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Chameleon inspires smart skin

Hydrogel studded with photonic crystals changes color in sunlight

by Bethany Halford
September 14, 2019 | A version of this story appeared in Volume 97, Issue 36


Two photos of the same fish-shaped object. The body of the fish in the top image is yellow and the body in the bottom image is green.
Credit: ACS Nano
When exposed to sunlight, a chameleon-inspired smart skin changes from yellow (top) to green (bottom).

Chameleons’ color-changing camouflage arises from photonic crystals embedded in their skin. By tensing or relaxing their skin, the reptiles can change the space between the crystals and generate different colors to blend into their surroundings. Scientists have tried to copy this structure by embedding photonic crystals in hydrogels, but these materials typically need dramatic deformations to change color, which can strain the materials. Chameleons, on the other hand, change color through subtle movements. That prompted Emory University’s Khalid Salaita and colleagues to study time-lapse images of chameleon skin in action. They noticed that only a fraction of the reptiles’ skin cells contain photonic crystals, and the rest are colorless. They reasoned that these colorless cells accommodate the strain that occurs as the photonic crystals move to change the chameleon’s color. Inspired by this, the researchers created arrays of photonic crystals in a hydrogel and then embedded those arrays in a second, colorless hydrogel that acts as a supporting layer (ACS Nano 2019, DOI: 10.1021/acsnano.9b04231). The resulting material shifts from yellow to green in sunlight, which could be useful for camouflage and anticounterfeiting applications.


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