A culmination of more than 15 years of work, researchers from the University of Texas at Dallas, together with collaborators from the US, South Korea, Australia, and China, have created unipolar carbon nanotube "muscles."

These artificial muscles are created by twisting coiling carbon nanotube or polymer yarn upon itself. When heat is applied, the carbon nanotube muscles actuate by contracting and relaxing back to their original length when cooled down. However, thermally-driven muscles still have their limitations.

Turning to Electrochemically Driven Muscles

To work around the limitations of thermally actuated artificial muscles, researchers turned to electrochemically driven carbon nanotube muscles. The alternative approach to meeting the growing demands for fast, powerful, and large-stroke artificial muscles can be used in a wide variety of applications from robotics, to heat pumps, to smart 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," explains Dr. Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute and Robert A. Welch Distinguished Chair in Chemistry at UT Dallas, in a news release from the university. He adds that contrary to thermally driven muscles and actual human muscles, their new artificial muscles requires significantly less energy input.

Researchers and their international collaborators report their powerful unipolar electrochemical yarn muscles that actuate faster, working around the limitations of conventional and thermally-driven muscles in the journal Science.

Electrochemically driven carbon nanotube yarn muscles are actuated by applying a voltage between the muscle and the counter electrode, which supplies ions to the surrounding electrolytes' muscles. However, researchers noted the restrictions to electrochemical carbon nanotube muscles. First, the artificial muscle is bipolar - muscle movements of expansion or contraction might switch the direction during a potential scan. This is affected by the potential of zero charges and the rate where this potential changes over time - known as the potential scan rate. Additionally, specific electrolytes are only stable over a corresponding voltage range. Outside this range, electrolytes break down.

READ MORE: Scientists Establish Relationship Between Nanotube Length and Friction, To Guide Creation of Stronger Fibers 

Optimizing Electrochemical Carbon Nanotube Muscles

To work around these problems, researchers observed that the interior surface of the coiled carbon nanotube yarns could be coated with an ionically conducting polymer that is doped to either be positively or negatively charged. 

Baughman explains that the polymer coating solves the carbon nanotube's bipolar nature and turns into a unipolar actuating muscle, keeping the same behavior across the entire stable voltage range of the electrolyte.

"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," explains chemistry doctoral student Zhong Wang, who is also the co-first author of the paper. He adds that this setup creates ions of only one sign to be injected electrochemically, compensating for the electronic charge.

The polymer material used was poly (sodium 4-styrene sulfonate), which has been approved for drug use and cost-efficient for water softening applications. Adding it to the carbon nanotube muscle allowed it to be usable from high temperatures to as cold as -30 degrees Celsius.


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