Colour-changing 'chameleon skin' powered by nanomachines

Researchers from the University of Cambridge have developed artificial ‘chameleon skin’ that changes colour when exposed to light and could be used in applications such as active camouflage and large-scale dynamic displays. Their work has been described in the journal Advanced Optical Materials.

In nature, animals such as chameleons and cuttlefish are able to change colour thanks to chromatophores: skin cells with contractile fibres that move pigments around. The pigments are spread out to show their colour or squeezed together to make the cell clear.

The researchers’ material, on the other hand, is made of tiny particles of gold coated in a polymer shell and then squeezed into microdroplets of water in oil. When the material is heated above 32°C, the nanoparticles store large amounts of elastic energy in a fraction of a second, as the polymer coatings expel all the water and collapse. This has the effect of forcing the nanoparticles to bind together into tight clusters. When the material is cooled, the polymers take on water and expand, and the gold nanoparticles are strongly and quickly pushed apart, like a spring.

“Loading the nanoparticles into the microdroplets allows us to control the shape and size of the clusters, giving us dramatic colour changes,” said co-first author Dr Andrew Salmon, from Cambridge’s Cavendish Laboratory.

The geometry of the nanoparticles when they bind into clusters determines which colour they appear as: when the nanoparticles are spread apart they are red and when they cluster together they are dark blue. However, the droplets of water also compress the particle clusters, causing them to shadow each other and make the clustered state nearly transparent.

At the moment the material is in a single layer, and so is only able to change to a single colour. However, different nanoparticle materials and shapes could be used in extra layers to make a fully dynamic material, like real chameleon skin.

The researchers also observed that the artificial cells can ‘swim’ in simple ways, similar to the algae Volvox. Shining a light on one edge of the droplets causes the surface to peel towards the light, pushing it forward. Under stronger illumination, high-pressure bubbles briefly form to push the droplets along a surface.

“This work is a big advance in using nanoscale technology to do biomimicry,” said study co-author Sean Cormier. “We’re now working to replicate this on roll-to-roll films so that we can make metres of colour-changing sheets. Using structured light we also plan to use the light-triggered swimming to ‘herd’ droplets. It will be really exciting to see what collective behaviours are generated.”

This story is a modified version of a news item published by the University of Cambridge under CC BY 4.0.

Image credit: ©stock.adobe.com/au/Jan

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