February 1, 2012 | 1
Spiders’ silk has been the envy of materials engineers for decades. Its combination of flexibility and durability has been difficult to match with even the most advanced technology.
“It is stronger than steel and tougher than Kevlar by weight,” Markus Buehler, an engineer at Massachusetts Institute of Technology, said in a prepared statement.
A new study by Buehler and his colleagues, published online Wednesday in Nature, is the first to use computer modeling to demonstrate how the molecular components and structures of the silk contribute to the astounding strength of spiders’ web designs (Scientific American is part of Nature Publishing Group). It explains how web patterns contribute to their impressive imperviousness to wind, weather and other assaults.
Many web-building spiders weave their webs primarily from two kinds of silk: dragline silk, which is relatively stiff, serves as the anchoring spokes, and more flexible viscid silk makes up the connecting spiral pieces. The research team based its computer model on detailed information about the molecular components of these substances and real-life observations of web constructions. “We could analyze the web in terms of energy and details of the local stress and strain,” Steven Cranford, a graduate student at MIT and co-author of the paper, said in a prepared statement. The play between these two types of silk enhanced the durability of the web.
Because of the molecular makeup of the silk and the macro patterns it has been weaved into, serious damage to the web remains confined to a small area. Local strands fail, but the overall structure stays intact. The spider has to fix only a bit of it—rather than conduct major repairs or redo the whole thing altogether.
This principle provides an excellent model for human engineers, who routinely struggle with the tradeoffs of making structures resilient. “Engineered structures are typically designed to withstand large loads with limited damage—but extreme loads are more difficult to account for,” Cranford said. Because spider webs are designed to allow for localized breaks, they can handle both types of situation within the same elegant structure: “It doesn’t matter if the load is just strong enough to cause failure or 100 times higher—the net effect is the same,” he said. “Allowing a sacrificial member to fail removes the unpredictability of ‘extreme’ loads from the design equation.”
The researchers subjected their digital web to the equivalent of hurricane-force winds. They were also able to change the web’s materials and see how that affected its behavior. “We were able to efficiently create ‘synthetic’ webs, constructed out of virtual silks that resembled more typical engineering materials,” Canford said. And that information could help scientists design stronger and more flexible materials.
Animation of computer-generated spider web courtesy of Cranford & M.J. Buehler/MIT