Nanotubes flex their wax-filled muscles

New breakthrough in robotics will let our mechanical friends lift more than 1,00,000 times the weight of their muscles and generate 85 times more power than their human counterparts

The artificial muscles are yarns constructed from carbon nanotubes, which are seamless, hollow cylinders made from the same type of graphite layers found in the core of ordinary pencils.Individual nanotubes can be 10,000 times smaller than the diameter of a human hair, yet pound-for-pound, can be 100 times stronger than steel, according to scientists at The University of Texas at Dallas and their international team from Australia, China, South Korea, Canada and Brazil.

   “The artificial muscles that we’ve developed can provide large, ultrafast contractions to lift weights that are 200 times heavier than possible for a natural muscle of the same size,” said Ray Baughman, team leader from UT Dallas. “While we are excited about nearterm applications possibilities, these muscles are presently unsuitable for replacing muscles in the human body.”
    Described in a study published in the journal Science, the new artificial muscles are made by infiltrating a volume- changing “guest,” such as the paraffin wax used for candles, into twisted yarn made of carbon nanotubes. Heating the wax-filled yarn, either electrically or using a flash of light, causes the wax to expand, the yarn volume to increase, and the yarn length to contract.
    The combination of yarn volume increase with yarn length decrease results from the helical structure produced by twisting the yarn. A child’s finger cuff toy, which is designed to trap a person’s fingers in both ends of a helically woven cylinder, has an analogous action. To escape, one must push the fingers together, which contracts the tube’s length and expands its volume and diameter.
    “Because of their simplicity and high performance, these yarn muscles could be used for such diverse applications as robots, catheters for minimally invasive surgery, micromotors, mixers for microfluidic circuits, tunable optical systems, microvalves, positioners and even toys,” Baughman said.
   Muscle contraction – also called actuation – can be ultrafast, occurring in 25-thousandths of a second. Including times for both actuation and reversal of actuation, the researchers demonstrated a contractile power density of 4.2 kW/kg, which is four times the powerto- weight ratio of common internal combustion engines.
   To achieve these results, the guestfilled carbon nanotube muscles were highly twisted to produce coiling, as with the coiling of a rubber band of a rubber-band-powered model airplane.
    When free to rotate, a wax-filled yarn untwists as it is heated electrically or by a pulse of light.

This rotation reverses when heating is stopped and the yarn cools. Such torsional action of the yarn can rotate an attached paddle to an average speed of 11,500 revolutions per minute for more than 2 million reversible cycles. Pound-per-pound, the generated torque is slightly higher than that obtained for large electric motors, Baughman said.
     Because the yarn muscles can be twisted together and are able to be woven, sewn, braided and knotted, they might eventually be deployed in a variety of self-powered intelligent materials and textiles. For example, changes in environmental temperature or the presence of chemical agents can change guest volume; such actuation could change textile porosity to provide thermal comfort or chemical protection. Such yarn muscles also might be used to regulate a flow valve in response to detected chemicals, or adjust window blind opening in response to ambient temperature.