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Want to walk on the ceiling? All it takes is a bit of fancy footwork

Article by Stephanie Pain from New Scientist magazine, Vol. 168 issue 2270, 23/12/2000, page 62.

If Kellar Autumn has his way, the first footprints on Mars won't be human. They'll belong to a gecko. Gecko toes have legendary sticking power-and Autumn would like to see the next generation of Martian robots walking about on gecko-style feet. A gecko can whiz up the smoothest wall and hang from the ceiling by one foot with no fear of falling. With feet like that, a roving robot could reach parts of the Red Planet other robots can't.

Autumn, an expert in biomechanics at Lewis and Clark College in Portland, Oregon, is one of a long line of researchers who have puzzled over the gecko's gravity-defying footwork. Earlier this year, he and his colleagues discovered that the gecko's toes don't just stick, they bond to the surface beneath them. Engineers are already trying to copy the gecko's technique-but reptilian feet aren't the only ones they're interested in.

Some of the most persistent hangers-on are insects. They can defy not just gravity, but gusts of wind, raindrops and a predator's attempts to prise them loose. Recent discoveries about how they achieve this could lead to the development of quick-release adhesives and miniature grippers ideal for manipulating microscopic components or holding tiny bits of tissue together during nanosurgery. "There are lots of ways to make two surfaces stick together, but there are very few which provide precise and reversible attachment," says Stas Gorb, a biologist at the Max Planck Institute of Developmental Biology in Tübingen, Germany. Geckos and insects have both perfected ways of doing it.

Scientists have been trying to uncover the secrets of insect sticking power for centuries. On rough surfaces insects can latch onto small projections with claws. But there's no such anchorage on smooth surfaces. Insects that have to tackle slippery slopes have specialised pads at the end of the tarsus, the last of the leg segments. Some have hairy pads, some have pads that are completely smooth, but what makes them stick? Engineers and materials scientists would dearly love to know.

Friction certainly plays a part in limiting the horizontal movement of the pads. But when the animal is running up a slope, climbing vertically or travelling upside down, it needs a more powerful adhesive. Just what that adhesive is has been hotly debated for years. Some people suggested that insects had microsuckers. Some reckoned they relied on electrostatic forces. Others thought that intermolecular forces between pad and leaf might provide a firm foothold.

Most of the evidence, though, suggested that insects rely on "wet adhesion", hanging on with the help of a thin film of fluid on the bottom of the pad. Insects often leave tiny trails of oily footprints. Some clearly secrete a fluid onto the "soles" of their feet. And they tend to lose their footing when they have their feet cleaned or dried. This year, Walter Federle, an entomologist at the University of Würzburg, showed experimentally that an insect's sticking power depends on a thin film of liquid under its feet.

Federle has spent plenty of time in tropical forests watching the ants that live in the treetops. For these ants, tumbling to the forest floor means almost certain death. Many species run around on shiny leaves and smooth, waxy plant stems yet never fall. When Federle put their staying power to the test in the lab he found that some species can withstand a pull of almost 150 times their own weight. "It's almost impossible to shake them off," he says.

To find out what makes them stick, he tried a simple experiment with Asian weaver ants. These ants can withstand a pull a hundred times their own weight, vital when they have to carry heavy prey and during nest-building, when they pull together living leaves and stitch them into a sort of tent.

Slipping and sliding

Federle placed an ant on a polished Perspex turntable inside the rotor of a centrifuge-and switched on. At slow speeds the ant carried on walking unperturbed. But as Federle slowly increased the speed, the pulling forces grew stronger and the ant stopped dead, legs spread out and all six feet planted firmly on the ground. At higher speeds still, the ant's feet began to slide slowly over the surface. "The stronger the pull, the faster they slide. This can only be explained by the presence of a liquid," says Federle. If the ant relied on some form of dry adhesion or a Velcro type mechanism, its feet would pop abruptly off the surface once the pull got too strong. But the liquid isn't the whole story. What engineers really find exciting about insect feet is the way they make almost perfect contact with the surface beneath. For a thin film to hold two solids together, they need to be close enough for both surface tension and the viscous forces of the liquid to come into play. The larger the area of contact, the stronger the stick. "Sticking to a perfectly smooth surface is no big deal," says Gorb.

But, in nature, even the smoothest-looking surfaces have microscopic lumps and bumps a few micrometres high. For a footpad to make good contact, it must follow the microscopic contours of the landscape beneath it. Flies, beetles and earwigs have solved the problem with hairy footpads. When the foot presses down, the flexible shafts of the hairs bend like the bristles of a miniature toothbrush to accommodate the wrinkles and troughs below.

Gorb has tested dozens of species with this sort of pad to see which had the best stick. Flies resist a pull of three or four times their body weight-perfectly adequate for crossing the ceiling. But beetles can do better. The champion of cling is a small, blue beetle with oversized yellow feet which lives on the leaves of palmetto trees in the south-eastern parts of the US.

Tom Eisner, a chemical ecologist at Cornell University in New York, has been fascinated by Hemisphaerota cyanea for years. Almost thirty years ago, he suggested that the beetle clung on tight to avoid being picked off by predators-ants in particular. When Eisner measured the beetle's sticking power earlier this year, he found it can withstand pulling forces of around 80 times its own weight for about 2 minutes and an astonishing 200 times its own weight for shorter periods. "The ants give up because the beetle holds on longer than they can be bothered to attack it," he says.

How does the beetle do it? The surface of a palmetto leaf is so hard that it can't dig in with claws and must rely on adhesion. Each of the beetle's enormous feet sports around 10,000 hairs-ten times as many as any of its close relatives. "Each hair is split into two at the tip, so that gives it 120,000 contact points," says Eisner. The tips are slightly expanded at the ends, which vastly increases the contact area. When the beetle walks, only a small proportion of its bristles touch the leaf. But at the first brush of an ant's antenna, the beetle presses its tarsi down flat, bringing all of its bristles into contact with the leaf.

The bristles make the contact, but they need to be moistened to achieve the sort of stick Eisner measured. The beetle leaves obviously oily footprints wherever it walks. Eisner and his colleague Daniel Aneshansley discovered that H. cyanea has a neat mechanism for sending fluid to the tips of its bristles. As the liquid leaves the pores and wets the shafts of the adjacent bristles, they clump together in groups of four or more-creating channels which draw the liquid to the tips. As the beetle walks around, the bristles remain in clumps. But when danger threatens and the beetle presses its feet firmly down, the tips separate out, each ready wetted to provide maximum stick.

Hairy footpads are one way of achieving close contact with leaf, branch or ceiling. But some of the best hangers-on, the ants, crickets and cockroaches, have soft, smooth footpads. Or so it seems. Gorb and his colleague Matthias Scherge from Ilmenau Technical University in Germany have made a close study of one particular smooth-padded insect, the great green bushcricket, Tettigonia viridissima. The bushcricket can walk on a smooth vertical surface, even upside down, with no trouble. Its secret lies in the cunning design of the material the pads are made of.

The cuticle of an insect's exoskeleton is usually made up of parallel layers, hardened on the outside, with softer material below. The cuticle of the pad consists mainly of the soft inner material. Gorb and Scherge found that just below the smooth exterior, the cuticle is sculpted into fine branching rods which slope forwards at an angle of around 60 degrees. "This is a very unusual design," says Gorb. As the foot presses down, the rods bend and the pad moulds itself around the irregular surface below, achieving the maximum contact. "If the hairy foot is like a toothbrush with soft bristles, the bushcricket's 'soles' are like toothbrushes covered in clingfilm," says Gorb.

The interior design of the bushcricket's footpads has another advantage: it allows the insect to pick its feet up fast and move at speed. Hairy-footed insects detach their feet by peeling their pads up a little at a time, rather as we peel off sticky tape. In smooth pads, the forward sloping rods provide a better grip as the foot pulls towards the body. But if the insect raises its legs, its feet come away easily. Industry could make good use of materials with similar properties-in designing tyres that hold the road better, for instance.

Ensured of intimate contact underfoot, the bushcricket has only to add a thin film of fluid to get all the adhesion it needs. Like the ant and the beetle, the bushcricket secretes a highly viscous liquid onto its pads-and leaves footprints where it walks. The nature of the fluid, though, remains a mystery.

Whatever liquid insects rely on, the Tokay gecko seems able to manage without it. Gekko gecko is a giant compared with an ant or a cricket, but it can hang upside down from the ceiling just as well. Like flies and beetles it relies on the sticking power of hairs, millions of them arranged in rows on the bottoms of its toes. Each foot has around half a million hairs, or setae, about a tenth the thickness of a human hair. And each of those hairs is finely divided into hundreds of "split ends" tipped by a spatula-shaped knob. According to Autumn and Robert Full of the University of California, Berkeley, these finest of points allow the hairs to get so close to the surface that they generate intermolecular forces quite strong enough to bond the foot to the surface.

Working out what makes a gecko stick was not easy. Autumn and Full teamed up with two engineers, Ron Fearing at Berkeley and Tom Kenny of Stanford University. Kenny designed a sophisticated device so the team could measure the sticking power of a single hair. First, they had to get the hair to stick to the sensor. "We tried for two months to get it to stick. Nothing happened," says Autumn. Frustrated, they went back to the real thing. They filmed geckos with a high-speed camera and looked to see what the lizards were doing that they might have missed.

They quickly found the answer. As a gecko plants its feet it unfurls its toes. This action pushes the setae into the wall and drags them slightly backwards, bringing more and more of the knob-like spatulae into contact with the wall. They tried the same action with a single hair, pushing it onto the sensor and dragging it back a few nanometres. "Then we got something really amazing," says Autumn. "It stuck 10 times as strongly as we expected. A single seta could resist a pull of 20 milligrams. If you took a whole gecko's worth of setae-which would fit on the area of a dime-it could hold a weight of 20 kilograms." Autumn and Full calculate that with such close contact even the weakest of the inter-molecular forces-van der Waals force-could provide this amount of stick.

A gecko can run up a wall at a metre a second, attaching and detaching its feet 15 times for each metre as it goes-so it clearly needs to come unstuck as fast as it sticks. High-speed video showed that as the lizard takes off it peels its toes upwards much as an insect peels off its footpads. By simply changing the angle of the setae the intermolecular forces disappear and the foot pulls free.

There is still a question mark over whether the gecko relies solely on van der Waals force. Geckos don't secrete any fluid onto their feet, but in the natural environment it's hard to escape films of water. "If I had to guess, I think we're likely to find a combination of mechanisms that change under different circumstances. We may find that water dominates in some environments but that van der Waals force dominates in others," says Autumn.

No one knows quite why the gecko needs so much sticking power. "It seems overbuilt for the job," says Autumn. "But maybe it's useful in a hurricane." Whatever the gecko's needs, its skills are in demand by humans. Autumn and his colleagues in California have already helped to create a robot that walks like a gecko. Mecho-Gecko, a robot built by iRobot of Massachusetts walks like a lizard-rolling its toes down and peeling them up again. At the moment, though, it has to make do with balls of glue to give it stick. The next step is to try to reproduce the hairs on a gecko's toes and create a robot with the full set of gecko skills. Then, we could build robots with feet that stick without glue, clean themselves and work just as well repairing underwater structures, mending satellites in the vacuum of space, or crawling over the dusty landscape of Mars.

Other industries are also keen to find new types of dry adhesive. "Adhesives that leave no residue are ideal for use in clean-room conditions-to pick up and place silicon wafers, for instance," says Autumn. Until technology comes up with a reliable method for making artificial setae, they can always work with the real thing, says Autumn. "A gecko sheds and regrows its skin every few months and we can harvest thousands of hairs without harming the gecko. We aren't talking about using them for sticky tape or Post-it notes but in micromanipulation or nanosurgery, so one gecko's-worth of hairs would go a long way."

Suspended animation

Adhesive foot pads let insects walk up slippery vertical surfaces or clamp down tightly for protection. But there's still that nagging question. How do they manage to walk upside down?

"People have been looking at how a fly walks up walls since the 18th century, but no one seems to have looked at how it moves upside down," says Stas Gorb of the Max Planck Institute for Developmental Biology in Tübingen. This year, Gorb did, and found that the answer was quite simple. "It changes its way of walking so it keeps more feet on the ground." Instead of walking with three feet on and three off, it clamps an extra foot to the ceiling, moving only two legs at a time. "They never do this walking up a wall, only when they cross the ceiling," says Gorb.

Intrigued, he watched to see how other insects perform the same trick. Bugs have a particularly hard time. "Their adhesion is worse than flies, and the way they walk is even stranger. They move one leg at a time, keeping the other five firmly on the ceiling. This makes them move in a very jerky way." His champion clinger, a little beetle related to the yellow-booted Hemisphaerota cyanea, has no trouble at all. "Its sticking power is so great it doesn't need to alter its gait to stay on," says Gorb. "It could hold on with one leg in contact with the ceiling."