While researchers have long known of the incredible strength of spider silk, the robust nature of the tiny filaments cannot alone explain how webs survive multiple tears and winds that exceed hurricane strength. Now, a study that combines experimental observations of spider webs with complex computer simulations has shown that web durability depends not only on silk strength, but on how the overall web design compensates for damage and the response of individual strands to continuously varying stresses.
Reporting in the cover story of the Feb. 2, 2012, issue of Nature, researchers from the Massachusetts Institute of Technology (MIT) and the Politecnico di Torino in Italy show how spider web-design localizes strain and damage, preserving the web as a whole.
“Multiple research groups have investigated the complex, hierarchical structure of spider silk and its amazing strength, extensibility, and toughness,” says Markus Buehler, associate professor of civil and environmental engineering at MIT. “But, while we understand the peculiar behavior of dragline silk from the ‘nanoscale up’—initially stiff, then softening, then stiffening again—we have little insight into how the molecular structure of silk uniquely improves the performance of a web.”
The spider webs found in gardens and garages are made from multiple silk types, but viscid silk and dragline silk are most critical to the integrity of the web. Viscid silk is stretchy, wet and sticky, and it is the silk that winds out in increasing spirals from the web center. Its primary function is to capture prey. Dragline silk is stiff and dry, and it serves as the threads that radiate out from a web’s center, providing structural support. Dragline silk is crucial to the mechanical behavior of the web.
Some of Buehler’s earlier work showed that dragline silk is composed of a suite of proteins with a unique molecular structure that lends both strength and flexibility. “While the strength and toughness of silk has been touted before—it is stronger than steel and tougher than Kevlar by weight—the advantages of silk within a web, beyond such measures, has been unknown,” Buehler adds.