For an arachnophobe, the sight of a spider making its way up a wall sparks fear. For a materials scientist, however, it can provide inspiration. In the current issue of the journal Smart Materials and Structures, researchers describe just how a spider manages to stick to ceilings in apparent defiance of gravity. The discovery could point the way to novel adhesives.
"We carried out this research to find out how these spiders have evolved to stick to surfaces, and found that it was all down to a microscopic force between molecules," says lead author Antonia Kesel of the Institute of Technical Zoology and Bionics in Germany. Using a scanning electron microscope, the team obtained images of the foot of a jumping spider (Evarcha arcuata). The spider¿s ability to cling, the team reports, stems from tiny hairs called setules that cover larger hairs comprising a tuft on each foot (see image). The researchers then used atomic force microscopy to measure the strength of adhesion between the spider¿s feet and a surface. "We found out that when all 600,000 tips are in contact with an underlying surface the spider can produce an adhesive force of 170 times its own weight," explains study co-author Andrew Martin, also at the Institute of Technical Zoology and Bionics. "That's like Spiderman clinging to the flat surface of a window on a building by his fingertips and toes only, whilst rescuing 170 adults who are hanging on to his back."
The results are the first quantification of the strength of spideybonds, but the mechanism at work is similar to that employed by geckos to scamper across practically any type of surface. Both types of amazing adhesion arise from so-called van der Waals forces, which create electrostatic attraction between the tiny hairs and a surface. The strength of these forces depends on the distance between the two objects and is independent of the surrounding environment. As a result, Kesel notes that "one possible application of our research would be to develop Post-it notes based on the van der Waals force, which would stick even if they got wet or greasy."