: It's a super strong polymer that’s also remarkably flexible and completely biodegradable, and it expands and contracts in response to humidity. Some researchers have studied its biomedical uses inside the body, too. But little work has been done on using it to send electrical signals, said lead author Eden Steven, a materials scientist at Florida State University in Tallahassee.
Spider silk is actually an extremely effective insulator – which means it blocks electricity, Steven said. Coating it with electrically conducting material like gold nanoparticles mostly solves that issue but generally makes it too stiff to take advantage of the spider silk’s flexible properties.
Steven decided to try coating spider silk with carbon nanotubes, which are basically rolled-up sheets of a very tough but lightweight crystalline carbon called graphene. But the scientists’ typical high-heat chemical methods made a carbon layer so thick that it was too stiff and inhibited the spider silk’s flexibility.
Steven wondered what would happen if he essentially coated the spider silk by hand. First, he gathered raw supplies.
"Our sample is taken literally from the tree outside of our lab," said Steven. "Fortunately I work in Florida where we basically have spiders everywhere. So we never run out of samples because we simply go out and pick them from the trees."
The researchers mixed some silk from a banana spider web with a powder of carbon nanotubes and a few drops of water, then pressed down and rubbed around between two Teflon pads.
This simple technique made a spider silk thread with an evenly coated carbon nanotube covering that was only about 80 nanometers thick – a tiny fraction of the 5-to-10-micrometer layer (equivalent to 5,000 to 10,000 nanometers) created using the industrial method. The thin layer of carbon nanotubes meant the spider silk could still move freely.
It’s a sign that sometimes simplicity holds the answer, Steven said.
"The simple step that we thought would not work turns out to be the best method," Steven said. "So there is a very good lesson for me. Sometimes we think too much – why not just try?"
The researchers were able to fashion spiral coils, knots and even letters ("FSU," for their university) out of these coated wires, showing how they could be useful for flexible electrodes. They even built a heart monitor as a proof of concept.
They could also be used as synthetic muscles, Steven said, given that spider silk has been shown to be 50 times stronger than a conventional muscle fiber.
There are a few drawbacks. Though the 80-nanometer level could be great for stretchy sensors, it’s less effective for conducting electricity. Conducting a strong current would require a thicker carbon nanotube casing – the kind that's harder to make and that would detract from the spider silk’s stretchability.
Still, these hybrid materials could become extremely useful as scientists try to make electronics with more biodegradable parts, rather than the plastics and other synthetic materials that ultimately end up in landfills or result in pollution.
"It’s biodegradable and bio-renewable, so … it will not produce this electronic waste as conventional devices [do]," he said. "It’s a very versatile material."