Researchers have once again found inspiration in nature to improve the design of materials, this time for programmable fibers for artificial muscles that demonstrate an unprecedented amount of pulling force, they said.

A team at MIT were inspired by the growth of a cucumber plant, which sprouts tendrils that seek support to pull the plant upward, enabling it to receive optimal sunlight exposure. Scientists—including MIT Professor Polina Anikeeva, MIT postdoc Mehmet Kanik, and MIT graduate student Sirma Örgüç—used this example to develop a new mechanism that alternately coils and pulls to produce contracting fibers that can be programmed for use in artificial muscles for robots, prosthetic limbs, or other mechanical and biomedical applications, they said.

Researchers already have used myriad approaches to create artificial muscles; some of these include hydraulic systems, servo motors, shape-memory metals, and polymers that react to external stimuli. However, so far all of these approaches are limited in various ways, including being too heavy or responding too slowly.

The new approach taken by the MIT team uses a fiber-drawing technique to combine two dissimilar polymers into a single strand of fiber, creating a system that is very lightweight and also highly responsive, researchers said.

Common Approach, New Invention

Researchers took an approach already used commonly to measure temperature in thermostats—combining two materials that have different thermal-expansion coefficients, or different rates of expansion when heated.

When these joined materials get hot, the side that wants to expand faster is held back by the other. This causes the material to curl up, bending toward the side that expands more slowly.

Specifically, MIT researchers used a very stretchable cyclic copolymer elastomer and a much stiffer thermoplastic polyethylene as their materials. They bonded them together to produce a fiber that--when stretched out to several times its original length--naturally formed a tight coil much like the cucumber tendrils.

Researchers discovered the potential for the fibers’ strength almost by accident, Anikeeva said in a press statement. “There was a lot of serendipity in this,” she said.

What happened was that researcher Kanik picked up the coiled fiber for the first time and noticed the warmth of his hand caused it to curl even more tightly. Upon further observation, researchers realized that increasing the temperature of the material made the coil tighten even more, creating a strong pulling force. Then, when the material cooled, the fiber returned to its original length.

Programmable and Versatile

Expanding on the concept further, researchers realized that they can program the degree of tightening that occurs when the fiber is heated by deciding how much of an initial stretch to give the fiber. This means they can tune the material to exactly the amount of force needed for an applications as well as the amount of temperature change needed to trigger that force.

Researchers published a paper on their work in the journal Science.

In tests, the fibers--which can span a wide range of sizes from a few micro-millimeters to millimeters--showed both longevity and extreme capability for lifting loads, researchers reported. Experiments proved that a single fiber can lift loads of up to 650 times its own weight, and the fibers maintained their ability to contract and expand for at least 10,000 cycles, they said.

Also, because the fibers are created on a fiber-drawing system, researchers can incorporate other components into the fiber itself, they said. For instance, in testing, researchers coated the fibers with meshes of conductive nanowires that can be used as sensors to reveal the exact tension experienced or exerted by the fiber, they said.

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The fibers also in the future could include optical fibers or electrodes that can heat the material internally without having to rely on any outside heat source to activate artificial-muscle contraction, researchers said. Moreover, they can be bundled together similarly to human muscle fibers--another aspect that makes them well-suited for use in artificial muscles for robots as well as human prosthetics, researchers said. 

Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.