As a cucumber plant grows, it sprouts tightly coiled tendrils that seek out supports in order to pull the plant upward. This ensures the plant receives as much sunlight exposure as possible. Now, researchers at MIT have found a way to imitate this coiling-and-pulling mechanism to produce contracting fibres that could be used as artificial muscles for robots, prosthetic limbs, or other mechanical and biomedical applications.
While many different approaches have been used for creating artificial muscles, including hydraulic systems, servo motors, shape-memory metals, and polymers that respond to stimuli, they all have limitations, including high weight or slow response times. The new fibre-based system, by contrast, is extremely lightweight and can respond very quickly, the researchers say.
The new fibres were developed by MIT postdoc Mehmet Kanik and MIT graduate student Sirma Örgüç, working with professors Polina Anikeeva, Yoel Fink, Anantha Chandrakasan, and C. Cem Taşan, among others, using a fibre-drawing technique to combine two dissimilar polymers into a single strand.
The key to the process is mating 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, Kanik, Örgüç and colleagues produced a fibre that, when stretched out to several times its original length, naturally forms itself into a tight coil, very similar to the tendrils that cucumbers produce. But what happened next actually came as a surprise when the researchers first experienced it. “There was a lot of serendipity in this,” Anikeeva recalled.
As soon as Kanik picked up the coiled fibre for the first time, the warmth of his hand alone caused the fibre to curl up more tightly. Following up on that observation, he found that 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 fibre returned to its original length. In later testing, the team showed that this process of contracting and expanding could be repeated 10,000 times “and it was still going strong,” Anikeeva said.
One of the reasons for that longevity, she said, is that “everything is operating under very moderate conditions,” including low activation temperatures. Just a 1 degree Celsius increase can be enough to start the fibre contraction.
The fibres can span a wide range of sizes, from a few micrometers (millionths of a meter) to a few millimetres in width, and can easily be manufactured in batches up to hundreds of meters long. Tests have shown that a single fibre is capable of lifting loads of up to 650 times its own weight. For these experiments on individual fibres, Örgüç and Kanik have developed dedicated, miniaturised testing setups.
Kanik said that the possibilities for materials of this type are virtually limitless, because almost any combination of two materials with different thermal expansion rates could work, leaving a vast realm of possible combinations to explore. He added that this new finding was like opening a new window, only to see “a bunch of other windows” waiting to be opened.