Nature gives up its secrects

Back in October we told you of the advances being made by material scientists in trying to copy the amazing ability of geckos to cling to almost anything. Now, the bioinspirationalists have managed to do just that – and they think it could revolutionise the way we think about materials 

SCIENTISTS at BAE Systems have created an artificial surface that grips incredibly tightly without glue or pressure. A sheet of this material just over 1m2 could be used to suspend the weight of an average family car.

Called Synthetic Gecko, the new nylon-like adhesive is inspired by the gecko lizard, whose ability to scurry up vertical walls and windows has intrigued people for centuries and inspired comic book characters such as spiderman.

“We wanted to mimic this ability”, commented Dr Jeff Sargent, research physicist at BAE Systems’ Advanced Technology Centre. “We recognised that a synthetic material could have tremendous engineering potential not only in our own aerospace and defence businesses, but also in other commercial applications”.
The gecko lizard gets its ability to stick to surfaces without glue from the soles of its feet which are patterned with millions of tiny hairs with split ends. At the tip of each split is a mushroom shaped cap less than one-thousand of a millimetre across. These ensure the gecko’s toes are always in very close contact with the surface beneath – so close that molecular forces of attraction create the grip. The grip is released by a peeling action when the animal lifts its foot to break the bond.

Using their micro-engineering clean room facilities, BAE Systems’ scientists, led by Dr Jeff Sergeant and Dr Sahad Haq, created layers comprising thousands of microscopic polyimide stalks with splayed tips, closely resembling the mushroom headed hairs on a gecko’s feet.

A number of potential applications for Gecko have been identified, ranging from instant repair patches for holes structures such as fuel tanks and aircraft skins, access panels without fasteners, or even the rapid attachment of armour panels.
Synthetic Gecko could also be used for new building materials, personal safety harnesses and for super grip tyres and training shoes.

Dr Haq and Dr Sargent believe that the material they are developing can be produced on a large scale and be used for various different and wide-ranging applications. One such application would be in the defence industry, more specifically in the area of robot crawlers. Robot crawlers, used for example in aviation to check for flaws or defects, currently use suction to attach themselves to the wing or fuselage, but they also have hoses and other peripherals attached. Synthetic Gecko would eliminate the need for these.

Other potential applications include medical applications, for example for use in skin graft operations. Further uses include sportswear and prosthetic devices.
Dr Haq comments: “One use is rapid patch repairs on military vehicles. Being able to patch a hole without having to prepare the surface or wait for adhesive to dry would be a huge advantage. It could also eliminate the need for rivets or fasteners”.

The material could potentially be mass produced in the future and would therefore be priced very competitively. As the material is still in the developmental stages, BAE Systems are as yet unable to comment on its price range or provide a direct comparison with other adhesive products.

However this is not the first time that material has been produced that has tried to copy gecko’s climbing ability. As far back as 2000, scientists in at the University of California discovered the secrets of the lizard’s seemingly gravity-defying ability. Researchers synthesised a material that mimicked the millions of tiny foot hairs, called setae, found on a gecko’s foot. The cumulative attractive force, van der Waals force, allows the lizard to move up walls and ceilings without falling.

In 2003 scientists at the University of Manchester, UK, produced a one centimetre patch of ‘gecko tape’, but neither the University of Manchester not the University of California managed to produce larger patches of the material.

The main characteristic of Synthetic Gecko which makes it unique is that it will only stick when the microscopic hairs are at a specific angle to the surface beneath, so a person wearing a glove covered in the adhesive material would only have to peel their hand from the surface to become unattached. Dr Haq explains: “It is only when you press the material to the substrate that it actually sticks. It is the molecular interaction hat causes it to stick.”

The obvious advantage of the material over current adhesives is that it can be used repeatedly, leaving no residues and is not sticky to touch. Only when pressure is added to the surface will it actually stick, so it is possible for a person to run their finger freely over it without getting stuck.

The material is manufactured by a modified version of a technique known as photo-lithography, commonly used to make silicon chips. The technique uses light to etch three-dimensional patterns into a material. Dr Haq explains: “The processes we use are modification of standard electronic fabrication processes. They are cheap, well known, well understood and can be scaled up to very large areas cheaply.”

Previous attempts at making similar material relied on more intricate techniques such as electron-beam lithography, which is expensive and difficult to scale-up to produce vast quantities of the material. So far, researchers at BAE Systems have manufactured several different materials with different sized mushrooms to try to optimise its stickiness.

The next step in the development programme comprises further research into the influence of surface roughness and water on the adhesive properties of the material, to ensure that it is effective even on a wide range of surface roughness.
Dr Haq concludes: “We fully believe that Synthetic Gecko can be in practical use within a year. Given the stringent testing procedures in the aerospace industry it almost seems likely that it will be fist seen in some form of commercial guise.”

However the gecko is not the only animal to have inspired scientists in their quest for adhesion. As recently as November 2006 German scientists announced the secret to their new adhesive material – the soles of the beetles’ feet.

Researchers found that, like the gecko, the mushroom-shaped microhairs found in the soles of a beetle’s foot allow it to stick to smooth walls without any adhesives. Scientists from the Max Planck Institute for Metals Research, Stuttgart, and their colleagues at the Gottlieb Binder GmbH in Holzgerlingen, a specialist company for fastener systems, have developed a material with a biomimetic microsctructure that exhibits excellent adhesive qualities, basing it on investigations of the foot soles of several types of beetle. Their particularly strong adhesive force is the result of very small, specially shaped hairs reminiscent of tiny mushrooms. So far, in tests carried out by the Max Plank Institute with measuring instruments developed specially for the purpose, the adhesive has shown to last hundreds of applications, does not leave any visible marks, and can be thoroughly cleaned with soap and water. The researchers found that five square centimetres of the material can hold objects weighing up to one hundred grams on the walls.

To manufacture the material, a mould is used in which the required surface is embossed as a negative image. This mould is filled with a polymerizing mixture which is allowed to cure and then is released from the mould. The researchers found the construction of the mould the most challenging process and exactly how it works remains a trade secret. Optimising the polymer structure is also fundamental to the process; if it is too fluid it runs out of the mould, and if it is too viscous it won’t even go in.

Potential applications range from protective foil for delicate glasses to reusable adhesive fixtures. The new material will soon be found in industrial production processes in the manufacture of glass components. Researchers are trying to improve the adhesion by refining the structures even further.

Maria Anguita
Maria is a freelance science and health writer. She has previously edited several science and health magazines. She holds a degree in biochemistry with medical biochemistry from Bristol University and an MA in international journalism from City University, London.