Chameleon inspires "smart skin" that changes color in the sun

A chameleon can change the color of his skin to blend in with the decor or detach to defend his territory and attract a partner. The chameleon makes this trick easy, by using photonic crystals in his skin. However, scientists have struggled to create a "smart skin" with photonic crystals that changes color depending on the environment, without changing size.

The newspaper ACS Nano publishes research conducted by chemists from Emory University who have found a solution to the problem. They have developed a flexible smart skin that reacts to heat and sunlight while maintaining almost constant volume.

"Watching a chameleon change color has given me the idea of ​​a breakthrough," says first author, Yixiao Dong, Ph.D. candidate at Emory's chemistry department. "We have developed a new concept for smart skin that changes color, based on observations of how nature does it."

"Scientists working in the field of photonic crystals have long been working to create smart, color-changing skins for a range of potential applications, such as camouflage, chemical detection and anti-counterfeit labels," adds Khalid Salaita. , lead author. paper and Emory chemistry professor. "Although our work is still in the fundamental stages, we have established the principles of a new approach to explore and develop."

The article's co-authors include Alisina Bazrafshan and Dale Combs (Emory Ph.D. students); Kimberly Clarke (Emory Postdoctoral Fellow); and Anastassia Pokutta, Fatiesa Sulejmani, and Wei Sun (from the Department of Biomedical Engineering Wallace H. Coulter of Georgia Tech).

In addition to chameleons, many other creatures have developed the ability to change color. For example, strips of a tetra neon fish go from deep indigo to blue-green when they swim in the sun.

Staining in these organisms is not based on pigments, but on tiny, repetitive particles called photonic crystals. The periodicity in these particles causes the interference of the material with the wavelengths of light. Although the particles themselves are colorless, their precise spacing allows certain light waves to pass through while rejecting others. The visible colors produced change depending on factors such as lighting conditions or offsets in the distance between particles. The iridescence of some butterfly wings and peacock feathers are among many other examples of photonic crystals in nature.

If you put strawberries in a blender, says Dong, the resulting liquid will be red because the strawberry color comes from a pigment. If you crush iridescent butterfly wings, the result will be a matte powder because the rainbow colors are not based on pigments, but on what is called a "structural color" ". The structure of photonic crystal arrays is destroyed when the wings of the butterfly are crushed.

To mimic chameleons and create artificial intelligent skin, scientists have experimented with the incorporation of photonic crystal networks into flexible polymers, or hydrogels. The enlargement or contraction of the hydrogel alters the spacing between the arrays, resulting in a color change. The problem, however, is that the accordion action needed to generate a visible hue change causes significant growth or reduction in the size of the hydrogel, resulting in structural instability and buckling of the material .

"Nobody wants a camouflage cloak that shrinks to change color," Salaita notes.

Dong pondered the problem by watching YouTube videos of a chameleon. "I wanted to understand why a chameleon does not get bigger or smaller when it changes color but stays in its original size," he says.

In close-up, shifted images of the dying chameleon, Dong noticed that the photonic crystal arrays did not cover the entire skin, but were distributed in a dark matrix. As the photonic crystals changed color, these color spots remained at the same distance. Dong hypothesized that the skin cells constituting the dark matrix are somehow adapted to compensate for shifts in photonic crystals.