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
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The Independent Online

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.

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.