Anyone who's got tangled in a spider web can't help but identify with the hapless victims that end up entombed in them. But what makes this remarkable material so sticky? Brent Opell explains that the spiral silk is coated in microscopic droplets of a glycoprotein adhesive and that the silk's stickiness increases as more and more droplets contact an ensnared object. Having already established that spider silk behaves almost like the suspension cable on a suspension bridge, with the adhesive droplets attaching the silk to an object in the same way that ropes connect the suspension cable to a bridge's deck [B. D. Opell and M. L. Hendricks (2008) J. Exp. Biol.210, 553-560], Opell wondered how much the spider silk's elasticity contributes to its stickiness. Teaming up with Brian Markley, Charles Hannum and Mary Hendricks, Opell set about stretching five different spider silks and measuring how their stickiness varied (p. 2243).
But Opell had some significant technical challenges to overcome. First, he had no quality control over the silk. Spiders adjust the way they spin their silk to each web's circumstances, so the team had no idea how much a sample had been stretched before they collected it. They overcame the problem by calculating the amount of elasticity remaining in each sample. Assuming that all silks have similar mechanical properties, Opell and his team were able to use the known stress/strain curve of Araneus didematus silk [T. Köhler and F. Vollrath (1995) J. Exp. Zool.271, 1-17]coupled with the silk's breaking and original lengths to estimate the silk's stiffness at different extensions.
Secondly, the team had to be sure that they had accurately measured the average stickiness per drop, as the spacing between droplets increases as the silk stretches. Pressing a suspended thread against a flat contact plate and pulling them apart, the team recorded the force at the moment the two separated. Repeating the experiment after stretching the silk by two and three times, the team used proportionately longer contact plates to ensure that the same number of adhesive droplets attached the silk to the plate at each length. Dividing the detachment force by the number of droplets in contact with the plate gave the team the average stickiness per drop.
Having convinced themselves that they could accurately determine the silk's stiffness and droplet stickiness, the team found that as four of the five silks were stretched and became stiffer, the stickiness per drop decreased.`Extensibility contributes positively to stickiness,' says Opell; in other words, the stretchier the silk the stickier it is. This is because more droplets contribute to stickiness on a stretchy thread than on a stretched and rigid thread. Opell suspects that the elasticity of an unstretched thread contributes as much as one-third of the silk's stickiness.