Replicating how chameleons do it, scientists develop colour-changing 'smart skin'

Imagine having skin that changes colour while reacting to light or other stimuli. This is very well seen as an ambition for some scientists who say that they have developed a flexible 'smart skin' that changes its colour when it comes in contact with sunlight and heat. The researchers had been conducting a study on animals like chameleons who can adjust their skin colour to blend in with an object it is in contact with.

Not colour pigments but photonic crystals

A study was published in the peer-reviewed journal ACS Nano which argued that hues of a chameleon's skin rely not on dyes or pigments as most colours do but instead on arrays of tiny structures known as photonic crystals. Light reflects from these microscopic surfaces and interferes with other beams of reflected light, producing a colour. When the distance between photonic crystals varies, the hue changes – for example when a chameleon relaxes or tenses its skin. The iridescence of some butterfly wings and the feathers of peacocks are among many other examples of photonic crystals in nature.

Yixiao Dong, a PhD candidate in Emory University's Department of Chemistry and one of the authors of the research, said that watching a chameleon change colours gave him the idea for the breakthrough. "We've developed a new concept for a colour-changing smart skin, based on observations of how nature does it."

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Scientists mimic natural action

To replicate this natural phenomenon, scientists have embedded photonic crystals in flexible water-containing polymers, or hydrogels and changed their colours by expanding or contracting the material like an accordion. However, these large fluctuations in size can lead to structural instability and buckling of the material. Another senior author of the paper, Khalid Salaita noted, "No one wants a camouflage cloak that shrinks to change colour."

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YouTube video helps crack the puzzle

Faced with this obstacle, Yixiao Dong began watching time-lapse images of chameleon skin on YouTube to crack the problem. He then observed that only a small fraction of skin cells actually contain photonic crystal arrays, while the rest are colourless. The researchers concluded that the colourless skin cells might help accommodate the strain when the photonic crystals expand and contract.

"I wondered if we could design something similar – a composite structure of photonic crystal arrays embedded into a strain-accommodating matrix," Dong said.

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Time for experiment

Inspired by this observation, the researchers patterned arrays of photonic crystals in a hydrogel and then embedded these arrays in a second, non-colour-changing hydrogel that acted as a supporting layer. When heated, this strain-accommodating smart skin (SASS) changes colour but maintains a near-constant size. The scientists experimented this by exposing the hydrogel to sunlight for 10 minutes and they were charmed to see the smart skin change colour from orange to green without changing in size, similar to how tetra-fish does.

Potential tech

"We've provided a general framework to guide the future design of artificial smart skins," Dong said. He added, "There is still a long way to go for real-life applications, but it's exciting to push the field another step further." Khalid Salaita noted that in future, smart skin can find applications in camouflage, signalling, and anti-counterfeiting.

(With agency inputs)

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