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Super-elastic conducting fibres make artificial muscles

25 July 2015

These novel electrically conducting fibres' conductivity increases 200-fold when stretched, and they can be reversibly stretched to over 14 times their initial length.

This illustration shows complex two-dimensional buckling, shown in yellow, of the carbon nanotube sheath/rubber-core fibre (image: UT Dallas Alan G MacDiarmid NanoTech Institute)

The research team, based at The University of Texas at Dallas, is using the new fibres to make artificial muscles, as well as capacitors whose energy storage capacity increases about tenfold when the fibres are stretched.

In a study published in the July 24 issue of the journal, Science, the scientists describe how they constructed the fibres by wrapping lighter-than-air, electrically conductive sheets of tiny carbon nanotubes to form a jelly-roll-like sheath around a long rubber core.

The new fibres differ from conventional materials in several ways. For example, when conventional fibres are stretched, the resulting increase in length and decrease in cross-sectional area restricts the flow of electrons through the material. But even a 'giant' stretch of the new conducting sheath-core fibres causes little change in their electrical resistance.

One key to the performance of the new conducting elastic fibres is the introduction of buckling into the carbon nanotube sheets. Because the rubber core is stretched along its length as the sheets are being wrapped around it, when the wrapped rubber relaxes, the carbon nanofibres form a complex buckled structure, which allows for repeated stretching of the fibre.

"Think of the buckling that occurs when an accordion is compressed, which makes the inelastic material of the accordion stretchable," said UT Dallas' Dr Ray Baughman. "We make the inelastic carbon nanotube sheaths of our sheath-core fibres super stretchable by modulating large buckles with small buckles, so that the elongation of both buckle types can contribute to elasticity. These amazing fibres maintain the same electrical resistance, even when stretched by giant amounts, because electrons can travel over such a hierarchically buckled sheath as easily as they can traverse a straight sheath."

Dr Zunfeng Liu, lead author of the Science paper says the structure of the sheath-core fibres has further interesting and important complexity. Buckles form not only along the fibre's length, but also around its circumference.

"Shrinking the fibre's circumference during fibre stretch causes this second type of reversible hierarchical buckling around its circumference, even as the buckling in the fibre direction temporarily disappears," says Liu. "This novel combination of buckling in two dimensions avoids misalignment of nanotube and rubber core directions, enabling the electrical resistance of the sheath-core fibre to be insensitive to stretch."

By adding a thin overcoat of rubber to the sheath-core fibres and then another carbon nanotube sheath, the researchers made strain sensors and artificial muscles in which the buckled nanotube sheaths serve as electrodes and the thin rubber layer is a dielectric, resulting in a fibre capacitor. These fibre capacitors had a capacitance change of 860 percent when the fibre was stretched 950 percent.

Adding twist to these double-sheath fibres resulted in fast, electrically powered torsional artificial muscles that could be used to rotate mirrors in optical circuits or pump liquids in miniature devices used for chemical analysis.

In laboratory conditions, the conducting elastomers can be fabricated in diameters ranging from about 150 microns to much larger sizes, depending on the size of the rubber core. Individual small fibres also can be combined into large bundles and plied together like yarn or rope.

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