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Nylon Fibers Could Help Your Robot Flex

This image shows the fabrication steps from raw circular filament to a fully functional bending artificial muscle. The bottom filament is a raw circular filament. Researchers press the filament using a rolling mill (the second sample from the bottom). Next, they add a mask in the middle of the surface (the third sample from the bottom). Then, they add the conductive ink (the second sample from the top). Finally, they remove the mask after the ink is dried (the sample on the top). Credit: MIT
This image shows the fabrication steps from raw circular filament to a fully functional bending artificial muscle. The bottom filament is a raw circular filament. Researchers press the filament using a rolling mill (the second sample from the bottom). Next, they add a mask in the middle of the surface (the third sample from the bottom). Then, they add the conductive ink (the second sample from the top). Finally, they remove the mask after the ink is dried (the sample on the top). Credit: MIT

Researchers at MIT have recently developed a new way to mimic muscle function and it could have profound implications for robotics. Using highly oriented nylon fibers, shaped in particular way and manipulated through the sequenced application and removal of a heat source, the researchers were able to make the fibers shrink in length but expand in diameter and ultimately bend in ways similar to the ways natural muscle tissue does when activated.

Previous research with other materials, such as coiled nylon fibers which are limited to producing linear motion and exotic materials like carbon fiber nanotube yarns which are very expensive, has demonstrated the benefits of artificial muscle over human muscle in terms of strength and longevity. However, these benefits come at a cost. In the case of nylon fiber, the range of motion is severely limited and can only produce a bending action through the use of additional components like a pulley and a take-up reel. More flexible carbon fiber nanotubes and other exotic materials are too expensive for widespread use.

Seyed Mirvakili, a doctoral candidate, and Ian Hunter, the George N. Hatsopoulos Professor in the Department of Mechanical Engineering at MIT came up with a simpler solution. As in previous applications, the nylon fiber is subjected to heating to cause the fiber to contract. However, in this case, rather than using a round fiber, they changed the shape to a square. This configuration has lower thermal conductivity, so that when heat is selectively applied to one side of the fiber, that side will then contract more quickly than the other side. This results in the fiber bending in the direction of the heat source.

By changing the direction of heating, the researchers were able to make the fibers move in circles and figure eights and even more complex motions could also be easily achieved with this method. Better yet, according to the press release from MIT, the material can maintain its performance after at least 100,000 bending cycles, and can bend and retract at a speed of at least 17 cycles per second.

Heat can be applied to the fibers in a variety of ways, including electric resistance, chemical reactions, or a laser beam. The researchers also successfully used a special conductive paint which was applied to the fibers and held in place by a resin binder; when they applied a voltage to the material, heating the portion of the fiber below the paint, the fiber bent in that direction, illustrating the type of precision response that can be achieved with this solution.

This method “is novel and elegant, with very good experimental data supported by appropriate physics-based models,” says Geoffrey Spinks, a professor at the University of Wollongong in Australia, who was not connected with this research. “This is a simple idea that works really well. The materials are inexpensive. The manufacturing method is simple and versatile. The method of actuation is by simple electrical input. The bending actuation performance is impressive in terms of bending angle, force generated, and speed.”

For more information, visit http://news.mit.edu/2016/nylon-muscle-fibers-1123

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