Flexible film can change colour like a chameleon
Chameleons can famously change their colour to camouflage themselves, communicate and regulate their temperature — a trait that scientists have been attempting to replicate for use in stealth technologies, anti-counterfeiting measures and electronic displays. Now, researchers from Sichuan University have developed a flexible film that changes colour in response to stretching, pressure or humidity.
By tensing or relaxing their skin, chameleons can change the way light reflects from guanine crystals under the surface, producing what’s known as structural coloration. These structural colours are different from the pigments that give many other creatures their hues. Scientists have mimicked the crystalline nanostructures of chameleon skin in various colour-changing materials, but they’re either difficult to produce or they rely on non-renewable petroleum resources. In contrast, cellulose nanocrystals are a renewable material that can self-assemble into a film with iridescent structural colours; however, the films are typically fragile and, unlike chameleon skin, can’t be stretched without breaking.
Fei Song, Yu-Zhong Wang and colleagues wanted to develop a highly flexible film made of cellulose nanocrystals that changes colour when stretched. They reported their results in the journal ACS Applied Materials & Interfaces.
To increase the flexibility of cellulose nanocrystals, the researchers added a polymer called PEGDA and used UV light to crosslink it to the rod-shaped nanocrystals, producing films with bright iridescent colours ranging from blue to red, depending on the PEGDA amount. The films were both strong and flexible, stretching up to 39% of their original length before breaking.
During stretching, the colour of one film gradually changed from red to green, and then changed back when relaxed. According to the researchers, this is the first time that stretching- and relaxing-induced, reversible structural colour changes that are brilliant and visible to the naked eye have been realised for cellulose nanocrystal materials. The film also changed colour with pressure and humidity, allowing the team to show or hide writing made by an inkless pen.
The new bio-based smart skin could find applications in strain sensing, encryption and anti-counterfeiting measures, the researchers say.
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