Researchers Create Artificial Muscles by Twisting and Coiling Carbon Nanotubes

Written by AZoNanoJan 29 2021

For more than 15 years, researchers at The University of Texas at Dallas and their collaborators in the U.S., Australia, South Korea and China have fabricated artificial muscles by twisting and coiling carbon nanotube or polymer yarns. When thermally powered, these muscles actuate by contracting their length when heated and returning to their initial length when cooled. Such thermally driven artificial muscles, however, have limitations.

Electrochemically driven carbon nanotube (CNT) muscles provide an alternative approach to meet the growing need for fast, powerful, large-stroke artificial muscles for applications ranging from robotics and heart pumps to morphing clothing.

"Electrochemically driven muscles are especially promising, since their energy conversion efficiencies are not restricted by the thermodynamic heat engine limit of thermal muscles, and they can maintain large contractile strokes while supporting heavy loads without consuming significant energy," said Dr. Ray Baughman, the Robert A. Welch Distinguished Chair in Chemistry and director of the Alan G. MacDiarmid NanoTech Institute at UT Dallas. "In contrast, human muscles and thermally powered muscles need a large amount of input energy to support heavy loads even when not accomplishing mechanical work."

In a study posted online Jan. 28 in the journal Science, the researchers describe creating powerful, unipolar electrochemical yarn muscles that contract more when driven faster, thereby solving important problems that have limited the applications for these muscles.

Electrochemically powered CNT yarn muscles are actuated by applying a voltage between the muscle and a counter electrode, which drives ions from a surrounding electrolyte into the muscle.

But there are limitations to electrochemical CNT muscles. First, the muscle actuation is bipolar, which means that muscle movement - either expansion or contraction - switches direction during a potential scan. The potential at which the stroke switches direction is the potential of zero charge, and the rate at which the potential changes over time is the potential scan rate.

Another issue: A given electrolyte is stable only over a particular range of voltages. Outside this range, the electrolyte breaks down.

"Previous yarn muscles cannot use the full stability range of the electrolyte," said Baughman, a corresponding author of the study. "Also, the muscle's capacitance - its ability to store the charge needed for actuation - decreases with increasing potential scan rate, causing the muscle's stroke to dramatically decrease with increasing actuation rate."

To solve these problems, the researchers discovered that the interior surfaces of coiled carbon nanotube yarns could be coated with a suitable ionically conducting polymer that contains either positively or negatively charged chemical groups.

"This polymer coating converts the normal bipolar actuation of carbon nanotube yarns to unipolar actuation, where the muscle actuates in one direction over the entire stability range of the electrolyte," Baughman said. "This long-sought behavior has surprising consequences that make electrochemical carbon nanotube muscles much faster and more powerful."

Chemistry doctoral student Zhong Wang, a co-first author of the study, explained the underlying science: "The dipolar field of the polymer shifts the potential of zero charge - which is where the electronic charge on the nanotubes changes sign - to outside the electrolyte's stability range. Hence, ions of only one sign are electrochemically injected to compensate this electronic charge, and the muscle's stroke changes in one direction over this entire useable potential scan range."

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This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement Nº 737882.



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