Artificial Muscles Made from Nanotube Yarn

artificial muscleIs sports as we know it about to make a giant leap forward? Think about this: scientists at The University of Texas at Dallas have developed new artificial muscles from cabon nanotube yarns infused with paraffin wax that can lift more than 100,000 times their own weight and generate 85 times more mechanical power than the same size natural muscle.

Carbon nanotubes 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. Imagine what Lance Armstrong could do with that.

“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 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.”

Detailed in the Nov. 16 issue of Science, the 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 yarn volume increase, in combination with the yarn length decrease results from the helical structure produced by twisting the yarn. The old trick child’s finger toy, designed to trap a person’s fingers in both ends of a helically woven cylinder, has a similar action. To escape, you need to push your fingers together, which contracts the tube’s length and expands its volume and diameter.

Diverse Range of Applications

“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 referred to as actuation, can be super 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 4.2 kW/kg, which is four times the power-to-weight ratio of common internal combustion engines. To get those results, the guest-filled 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 not rotationally constrained, a wax-filled yarn untwists as it is electrically heated or heated from a pulse of light. Rotation reverses when heating is stopped and the yarn cools. That 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.

Since the yarn can be twisted together and woven, sewn, braided and knotted, the muscles might eventually be used in a range of self-powered intelligent materials and textiles. For instance, 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.

Benefits of Negative Thermal Expansion Coefficient

Even without the addition of a guest material, the paper’s authors found that introducing coiling to the nanotube yarn increases tenfold the yarn’s thermal expansion coefficient. This thermal expansion coefficient is negative, which means 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 to temperatures 1,000 C above the melting point of steel, where no other high-work-capacity actuator can survive.

“This greatly amplified thermal expansion for the coiled yarns indicates that they can be used as intelligent materials for temperature regulation between 50 C below zero and 2,500 C,” said Dr. Márcio Lima, research associate in the NanoTech Institute at UT Dallas and co-lead author.

“The remarkable performance of our yarn muscle and our present ability to fabricate kilometer-length yarns suggest the feasibility of early commercialization as small actuators comprising centimeter-scale yarn length,” Baughman said. “The more difficult challenge is in upscaling our single-yarn actuators to large actuators in which hundreds or thousands of individual yarn muscles operate in parallel.”


Electrically, Chemically, and Photonically Powered Torsional and Tensile Actuation of Hybrid Carbon Nanotube Yarn Muscles

Márcio D. Lima, Na Li, Mônica Jung de Andrade, Shaoli Fang, Jiyoung Oh, Geoffrey M. Spinks, Mikhail E. Kozlov, Carter S. Haines, Dongseok Suh , Javad Foroughi, Seon Jeong Kim,Yongsheng Chen, Taylor Ware, Min Kyoon Shin, Leonardo D. Machado, Alexandre F. Fonseca, John D. W. Madden, Walter E. Voit, Douglas S. Galvão, Ray H. Baughman

Science 16 November 2012: Vol. 338 no. 6109 pp. 928-932 DOI: 10.1126/science.1226762