Twisting a thin plastic film changes the spacing between tiny embedded silicon beams, altering the color of light they reflect.

Twisting a thin plastic film changes the spacing between tiny embedded silicon beams, altering the color of light they reflect.

Image courtesy of The Optical Society (OSA) in conjunction with UC Berkeley

New film changes colors when you stretch it

Materials scientists often look to the natural world for inspiration, but usually it takes their inventions a while to catch up with biological discoveries. Not this time. Earlier this week, scientists in Switzerland revealed that chameleons change colors by expanding a lattice of tiny crystals just under their skin. Now, researchers in California are reporting that they’ve made a thin film that changes colors when they tug on it. Such films could produce color-changing sensors that give engineers a way to monitor potentially dangerous structural changes to everything from bridges to airplane wings.

Both studies show how nature and technologists often converge on the same solution to challenges they face, says Richard Prum, an ornithologist at Yale University who specializes in how animals change color. The new thin film “has grand prospects in technology,” Prum says. “It also shows the value of studying organisms.”

The colors we typically see in everything from grass and flowers to paints and fabrics occur when white, broad-spectrum light strikes their surfaces. The unique chemical composition of each surface absorbs various bands, or wavelengths, of light. The wavelengths that aren’t absorbed are reflected back, with shorter wavelengths giving objects a blue hue and longer objects appearing redder, and the entire rainbow of possible combinations in between. Changing the color of a surface, such as the leaves on a tree in autumn, requires a change in chemical makeup.

But that’s not the way chameleons shift their colors. Instead of changing the composition of the light-absorbing pigments in their skin, chameleons change the way materials embedded in the skin reflect light. In this case, the reptiles have a layer of cells called iridophores, which lie just below the skin’s surface. These cells contain tiny spherical crystallites made from guanine, one of the nucleic acid building blocks of DNA. The crystallites are arranged in a regular 3D lattice, like oranges on a fruit stand. This arrangement causes them to strongly reflect one set of wavelengths, making them appear green, for example. But this week in Nature Communications, researchers led by geneticist Michel Milinkovitch from the University of Geneva reported that chameleons rapidly shift colors by changing the salt balance in their iridophores. This causes the cells to swell with water, increasing the distance between guanine crystallites, in turn causing the array to reflect a longer wavelength of light, such as yellow.

Today, researchers led by Connie Chang-Hasnain, an electrical engineer at the University of California, Berkeley, report in the journal Optica that they’ve followed a related strategy to produce color changes from green to orange in a thin plastic film. To do so, they etched a 1 cm2 array of beams, each thinner than the wavelength of light, into a thin silicon film. They then embedded the silicon array within a flexible plastic film. And when they flexed, bent, or stretched the film, this changed the space between the beams, producing brilliant color changes. The setup worked so well that it reflected up to 83% of the light that hit it.

That efficiency bodes well for potential future applications. One possibility, Prum says, is that such films could lead to a new class of energy-efficient full-color displays for e-readers and other electronic devices. Conventional lighted color displays in devices such as iPads and Kindles are power-hungry, because they use electricity to force elements of the display to emit particular colors of light. However, displays that make use of tiny movements to manipulate colors might not require this continuous electrical input, giving them additional hours between charges. The same color-changing ability could also enable the films to work as novel sensors, revealing structural changes in bridges, buildings, and even the wings of aircraft. That way, a simple shift from green to orange could signal dangerous wear and tear, and possibly even save lives.