Artificial Muscles Work Graphic
Different levels of stretch and twist result in different types of complex non-planar shapes. (Credit: Harvard John A. Paulson School of Engineering and Applied Sciences)

Artificial muscles will power the soft robots and wearable devices of the future. But more needs to be understood about the underlying mechanics of these powerful structures in order to design and build new devices. Now, researchers have uncovered some of the fundamental physical properties of artificial muscle fibers.

Soft fibers, or filaments, can be stretched, sheared, bent or twisted. How these different actions interact to form knots, braids, and helices is important to the design of soft actuators. Imagine stretching and twisting a rubber band as tight as you can. As the twist gets tighter and tighter, part of the band will pop out of the plane and start twisting around itself into a coil or knot. These coils and loops, in the right form, can be harnessed to actuate the knotted fiber.

The researchers found that different levels of stretch and twist result in different types of complex non-planar shapes. They characterized which shapes lead to kinked loops, which to tight coils, and which to a mixture of the two. They found that pre-stretch is important for forming coils, as these shapes are the most stable under stretching, and modeled how such coils can be used to produce mechanical work.

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