Greg Sotzing of the University of Connecticut in Storrs has developed threads of electrochromic polymers, which change colour in response to an applied electric field. The knittable, washable thread could be used to create T-shirts that change colour or pattern to suit your mood or outfit.
A mixture of differently coloured threads would be knitted or woven into a T-shirt or blanket, along with a small number of thin metal wires connected to a battery pack and a microcontroller. The crisscrossing wires effectively divide the shirt into pixels (see Diagram).
Each coloured thread changes its state at different voltages, so by varying the voltage between different pairs of wires you can change the colour of each pixel. By connecting the controller to a camera, it could even be made to switch the pixels to display a pattern matching your surroundings.
Electrochromic polymers are coloured because the electrons in their chemical bonds can absorb light across a range of visible wavelengths. When a voltage is applied it changes the energy levels of these electrons, causing them to absorb light of a different wavelength, and changing the material's colour. When the voltage is reversed, the electrons return to their usual energies and the original colour returns.
Previously researchers have used dissolvable moulds to shape electrochromic polymers into thin cylinders. However, this only allowed them to create "fibres" a tenth of a millimetre long, far too short to be woven or knitted into a fabric.
Sotzing's secret is to spin the polymer, allowing him to make continuous threads up to 1 kilometre long. He and his colleagues used a process called electrospinning, in which a solution of a polymer is squirted out of a nozzle and drawn out into a thread by its electrostatic attraction to a nearby charged target. While in solution at high pressure each polymer molecule remains separate from the others, but as the solution leaves the nozzle, the pressure drops and the solvent evaporates. This causes the polymer chains to tangle together into a single slender thread, just like a rope made of intertwined fibres.
However, while this process works well for long, flexible polymer chains such as polyester, electrochromic polymer molecules are short and rigid and so do not entangle as the solvent evaporates. Sotzing's answer is to start with a long, flexible, nonelectrochromic polymer and add groups of carbon and sulphur atoms to it. Once the polymer has been spun into strands, an oxidant is added that causes these groups of atoms to react with each other. This forms cross-linking bonds between strands, giving the thread colour and, crucially, making it electrochromic. "These are the first long fibres that have the ability to change colour," says Sotzing.
At the moment the fibres change from orange to blue and from red to blue, but Sotzing also hopes to create threads that change from red, blue and green to white.
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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 8 APRIL 2006
Author: Celeste Biever
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