Living creatures have a wide variety of ingenious ways of sticking to things,1 which we have written about in this magazine. These include: the fine hairs on gecko2 and spider3 feet that exploit fine chemical forces, and have inspired self-cleaning adhesive tape,4 the mechanical and hydraulic machinery of ant and bee feet5 and mussels that secrete special proteins that can even stick to “non-stick” frying pans.6
One of the best-known sticky things in nature is the spider’s web. We have written before on the amazing strength of the fibres—they are stronger than any man-made fibre of the same thickness, including Kevlar and steel,7 and are heat resistant as well.8 However, strength is not enough to trap insects—spiders also deposit tiny droplets of glue over the fibres, about 20 per mm of length. How this worked was a mystery—until biologists at the University of Akron, Ohio, USA, discovered its secret.9
Most glue is uni-functional: its one function is to stick things together. But the spider glue is multi-functional—a ‘smart material’.10 It is composed of polymers—long molecules composed of small molecules joined together—called glycoproteins, where the small molecules are amino acids and sugars. These are cross-linked, which means that forces can be transmitted efficiently—as one of the researchers said, “The success of an adhesive, however, depends on how efficiently the force is transmitted through the adhesive.”11 The crosslinking makes the droplets a hundred times stickier.
But this might be a problem when the spider wants to remove a trapped insect from the web. The solution is a variable-strength glue: much stronger at high speeds than at low speeds. Thus it can stick strongly to a flying insect, but when the insect struggles, with slower movements than flight, then the glue behaves more like a rubber band. When the insect stops moving, the spider can remove it fairly easily because the sticky force is lower still….
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