Take a rubber band and twist it. Keep twisting it until it starts to collapse onto itself and form larger loops—it's something you can do with almost any strand-like structure. Now, scientists from the University of Texas at Dallas in Richardson are taking advantage of this property in everyday materials such as fishing line and sewing thread and using it to make artificial muscles.
The scientists took pieces of fiber that were a few hundred micrometers long and twisted them until they began to coil. As the pieces coiled, the twisted fibers became shorter and thicker; once tightly coiled, the scientists heat-treated them to prevent the fibers from unfolding. If heat is applied to the finished coil after this procedure, the individual fibers try to untwist. The untwisting causes the coils to expand in volume as they shorten in length, just like a muscle.
The researchers found that if they made the fiber form larger coils in the same direction as the initial twists, the fibers contracted. If the fibers were made to coil in the opposite direction from the twist, the fibers expanded. By combining large quantities of these twisted fibers, the team could produce artificial muscles with above-average characteristics.
In their study, the scientists compared their artificial muscles to natural ones. Biological muscles contract to only about 20 percent of their length, while these artificial muscles contract to over 50 percent of their length. In addition, the synthetic versions can lift loads over 100 times heavier than human muscles of the same length and weight can handle. The twisted fibers can generate over 5 kilowatts of mechanical work per kilogram of muscle weight, which is similar to the output of a jet engine.
These are not the first artificial muscles to have been created, but they are among the first that are inexpensive and store large amounts of energy. The team that developed them believes the heat-dependent contraction, low cost, and the ability to store large amounts of energy make these fibers ideal candidates for a huge range of applications, including medical devices, clothing, prosthetic limbs, and even home automation. Some day, your blinds may open and close on their own as coiled fibers respond to the weather.