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Shutterstock | Jason Mintzer
Nature is so often science’s muse and over the years it has provided inspiration in many ways, whether it be the gecko’s foot triggering an idea for new adhesives or butterflies inspiring new solar cells. Chameleons too have been a source of inspiration, with scientists attempting to mimic their colour-changing skin.
Chameleons are amazing animals: they can change their colour in response to their surroundings, the temperature and how they are feeling – when they are frightened or feeling amorous for example. Research revealed their skin comprises of an array of non-close-packed guanine nanocrystals in iridophores, making them reflective or iridescent. They have the ability to alter their colour to any found in the full visible spectrum, a trait which could be incredibly useful in real-world applications such as biosensors, communication and in the military.
As a result, scientists have been developing materials capable of mimicking chameleon skin in response to various stimuli. As recently as November 2017, Korean researchers reported that they had designed photonic films containing a non-close-packed face-centred-cubic array of silica particles implanted in an elastomer – the result is a material which changes from red to blue under stretching or compression.
In 2015, a team from Stanford University in America stated they had developed a similar material – a stretchable electronic skin that changes colour when touched. The pressure-sensitive e-skin has been called the ‘closest thing yet to artificial chameleon skin’. Applying varying amounts of pressure to the e-skin causes a change in colour – the pressure indirectly alters the chemical structure and optical properties of the electrochromic materials. The two-layer skin consists of a stretchable microstructured polymer that changes its voltage upon the application of pressure, and a stretchable electronic polymer that is a shade of red or blue depending on the applied voltage.
The multi-step process alters the structure of the electrochromic polymer into a different chemical structure that emits light at a different wavelength, and therefore different colour. When pressure is applied, there is a decrease in electrical resistance on the pressure-sensitive polymer, which causes an increase in voltage to the electrochromic polymer and oxidises the material, slightly changing its chemical structure. This change is small but causes a large alteration in the material’s light absorption spectrum. The change is reversed simply by releasing the pressure.
Although the electrochromic polymer in this research only switched between shades of blue and red, researchers expect other electrochromic polymers could be designed to allow exchanging between any colour in the visible spectrum when differing pressure is applied. They believe that the e-skin could have applications smart robots and prosthetics where it could function as camouflage. It could also be used in systems where pressure is unknown as the colour change would correspond to a specific application of pressure. In the wider world, the e-skin could be incorporated into things we wear – clothes, smart phones/watches and other wearable devices – and the user could change the colour interactively for decoration or to express emotion.
A group of researchers in China announced in 2016 that they too had developed a material capable of mimicking the chameleon’s skin. The researchers designed a thermo-photochromic biomimetic material consisting of a photonic poly (N-isopropylacrylamide-co-methacrylic acid) copolymer and plasmonic nanoparticles. The use of gold and silver nanoparticles served to increase the sensitivity of responsive behaviour and to control the lower critical solution temperature of a thermosensitive film by tuning the polymer chain conformation transition.
The polymeric film with plasmonic nanoparticles is sensitive to the surrounding temperature, infrared and visible light. Light works to couple the thermosensitive film with the nanoparticles, causing a light to thermal conversion effect. The use of different types, size and morphology of the nanoparticles used to tune the plasmonic excitation wavelength means that the Chinese researchers tbelieve it will be possible for their material to respond across the entire visible spectrum. Their material could find uses in biosensors, military camouflage and stealth and information transmission, they state.
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