Imagine you had a pair of trainers in which you could walk up walls, hang upside-down from the ceiling or even stroll underwater along the side of a submarine.
Just think about that. The soles of those shoes would have to stick so tightly that they could support your entire body weight, but simultaneously stick so loosely that you could walk in them. And, of course, they would have to be waterproof, too.
It may sound impossible, but this "ultimate superglue" has now been created, inspired by two animals that are naturally designed to perform superhuman feats of stickiness.
Geckos are renowned for their ability to scurry up vertical surfaces and even crawl along upside-down. Their feet stick tight, but only temporarily, coming on and off surfaces again and again, like an infinitely re-usable Post-it note.
Any scientist who managed to mimic the gecko's foot could become a real life Spider-Man. So it's no surprise that many have tried to follow in the little lizard's footsteps.
Last year, a team from Manchester University invented a "gecko tape" so sticky that if both your palms were covered with it, you could support your own body weight. Slip on a pair of gecko gloves and you could change your lightbulbs without using a stepladder.
It sounds like the ultimate superglue, but there was still one major sticking point. The gecko's foot has an achilles heel - it does not stick underwater.
Water is an enemy of adhesives. Just think of how long a bandage on your finger lasts in the bath. To create the ultimate plaster, you need a material that can stick tightly to a your skin underwater, then peel off smoothly after the wound has healed.
Today, finally, that problem has finally been overcome, in the most ingenious fashion, by two US biomedical engineers.
They have created the ultimate underwater glue, by marrying the secrets of the gecko's foot with the suction techniques of another animal well known for its underwater sticking power: the mussel.
The new adhesive material, called "geckel", combines the properties of the two. It is strong yet reversible - re-sticking up to 1000 times - and it works in both air and water.
Geckel was created by a team led by Phillip Messersmith, professor of biomedical engineering at Northwestern University, Illinois. "I envision that adhesive tapes made out of geckel could be used to replace sutures for wound closure and may also be useful as a water-resistant adhesive for bandages and drug-delivery patches," he said. "Such a bandage would remain firmly attached to the skin during bathing, but would permit easy removal upon healing."
Messersmith had his first brainwave when reading a research paper about the way adhesion in geckos was reduced underwater. Earlier, he and his research group had studied the "glue" proteins of mussels.
"It hit me - maybe we could apply what we know about mussels to make gecko adhesion work underwater," he said.
The gecko's strong but temporary adhesion comes from a mechanical principle known as "contact splitting". Each of its feet has a flat pad that is densely packed with very fine hairs that are split at the ends. The split ends allow a greater number of contact points between foot and surface, and thus create a greater adhesion force. Flies, bees and other insects also use this strategy.
To imitate the gecko's foot, Messersmith and his team assembled arrays of microscopic silicone pillars just nanometres wide, but with enough flexibility to adapt to rough surfaces. Next, they brought in the mussel power, coating the pillars with a very thin layer of a synthetic polymer, designed to mimic the mussel's "glue" proteins.
They found that the mussel polymer improved wet adhesion fifteen-fold over the uncoated pillar arrays. The findings are published today in the journal Nature.
In future, geckel could have countless applications in medical, industrial, consumer and military settings, Messersmith believes. "The challenge will be to scale up the technology and still have the geckel material exhibit adhesive behaviour," he said. In fact, he believes they could yet produce a material with even better adhesion.
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