Carbon Nanotube Artificial Muscles for Extreme Temperatures Invented at UT Dallas

Researchers at the UT Dallas Alan G. MacDiarmid NanoTech Institute have demonstrated a fundamentally new type of artificial muscle, which can operate at extreme temperatures where no other artificial muscle can be used — from below the temperature of liquid nitrogen (-196° C) to above the melting point of iron (1538° C).

The discovery is reported in the March 20 issue of Science under the title “Giant Stroke, Superelastic Carbon Nanotube Aerogel Muscles.”

Once actuated (or put into motion) in a certain direction, these new artificial muscles can elongate 10 times more than natural muscles and at rates 1,000 times higher than a natural muscle. In another direction, when densified, they can generate thirty times the force of a natural muscle having the same cross-sectional area. While natural muscles can contract at about 20 percent per second, the new artificial muscles can contract at about 30,000 percent per second.

A. An artificial muscle strip with no voltage applied. B. The above artificial muscle strip with 5 kV applied. C. An artificial muscle strip actuated at 1500 K using 5 kV applied voltage.

These artificial muscles are carbon nanotube aerogel sheets made by a novel solid-state process developed at UT Dallas. Aerogels are comprised mostly of air. The starting material is an array of vertically aligned carbon nanotubes manufactured by decomposing hydrocarbons. Because of the special arrangement of these nanotube arrays, which are called forests because they look like a bamboo forest, the carbon nanotubes can be pulled into sheets at speeds of up two meters per second. The sheets have such low density that an ounce would cover an acre.

When scientists apply a voltage to the carbon nanotube aerogel sheets, the nanotubes push away from one another, which in effect works the muscle. These transparent sheets have strange properties that are important for muscle operation. While having about the density of air, in one direction, they have higher specific strength (strength/density) than a steel plate. When stretched in another direction, they provide rubber-like stretchability, but by a mechanism quite different than for ordinary rubber. Because of their nanoscale and microscale structure, they amplify a percent stretch in the nanotube orientation direction to a 15 percent lateral contraction.

“Our discovery of methods for producing these carbon nanotube sheets, their strange properties, and their corresponding remarkable performance as artificial muscles is just the beginning of a story, which will likely be taken in new directions by researchers around the world,” said Dr. Ray H. Baughman, one of the article’s authors, who is the Robert A. Welch Professor of Chemistry and director of the NanoTech Institute. “My guess is that this story will have a happy ending in terms of new products that benefit humankind.”

The ability to either permanently or reversibly tune the nanotube sheet density by using artificial muscles to expand it, and the ability to change the shape of the sheet will likely provide the earliest applications of this technology. Due to their extremely light weight and variable density, carbon nanotube aerogels may become highly desired for use in artificial muscles and solar cells. The conductive properties of these nanotube aerogels, along with their ability to drastically expand their surface area, can improve solar cells by making them more efficient at collecting energy from the sun.

In addition, because no other artificial muscle can actuate at such extreme low and high temperatures, applications for these muscles might develop for use in space exploration, where a hostile environment prohibits use of any other actuating material.

The described breakthroughs resulted from the diverse expertise of the article’s co-authors, who are research scientists affiliated with the NanoTech Institute: Dr. Ali Aliev, Dr. Mikhail Kozlov, Dr. Jiyoung Oh, Dr. Yuri Gartstein, Dr. Alexander Kuznetsov, Dr. Shaoli Fang, Dr. Mei Zhang, Dr. Raquel Ovalle, Dr. Márcio Lima, Dr. Alexandre Fonseca, Dr. Anvar Zakhidov, graduate student Mr. Mohammad Haque, and Dr. Ray Baughman.

This research was funded by the Air Force Office of Scientific Research, the National Science Foundation, the Office of Naval Research, the Robert A. Welch Foundation, Honda Corporation, Lintec Corporation and the Brazilian government.

To obtain a copy of the Science article, please contact the journal at (202) 326-6440 or scipak@aaas.org. A supplemental information file and figures describing applications evaluations that go beyond the scope of the Science article can also be obtained by emailing scipak@aaas.org.