Brussels, 24 October 2006
German researchers have taken inspiration from nature to create a dry adhesive material which sticks to the smoothest of surfaces.
People have long marvelled at the ability of creatures such as insects and certain lizards to climb up vertical walls and even walk across ceilings without falling off. Recently research has shown that these animals owe their 'stickiness' to the presence of extremely fine hairs on their feet which optimise contact with the surface. Many people have tried to reproduce these systems, but until now the resulting artificial materials have had a limited life cycle and were simply not as sticky as the natural versions.
Now researchers from the Max Planck Institute for Metals Research and Gottlieb Binder, a company specialising in attachment systems, have successfully produced an adhesive material whose microstructure is based on the soles of insects' feet. Their findings are reported in the early online edition of Journal of the Royal Society Interface.
In this case the insects in question were beetles of the family Chrysomelidae, which are known to be experts in the art of sticking to very smooth surfaces.
The 'hairs' of the new material resemble tiny mushrooms; a thin plate 40 micrometres in diameter sits atop a pillar 100 micrometers high which has a sturdy base but a narrow neck. The 'mushrooms' are arranged hexagonally on the surface of the material.
The material was made by pouring a polymer mixture into a mould and waiting for it to set before carefully removing the material from the mould. According to the researchers the creation of the mould, with its delicate microstructure, was incredibly challenging and the details of the process are a closely guarded secret. Creating the polymer mixture was also difficult; mixtures which were too liquid simply flowed out of the mould, while mixtures which were too viscous were unable to penetrate into the fine microstructure.
A battery of tests revealed that five square centimetres of the material are able to stick objects weighing up to 100 grammes to a smooth wall. The material can be used several times without losing stickiness, and if it gets dirty, simply washing it in soapy water will restore its adhesive qualities. Furthermore, when removed from a surface it does not leave behind visible marks.
The material does not perform as well on rougher surfaces, something which does not surprise the researchers. 'Insects also have difficulties walking on surfaces which are slightly rough,' explained project leader Stanislav Gorb of the Max Planck Society. 'This is a fundamental problem of the adherence mechanism.'
The researchers put the success of their system down to a number of factors. The thin plate at the top of the 'mushroom' is flexible enough to form a reliable contact with the surface, even in the presence of small particles of dirt. Meanwhile larger particles are able to slip into the space between the pillars where they do not affect the ability of the plates to stick to the surface. The thin neck of the pillar also offers flexibility in the face of uneven surfaces. Finally, broken pillars do not alter the effectiveness of neighbouring pillars.
The scientists describe their work as 'a considerable step towards the development of an industrial dry adhesive.' The new material could be used in a protective tape for sensitive glass surfaces, or in reusable pads for the attachment of small objects to smooth surfaces. It has already been attached to the feet of a 120 robot which was subsequently able to walk up a vertical glass wall.
The researchers now plan to improve the 'stickiness' of their material by refining its structure.