Carbon nanotubes used to create conducting fibers for artificial muscles

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An electric current can flow even when the fiber is stretched to over 14 times its initial length(Credit: UT Dallas Alan G. MacDiarmid NanoTech Institute)

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A new kind of conducting fiber developed at the University of Texas at Dallas is being used to develop artificial muscles and capacitors that store more energy when stretched. The fiber, which is composed of carbon nanotube sheets wrapped around a rubber core, may one day also find use in morphing aircraft, stretchy charger cords and exoskeleton limbs, along connecting cables for a wealth of other devices.

Unlike previous carbon nanotube fibers, which emphasized rigid strength that could be applied to bulletproof vests or offer an alternative to traditional carbon fiber, or that combined strength, flexibility, and conductivity, this new fiber is super stretchy.

The nanotube sheets are wrapped around the rubber while it is stretched, which results in buckling when the wrapped rubber relaxes – kind of like an accordion. Unlike an accordion, however, the fiber buckles in two dimensions – not only along its length, but also around its circumference, because stretching causes the rubber to shrink in diameter.

This buckling is key to the fiber's design. It makes the electrical resistance insensitive to stretching, to the extent that an electric current can flow even when the fiber is stretched to over 14 times its initial length.

The researchers didn't stop there. They layered rubber and carbon nanotube sheaths atop the fibre to create a stretchable fiber capacitor, with the second layer of rubber acting as a dielectric between the two buckled nanotube sheath electrodes.

The double-sheath fibers could then be twisted to form artificial muscles that can rotate mirrors in optical circuits or pump liquids in miniature devices.

The researchers found that their fiber technology can be scaled up or down – even as small as 150 microns, which is twice the width of a human hair – depending on the circumference of the rubber core. "Individual small fibers also can be combined into large bundles and plied together like yarn or rope," said research associate Nan Jiang.

Co-author Raquel Ovalle-Robles also noted that the technology could see rapid commercialization because the bulk of the material required is inexpensive rubber and the procedure is not particularly complex.

The researchers found that the carbon-nanotube-sheathed rubber fiber could be used as a conductive wire in a pacemaker cable(Credit: UT Dallas)

The researchers believe that carbon-nanotube-sheathed rubber fiber could find a multitude of practical applications besides artificial muscles, including as a conductive wire in a pacemaker cable, super-elastic electronic circuits, super-stretchy cables that extend up to 31 times their initial length, optical circuits, morphing structures in space, and robotic arms with extreme reach.

You can watch a video demonstration of the fiber's stretchiness below.

A paper describing the research was published in the journal Science.

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