Chameleon Skin Powered by Nanomachines

The artificial material is made of tiny particles of gold coated in a polymer shell, squeezed into microdroplets of water in oil. (Source: U. Cambridge)

Researchers 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. The material, developed by researchers from the University of Cambridge, is made of tiny particles of gold coated in a polymer shell, and then squeezed into micro­droplets of water in oil. When exposed to heat or light, the particles stick together, changing the colour of the material.

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 arti­ficial chromato­phores are built on the same principle, but instead of contrac­tile fibres, their colour-changing abilities rely on light-powered nano-mechanisms, and the cells are micro­scopic drops of water.

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 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, 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 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.” (Source: U. Cambridge)

Reference: A. R. Salmon et al.: Motile Artificial Chromatophores: Light‐Triggered Nanoparticles for Microdroplet Locomotion and Color Change, Adv. opt. Mat., online 20 August 2019; DOI: 10.1002/adom.201900951

Link: Nanophotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK

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