Stretchy nanotube yarn could power wearable tech

‘Twistrons’ built from carbon nanotubes harvest energy from small movements of breathing or ocean waves to provide electricity, writes Andrew Masterson.

A scanning electron micrograph of the ‘twistron’ coiled carbon nanotube yarn.
A scanning electron micrograph of the ‘twistron’ coiled carbon nanotube yarn.
University of Texas at Dallas

It’s a standard rite of passage for high school physics students to be told to twist a length or string and then let it go, or stretch an elastic band and twang it, to demonstrate how potential energy converts to kinetic energy.

Perhaps Carter Haines of the University of Texas, Dallas, in the US, was particularly impressed by this evergreen lesson. It would happily explain the inspiration for an invention he and colleagues describe in the journal Science.

A twistron harvester powering an LED bulb.
A twistron harvester powering an LED bulb.
S. H. Kim et al., Science (2017)
Haines and his team have created a new type of yarn, constructed from carbon nanotubes. Dubbed the “twistron”, the yarn generates electricity when stretched or twisted.

Twistrons, the scientists suggest, have a wide range of potential uses. They could be deployed to harvest power from ocean waves. Equally, they could be woven into shirt fabrics and designed to function as a wearable heart monitor.

To create the basic structure for their invention, Haines and his team compiled component nanotubes – tiny carbon tubes around 10,000 times smaller than the width of a human hair – and twisted them together, not unlike a strand of wool. The tight twists give the nano-yarns significant elasticity.

Beforehand, though, the yarns were coated with a conductive substance – an electrolyte. At this stage, high tech met low, because a combination of salt and water was all that was needed, at least to establish proof of concept.

The tighter the yarns were twisted, the closer together the electric charges embedded in them became, increasing their energy output.

“Fundamentally, these yarns are super-capacitors,” says co-author Na Li.

“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.”

For such a simple machine, the twistron’s electrical output is impressive. The scientists calculated that stretching them 30 times a second produced around 240 watts of power per kilogram of yarn.

A stretched piece weighing less than a house-fly was able to light a small LED.

The scientists also showed that a twistron connected to an artificial muscle made from polymer was able to convert its contractions into electricity.

Haines and his colleagues have already filed a patent on twistrons, and see them as a possible solution to one of the major hurdles facing manufacturers of “smart” clothing and other wearables.

“Electronic textiles are of major commercial interest, but how are you going to power them?” team member Ray Baughman says.

“Harvesting electrical energy from human motion is one strategy for eliminating the need for batteries. Our yarns produced over a hundred times higher electrical power per weight when stretched compared to other weavable fibers reported in the literature.”

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