Pound for pound, spider silk is stronger than steel and tougher than Kevlar. But it doesn’t start out that way.
The silk starts out in a liquid form called dope. But in fractions of a second, this goopy, liquid slurry of proteins is transformed. And it doesn’t just turn into a solid. On their way out of a spider’s bottom, the protein building blocks in silk, called spidroins, fold themselves and interlace, creating a highly organized structure without guidance from any outside force.
“You can really generate materials with unique properties by exploiting this self-assembly process,” said Ali Malay, a structural biologist and biochemist at the Riken Center for Sustainable Resource Science in Japan.
Malay doesn’t yet have the entire process figured out. But in a paper published in Science Advances, he and his colleagues lay out a new way to tackle the spider silk puzzle, mimicking its orderly exit from the spinneret with chemical tools in the lab.
A crucial part of spinning, the researchers found, requires the spidroins to separate themselves from the watery buffer that swaddles them inside silk glands — a step that hyper-concentrates the proteins. An influx of acid then prompts the proteins to securely interlock.
The metamorphosis spider silk must undergo as it exits an arachnid cannot be overstated, said Anna Rising, a spider silk expert at the Karolinska Institute in Sweden who was not involved in the study. While still in the gland, spidroins have to stay suspended in a liquid form at “really extreme concentrations,” Rising said.
Millenniums of evolution have made spidroins versatile. The proteins, Rising explained, are structured like barbells. In the silk glands, these barbells are thought to naturally pair up at one end, creating V-shaped duos that slosh around in the dope.
To form the more stable architecture required of solid silk, the spidroins need to link up in chains. That seems to happen under the influence of a couple of chemical cues, said Jessica Garb, a spider silk researcher at the University of Massachusetts, Lowell who was not involved in the study. As the spidroin slurry is extruded through a labyrinth of increasingly narrow ducts, the spider cells pump acid into the mixture, making the free ends of the barbells stick together.
Malay and his colleagues found this sculpting and self-assembly could not happen if the liquidy spidroins weren’t dehydrated as they moved through the spider’s anatomy.