New 'twistron' yarns can generate electrcity when stretched

Scientists have developed high- tech "twistron" yarns that generate electricity when stretched or twisted, an advance that may lead to self-powered wearable health monitors as well as smart clothes in future.
The yarns have various possible applications such as harvesting energy from the motion of ocean waves or from temperature fluctuations, researchers said.
When sewn into a shirt, these yarns served as a self- powered breathing monitor, they said.
"The easiest way to think of twistron harvesters is, you have a piece of yarn, you stretch it, and out comes electricity," said Carter Haines, associate research professor at University of Texas at Dallas in the US.

The yarns are constructed from carbon nanotubes, which are hollow cylinders of carbon 10,000 times smaller in diameter than a human hair.
The researchers first twist-spun the nanotubes into high-strength, lightweight yarns. To make the yarns highly elastic, they introduced so much twist that the yarns coiled like an over-twisted rubber band.

In order to generate electricity, the yarns must be either submerged in or coated with an ionically conducting material, or electrolyte, which can be as simple as a mixture of ordinary table salt and water.
"Fundamentally, these yarns are supercapacitors," said Na Li, a research scientist at UT Dallas and co-lead author of the study published in the journal Science.

"In a normal capacitor, you use energy - like from a battery - to add charges to the capacitor. But in our case, when you insert the carbon nanotube yarn into an electrolyte bath, the yarns are charged by the electrolyte itself. No external battery, or voltage, is needed," said Li.
When a harvester yarn is twisted or stretched, the volume of the carbon nanotube yarn decreases, bringing the electric charges on the yarn closer together and increasing their energy, Haines said.
This increases the voltage associated with the charge stored in the yarn, enabling the harvesting of electricity.
Stretching the coiled twistron yarns 30 times a second generated 250 watts per kilogram of peak electrical power when normalised to the harvester's weight, said Ray Baughman, from UT Dallas.

Researchers showed that a twistron yarn weighing less than a housefly could power a small LED, which lit up each time the yarn was stretched.
To show that twistrons can harvest waste thermal energy from the environment, Li connected a twistron yarn to a polymer artificial muscle that contracts and expands when heated and cooled.
The twistron harvester converted the mechanical energy generated by the polymer muscle to electrical energy.
"There is a lot of interest in using waste energy to power the Internet of Things, such as arrays of distributed sensors," Li said.
"Twistron technology might be exploited for such applications where changing batteries is impractical," said Li.