Researchers have created a chameleon-inspired smart skin that changes color when exposed to heat and sunlight, hoping that the new flexible material could find applications in camouflage, signaling, and anti-counterfeiting.
"Watching a chameleon change colours gave me the idea for the breakthrough," said Yixiao Dong, first author of a study published in the journal 'ACS Nano'.
"We've developed a new concept for a colour-changing smart skin, based on observations of how nature does it."
Besides chameleons, many other creatures such as the tetra fish have evolved the ability to change colour. But the colouration is not based on pigments, but on tiny particles in a repeating pattern, known as photonic crystals.
When light reflects off of these microscopic surfaces, it interferes with other beams of reflected light and produces a color, whose shade is altered when the distance between photonic crystals fluctuates.
For their study, researchers at the American Chemical Society embedded photonic crystals in flexible materials like hydrogels and managed to change their colors by contracting or expanding the material like an accordion.
However, such abrupt changes often compromised their performance due to too much strain.
"No one wants a camouflage cloak that shrinks to change colour," Salaita, senior author of the study, said.
That’s why the researchers led by Salaita set out to create a smart skin that was more flexible and resilient under changing conditions.
Studying time-lapse images of chameleon skin, the team of researchers found that only a small fraction of skin cells actually contains photonic crystals.
The colorless cells, the experts hypothesized, may help ease the strain on photonic crystals as they contract and expand.
"I wondered if we could design something similar -- a composite structure of photonic crystal arrays embedded into a strain-accommodating matrix," said Dong.
Although the new material changed color when it was heated, it was able to maintain its integrity and size.
Dong and his team also tested the material in sunlight, moulding SASS films into the shapes of a fish and a leaf.
When exposed to natural sunlight for 10 minutes, the SASS films shifted from orange to green, without changing in size.
"We've provided a general framework to guide the future design of artificial smart skins," Dong said. "There is still a long way to go for real-life applications, but it's exciting to push the field another step further.”