While different approaches have been used to create artificial muscles — including hydraulic systems, servomotors, shape-memory metals, and polymers that respond to stimuli — they all have limitations such as high weight or slow response times. A fiber-based system was developed that is extremely lightweight and can respond very quickly.
The new fibers were developed using a fiber-drawing technique to combine two dissimilar polymers into a single strand of fiber. The process mates together two materials that have very different thermal expansion coefficients — meaning they have different rates of expansion when they are heated. This is the same principle used in many thermostats, for example, using a bimetallic strip as a way of measuring temperature. As the joined material heats up, the side that wants to expand faster is held back by the other material. As a result, the bonded material curls up, bending toward the side that is expanding more slowly.
Using two different polymers bonded together — a very stretchable cyclic copolymer elastomer and a much stiffer thermoplastic polyethylene — the team produced a fiber that, when stretched out to several times its original length, naturally forms itself into a tight coil. When the coiled fiber was picked up for the first time, the warmth of the researcher’s hand caused the fiber to curl up more tightly. Even a small increase in temperature could make the coil tighten up, producing a surprisingly strong pulling force. Then, as soon as the temperature went back down, the fiber returned to its original length. In later testing, the process of contracting and expanding could be repeated 10,000 times. Just a 1 °C increase can be enough to start the fiber contraction.
The fibers can span a wide range of sizes, from a few micrometers to a few millimeters in width and can easily be manufactured in batches up to hundreds of meters long. Tests have shown that a single fiber is capable of lifting loads of up to 650 times its own weight. The degree of tightening that occurs when the fiber is heated can be “programmed” by determining how much of an initial stretch to give the fiber. This allows the material to be tuned to exactly the amount of force needed and the amount of temperature change needed to trigger that force.
The fibers are made using a fiber-drawing system, which makes it possible to incorporate other components into the fiber itself. Fiber drawing is done by creating an oversized version of the material, called a preform, which is then heated to a specific temperature at which the material becomes viscous. It can then be pulled to create a fiber that retains its internal structure but is a small fraction of the width of the preform.
Such fibers could find uses as actuators in prosthetic limbs, where their slight weight and fast response times could provide a significant advantage. Some prosthetic limbs can weigh as much as 30 pounds, with much of the weight coming from actuators that are often pneumatic or hydraulic; lighter-weight actuators could make life easier for those who use prosthetics. Such fibers might also find uses in tiny biomedical devices such as a medical robot that works by going into an artery and then being activated.
To provide greater strength for lifting heavier loads, the fibers can be bundled together, much as muscle fibers are bundled in the body. The team successfully tested bundles of 100 fibers. Through the fiber drawing process, sensors could also be incorporated in the fibers to provide feedback on conditions they encounter such as in a prosthetic limb. Bundled muscle fibers with a closed-loop feedback mechanism could find applications in robotic systems where automated and precise control are required.