Elastic conducting fibers could be used in everything from robotics to aerospace and consumer electronics. The challenge is that stretching conventional materials changes their electrical resistance. Researchers from the University of Texas at Dallas have solved this problem by developing new conducting fibers that can be stretched to over 14 times their length with minimal change to resistance. In fact, electrical conductivity increases 200-fold when the fibers are stretched.
The scientists created the fibers by wrapping electrically conductive sheets of tiny carbon nanotubes around stretched rubber cores. When the wrapped rubber relaxes, the nanotubes buckle, like when an accordion is compressed. Dr. Ray Baughman described the effects of this complex buckled structure in a UT Dallas release. “We make the inelastic carbon nanotube sheaths of our sheath-core fibers super stretchable by modulating large buckles with small buckles, so that the elongation of both buckle types can contribute to elasticity. These amazing fibers 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.”
It gets better: when the fiber is stretched, the buckles along its length disappear, but then secondary buckles form around the rubber core’s shrinking circumference. Researcher Dr. Zunfeng Liu said, “This novel combination of buckling in two dimensions avoids misalignment of nanotube and rubber core directions, enabling the electrical resistance of the sheath-core fiber to be insensitive to stretch.”
The team is already using the new fibers to make artificial muscles, strain sensors, and fiber capacitors. By including other rubber and carbon nanotube sheath layers to make these new capacitors, the energy storage capacity increased by 860 percent when the fibers are stretched. The fibers can be as small as 150 microns (twice the width of human hair) or they can be made much larger by increasing the size of the rubber core. They could also be combined into bundles like rope.
The fibers can be twisted, bent, and stretched, as the researchers demonstrated in this video of the fibers being used as a conductive wire in a pacemaker cable. An oscilloscope shows minimal changes in voltage pulses, even when the cable is twisted.
The research is described in the July 24 issue of Science in a paper titled “Hierarchically buckled sheath-core fibers for superelastic electronics, sensors, and muscles.”