The animal kingdom boasts many an impressive form, from arching giraffe necks to spoon-shaped bird beaks to gigantic beetle claws. But evolution has worked on much smaller scales too, producing finely honed nanostructures--parts less than a millionth of a meter across, or smaller than 1/20th of the width of a human hair--that help animals climb, slither, camouflage, flirt, and thrive.
Consider an insect's compound eye, which has anywhere from 50 to 10,000 individual facets, each with its own set of optical machinery. Zoom in on the seemingly smooth curves of those facets and, in many insects--like the robber fly seen here--you'll find they're studded with an array of nanoscale protuberances called "corneal nipples." The tiny bumps, which range in diameter from 50 to 300 nanometers, help the insects camouflage: by breaking up the cornea's even surface, they cut down the glare that reflects off the eye, which could potentially alert a predator to the bug's presence. The nanoscale nipple pattern on moth eyes has inspired new anti-reflective coatings for solar cells.
In 2010, German scientists discovered another useful function of corneal nipples: they help keep pollen grains, dust particles, and other microscopic crud out of the insects' eyes. The bumpy texture means less contact area for a small particle to cling onto, so even when the rest of the bugs' bodies get grimy, the eyes stay clean.
Many of the shimmering colors in a butterfly's wings are produced not with pigments, like the melanin that tints our skin, but with nanostructures (pdf). The scales on their wings are patterned with nanoscale channels, ridges, and cavities made of a protein called chitin. Unlike pigments, which create color by absorbing some wavelengths of light and reflecting the rest, the nanostructures are shaped so that they physically bend and scatter light in different directions, sending particular colors back to our eyes. That scattering can also make the wing scales iridescent--meaning the color changes with the angle you see it from.
When heat, in the form of invisible infrared radiation, hits the chitin nanostructures, they expand, changing their shape and therefore the colors they display. Scientists at GE are working to harness this property to make hypersensitive thermal imaging sensors, useful for night vision. By coating the wings of a Blue Morpho butterfly with carbon nanotubes that magnify the effect, researchers there made an insect into a sensor that changes color when its temperature changes a mere 1/25th of a degree.
Butterflies aren't the only animals who harness nanotech for cosmetic purposes; so do birds, whose dazzling array of colors comes from a combination of pigment-producing cells and nanoscale design.
In Australia and New Zealand, the little penguin Eudyptula minor sports a tuxedo of dark blue feathers instead of the more traditional (and formal) black. Last year, scientists at the University of Akron in Ohio used X-ray imaging and other techniques to discover that the penguins produce the blue color in an entirely new way: with bundles of parallel nanofibers, like handfuls of uncooked spaghetti, that scatter light so as to produce the rich blue. The 180-nanometer-wide fibers are made of beta-keratin, a protein similar to the one in human hair. Similar fibers had previously been found in some birds' blue skin, where they are made of collagen rather than keratin, but never before in blue feathers.
Most wasps are most active in the morning and slow down considerably at midday, when the sun's heat is most oppressive. Not so oriental hornets, who build nests underground: their workers do more digging the more they're bombarded with sunlight. That's probably because, as researchers at Tel Aviv University revealed, nanostructures in the insect's exoskeleton form a kind of solar cell, harvesting light energy that could power the hornet's work.
In the brown section of the hornet's abdomen, the layers of cuticle that make up the exoskeleton are embossed with grooves about 160 nanometers high. The grooves are arranged into a sort of grating, which helps trap the light that hits the hornet and bounce it around within the cuticle. The yellow section, which has small, interlocking protrusions about 50 nanometers high, also absorbs light--and the researchers showed that xanthoperin, the pigment that gives it its yellow color, can be used to convert light into electricity. It's likely doing just that inside the insect, which would explain why they're busiest when it's sunniest--and why, as a previous study found, anesthetized Oriental hornets wake up faster when they're pounded with UV light.
Snakes like the ball python seem to slither effortlessly, but their movement is a actually a complex interaction of muscle movement and small-scale physics. On a nanoscale level, the scales on a snake's belly are covered in minuscule hairs, called microfibrils, which are less than 400 nanometers wide. They all point in the same direction--toward the tail end of the snake--and their ends are raised about 200 nanometers off the skin, allowing for a smooth glide forward but stopping any backward motion, like a row of one-way traffic spikes. The extra friction in only one direction helps prevent sideways slipping, even if the snake is inclined on a plane.
The tokay gecko uses nanotechnology to stick itself to trees, walls, windows, and even ceilings. The gecko's feet are covered in microscopic hairs, called setae, which branch into thousands of smaller hairs with paddle-shaped ends. Those branches, or spatulae, are a mere 200 nanometers wide at the tip.
The extra surface area of the spatulae maximizes the effect of van der Waals forces, the weak electrical pull between every molecule in the gecko and every molecule in whatever it's sticking to. The combined force is so strong that a gecko can hang its whole weight from a single toe, even on a sheer piece of glass. Engineers have used carbon nanotubes mimicking gecko setae to create super-sticky tapes, glues, and even a wall-climbing gecko robot.
Spider silks are some of the toughest materials known to man--pound for pound, they're stronger than steel, and their webs can stand up to gusts of wind and catch hurtling insects without falling to pieces.
The silks get their strength from thin crystal proteins only nanometers wide, which are stacked together like pancakes. On the atomic level, the layers are joined together by hydrogen bonds. Those bonds actually aren't particularly strong, but that turns out to be an advantage, because they can easily pull apart and reform, allowing the silk to stretch and flex under pressure instead of snapping like a twig.
In February, Italian scientists found what they think is the stretchiest silk yet in the egg sac of the European cave spider, Meta menardi--which also just so happens to be the European Society of Arachnology's 2012 Spider of the Year. Call that one a win for animal nanotechnology.
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