Artificial muscles, once a tangle of elaborate servomotors, and hydraulic and pneumatic actuators, is now a thing of shape-memory alloys and hair-thin carbon nanotube (CNT) fibers. Bionics are, in brief, getting smaller—though perhaps not simpler.
Electrochemical CNT muscles are also energy efficient, and they provide larger muscle strokes as well. Recently, a group of researchers from the U.S., Australia, South Korea and China, working with polymer-coated CNT fibers twisted into yarn, have effectively demonstrated how these muscles can be faster, more powerful and more energy efficient.
Electrochemically driven CNT muscles actuate when a voltage is passed between the muscle fiber and a counter-electrode, causing a movement of ions to and from the surrounding electrolyte and the muscle. Generally speaking, this results in the muscles either contracting or expanding, until the potential reaches zero charge—after which it changes direction. In other words, a bipolar muscle stroke ensues. The bipolar movement, however, results in a smaller muscle stroke, reducing the muscle's efficiency.
The research team devised a way to circumvent this limitation. “When we coated the internal surface of the yarns with about a nanometer thicknesses of special polymers, we could shift the potential of zero charge of the muscle to outside the [stability window of the electrolyte, a voltage range beyond which it breaks down]," says Ray Baughman, director of the Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, one of the authors of the paper.
These polymers are ionically conducting materials with either positively or negatively charged chemical groups. In other words, they can accept either positive (cations) or negative ions (anions). With the potential of zero charge outside the electrolyte's stability window, only one kind of ion (either cations or anions) infiltrates the muscle, and the muscle actuates in a unipolar direction.
Electrochemically driven CNT muscles actuate when a voltage is passed between the muscle fiber and a counter-electrode, causing a movement of ions to and from the surrounding electrolyte and the muscle
In the lab, the researchers used a CNT yarn muscle with a counter-electrode that was non-actuating to demonstrate their concept. But Baughman says that this doesn't have to be the case. They found that by using two different types of their polymer-coated carbon nanotube yarns—one with positive substituents and the other with negative—they could create a dual-electrode unipolar muscle. “You can use the mechanical work being done by each muscle [additively]… [by putting] an unlimited numbers of these muscles together."
The team were also able to make a dual-electrode CNT yarn muscle with a solid-state electrolyte, eliminating the need for a liquid electrolyte bath. “These dual electrode, unipolar muscles were woven to make actuating textiles that could be used for morphing clothing," said Zhong Wang, a doctoral student and co-author, in the press release.
The group's electrochemical unipolar muscles generate an average mechanical power output that is 10 times the average capability of human muscles, and about 2.2 times the weight-normalized power capability of a turbocharged V-8 diesel engine.
As such, it has a wide range of applications—including robotics and adaptable clothing. Of the former example, Baughman says robotic motors can be heavy and difficult to coordinate in a device with broad freedom of movement. By contrast, the artificial muscles could power electric robotic exoskeletons, which, could enable a person to work in a warehouse and move heavy items around with ease.
Clothing that could adjust according to comfort is another application, allowing wearers to change the porosity of textiles depending on the weather. Medical implants, like a heart assist apparatus, could also use compact and lightweight artificial muscles, as could prosthetics. “We are [currently]… writing a proposal for doing studies that will actually involve patients," Baughman adds.
Before real-world applications become possible, the challenge is to produce cost-effective, high-quality carbon nanotube yarn at scale. Baughman and his team also hope to adapt the unipolar CNT muscles to make more powerful mechanical energy harvesters.