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Greta Oto or Glasswing Butterfly! |
The stickiest stuff in the world
Animals use natural 'superglues' to climb up smooth surfaces, hold fast to trees
in a cyclone or hitch rides on their hosts. Kate Ravilious reports on exciting
new applications for these extraordinary adhesive powers
18 February 2004
Being able to cling on to something is a very useful skill in the natural world.
Geckos rely on having special sticky feet to enable them to scuttle up walls,
while wasps have evolved hooks in their wing tips so that they can clip their
wings together during flight and cruise along at high speed. Tapeworms can cling
firmly to their victim's intestine, while leeches get their dinner by glueing
themselves to a passing animal.
Until recently, superglue might have been the strongest adhesive you could hope
to find, but all that is about to change as scientists take a design tip from
nature. Andrew Parker and Abigail Ingram from the department of zoology at
Oxford University have been studying some of these clever natural hooking
mechanisms to help solve a few of mankind's sticky problems. By copying
caterpillars they have managed to develop a new, extra-strong Velcro-like
material, while observing fish parasites has led to a novel way of tagging
animals and fish.
Initially, Parker and Ingram studied a selection of sticky things, including
geckos' feet, flies' feet, various fish parasites and a butterfly chrysalis.
Very soon they narrowed this down to the two natural hooking mechanisms with the
greatest potential to be mimicked: the butterfly chrysalis and a fish parasite.
Like many butterflies, the dainty, transparent winged, Greta oto (or Glasswing)
butterfly from Costa Rica emerges from a chrysalis that dangles underneath a
leaf. During the pre-chrysalis stage, the caterpillar spins a complex,
three-dimensional silk mesh underneath the leaf. When the caterpillar is ready
to pupate, its skin splits open and a chrysalis emerges. One end of the
chrysalis is covered with a bunch of hooks, which it thrusts into the silk mesh,
enabling it to hang underneath the leaf until the butterfly is ready to fly
away. "The attachment is incredibly strong," says Ingram. "It really is very
difficult to pull it apart."
To discover how G. oto creates its extra-strong
bond, Ingram spent many hours watching the
caterpillars pupate. She set up a digital camera
and used time-lapse photography to record their
actions. Most of the caterpillars were not
natural film stars and they weren't always
co-operative for the cameras. "Catching the
right caterpillar on camera at the precise
moment was very tricky," says Ingram. It took
her several weeks to capture the silk-spinning
process and see exactly how the chrysalis
attached itself to the silk mesh. Nonetheless,
after recording and watching many hours of
caterpillars in action, she got the shots she
needed to see just what was going on.
Parker and Ingram noticed that one end of the
chrysalis was hemispherically shaped and had
hooks coming out of it in all directions. When
this bundle of hooks was inserted into the silk
mesh under the leaf it created an incredibly
strong attachment. They tested the strength of
this connection and used a high-speed video
camera to film it being pulled apart. "It turned
out to be 40 times stronger than needed to
support the weight of the chrysalis," says
Ingram. The reason for the excessive strength of
the bond is still a bit of a puzzle, but one
idea they have is that it allows the chrysalis
to ride out the hurricanes that hit Costa Rica.
Working together with Chris Lawrence at QinetiQ,
the science and technology solutions company,
they are borrowing the hook and mesh design from
the Greta oto butterfly to develop an
extra-strong, three-dimensional material similar
to Velcro, which could even be used underwater.
"One major advantage of the caterpillar silk
bond is that the caterpillar can swing freely,"
says Lawrence. An artificial product with
similar properties could be used to attach
objects that need to be able to respond to
underwater currents, such as sensors hanging
beneath a boat. This could be a very handy tool
for scientists who want to take measurements of
underwater variables like temperature, current
flow and chemical concentration, enabling them
to gain valuable data from previously
inaccessible locations.
Another natural attachment mechanism that
provided inspiration came from a parasite that
clings to the skin of marlin. P. instructa is a
stringy, worm-like crustacean that can grow more
than half a metre long. After burrowing its way
in, it embeds one end of its body firmly into
the marlin's skin, while the remainder of the
body dangles from the fish.
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Ingram collected samples of these wormy parasites at one of the largest marlin
fishing tournaments in the world, in Port Stephens, Australia. It wasn't the
most relaxing of experiences; working frantically to keep up with the fishermen
as they hauled in their catch and gathering parasites from a large, slippery
fish was no easy task. Nonetheless, she managed to collect a good number of both
live and dead parasite samples. Back home, she investigated the attachment
between the parasite and the fish by studying it under the microscope. She found
that the secret of their grip lay in how the parasite grew.
Pennella instructa turned out to have a sophisticated way of attaching itself to
the fish. "First, it burrows into the skin of the fish and then it grows an
anchor when it is in place," says Ingram. The advantage of this is that it
causes minimal damage to the fish skin, making it less likely to rip out later.
Parker and Ingram realised that this idea could be very useful for scientists
who tag animals and fish to monitor their numbers and follow their movements.
Currently, fish and animal tagging is problematic because inserting the tag
damages the skin and no one can be sure that the tag remains in place. "If the
tags are falling out, then the data from fish and animal tagging becomes
useless," says Parker. Research projects often want to monitor animals and fish
for many months, so they need tags that will remain attached for the duration of
the project. In addition, tagging can be very expensive, with some of the more
sophisticated satellite tags costing as much as £2,000 each. At these kinds of
prices no one wants their fish tag to drop to the bottom of the sea floor.
By putting their heads together, Parker, Ingram and Lawrence have come up with a
new kind of fish and animal tag that mimics Pennella. The tag uses a
revolutionary technology to change shape when it enters the fish or animal,
ensuring that is stays firmly in place and causes minimal harm to the skin. At
the moment they can't reveal the full details of how it is made because they are
still patenting the design, however, they hope to run trials very soon. If all
goes well, the new tags should be available to scientists later this year.
As ever, nature seems to have the best ideas. Both of these examples of natural
hooking mechanisms are beginning to change the way we think about sticking
things together and making attachments. The G. oto chrysalis has opened up a
host of new possibilities, with the development of a strong, underwater sticking
mechanism. Meanwhile the wormy P. instructa has led the way forward to huge
improvements in an established technology. Many more natural sticky solutions
may be waiting in the wings, with the potential to affix objects in places we
would have never dreamed possible. Conventional glues and adhesives may soon
become a thing of the past as we turn towards these superior natural solutions.
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