快猫短视频

IF KELLAR AUTUMN has his way, the first footprints on Mars won鈥檛 be human.
They鈥檒l belong to a gecko. Gecko toes have legendary sticking power鈥攁nd
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鈥檛.

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鈥檚
gravity-defying footwork. Earlier this year, he and his colleagues discovered
that the gecko鈥檚 toes don鈥檛 just stick, they bond to the surface beneath them.
Engineers are already trying to copy the gecko鈥檚 technique鈥攂ut reptilian
feet aren鈥檛 the only ones they鈥檙e 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鈥檚 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. 鈥淭here are lots of ways to make two surfaces stick together,
but there are very few which provide precise and reversible attachment,鈥� says
Stas Gorb, a biologist at the Max Planck Institute of Developmental Biology in
T眉bingen, Germany. Geckos and insects have both perfected ways of doing
it.

快猫短视频s 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鈥檚 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 鈥渨et adhesion鈥�,
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 鈥渟oles鈥� 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 W眉rzburg, showed experimentally that an insect鈥檚 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. 鈥淚t鈥檚 almost impossible to shake them off,鈥� 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鈥攁nd 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鈥檚 feet began to slide
slowly over the surface. 鈥淭he stronger the pull, the faster they slide. This can
only be explained by the presence of a liquid,鈥� 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鈥檛 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. 鈥淪ticking to
a perfectly smooth surface is no big deal,鈥� 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鈥攑erfectly 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鈥攁nts in particular. When Eisner measured the beetle鈥檚 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. 鈥淭he ants give up because the beetle holds on longer
than they can be bothered to attack it,鈥� he says.

How does the beetle do it? The surface of a palmetto leaf is so hard that it
can鈥檛 dig in with claws and must rely on adhesion. Each of the beetle鈥檚 enormous
feet sports around 10,000 hairs鈥攖en times as many as any of its close
relatives. 鈥淓ach hair is split into two at the tip, so that gives it 120,000
contact points,鈥� 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鈥檚
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鈥攃reating 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鈥檚 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. 鈥淭his is a very unusual
design,鈥� says Gorb. As the foot presses down, the rods bend and the pad moulds
itself around the irregular surface below, achieving the maximum contact. 鈥淚f
the hairy foot is like a toothbrush with soft bristles, the bushcricket鈥檚
鈥榮oles鈥� are like toothbrushes covered in clingfilm,鈥� says Gorb.

The interior design of the bushcricket鈥檚 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鈥攊n 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鈥攁nd 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 鈥渟plit ends鈥� 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. 鈥淲e tried for two months to get it to stick. Nothing happened,鈥� 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. 鈥淭hen we got something really
amazing,鈥� says Autumn. 鈥淚t stuck 10 times as strongly as we expected. A single
seta could resist a pull of 20 milligrams. If you took a whole gecko鈥檚 worth of
setae鈥攚hich would fit on the area of a dime鈥攊t could hold a weight
of 20 kilograms.鈥� Autumn and Full calculate that with such close contact even
the weakest of the inter-molecular forces鈥攙an der Waals force鈥攃ould
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鈥攕o 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鈥檛 secrete any fluid onto their feet, but in the
natural environment it鈥檚 hard to escape films of water. 鈥淚f I had to guess, I
think we鈥檙e 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,鈥� says Autumn.

No one knows quite why the gecko needs so much sticking power. 鈥淚t seems
overbuilt for the job,鈥� says Autumn. 鈥淏ut maybe it鈥檚 useful in a hurricane.鈥�
Whatever the gecko鈥檚 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鈥攔olling 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鈥檚 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. 鈥淎dhesives
that leave no residue are ideal for use in clean-room conditions鈥攖o pick
up and place silicon wafers, for instance,鈥� says Autumn. Until technology comes
up with a reliable method for making artificial setae, they can always work with
the real thing, says Autumn. 鈥淎 gecko sheds and regrows its skin every few
months and we can harvest thousands of hairs without harming the gecko. We
aren鈥檛 talking about using them for sticky tape or Post-it notes but in
micromanipulation or nanosurgery, so one gecko鈥檚-worth of hairs would go a long
飞补测.鈥�

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

鈥淧eople 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,鈥� says Stas Gorb of
the Max Planck Institute for Developmental Biology in T眉bingen. This year,
Gorb did, and found that the answer was quite simple. 鈥淚t changes its way of
walking so it keeps more feet on the ground.鈥� 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. 鈥淭hey never do this walking up a wall, only when they cross the
ceiling,鈥� says Gorb.

Intrigued, he watched to see how other insects perform the same trick. Bugs
have a particularly hard time. 鈥淭heir 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 飞补测.鈥� His champion
clinger, a little beetle related to the yellow-booted Hemisphaerota
cyanea, has no trouble at all. 鈥淚ts sticking power is so great it doesn鈥檛
need to alter its gait to stay on,鈥� says Gorb. 鈥淚t could hold on with one leg in
contact with the ceiling.鈥�