By Jonathan Fildes
Science and technology reporter, BBC News
The remarkable adhesive abilities of geckos and mussels have been combined to create a super-sticky material.
Setae allow geckos to scurry up walls and hang upside down
Unlike other adhesives inspired by the nimble reptiles, "geckel" can attach to both wet and dry surfaces, the team that developed the material says.
Its staying power comes from coating fibrous silicone, similar in structure to a gecko's foot, with a polymer that mimics the "glue" used by mussels.
Writing in Nature, the researchers say it could have medical applications.
"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," said Professor Phillip Messersmith from Northwestern University in Evanston, near Chicago, US.
"Such a bandage would remain firmly attached to the skin during bathing but would permit easy removal upon healing."
Other research teams claim they have already produced a gecko-inspired material that works underwater.
Geckos have an incredible ability to stick to surfaces. Some studies suggest the over-engineered reptiles can hold hundreds of times their own body weight.
In 2000, a University of California team showed that the adhesion was due to very weak intermolecular forces produced by the billions of hair-like structures, known as setae, on each gecko foot.
These "van der Waals" forces arise when unbalanced electrical charges around molecules attract one another.
Geckos can support hundreds of times their own body weight
The cumulative attractive force of billions of setae allows geckos to scurry up walls and even hang upside down on polished glass.
The reptile's grip is only released when it peels its foot off the surface.
The new geckel material exploits this ability but also combines it with the sticking power of mussels.
It consists of a base of densely packed silicone setae coated with a polymer that mimics amino acids found in the glues of mussels.
"I was reading a research paper about the drop of adhesion in geckos when [they go] under water, and it hit me: maybe we could apply what we know about mussels to make gecko adhesion work under water," said Professor Messersmith
Tests showed that the material could be stuck and unstuck more than 1,000 times, even when used under water. The researchers said that other materials had only demonstrated "a few contact cycles".
Removing the polymer coating drastically reduced its efficiency.
Creating a cheap, mass produced adhesive that mimics the sticking power of the cold-blooded gecko has long been a goal of scientists.
In 2003, a team from the University of Manchester, UK, produced small quantities of a sticky gecko tape.
It was produced using electron-beam lithography, a process in which a beam of electrons etches patterns in a surface.
The same technique is used to make geckel but is expensive and difficult to scale-up for mass production.
For example, the pieces of geckel used in the latest experiments were just 60 nanometres (billionths of a metre) in diameter.
"We have demonstrated a proof of concept," said Professor Messersmith.
"The challenge will be to scale up the technology and still have the geckel material exhibit adhesive behaviour."
But last year, researchers at aerospace and defence firm BAE Systems raised hopes of mass production when they showed off centimetre length strips of a plastic, known as Synthetic Gecko.
Using a technique known as photo-lithography, common in the silicon industry, they have since been able to scale up production.
"We've now got large pieces," said Dr Sajad Haq, a research scientist at the company's Advanced Technology Centre in Bristol, UK.
He was unable to reveal the exact size of the sheets as the company has applied for patents on the material.
He also said that they had optimised the design of the nylon-like material, which is covered with millions of tiny mushroom-like hairs.
Synthetic Gecko is composed of millions of mushroom-shaped hairs
"We've now got the material working on rough surfaces and wet surfaces; so it does work underwater for example." he said.
Crucially, he said, his team had not had to tweak the design too much to make it work when wet.
"The material we use is still a simple system," he said. "We haven't had to do anything complex to ensure it works underwater."
He also said that, like geckel, Synthetic Gecko could be re-used over and over again.
Once patented, the firm plans to use the material for a range of applications from repair patches for tanks, aircraft and submarines to crawler robots.
"It's becoming more and more practical," he said. "It's getting very close to a high maturity level."
As the Synthetic Gecko research is commercially sensitive, specific details have not yet been published.