Chameleons and photonic crystals

Using photonic crystals, chemists recently developed a flexible smart skin which reacts to heat and sunlight while maintaining a near constant volume. By using photonic crystals in its skin, a chameleon can alter the colour of its skin so it either ‘blends into the background to hide or stands out to defend its territory and attract a mate’. However, until now, scientists have struggled to mimic this phenomenon, to make a photonic crystal ‘smart skin’ which changes colour in response to the environment, without also changing in size.

The colouration in organisms such as chameleons is not based on pigments, but instead on tiny particles in a repeating pattern, known as photonic crystals. The periodicity in these particles is said to cause the material ‘to interfere with wavelengths of light’. Although the particles themselves are colourless, the precise spacing between them allows certain light waves to pass through them while rejecting others. The visible colours which are produced change depending on factors such as lighting conditions or shifts in the distance between the particles.

By analysing close-up, time-lapsed images of the chameleon changing hues, the researchers noticed that the arrays of photonic crystals did not cover the entire skin but were spread out within a dark matrix. As the photonic crystals changed colours, these patches of colour remained the same distance apart. By using magnets to arrange patterns of photonic crystals containing iron oxide within a hydrogel, they embedded these arrays into a second, non-colour-changing hydrogel. This second, springy hydrogel was mechanically matched to the first hydrogel as to compensate for shifts in distances between the photonic crystals. When heated, this ‘strain-accommodating smart skin’ (SASS) successfully changes colour while maintaining a near-constant size.

Students applying for Chemistry, as well as applicants for Materials Science, can consider how this novel method provides a general framework which may ‘guide the future design of artificial smart skins’, advancing this scientific field another step further.