From hypercolor to structural colour: fabrics that change hue when stretched

The 1990s brought us heat-sensitive ‘hypercolor’ t-shirts, a technology which highlighted sweat patches and invited unwanted physical contact.

Thirty years on, and a team of engineers at Massachusetts Institute of Technology are gearing up for a new fad in fabrics which change colour when stretched.

By combining a 19^th century colour photography technique with modern holographic materials, the MIT research team has printed colours and images onto elastic fabric using ‘structural colour’, instead of chemical additives or dyes. Their results are detailed in Nature Materials.

Structural colour is a concept inspired by the light-reflecting properties in nature, in organisms like mollusc shells and butterfly wings which appear to shimmer and shift their colour. The effect is due to microscopic angled and layered surface structures which reflect light like miniature-coloured mirrors.

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Read more: How to print colours without ink

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Many researchers have tried to replicate structural colour in materials but achieving precision at a larger scale has been challenging.

“Scaling these materials is not trivial, because you need to control these structures at the nanoscale,” says the paper’s author and MIT graduate student Benjamin Miller.

The new technique represents the first scalable technique for printing detailed, large-scale materials using structural colour.

The research was inspired by a trip to the museum. After visiting an exhibition on holography, Miller was inspired to learn about holographics and early colour photography, including a technique called Lippmann photography invented in the late 1800s.

Physicist Gabriel Lippmann generated coloured photos by setting a mirror behind a very thin, transparent emulsion — a material concocted from tiny light-sensitive grains. He then exposed the setup to a beam of light, which the mirror reflected back through the emulsion. The interference of incoming and outgoing light waves reconfigured the emulsion’s grains to change position, which like tiny mirrors, reflected the pattern and wavelength of the exposing light.

Using his technique, Lippmann was able to produce structurally coloured images of flowers and other scenes. But despite earning its creator a Nobel Prize in Physics, the method was so laborious it had largely faded into history. Until recently.

Benjamin Miller wondered if, paired with modern, holographic materials, Lippmann photography could be sped up to produce large-scale, structurally coloured materials? Like Lippmann’s emulsions, holographic fabrics consist of light-sensitive molecules that behave in a similar way when exposed to incoming light.

To test the concept, the MIT team adhered elastic, transparent holographic film onto a reflective, mirror-like surface (in this case, a sheet of aluminium). They then placed an off-the-shelf projector several feet from the film and projected images onto each sample, including Lippman-esque bouquets.

Within several minutes, the films were printed with detailed pictures vividly reproducing the full spectrum of colours in the original images.

After peeling the film away from the mirror and sticking it to a silicone backing for support, the researchers found they could stretch the film and watch its appearance change — an effect arising from the material’s structural colour.

In further testing, the MIT researchers printed other films with flower bouquets capable of morphing from warm to cooler shades when stretched.

They found they could also print hidden images – like a strawberry or a fingerprint – onto the film by tilting it at an angle to the incoming light. When stretched, the fabric’s secrets can be revealed.

Beyond creating new fads and fabrics, materials painted with structural colour could be used to monitor bandage pressure levels when treating conditions such as venous ulcers and certain lymphatic disorders.

“Imagine checking an arm band’s colour to gauge muscle mass. Or sporting a swimsuit that changes hue as you do laps,” the paper’s press release enthuses. And after the experience with hypercolor, one can also forsee the potential for colour-changing stretch fabric to highlight body parts with flattering – or more likely unflattering – results.