Researchers have spun fishing line and sewing thread into strong actuators, or artificial muscles, that are capable of lifting loads 100 times heavier than human muscles of the same length and weight can manage.
These new actuators also respond to a number of stimuli, including temperature, and they may find applications in a wide range of robotics and prosthetics in which various degrees of strength and precise control are needed.
Carter Haines from the University of Texas at Dallas and colleagues from around the world, who designed these new actuators, demonstrated their potential by making textiles and window shutters. Lined with these coiled muscles, the shutters and textiles gauge heat and respond accordingly. The shutters, for example, open and close without electricity to keep the temperature inside a building or greenhouse comfortable.
The researchers' findings are published in the 21 February issue of Science.
"Taking these fibers and converting them into muscles is simple," explained Ray Baughman, a co-author of the Science report. "Just connect one end to a motor and the other end to a weight so that it doesn't move, and insert some twist. As the fiber coils, the 'muscle' will dramatically contract."
Specifically, the researchers show that high-strength polymer fibers, like those used for fishing lines and sewing threads, can be transformed into efficient actuators by simply twisting them until they coil up. This extreme twisting allows the fibers to function as torsional muscles, they say. The muscles contract when they are heated and relax when they are cooled down.
These new, simple actuators are much cheaper—but comparably efficient—to shape memory polymers and shape memory alloys, which can switch quickly back and forth between their deformed and original shapes and are often used as artificial muscles.
"You can buy a kilogram of ultra-high-strength fibers in bulk for five dollars," Baughman said. "High school students who want to do an experiment in their living room can get these fibers at a local store."
Haines and his team of researchers suggest that their actuators might be used to control the small, sensitive muscles in a prosthetic limb or the subtle facial expressions of a humanoid robot.
"Prosthetic limbs and exoskeletons are clunky devices because they use motors and hydraulic systems," Baughman continued. "But we can take many fibers and put them together when a lot of force is needed, or just use a single muscle strand, or even 50 strands that are singly controlled, when only a little bit of force is needed."
The detailed control these new actuators provide may help someone with a prosthetic hand to catch a ball, for example. "You can't catch a ball with a rigid hand; it bounces from your glove," Baughman said. "But we can change the stiffness of these muscles by a factor of 20, so we can generate a range of muscles with a range of compliance and actuation at the same time."