Flies caught in webs with hungry spiders bearing down on them don't have the time to appreciate good engineering. Luckily, researchers aren't so constrained. A new analysis reveals the intricacies of spider web design, showing how the unique properties of its silk turn webs into flexible yet strong traps.
"This is very innovative," says Joyce Wong, a biomaterials researcher at Boston University, who was not involved with the study.
Spider silk is a remarkable material, says study co-author Markus Buehler, a materials scientist at the Massachusetts Institute of Technology in Cambridge. Take the single thread that suspends a dropping spider. When pulled, the thread will begin to stretch, sometimes to twice its original length, but eventually it will stiffen again.
Buehler and colleagues previously discovered that, during the elastic stage, the proteins in spider silk are scrunched into intricately folded structures. Pulling disentangles these convolutions, he says. When there are no more knots to untie, the proteins reconfigure into tough structures called beta-sheet nanocrystals.
Spiders do more than just dangle, however, and Buehler and colleagues wanted to see how these molecular properties impact their entire web. So they sought out a wide, radial spider web outdoors. With the spider still on board, the team hung tiny weights made from metal wires at various places, mimicking a fly pinging into the trap. When tugged, individual silk spokes would stretch and snap, but other threads wouldn't break with them, Buehler says. And spider webs can stand to lose a few threads. These traps seem to retain their original strength even if 10% of the spokes at various locales are snipped, the group reports today in Nature.
To test if spider silk's unique stretchability might be responsible for this structural feat, the team turned to computer simulations. Here, bending reality is easy. The team designed generic-looking webs that were constructed, for instance, of a type of material called dragline silk that was modified to be entirely stretchy. In other words, no beta-sheets. When an imaginary finger pulled on these simulations, whole portions of the web bulged out then eventually ruptured. Buehler explains that totally elastic spider silk would distribute weight widely across the net, which means that pulling on one thread can damage many others. Real orb weaver's silk, however, can be either stretchy or stiff at different times, which produces threads that flex and then snap in just the right way to avoid wrecking nearby spokes.
The findings highlight the unique ecology of spiders. In fact, such self-sacrificing by a unit is highly unusual among natural materials, Buehler notes. Other silk spinners, such as silk worms, produce more elastic silk, which dissipates forces over an entire structure such as a cocoon, making it difficult for predators to bite through.
Philip LeDuc, a mechanical engineer at Carnegie Mellon University in Pittsburgh, Pennsylvania, is impressed: "It's just fantastic work." The molecular explanations for why webs stretch and snap will not only help engineers mimic these materials but also, potentially, make them better, he says.
Buehler is already working with biologists to genetically engineer arachnids that can spin threads with properties not seen in nature. Now that's something that should scare a fly.