Fabrics that change color on demand could help soldiers blend into their environments and could enable wearable displays and sensors. In a step toward such textiles, researchers have made flexible fibers that turn red, green, blue, or yellow when triggered by an electric voltage (ACS Appl. Mater. Interfaces 2014, DOI: 10.1021/am502929p). Unlike previous color-switching threads, the new fibers work with low voltages, change color in milliseconds, and retain their hue for about a half hour. What’s more, they work when twisted, knotted, and braided.
The new fibers use so-called electrochromic materials that reversibly change color under an applied voltage. Smart glass used in building windows sometimes use electrochromic metal oxides, such as tungsten trioxide, to switch between transparent and sun-blocking dark colors.
For color-changing fibers in textiles, some researchers want to use electrochromic polymers such as polyanilines and polythiophenes. One group recently reported coaxial polymer filaments with an outer layer that changes colors when the conductive core heats up. But these materials ran on 50 V, which would require bulky batteries. The new color-switching fibers, by contrast, need between 1.4 and 2.6 V, which can be produced by coin cells commonly used in watches.
To make the new fibers, Yaogang Li, Hongzhi Wang, and their colleagues at Donghua University, in China, picked three different electrochromic polymers that are known to respond quickly to low voltages and can make many color changes over a long period of time. When constructing the fibers, the team used a spiral-wound design that has been used for fiber-based photovoltaics. They first coated 0.1-mm- and 0.3-mm-thick stainless steel wires with one of the three polymers: poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3-methylthiophene) (P3MT), and poly(2,5-dimethoxyaniline) (PDMA). The researchers then dipped the wires in a polymer gel electrolyte containing perchlorate ions. Finally, they twisted finer stainless steel wires around the coated ones and dipped the whole fibers in the electrolyte again.
When a positive or negative voltage is applied to the wires, perchlorate ions from the electrolyte move into or out of the electrochromic polymer, doping or undoping it. This causes the polymer to change color. The PEDOT-coated fibers switch between gray and dark blue, P3MT fibers change between blue and red, and the PDMA ones turn green and yellow.
The fine steel wire and the gel polymer give the fibers strength and flexibility, Wang says. He adds that the fibers could be combined to produce multicolor fabrics that show high-resolution color change.
Producing color changes with a low driving voltage is impressive, says Huisheng Peng, a macromolecular scientist at Fudan University, in China. The low voltage makes the fibers safe for use in clothing, he says.
Alexis Laforgue, a materials researcher at the National Research Council Canada, says that the fibers need further processing to make them robust and long lasting. What’s exciting, he says, is that other research teams have developed fiber-based batteries with similar structures, paving the way for both the energy source and the electrochromic system to be sewn into fabric. Such combinations could enable interesting applications, he says.