“The hydrogel rapidly changes color based on light or heat input and it does so in a fully reversible manner across the visible or near-infrared spectrum”Khalid Salaita
Smart skin material that changes color in response to heat and lightScientists at Emory University and Georgia Institute of Technology in the US have developed a flexible smart skin based on how the chameleon changes color to hide, intimidate predators or attract a mate. The new material mimics this natural ability in response to both heat and sunlight, which could lead to a range of applications in camouflage, chemical sensing, signaling and anti-counterfeiting.
The chameleon can alter the color of its skin on demand due to arrays of tiny photonic crystals in their skin, which are colored because they diffract specific wavelengths of light, rather than the dyes and pigments that most colors are based on. The hue changes as the distance between photonic crystals varies, such as when the skin tenses or relaxes. However, when researchers have embedded photonic crystals in thin films of a responsive hydrogel and then changed their color by expanding or contracting to mimic these properties, the large fluctuations in size puts a strain on the materials, resulting in them becoming buckled.
However, as described in ACS Nano [Dong et al. ACS Nano (2019) DOI: 10.1021/acsnano.9b04231], this new smart skin is strain-accommodating, and can respond to natural sunlight and change color within minutes without detectable change in the size of the film. Based on time-lapse imaging of chameleon skin, the team found that just a small number of skin cells contain photonic crystal arrays and that the rest are without color, which helps contain the strain as the photonic crystals expand and contract. The colored photonic crystal cells were actually found to be surrounding the non-colored cells, and acting like a spring – filling in the void to maintain a constant size of the skin as the photonic crystals swelled or contracted.
The design of the material requires integration of two hydrogels, one to contain the responsive hydrogel photonic crystals and the other being mechanically robust and able to be stretched or collapsed to maintain constant volumeas a supporting layer. The work provides a framework for multi-component materials that combine to offer new properties to optimize responsive color change and mechanical robustness.
As team leader Khalid Salaita told Materials Today, “The hydrogel rapidly changes color based on light or heat input and it does so in a fully reversible manner across the visible or near-infrared spectrum”. The next step could be to develop hydrogel materials that produce a permanent color change, with uses in sensing applications, and to explore how small the segments could be made while maintaining photonic crystal coloration.