Nylon fibers used to make artificial muscle

MIT mechanical engineers have developed an easy and cheap method of creating artificial muscle fibers.

In the new study, published in the journal Advanced Materials, the scientists used nylon fiber to replicate natural muscle fibers. Artificial muscles are materials or devices that contract and expand in response to an external stimulus like voltage, pressure or temperature.

Researchers have previously shown that artificial muscles could extend and retract further than normal muscles. Artificial muscles are an emerging technology that have many uses from biomedical robotics to the aviation and automobile industries.

Nylon is cheap and easily accessible, and scientists have previously used twisted coils of nylon to create artificial fibers. The advancement of the new study lies in the ability of the nylon to reproduce the bending motions of muscle fibers.

When heated, fibers shrink in length but increase in diameter. To turn that quality into a bending motion would previously have required a pulley and other extraneous mechanical parts. But the new study uses nylon heating to their advantage. The team figured out that they could produce a bending motion by using voltage to heat one side of the fiber.

They began by modifying the cross-section of nylon fishing line fibers, changing its shape from round to square. Then they tried controlling from which direction they heated the fiber. Depending on the direction, one side would begin contracting before the heat could reach the other side, producing a bending motion. They were able to make more complex motions with the fibers, including figure eights and circles.

“The cooling rate can be a limiting factor. But I realized it could be used to an advantage,” Seyed Mirvakili, a Ph.D. student at MIT and the lead on the study, said in a press release.

In the past, scientists have used materials that were very expensive and very difficult to make. The materials should also be long-lasting and able to go through many contraction cycles. Researchers have tried to use carbon nanotubes, which are incredibly long-lasting but are very expensive. Others have tried using shape-memory alloys which do not last very long. Nylon is both long-lasting and can expand and contract quickly.

According to Ian Hunter, a mechanical engineering professor at MIT, the fibers might be used to make self adjusting shoes that tighten when you put them on or change in shape and stiffness as you walk. He further suggested that we might be able to use fibers for clothes that contract to fit the contours of an individual body, to reduce the number of sizes a manufacturer has to make.

“[This method] is novel and elegant, with very good experimental data supported by appropriate physics-based models,” Geoffrey Spinks, a professor at the University of Wollongong in Australia, said in a press release. “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.”

“Bending-type actuators are needed for robotic grippers, microscopic tools, and various machine components. These new bending actuators could have immediate application,” Spinks adds.

The new nylon fibers can be used in biomedical devices like self-adjusting catheters. In the automobile and airplane industries, mechanical systems can be fundamentally changing. Tracking systems for solar panels may use excess heat to keep the panels aimed the sun. And the outside of cars may adjust their aerodynamic shape to adapt to changes in speed and wind conditions.

“[These are] exciting and game-changing findings,” Andrew Taberner, an associate professor of bioengineering at the University of Auckland in New Zealand, said in a press release. “One can imagine many applications for this type of actuator in the medical and instrumentation fields. I expect that this work will become highly cited.”