Fasteners and Sealing
Conventional adhesives have a lot going for them, as they can support enormous forces. When a glued structure fails, it is often the material itself that shears - while the adhesive join itself remains intact. But adhesives are not effective in all environments. The vacuum of space is a particular weak point, as are aqueous environments - such as under the sea or even inside the human body.
professor in the chemical and biomolecular engineering department at Tulane University in the US, who led the research. The other advantages of the gecko’s foot include:
it leaves no sticky residue behind; and, while it sticks strongly, it peels off very easily. Mimicking this latter ability could allow structures stuck in this way to be easily ‘repositioned’ if applied incorrectly. To create the adhesive material, Pesika has used
a technique normally used in the production of microchips - called photolithography - to create special adhesive surfaces that mimic the gecko’s foot. These tiny polyurethane structures have microscopic features on the surface that stick through ‘attraction’ only. The bulk of the structure is a cylinder; a series of
Fig. 2. Rotite’s novel connector technology could be used across a range of industries – from aerospace and defence to cosmetics and medical.
Because of this, scientists are looking to develop adhesives that work in new ways. Copying the gecko’s foot is a good starting point. The pads on a gecko’s feet are composed of millions of tiny ‘hairs’, which make such intimate contact with the floor - or ceiling - that forces of molecular attraction (called Van der Waals forces) come into effect. “You can’t usually get close enough contact with a surface for this to happen,” says Noshir Pesika, assistant
Underwater curing O
ne of Noshir Pesika’s aims, with his ‘gecko’ research’, is to develop an adhesive that will work
underwater – and this is something that the natural world is very good at. Another example is the buoy barnacle
(Dosima fascicularis), which produces a special adhesive that it uses to attach itself to flotsam. The adhesive is so strong that it is almost impossible to break down into its constituent parts using ordinary solvents. The adhesive can also cure underwater. Now, researchers at the Fraunhofer
Institute for Manufacturing Technology and Advanced Materials (Ifam) in Bremen are trying to discover which particular amino
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acids make up the relevant proteins in this adhesive. “Once we’ve done that, the next step will
be to recreate the adhesive proteins in the laboratory,” said Ingo Grunwald, an expert in biological adhesives at Ifam. These types of ‘bioadhesive’ are mainly
of interest in medical applications, such as for closing incisions or replacing and supporting the pins and screws used to treat bone fractures. A 2011 book called ‘Biological Adhesive
Systems’, which Grunwald co-edited, lists a host of potential applications of ‘biological’ glues, ranging from cardboard packaging and labels for glass bottles, through to ways
tiny ‘fibres’ protrude from its surface. These measure around 20 x 20 x 7 microns. It is these tiny fibres, or pads, that make intimate contact with a surface and stick to it via molecular attraction. Despite the microscopic nature of Pesika’s synthetic structures, they are still around 1,000 times larger than the equivalent one on the gecko’s foot. However, the tiny fibres on the gecko’s foot are quite stiff; Pesika’s synthetic structures have been made more pliable, to improve surface contact. He estimates that his structures are around 100 times
weaker than a real gecko’s foot. But he says that a 1ft x 1ft pad of his material could support the weight of a human. So far, his structures - which were made at the Naval Research Laboratory in Washington DC, US - are about 1.5 inches square. A key attribute of the gecko’s foot is its anisotropy
- meaning that it sticks firmly when sheared in one direction, but weakly in the other. It is this quality that allows the gecko to run quickly across the ceiling. Pesika has reproduced this effect by mimicking the structure of
Noshir Pesika has created micro-scaled polyurethane structures that mimic the way that a gecko’s foot sticks to surfaces.
to heal parts of the body such as skin, teeth and internal organs. ●
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