Carbon Nanotube Yarns Flex Their Muscles

The power of carbon nanotubes is being marshaled to develop artificial muscles. Capable of being twisted together, woven, sewn, braided, or knotted, the yarn-like material can lift more than 100,000 times its own weight and produce 85 times more mechanical power than natural muscles of the same size. Developed by researchers at the University of Texas at Dallas (UT Dallas), together with scientists from Australia, China, South Korea, Canada, and Brazil, the material could eventually be used to manufacture a range of products, including such medical devices as catheters for minimally invasive surgery.

The artificial muscles are made by infiltrating a volume-changing 'guest,' such as paraffin wax, into the twisted carbon nanotube yarn. After the wax-filled yarn is heated using either electricity or light, it increases in volume while contracting in length--a result of the helical structure produced by twisting the yarn.

"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," remarks team leader Ray Baughman, professor of chemistry and director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas. "While we are excited about near-term applications possibilities, these artificial muscles are presently unsuitable for directly replacing muscles in the human body."

Muscle actuation can occur in 25/1000 of a second, and the contractile power density of both actuation and the reversal of actuation is 4.2 kW/kg. To achieve these results, the researchers twist the guest-filled carbon nanotube muscles to produce coiling. When free to rotate, the wax-filled yarn untwists as it is heated. And when heating is stopped, this rotation reverses. 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. Pound-per-pound, the generated torque is slightly higher than that obtained for large electric motors, Baughman comments.

"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 notes. "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."