THERE is something odd about Laetitia鈥檚 appearance. As she speaks, her lips change colour 鈥� from pink to blue and back again. Her eyelids shimmer as she turns her head. And when she blinks, her lashes flutter in silver and black. Laetitia is modelling high-tech make-up that has been carefully designed to make light play the same tricks that produce the brilliant iridescence of butterfly wings and peacock feathers. Its developers hope it will be the next big thing in cosmetics.
Ever since the ancient Egyptians dyed their hair with henna and painted their eyes with kohl, people have been coming up with some crazy fashions in cosmetics. From symbolic tribal face markings to disco glitter, you might think there is little else left to stick, paint or brush onto our bodies. But we have yet to come close to some of the wonderful displays seen in the natural world, such as the blue morpho butterfly found in the rainforests of Central and South America. With its intense, iridescent blue colouring the morpho is the ultimate in eye candy. And it is what started physicist Peter Vukusic at the University of Exeter in the UK on an unlikely journey from the lab to the cosmetics studio.
In 1997 Vukusic was a young postdoc looking for a research project when he met Roy Sambles at Exeter. Vukusic remembers that as they chatted, sunlight streamed through the window, lighting up a blue morpho that was framed on Sambles鈥檚 office wall. He was so taken with the giant butterfly that when Sambles went away on a business trip, Vukusic snuck back to the office, snipped off a corner of the wing and started studying its optical properties. Astonished by what he saw, Vukusic teamed up with Sambles and they spent the next three years characterising the structures of butterfly and moth wings.
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鈥淧hotonic mascara, eyeshadow and nail varnish could be on the shelves next year鈥�
They found that the morpho鈥檚 iridescence 鈥� the stunning change of hue as you look at it from different angles 鈥� is an example of 鈥渟tructural colour鈥�. Most colours come from chemical pigments in a material that absorb certain wavelengths of light. But structural colour is different: it is due to light undergoing multiple reflections within the material itself (New 杏吧原创, 26 June 1999, p 34).
It was the 18th-century physicist Thomas Young who first explained this effect. He showed that light hitting a thin film, such as oil on water, is partly reflected by the top surface and partly transmitted through the film. When the transmitted light reaches the bottom surface of the film, some of it is reflected back up. Young鈥檚 calculations showed that under the right conditions the two reflected beams can interfere, resulting in a strong reflection at one wavelength. Place a number of films on top of each other and the colour can become intense.
And that鈥檚 exactly how butterflies, beetles and other creatures create their shimmering displays. Vukusic and Sambles discovered that butterfly wings are made of layers of scales just 3 to 4 micrometres thick that overlap like microscopic roof tiles. But when they sliced through a blue morpho scale and examined it under an electron microscope, they found its surface was covered with even more intricate structures. 鈥淭hey look a bit like Christmas trees,鈥� says Vukusic. Each one contains eight or nine layers of cuticle about 40 nanometres thick, separated by air gaps. This orderly arrangement forms what is known as a photonic crystal 鈥� a material that contains regular structures similar in size to the wavelength of incoming light, which trap light within it so that only certain wavelengths can pass through.
Vukusic and Sambles鈥檚 papers soon attracted the interest of French cosmetic giant L鈥橭r茅al whose researchers were already trying to achieve structural colour using thin films. 鈥淭o reproduce it, we just needed to understand how nature manages it,鈥� says Patricia Pineau of L鈥橭r茅al. Their work has helped the company master the art of synthesising photonic crystals to incorporate into their products. To produce their photonic cosmetics, L鈥橭r茅al researchers create a single sheet of alternating layers of metal oxide and mica and fracture it into countless thin flakes. These multilayered flakes are then added to a transparent wax or oil. One of the results, says Pineau, is a lipstick that should stay put better than traditional lip colours because they use less oil to achieve the reflective finish, so they won鈥檛 come off on your teacup.
The prototypes are stunning. Laetitia鈥檚 photonic lipstick exploits the curvature of her lips to create a shimmering colour. L鈥橭r茅al has also developed photonic mascara, eyeshadow and nail varnish, and is hoping to launch the first products next year.
But the company has some challenges to overcome. While the prototype make-up looks stunning in daylight, the photonic effect is barely visible in a dim room. So it won鈥檛 be much use if you鈥檙e trying to impress someone over a candlelit dinner.
Perhaps the biggest challenge is how to apply the make-up to maximise the photonic effect. The colour relies not only on the internal structure of the thin flakes, but also on the flakes being oriented the right way on the body. One solution might be to introduce magnetic particles into the cosmetics. So if you wanted to touch up your eyeshadow, you could just wave a magnetic wand across your face.
And there is something else about photonic eyeshadow that could prove a marketing challenge. Every shade, no matter how brilliant it looks when applied to your skin, is a plain white powder in the pot. Maybe some photonic packaging would do the trick.
