From Flat to Fat with a Twist
A coiled polymer fiber self-expands to become insulation depending on the temperature. Now researchers at the University of Texas at Dallas have found a way to make them on the cheap—by wrapping them around themselves.
Written by Michael Abrams
THE U.S. OLYMPIC TEAM WORE some pretty cool jackets to the Beijing Winter Olympics in 2022—or, rather, they were cool when they needed to be. Filled with coiled, high spring index, polymer, artificial muscles—essentially tightly wound fishing line—they stayed flat when warm and expanded when cold. In balmy weather the jackets were as thin as windbreakers, but when temperatures dropped, they swelled into puffer jackets. All without a sensor or a power source.
The jackets were perfect for environments where temperatures might vacillate between warm and extreme cold. But unfortunately, they had to endure another extreme: the price. Each Ralph Lauren jacket went for about four hundred dollars. Now, though, the same researchers who came up with the self-actuating thermally morphing springs have a new method for making them that is more efficient and far less expensive.
The tiny, coiled springs and the subsequent jackets were the result of research done at the Alan G. MacDiarmid NanoTech Institute at The University of Texas at Dallas. To create springs with the power to expand so dramatically, they twisted fishing line till it doubled on itself—much as the rubber band on a rubber-band-powered airplane begins to coil after the propeller has been turned enough times.
How the bilateral textile—featuring two inter-layer homochiral high-spring-index nylon PEC muscles in each buckled area—morphs when in contact with a hot plate at 43 °C. Video: Mengmeng Zhang
An infrared wavelength view that shows the bilateral textile going through thermal morphing. Video: Mengmeng Zhang
“A self-coiled muscle means you can just insert twist into a fiber, and if you over twist, you get a self-coiled structure—but this fiber is not useful for textiles,” said Mengmeng Zhang, a research scientist at the institute and lead author of the paper “Mandrel-free fabrication of giant spring-index and stroke muscles for diverse applications,” which appeared in Science earlier this year. “If you want to get an activation, you need to apply a very, very high-tension load to give the space between neighboring coils.”
To achieve that, the researchers initially wrapped fibers around a mandrel to produce their first coils, which had potential applications in energy storage, robotics, and morphing textiles. But the mandrel inside the coils was tightly sheathed—too tightly to be removed mechanically.
To separate shaft from coiled fiber, the researchers had to resort to chemical means. “The only way you can remove the mandrel is to use a chemical solvent to dissolve it,” Zhang said. “It’s not possible for you to just pull the mandrel from this structure.” The need to submerge each coiled fiber in a solvent is what kept production speed low, and the price high.
“Right now, this morphing textile is only used for cold temperatures. So now we can also incorporate our muscle in a morphing textile for insulation for hot temperatures.”
—Mengmeng Zhang, a research scientist at the University of Texas at Dallas
But now Zhang has found a way to make the coils without using a mandrel at all. In essence, the fibers are coiled around each other—one fiber becomes the mandrel for another. The spring index, or rigidity of the coil, can be controlled by how much twist is added during the process.
Once the fibers have been twisted and coiled, the plied yarns are put in an oven and subjected to high heat to set the plied coils. A second thermal annealing is then applied to further control the spring index of the pulled-out individual fiber muscles. Then, individual fibers can be easily pulled from the plied yarns.
These mandrel-free-fabricated coiled polymer fibers have spring indexes of: (A) 2.5, (B) 3.3, (C) 7.7, (D) 45.2, (E) 2.6 at the ends and 9.6 in the middle, and (F) where the spring index increases from 2.4 to 18.1. Individual fiber diameter is 0.28 millimeters. Image: Mengmeng Zhang
The end result is a self-expanding fabric inexpensive enough to soon be in jackets, blankets, and other forms of insulation, everywhere. And a less costly flat-to-puffy, self-controlling coat will likely find applications in unexpected places.
“Right now, this morphing textile is only used for cold temperatures,” Zhang said. “So now we can also incorporate our muscle in a morphing textile for insulation in hot temperatures. It’s very useful for firefighters’ clothes, since normally they work very close to the fire, where the temperature is really high. If they have this textile inside this jacket, or suit, it may actually protect their lives.”
Michael Abrams is an independent writer in New York.
As the temperature goes up, the textile contracts, and as the temperature drops, it expands. Image: Mengmeng Zhang
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