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A New Twist on Artificial Muscles

Even as electronics have shrunk more and more, motors, hydraulics and other gadgets used to drive motion have stubbornly resisted the trend. It is difficult to make and assemble minuscule mechanisms that can provide the forces and handle the stresses needed to drive exceptionally small moving parts. This week in Science, several teams of researchers present studies describing advances in making small artificial muscles—all of which use tiny twisted fibers to store and release energy. The fibers could be employed in everything from miniature robots to valves in medical devices.

These fibers, which often include lightweight polymers such as nylon or high-density polyethylene, can be more powerful, based on their weight, than human muscles. As they contract, some can lift more than 1,000 times their own mass, says Sameh Tawfick, a mechanical engineer at the University of Illinois at Urbana-Champaign. The fibers enable engineers to store a lot of energy in a small space, which “lets them do things they can’t otherwise do,” notes Tawfick, who co-authored a perspective on the studies published in the same issue of Science.

One of the new artificial muscle designs is, in essence, a small, high-tech version of the rubber bands used to propel balsa-wood airplanes. But these fibers do not require winding each time they are used, says Jinkai Yuan, a materials scientist at the University of Bordeaux in France and a co-author of one of the studies. Instead they are made of a “shape memory” polymer that twists and untwists as the temperature of the material changes.

Weight-lifting performance of individual and bundled fiber muscles actuated via a heat gun. Credit: Mehmet Kanik and Sirma Orguc Massachusetts Institute of Technology

Here is how Yuan’s team made its muscles: First, the researchers heated a two-centimeter-long, 40-micron-diameter fiber of a material called polyvinyl alcohol (PVA) above its so-called programming temperature. (Above this temperature, the material naturally takes one shape; below it, the material can take another. If temperatures fluctuate about this threshold, the material alternates between the two shapes.) After twisting the fiber to store energy, they cooled it to freeze its shape. When the fiber was once again heated above its programming temperature, it quickly untwisted to its original shape, Yuan says.

Although a PVA fiber could store a substantial amount of energy, the team found that adding three-to-five-micron-size flakes of graphene oxide to the material allowed it to lock away even more. That is because those flakes would flex—and thus store energy, as a spring might—when the fiber was first twisted but then release that energy as it untwisted. In the team’s lab tests, an untwisting fiber spun a bit of paper at 600 revolutions per minute for a full five seconds. To demonstrate the fiber’s energy-storage capability, the team used one to propel a toy boat. On a more practical note, this sort of artificial muscle could also open and shut tiny valves in medical devices, Yuan suggests.

Whereas the fibers made by Yuan and his colleagues provide torque as they twist and untwist, the artificial muscles developed by other teams work more like real muscles: they do work by pulling on or lifting objects. A team led by researchers at the Massachusetts Institute of Technology created fibers that can stretch more than 1,000 percent of their initial size and lift more than 650 times their own weight. They operate on a principle similar to the bimetallic strips in early thermostats: the fiber is made by bonding two materials that expand at radically different rates as the temperature of their environment changes, says Polina Anikeeva, a materials scientist at M.I.T. and senior author of that study.

Stretching a single fiber-based artificial muscle and an artificial bicep made of 100 fiber muscles. Credit: Mehmet Kanik and Sirma Orguc Massachusetts Institute of Technology

Her team’s new artificial muscle contains a high-density polyethylene (HDPE), the same sort of plastic used to make recyclable bottles. It also has another material, a stretchy type of polymer known as an elastomer, Anikeeva says. As small blocks of these substances are heated and drawn through a narrow nozzle, they bond and are stretched into a long, thin fiber. When tension in the fiber is released, the elastomer shrinks back to its original size. That change, in turn, causes the fiber to coil into a springlike shape resembling an old phone cord. As the fiber is heated or cooled, the HDPE expands or contracts about five times faster than the elastomer to which it is bonded, which tends to shorten or increase the overall length of the coiled fiber, respectively.

When Anikeeva and her colleagues heated one of their fibers by 14 degrees Celsius over four seconds, the artificial muscle shrank in overall length a whopping 50 percent. In other tests, the team heated and cooled fibers to lift light weights or flex a small robotic arm. Although those tests lifted gram-size weights, massive bundles of such fibers could be used to perform heavier lifting or tugging, Anikeeva says. Larger-diameter fibers, or bundles of them, could find uses in robotics or prosthetic limbs, she notes.

Another team reporting its work in this week’s Science tackled artificial muscles in a totally different way. Although its devices were built around a core of twisted fibers, the active part of the muscle was actually a thin sheath of material surrounding the core. Using such a sheath had several benefits, says Ray Baughman, team leader and a materials scientist at the University of Texas at Dallas. For one thing, he notes, it allows engineers to use cheaper materials for a fiber’s core. He and his colleagues have developed sheath-driven muscles built around cores made of nylon, silk and bamboo yarns. Their tests show that the choice of material for a fiber’s core does not dramatically impact its performance.

Artificial limb is driven by two fiber-based muscles actuated via a heat gun. Credit: Mehmet Kanik and Sirma Orguc Massachusetts Institute of Technology

There are other reasons to build sheath-driven muscles, Baughman says. The outside of the fiber is where environmental stimuli, such as humidity or the presence of certain substances driving its motion, will be more quickly felt, he explains. Also, swelling and shrinkage in the sheath, which is farthest from the center of the fiber, will exert more leverage than equivalent changes near the fiber’s core.

Unlike the other teams, Baughman and his colleagues developed fibers that respond to more than just changes in temperature. Some sported muscle sheaths that swell when exposed to ethanol vapor; others were veneered with a material that shrinks when soaked in a glucose solution. These sorts of fibers could be used to open or close valves in medical devices or to squeeze a small pouch and dispense a drug. Fibers that respond to sweat or water vapor could be woven into “smart fabrics” that adjust the tightness of their weave to become more breathable in hot, humid conditions, Baughman says. Alternatively, coatings that respond to noxious vapors could tighten a fabric’s weave to protect people responding to a chemical spill.

“I’m extremely excited about the developments” reported by these teams, Tawfick says. “This technology has a very bright future.”

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