Understanding seven sophisticated tactics that animals use to create spectacular color tones could lead to sophisticated new technologies
The changing hues of the magnificent peacock tail feathers have always stimulated the imagination of inquisitive minds. Robert Hooke, the 17th century English scientist, called them "creatures of fantasy", in part because wetting the feathers caused the colors to disappear. Hooke studied the feathers with a microscope, which had been invented not long before, and discovered that they were covered with tiny ridges. He concluded that it may be these ridges that produce the bright yellow, green and blue colors.
Credit: Jen Christiansen
Hook was on the right track. The vivid colors of bird feathers, butterfly wings, and squid bodies are often created not by colored substances, dyes, or light-absorbing pigments, but by arrays of tiny structures, only hundreds of nanometers across. The size of these structures and the spacing between them result in the isolation of certain wavelengths from the full spectrum of sunlight. These shades often also change, changing as if by magic from green to blue or orange to yellow, depending on the angle from which we look at the animal. Since the colors are created by the reflection of light only, and not by the absorption of a part of it as in dyes, they are brighter: you can spot the Blue Morpho butterfly, which lives in South and Central America, even from a kilometer away. It seems to literally glow as sunlight penetrates through the rainforest foliage and is reflected from its wings.
Scientists are beginning to get a more complete picture of how the carefully arranged nanostructures in animals manipulate light, inspiring engineers to mimic the biological designs in new man-made optical materials. These materials could lead to brighter computer displays and new chemical sensors, as well as more efficient storage, transmission and processing of information.
We don't know much about how these biological structures developed during evolution, but at least we are learning how they form and how they produce the spectacular colors. Teva doesn't have sophisticated technologies like electron beams that can burn grooves into thin layers of material, so instead it relies on creative tricks. If engineers can master this skill, they may be able to produce fabrics that change color like the squid's camouflage layer, or computer chips that transmit information by optical means instead of electrical, and at breakneck speeds. In this article we will review some of nature's tricks to create structures that produce colors, and the ways in which human inventors try to take advantage of them.
1 layers upon layers
The ridges that Hooke discovered on the peacock's feathers do scatter the light, but the bright colors come mainly from nano-structures that are below the surface and that Hooke was unable to see. Colorful feathers of birds and scales of fish and butterflies often contain microscopic, arranged layers or rods of compressed light-scattering material. Since the distance between the layers or rods is close to the size of the wavelengths of visible light, these structures cause a phenomenon known as "diffraction". Rays of light of certain wavelengths are reflected from the layers and fight with each other, the fight "constructs" or "destroys" in such a way that some of the colors in the reflected light are strengthened and others are canceled. This process is responsible for creating the rainbow of colors we see when we tilt at different angles from the shiny surface of a CD.
In butterfly wings, the light-reflecting layers on the hard outer surface (cuticle) of the wing scales are made of the natural polymer chitin, separated by air-filled spaces. In bird feathers, the layers or bars are made of melanin and embedded in keratin - the protein from which our hair and nails are also made. The optics industry already uses diffraction gratings, made of ultra-thin layers of two different alternating materials, to isolate and reflect light of a particular color in a variety of products, from telescopes to solid-state lasers.
The males of the bird of paradise species Parotia Lawesii A genius change is put into this trick, as discovered in 2010 by Dukele K. Stavenga from the University of Groningen in the Netherlands. Thin hair-like bristles on their breast feathers, called "cilia", contain layers of melanin, spaced to create bright orange-yellow reflections, but the cross-sectional area of each cilia is V-shaped, with diagonal surfaces that also reflect blue light. Light movements of the feathers during the bird's courtship ritual can quickly change the color from orange-yellow to blue-green and back again, ensuring the attention of the females.
The researchers haven't tried to replicate this stunt, but Stabenga believes the fashion and automotive industries will eventually try to replicate such color changes. Tiny V-shaped lures in the fabric will cause the dress to change color as the wearer moves, and similar lures in the color of a passing car will cause the car to change color from end to end.
2 Christmas tree effect
The dazzling blue color of the butterflies of a species morpho didius and-M. rethenor It is not obtained from multiple layers of chitin, but from more complex nanostructures in the wing scales: chitin arrays built like Christmas trees and emerging from the outer surface of the scales. The parallel branches of each "tree" act as a diffraction grating of a different kind: these arrays may return up to 80% of the blue light that hits them. Because they are not flat, they are able to reflect a single color over a wide range of viewing angles, which slightly reduces glare: animals don't always want their color to change when viewed from different directions.
As Hook observed in the peacock feathers, when water is spilled on the blue morph wings it changes the refraction of light. Different liquids with different refractive indices therefore create different light reflections. Researchers at GE Global Research from Niscone, New York, in collaboration with researchers at the University of Albany and butterfly wing expert Pete Vukusik from the University of Exeter in England, are developing artificial morpho-like structures to create chemical sensors that can detect different liquids. These sensors will be painted in unique colors depending on the liquid they come in contact with. The researchers use microlithography techniques, taken from the semiconductor industry, to etch the structures into a solid material. Such sensors will be able to detect emissions of certain substances from power plants and contamination in drinking water.
3 reflective bowls
The bright green color of the "emerald swallowtail" butterfly (Papilio palinurus), common in Southeast Asia, does not result from green light at all. The wing scales are covered with a lattice of tiny bowl-like pits, a few microns in diameter. The dimples are lined with chitin layers separated by air, which act as selective mirrors. The bottom of each bowl reflects only yellow light, while the sides around the yellow center reflect only blue light (see illustration). Our eyes are not able to separate the yellow from the blue on such a small scale, so our brain sees their combination as a green color.
Christopher Summers and Mohan Srinivasrao of the Georgia Institute of Technology copied this method to produce paint. To create the tiny swirls, they allowed water vapor to condense into microscopic droplets on the surface of a polymer as it turned from a liquid to a solid. The water drops clump together on the surface like rows of eggs in a carton and sink into the canvas. When the polymer solidifies, they evaporate and form a surface with bowl-like bumps. The researchers then deposit thin layers of titanium dioxide and aluminum oxide (alumina) alternately in each bowl, to create a light reflector that mimics the natural coating of the bowls in the butterflies.
Light that shines over the surface of this structured sheet appears green. However, when the sheet is placed under a set of polarizing filters, the yellow light emitted from the center of the bowls disappears, while the blue light from the edges remains. This mechanism can create a distinct authentication mark for credit and other debit cards: a reflective coating that looks plain green will actually covertly include a polarized yellow and blue signature that will be difficult to forge. Nevertheless, Srinivasrao admits that the main reason for trying to imitate the green color of the butterfly is that "it is beautiful in itself."
4 nano-sponges
Another butterfly, known in English as a "patched green butterfly" (Parides sesostris), creates a green color using another nano-structure, also without any dyes. Its wing scales boast microscopic crystal-like arrays of holes. These arrays, called "photonic crystals", completely block light in a certain band of wavelengths and cause it to return. The Leshem gemstone (opal) is a photonic crystal, which is made of tiny, dense globules of silicon dioxide (silica) that scatter light and give the stone its bright rainbow colors. It is possible to use photonic crystals to limit light to narrow channels and create waveguides ("Galvo") that could, perhaps, steer light in the intricacies of computer chips.
With the help of an electron microscope, you can see that the wing scales of the bull heart butterfly show arrays organized in a zigzag pattern: patches of sponge made of chitin with neat patterns of holes about 150 nanometers in diameter. Each patch is a photonic crystal fixed at a slightly different angle than its neighbors, a structure that allows it to reflect light in the green part of the spectrum from a wide range of angles of incidence. Some other wasps and beetles also create their bright colors using photonic crystals made of chitin.
Biologist Richard Fram of Yale University and his colleagues discovered how these photonic crystals grow as the young butterfly's wings develop. Essentially, lipids found in the embryonic cells of the wing scales spontaneously form a three-dimensional pattern, and the chitin hardens around them. With the death of the cell, the lipids break down, and a template with neat empty spaces remains.
Some researchers are trying to create similar structures from scratch. For example, fat-like molecules called "surfactants" form ordered sponges, as do the molecules called "block copolymers." Ulrich Wiesner of Cornell University used such copolymers to organize niobium and titanium dioxide nanoparticles into mineral-like "nano-sponge" structures.
These porous solids can have many uses, for example efficient and cheap solar cells. Moreover, Wiesner calculated and found that nano-sponges made of metal, such as silver or aluminum, would have a strange property of negative refractive index, meaning that they would bend light "in the wrong direction". If it is possible to produce such materials, it will be possible to create super lenses for optical microscopes, which can show objects smaller than the wavelength of light - something that is not possible with ordinary microscopes.
5 crystalline fibers
Animals are able to shape photonic crystals in a variety of ways. The spines on certain marine worms, such as Aphrodita ("the sea mouse") contain hexagonal arrays of hollow fibers, several hundred nanometers wide. These arrays are made of chitin, and they block light in the red part of the spectrum, giving Aphrodita's spines a bright red color.
It is not known whether these optical properties serve any biological function in the sea mouse, but it is certainly possible to find technological applications for fibers that have such an effect on light. Philip Russell, who now works at the Max Planck Institute for the Science of Light in Erlangen, Germany, heated bundles of glass filaments and stretched them to form thin fibers with densely packed hexagonal holes running through them. If a wider nim, or a solid rod, is added to the center of the original bundle, this creates a defect in the array of holes through which light can pass that will be blocked by the surrounding photonic crystals. This creates a protected optical fiber with a layer that is practically impermeable to light in a certain band of wavelengths.
The light "leaks" less from photonic crystal fibers compared to normal optical fibers, so they can replace the normal fibers in communication networks. They will consume less energy, which will eliminate the need for expensive amplifiers to strengthen signals transmitted over long distances. The normal fibers leak especially when they are bent sharply, due to less efficiency of the reflections that keep the light inside the fiber. Photonic crystals do not have such a problem, because the light capture in them is not based on reflection. Therefore, they should work better in small and dense environments, and thus it will be possible to create much faster microprocessors than the electronic chips in our computers and cell phones.
6 distorted arrays
To produce color, some animals create spongy arrays that have a disordered pattern instead of an organized pattern. This structural variation is responsible for the magnificent blue-green feathers of many birds, which are not endowed with the bright colors seen on the hawk or peacock. Because the sponge-like nanostructures, made of keratin, are disordered, the light scattering is unfocused and resembles the blue of the sky more than a mirror-like glow. This is why the colors look uniform from every angle.
In the blue-yellow macaw (Ararauna route(and the black-capped kingfisher)Halcyon pileata), the empty spaces in the arrays of the feather sections form winding channels that are about a hundred nanometers wide. A similar random network in the cuticle of the beetle Cyphochilus Gives her a dazzling white exoskeleton. In a blue-crowned mannequin bird (Lepidothrix coronata), the air holes are not channels but tiny bubbles connected to each other.
Richard Fram of Yale University believes that the canals or bubbles are formed when the keratin separates itself, like oil from water, from the fluid in the feather-forming cells during early development. He also believes that birds have developed a way to control the rate at which the keratin separates, so that the formation of the canal or bubble stops when the empty spaces reach a certain size. This size determines the wavelength of the scattered light and therefore the color of the feather.
Unordered light scattering also exists in other natural materials and man-made materials. In milk, tiny fat droplets in a wide range of sizes scatter all visible wavelengths and create an opaque layer. Pete Vukosik of the University of Exeter simulated the beetle's cuticle Cyphochilus Using randomly perforated arrays made of calcium carbonate or titanium dioxide mixed with a polymer, creating thin bright white coatings. Fram and biotech engineer Eric Dufrain, also of Yale University, mimicked the disordered sponginess of bird feathers by creating sheets of randomly agglomerated microscopic polymer beads with blue-green colors. Such approaches may lead to the creation of coatings with bright and very opaque colors despite their thinness that will never fade, because the sheets do not contain organic dyes.
7 reversible proteins
One of nature's most enviable optical tricks is the creation of reversible color changes. Squids from the Lolig family (Loliginidae) use the protein called reflectin to create colors in their skin and change them. The protein molecules are arranged in stacks of plates inside cells called iridiophores, which reflect unique colors. The biologists believe that the color changes are intended for camouflage and as a means of communication - for courting and showing aggression.
Daniel Morse of the University of California, Santa Barbara studies how iridiophores change color. The reflectin proteins fold and become nanoparticles that make up the plaques. The plates are placed between folds in the membrane of the iridiophore cell. The proteins clump together when a neurotransmitter activates a chemical process that cancels the electrical charge of the reflectins. This change increases the light reflecting capacity of the plates and changes the spaces between them, and thus the color changes. The situation can be restored by reloading the reflectins.
Morse believes he is able to mimic this mechanism in optical devices, perhaps even using real reflectins. The team led by him inserted into Escherichia coli bacteria the gene that codes for the reflectin protein in squids of the species Loligo pealeii. When this protein is expressed, it collapses into nanoparticles, the size of which can be adjusted with the help of salts that regulate the interactions between the electrical charges of the proteins. The materials produced may swell or shrink in response to chemical signals and change the reflected wavelengths.
Morse also developed a polymer that instantly turns from transparent to opaque in response to electrical voltages, which change the polymer's resilience and cause the polymer sheet to swell through salt adsorption. Devices that use these materials can be produced with simple manufacturing methods. Morse's team is working with Raytheon Vision Systems of Goleta, California, to create from this material fast shutters for infrared cameras, which will enable high-speed night photography based on detecting heat instead of light.
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About the author
Philip Ball (Ball) is a science reporter from London. His latest book "Curiosity: How Science Became Interested in Everything" was published in May 2012 by Bodley Head.
More on the subject
Photonic Structures in Biology. Pete Vukusic and J. Roy Sambles in Nature, Vol. 424, pages 852-855; August 14, 2003.
Natural Photonics. Pete Vukusic in Physics World, Vol. 17, no. 2, pages 35-39; February 2004.
Optical Filters in Nature. HD Wolpert in Optics and Photonics News, Vol. 20, no. 2, pages 22-27; February 2009.
A Protean Palette: Color Materials and Mixing in Birds and Butterflies. Matthew D. Shawkey et al. in Journal of the Royal Society Interface, Vol. 6, Supplement no. 2, pages S221-S231; April 6, 2009.
For photos of bright, color-changing paintings by artist Franziska Schenk using nano-particle paints
