Coiled carbon fiber artificial muscle lifts 12,000 times its own weight

No matter how regularly you exercise, artificial muscles have long since surpassed their natural counterparts. Now, researchers from the Department of Mechanical Science and Engineering at the University of Illinois have developed an artificial muscle made of carbon fiber and rubber that can lift over 12,000 times its own weight.

The MechSE Illinois team started with the goal of making coiled artificial muscles – a relatively new design – stronger and as a result, more practical. With that in mind, carbon fiber was chosen, a very strong but lightweight material. To make it more deformable, the researchers mixed the carbon fiber with polydimethylsiloxane (PDMS) rubber and twisted it into a coiled shape.

"Coiled muscles were invented recently using nylon threads," says Sameh Tawfick, an author of the study. "They can exert large actuation strokes, which make them incredibly useful for applications in human assistive devices: if only they could be made much stronger. To use carbon fibers, we had to understand the mechanism of contraction of coiled muscles. Once we uncovered the theory, we learned how to transform carbon fibers into ultra strong muscles. We simply filled carbon fiber tows with the suitable type of silicone rubber, and their performance was impressive, precisely what we had aimed for."

The muscles can be made to flex by applying a small electric current to the ends, which heats up the silicone rubber. That pushes the carbon fibers apart, making the diameter of the muscle expand and the length contract, pulling up a load attached to the bottom. This longways contraction could also be achieved by delivering liquid hexane to the coiled muscle.

In tests, the team found that its creations were very strong with even a mild input voltage. An artificial muscle bundle measuring just 0.4 mm across was able to lift a half-gallon of water by 1.4 in (3.6 cm), with an applied voltage of only 0.172 volts per cm. It was able to lift up to 12,600 times its own weight, support up to 60 megapascals of mechanical stress, was capable of tensile strokes over 25 percent, and produced specific work (work per unit weight) of up to 758 joules per kg.

The team also developed a mathematical model to describe how the artificial muscle would function under different parameters. The researchers say this could be used to design new artificial muscles with specific properties, tailored to given applications.

"The range of applications of these low cost and light weight artificial muscles is really wide and involves different fields such as robotics, prosthetics, orthotics, and human assistive devices," says Caterina Lamuta, an author of the study. "The mathematical model we proposed is a useful design tool to tailor the performance of coiled artificial muscles according to the different applications. Furthermore, the model provides a clear understanding of all the parameters that play an important role in the actuation mechanism, and this encourages future research works toward the development of new typologies of fiber-reinforced coiled muscles with enhanced properties."

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Coiled carbon fiber artificial muscle lifts 12,000 times its own weight

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