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Wax-filled nanotech yarn behaves like super-strong muscle

19 November 2012

Artificial muscles made from wax infused carbon nanotube yarns can lift more than 100,000 times their own weight and generate 85 times more mechanical power than natural muscle.

The diameter of this coiled yarn is about twice the width of a human hair (image courtesy of the University of Texas at Dallas)

The work was carried out by scientists at The University of Texas at Dallas and a team of international co-workers 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,” says team leader Dr Ray Baughman. “While we are excited about near-term applications possibilities, these artificial muscles are presently unsuitable for directly replacing muscles in the human body.”

The new artificial muscles are made by infiltrating a volume-changing 'guest' such as paraffin wax 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.

“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 can be ultra-fast, occurring in 25-thousandths of a second. Including times for both actuation and reversal of actuation, the researchers demonstrated a contractile power density of 4.2kW/kg, which is four times the power-to-weight ratio of common internal combustion engines.

To achieve these results, the guest-filled carbon nanotube muscles were highly twisted to produce coiling. 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 can rotate an attached paddle to an average speed of 11,500 revolutions per minute for more than 2 million reversible cycles.

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.

Even without the addition of a guest material, the co-authors found that introducing coiling to the nanotube yarn increases tenfold the yarn’s thermal expansion coefficient. This thermal expansion coefficient is negative, meaning that the unfilled yarn contracts as it is heated.

Heating the yarn in inert atmosphere from room temperature to about 2,500 degrees Celsius provided more than 7 percent contraction when lifting heavy loads, indicating that these muscles can be deployed at elevated temperatures where no other high-work-capacity actuator can survive.


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