Friday, October 12, 2018

Raising Spiders in a Physics Lab Reveals There's More than Strength Behind a Spiderweb's Sturdiness

Spider silk has been seen as "the material of the future"...for about 300 years now. Since the 1700s, people have been so anxious to harness its strength, durability, and flexibility that they’ve coordinated massive spider-catching operations, painstakingly harvested threads from hundreds of spiders in silk factories, and even genetically modified goats to produce it in their milk. We're wooed by images of Spiderman and giant helicopter-snaring nets, or bridges supported by pearly white cables stronger than steel. The New Yorker claims, “In the Future, We’ll All Be Wearing Spider Silk”. We love the stuff, even if making use of it has turned out to be a practical impossibility.

(Click to Enlarge)
This 11-foot-long cloth is one of the only actual textiles ever made from spider silk.
It is undyed; the golden hue is the color of the silk itself.
Image Credit: Courtesy Simon Peers and Nicholas Godley, via the New York Times.
It's worth noting that, although spider silk is often touted as being "five times stronger than steel", that’s not quite correct: its tensile strength—the strength of a strand with a given thickness—is about on par with steel and carbon fiber. What makes silk so special is actually its lightness relative to its strength; this is where the "5x" figure comes from: a given weight of spider silk is five times stronger than steel, but it’s also much bulkier. Still, this strength-to-weight ratio, along with its resistance to snapping, are far greater than those of any other material used in construction.

Of course, spider silk has to be able to stretch without breaking, since it exists primarily to ensnare fast-moving insects. Grasshoppers, for example, can reach 6 m/s while jumping, which is equivalent to 9 mph—a pretty fast clip! When an insect like that flies into the web, the silk must be able to slow it down to a stop immediately, without snapping or bending so much that it creates a slingshot effect and catapults the insect back out into the void. On top of that, prey doesn't usually sit still once it's trapped, so the web has to be able to absorb and disperse the insect's flailing attempts to escape.

Traditionally, this incredible functionality of spiderwebs has been attributed to the remarkable qualities of the silk itself. However, new research out of the Tsinghua University in Beijing indicates that there’s more to the story.

First, a quick refresher on spider webs (arachnophobes, take a deep breath). To begin with, although we often use the term “spider silk” as a blanket category, there are lots of different varieties—some spiders can produce seven different kinds of silk! Each type of silk has its own properties and its own uses within the structure of a web. These different materials are interwoven to create a work of engineering genius, capable of leaping out to snatch an insect from the air, then telling the spider about the prey it's caught just through the vibrations.

You’re probably familiar with the basic structure of an orb web: straight spokes extend outwards from a center (these are made of radial silk) while long strands spiral around them fairly evenly (the spiral silk). As it turns out, the radial silk is quite stiff and tough as far as spider silk goes. It’s also usually not sticky, which is part of how spiders navigate their own webs without getting stuck. Spiral silk, on the other hand, is much more stretchy and flexible, in addition to being coated with dozens of glue droplets per millimeter.

The sketch at left shows the radial silk extending outward from the center of the web, while the spiral silk winds around. In this web, only the spiral silk is coated with glue. Although the spiders brush the glue onto the silk strands evenly, surface tension causes the glue to bunch up into a beadlike pattern (right).
Image Credit: Xi-Qiao Feng.
These differences have been well-documented, along with the variations among webs and spider species. But what Yang Guo—the lead author on this new work—and his colleagues wanted to learn was whether or not each type of silk varied within the same web. Guo ended up raising spiders in his laboratory, where he provided them with wooden frames perfect for web-building. Once they’d completed the web, the hapless spiders were removed and the web was studied for tensile stiffness and glue drop density.

Surprisingly, the team found that, while the radial silk is pretty much the same all across, the spiral silk varies in stiffness along the radial direction: as the spider worked its way outward, it produced stiffer and stronger silk. Further analysis showed that this stiffness gradient plays a key role in dispersing the energy of an incoming insect. Greater stiffness near the edges—where silk strands are spread farther apart, and have more distance to cross between radial strands—helps the web absorb evenly across the structure, regardless of whether the insect lands in the center or at the periphery!

To investigate the reasons behind the observed variations in silk properties, the Tsinghua team turned to computational models, analyzing the stresses imposed on the rest of the web by an applied force.
Image Credit: Xi-Qiao Feng.
Previous work had found that almost all of the work in stopping incoming insects was done by the stiff radial silk, while the spiral silk was assumed to be primarily responsible for entrapping the prey, but Guo’s research shows that it’s a lot more complicated than that. Instead, he concludes that “The spiral and radial silks work synergistically to absorb kinetic energy when a flying insect is trapped.”

While this research is exciting in and of itself, and maybe will make you think twice before squashing that eight-legged visitor, Guo thinks that a greater understanding of spiderweb energy absorption will help us as well. Even without a true synthetic spider silk, we can use the geometric properties he’s discovered—particularly the stiffness gradient—to develop new anti-impact structures, like bulletproof vests.

The key point of this work is that an orb web’s geometric structure is crucial to its insect-stopping abilities, and not just a convenient shape for a net. As incredible as spider silk is, the web itself is a critical part of the design; if we want to truly harness the power of spider silk, we’ll have to pay attention not just to the material itself but how it’s used by spiders.

Eleanor Hook

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