Stuck on you

A close study of geckos has led to a way of sticking objects together without conventional adhesives. Scientists at defence and aerospace supplier BAe Systems have developed a plastic film whose surface is modelled on a gecko’s foot. Instead of having a ‘sticky’ surface, it has millions of tiny artificial ‘hairs’. These make very close contact with a surface, and stick to it through molecular attraction. Although each force is very tiny, the cumulative effect is comparable to an adhesive join. In the same way that a gecko can repeatedly unpeel and re-attach its feet during an ascent up a wall, this new material can do the same. Because it uses no adhesive coating, it will not leave a surface residue. BAE believes the technology has a number of defence-related applications – such as fast repair of aircraft bodies, or attaching temporary armour plate to tanks “Battle damage to an aircraft could be repaired more rapidly than with conventional adhesives,” says Sajad Haq, of BAe Systems’ Advanced Technology Centre in Bristol. With Jeffrey Sargent, he is leading the project. “This might normally take anything from several hours to a few days. This could save time and money.” Access panels might also use this material in place of fasteners, bolts and screws. Civil service BAE has also identified many civil applications. “Applications in the civil market are immense,” says Haq. “We are considering civil and defence applications in parallel.” Work began on the material – which as been dubbed Synthetic Gecko – in 2004. BAe was aware of earlier research in this field, in both the US and UK, but says that it was very academic. “A lot more work was needed to mature the technology,” says Haq. “The material was not available from these sources, because they did not manufacture it. We recognised that if it could be scaled up and made to work, it would be relevant to typical BAe applications.” The company has carried out representative tests – which was one of the main reasons for manufacturing samples in the first place. “We had to see if the material would do what we though it would do,” says Haq. So far, it has produced pads of “thin, flexible plastic sheet” that are 10cm square. It has the feel of Sellotape – rather than clingfilm – but has no sticky residue. BAE has used a number of plastics, including polyimide, to make different formulations. The material and manufacturing technique used can create different properties, such as its adhesive power, and how much pressure is needed to make it stick. “We are now trying to understand how the material will behave under non-ideal conditions, such as dusty, dirty or wet conditions,” says Haq. Among a long list of potential applications, both civil and military, BAe cites: skin grafts, repair patches, supergrip sportswear, prosthetic devices, rock climbing shoes, medical devices and ‘Spiderman’ gloves. The company is guarded on the subject of exact manufacturing techniques. “We use cleanroom processing techniques, similar to those used to make electronic circuitry,” he says. “We’ve used other techniques too, which are scalable and relatively low cost. Eventually we want to use this for BAe products.” One of the most promising applications is to use the material as pads on the feet of ‘crawler robots’ to look for defects on the surface of aircraft. This job is usually done manually, but robots are being developed for the purpose. “These robots currently use hefty suction pads with vacuum pumps, because you do not want to contaminate the surface of the aircraft,” says Haq. “You could replace the suction pads with this material.” Regarding further commercialisation, BAe is unwilling to commit to a definite date. “Time frames are variable,” says Haq. “In some cases, we have material that could be used tomorrow. For harsher environments, it would depend on the current R&D.” Feel the force The gecko’s foot – and BAe’s new material – rely on the close proximity of two surfaces. Millions of stalks on the gecko’s foot make intimate contact with a surface such as a sheer wall. “When structures are brought very close together – a few tens of nanometres – they are attracted by Van der Waals forces,” says BAe’s Jeffrey Sargent. “We think that’s probably happening here. It’s the right order of strength, and still sticks in a vacuum.” The bond strength, of around 30kPa, cannot compare with a super-strength bond of more than 30MPa, but Sargent says: “It’s enough for some of the applications we have in mind. The added effect of millions of tiny forces is one that is large enough to support the gecko as it climbs a wall. BAe says that its material could do the same – and calculates that a pad of 1m square could support the weight of a family car. BAe has tried to mimic the structure of the gecko’s foot as accurately as possible, copying characteristics such as geometry, spacing, length and mechanical properties of the surface. The keratin of the gecko’s hairs is reproduced using engineering plastics such as polyimide. The gecko’s foot has a ‘spatula’ that measures around 200nm. It sits at the end of each hair, which gives it the ‘compliance’ to stick to rough surfaces. Pointers Plastic pad mimics gecko’s foot to stick to any surface – without an adhesive. Micro-engineered material is likely to use Van Der Waals forces to make surfaces stick together. BAe is aiming the material at both military and civil markets. Potential applications include repairing battle damage to aircraft, attaching temporary armour plating and ‘supergrip’ sportswear