Geckos are able to walk upside down across glass ceilings because of the arrays of submicron-scale hairs on their feet. Now, a team of scientists from the Centre for Mesoscience and Nanotechnology at the University of Manchester, UK, and the Institute for Microelectronics Technology in Russia have mimicked these gecko hairs to produce a self-cleaning, re-attachable adhesive "gecko tape" (A K Geim et al. 2003 Nature Materials to be published).
“Geckos can run on a dirt road and then immediately climb a glass ceiling,” said Andre Geim of the University of Manchester. “But try to use scotch tape on a pavement or a sand pitch – you would not be able to attach it to anything after that. This is why the gecko tape is called self-cleaning. Moreover, adhesive tapes use glue, which eventually runs out and so they stop sticking. Gecko tape is the first without a glue (i.e. re-attachable).”
Gecko foot hairs typically have diameters of 200 to 500 nanometres. At this size of hair the geckos can exploit both van der Waals and capillary forces to climb surfaces, depending on the nature of the surface. Each hair produces a force of about 10-7 Newtons, but because so many are present in total they can produce an adhesion of roughly 10 Newtons per square centimetre.
While gecko foot hairs are made of keratin, to make their gecko tape Geim and colleagues prepared flexible fibres of the polymer polyimide on the surface of a 5 microns thick polyimide film, using electron-beam lithography and dry etching in oxygen plasma. They made fibres 2 microns long, with a diameter of around 500 nanometres and a periodicity of 1.6 microns, covering an area of roughly 1 square centimetre.
Initially, the team used a silicon wafer as a substrate for the polyimide film, but found that the tape’s sticking power increased by almost 1000 times if they used a soft bonding substrate such as scotch tape. “You never have an ideally flat surface because there are always bumps and dust,” said Geim. “Only hairs that reach these few points [the bumps and dust] stick, leaving other hairs idle. To overcome the obstacle, we eventually learnt how to place the hairs on a flexible base, a plastic ribbon like scotch tape. This compensates for the surface unevenness.”
According to Geim, although the team did consider producing enough gecko tape to hang a team member by their palm out of the window of a tall building, they felt it would be a waste of resources – costing tens of thousands of pounds for little scientific purpose. “Therefore, we limited our demonstration to a spiderman toy,” he said. Attaching gecko tape to the hand of a 15 cm high plastic spiderman figure weighing 40 grams enabled it to stick to a glass ceiling. In fact, the tape, which had a contact area of around 0.5 square centimetre with the glass, was able to carry a load of more than 100 grams.
The team still has to make the material durable, that is, re-attachable more than a few times. “You don’t want to start climbing [up a building] and after a few floors find that your gecko gloves have worn out completely,” said Geim. He reckons that means finding a different material for the hairs, perhaps keratin itself. “This is the real problem at the moment and we do not know how to deal with it,” he added.
