Waiting in the wings

Scientists shed light on crystals that illuminate butterfly wings. Are butterfly cosmetics next?


Just where does a butterfly get its vivid coloring?

From microscopic crystals that capture light in its wings and reflect it back, a process that scientists are trying to understand and mimic.

The push to learn how those crystals work is more than mere curiosity. The knowledge could produce brighter, man-made shades of everything from paints to cosmetics, researchers say.

Scientists know that crystals on the wings of some butterflies produce what's known as constructive interference - where waves of light combine to selectively produce a color. The color is determined by the tiny "nanostructures" that make up the crystals, some of which are only a few billionths of a meter across.

"What we're looking at is the presence of nanostructures that control - and the key word is control - the way specific colors are reflected," said Pete Vukusic, a physicist at England's University of Exeter and lead author of a study on butterfly-wing coloration published in the Nov. 18 issue of Science.

Vukusic has been studying light-producing, or photonic, crystals in butterflies since the late 1990s, and his work has attracted the attention of L'Oreal, the Paris-based cosmetics company that now employs him as a consultant.

The company had been working on ways to replicate the butterfly's use of light in its products when Vukusic began publishing his work a few years ago. L'Oreal plans to launch its first lines of lipstick, mascara, eye shadow and nail varnish - using colors created by photonic crystals of silica and mica - sometime in the next few years.

"We were looking for new color effects, and structural color was obviously a huge field of interest. The depth, the intensity and the brightness of those colors obtained without pigment is surprising," said Patricia Pineau, a spokeswoman for L'Oreal's research division.

Scientists say photonic crystals are one source of color in butterflies. The other source involves the same system that works for the rest of us: pigments produced by biological chemicals that absorb light in specific wavelengths and reflect other wavelengths. Such pigments produce much of the world's natural color, from a cardinal's red coat to the stripes on a zebra. Melanin, for instance, is a natural pigment that helps determine the color of our skin.

But the startling blue, green and yellow tints of a butterfly's wing come from photonic crystals that reflect light at different angles and depths and produce color the way that an oil-covered puddle creates rainbows on its surface.

Butterflies aren't the only creatures that work this way.

Vukusic and his colleagues have also studied beetles, birds, fish and moths that use photonic crystals of various shapes and sizes to control light and produce the colors they need to attract mates and hide from predators.

Researchers still aren't sure why some animals have developed photonic crystals, while most are content to make colors with pigment.

But they do know that many animals with photonic crystals use them to produce vivid blues and blue-greens, which are hard to produce in nature with chemical pigmentation. So it could be that animals developed photonics because they needed to stand out in a crowd - to spot each other.

"It's something you see all over in nature. In fish, the neon tetra is a perfect example," said Helen Ghiradella, a biologist at the University at Albany who has spent 30 years researching how animals such as fireflies produce light.

Teams in England, Australia, Japan and the United States are studying how light interacts with tiny biological structures to produce color in beetles, birds and fish.

Vukusic and other British researchers reported in Science that a type of swallowtail butterfly from eastern and central Africa has a sophisticated system of tiny, honeycomb-shaped photonic crystals and reflectors on its wings that control the type of light - and thus the colors - produced.

The crystals on the Princeps nireus are spaced so precisely that only certain wavelengths of light can pass through them. The reflectors work to allow some fluorescent pigments to pass through, along with some of the other natural light, giving the wings their distinctive blue and blue-green markings

Vukusic reported that the Princeps species they examined is the only one known to combine photonic crystals with fluorescent pigments. Most other butterflies use either chemical pigments or other types of photonic crystals to produce color.

"What we found is a one-of-a-kind thing," Vukusic said.

Vukusic began researching butterfly wings in the late 1990s, after he was struck by the colors of a Morpho butterfly in the office of his former Exeter professor, Roy Sambles.

"I asked how it worked, and why it was colored so brightly, and all these questions popped into my head. He said, `Why don't you go for it, apply for a grant and research it?' " Vukusic recalled.

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