ACADEMIA

It’s not just the strength of the silk itself, a new study finds; the silk’s way of stretching and the structure of the whole web help it resist damage.

The silk that spiders use to build their webs, trap their prey and dangle from your ceiling is one of the strongest materials known. But it turns out it’s not simply the material’s exceptional strength that makes spider webs so resilient; it’s the material’s unusual combination of strength and stretchiness — silk’s characteristic way of first softening and then stiffening when pulled. These properties, scientists have found, vary depending on the forces applied, as well as on the overall design of the web.

Markus Buehler, an associate professor of civil and environmental engineering (CEE) at MIT, has previously analyzed the complex, hierarchical structure of spider silk and its amazing strength — on a pound-for-pound basis, it’s stronger than steel. Now, Buehler and his colleagues have applied their analysis to the structure of the webs themselves, finding evidence of the key properties that make webs so resilient and relating those properties back to the molecular structure of silk fibers. 

The lessons learned from this work, Buehler says, could not only help develop more damage-resistant synthetic materials, but could also provide design principles that might apply to networked systems such as the Internet or the electric grid.

A paper describing the new findings is published this week in Nature. In addition to Buehler, the study was carried out by CEE graduate students Steven Cranford and Anna Tarakanova, and Nicola Pugno of the Politecnico di Torino in Italy.

It turns out that a key property of spider silk that helps make webs robust is something previously considered a weakness: the way it can stretch and soften at first when pulled, and then stiffen again as the force of the pulling increases. 

This stiffening response is crucial to the way spider silk resists damage. Buehler and his team analyzed how materials with different properties, arranged in the same web pattern, respond to localized stresses. They found that materials with other responses — those that either behave as a simple linear spring as they’re pulled, or start out stretchy and then become more “plastic” — perform much less effectively.

Spider webs, it turns out, can take quite a beating without failing. Damage tends to be localized, affecting just a few threads — the place where a bug got caught in the web and flailed around, for example. This localized damage can simply be repaired, rather than replaced, or even left alone if the web continues to function as before. “Even if it has a lot of defects, the web actually still functions mechanically virtually the same way,” Buehler says. “It’s a very flaw-tolerant system.”

Buehler’s research is mostly theoretical, based on supercomputer modeling of material properties and how they respond to stresses. But in this case, to test the findings, he and his team literally went into the field: They tested actual spider webs by poking and pulling at them. In all cases, damage was limited to the immediate area they disturbed.

The effect was somewhat surprising, Buehler says: The initial response was a deformation of the entire web, since the strands are initially relatively easy to deform. But then, because of the fibers’ nonlinear response, only the threads where the force was applied carried the load — by stretching out and then becoming stiff. As the force increased, they eventually broke. 

“No matter where you pull, the web always fails exactly at that location,” Buehler says. Anyone can try this simple experiment, he adds: Simply pluck a single silk thread from a spider web, and it should break only where it’s pulled. In a web made of material with a more uniform stretching response, by contrast, local stresses cause much more widespread damage.

In a strong wind, on the other hand, it’s the initial stiffness of the silk that helps a web survive. Webs in Buehler’s simulation were able to tolerate winds up to almost hurricane strength before tearing apart.

Engineers tend to focus on materials with uniform, linear responses, Buehler says, because their properties are so much easier to calculate. But this research suggests that there could be important advantages to materials whose responses are more complex. In the unusual response of spider silk, for example — initially stiff, then stretchy, then stiff again — “each little piece of that funny behavior has a fundamental role to play” in making the whole web so robust, he says. Materials with the same ultimate strength, as measured by their breaking point, often perform very differently in real-world applications. “The actual strength is not so important, it’s how you get there,” he says.

The basic principle of permitting localized damage so that an overall structure can survive, Buehler says, could end up guiding structural engineers. For example, earthquake-resistant buildings are generally designed to protect the whole building by dissipating energy, reducing the load on the structure. When they fail, they tend to do so in their entirety.

A new design might allow the building to flex up to a point, but then certain specific structural elements could break first, allowing the rest of the structure to survive; this might ultimately allow the building to be repaired rather than demolished. Similar principles might apply to the design of airplanes or armored vehicles that could resist localized damage and keep functioning.

Such “sacrificial elements” might be used not just for physical objects but also in the design of networked systems: For example, a computer experiencing a virus attack could be designed to shut down instantly, before its problems propagate. Someday, then, the World Wide Web might actually be strengthened thanks to lessons learned from the backyard version that inspired its name.

“It’s a real opportunity,” Buehler says. “It opens a new design variable for engineering.”

David Kaplan, a professor of engineering at Tufts University and director of its Center for Biological Engineering, calls these findings “quite exciting.” He says, “The combination of modeling and experiment makes this particularly attractive as a platform for study and inquiry into materials designs and failure modes in general, with structural hierarchy in mind.”

“These principles, I believe, will have an impact in a wide range of fields such as medicine, future materials and architecture,” adds Philip LeDuc, a professor of mechanical engineering at Carnegie Mellon University.

This work was supported by the Office of Naval Research, the National Science Foundation, the Army Research Office and the MIT-Italy Program.CIV.