Spider Silk Stretches for Strength

As Peter Parker looked to create fluid for his web shooters in the Spider-Man comic book, he determined he needed a synthetic material that was “load-bearing and strong, [could] hold with extreme amounts of tension and stress, yet have a little stretch to it.” Basically, a material much like natural spider silk.
 
Even outside the Marvel Universe, laboratories focused on bio-inspired materials have long been interested in trying to mimic the natural properties of spider silk. Web threads may be thin, but they have remarkable strength and toughness—not to mention boast significant stretching capabilities. Sinan Keten, Jerome B. Cohen Professor of Engineering at Northwestern University’s McCormick School of Engineering, said spider silk’s properties are so interesting that it was the subject of his doctoral work at MIT more than a decade ago.
 
“Back then, we were interested in using molecular simulation to understand the molecular structure of spider silk, what makes it unique as a tough material that we can’t easily reproduce with manmade materials,” he said. “We looked at the sequence of spider silk to see which regions would form crystals, which regions would be more amorphous, and how the strength of hydrogen bonds might contribute to the mechanism that toughens it.”
 
Keten said there were significant challenges in trying to untangle the mechanical properties of the material at the time. Yet, he continued to think about how, when spiders spin the silk, the end material is not just the byproduct of the chemistry involved, but the mechanical forces stretching protein chains as the spider uses their hind legs to pull silk threads from its spinnerets. So, when colleague Fuzhong Zhang, the Francis F. Ahmann Professor at Washington University in St. Louis, successfully engineered microbes to produce artificial fibers similar to the silk produced by the golden silk orb weaver spider, Keten said there was an opportunity to return to those ideas.
 
To start, Keten and graduate student Jacob Graham created a computational model to better elucidate how the spinning process itself supports structural changes that can affect the fiber’s strength.
 
“I had wanted to create a larger model where you could see the interconnectivity between the crystallite in the fiber,” Keten explained. “[Graham] built a very nice one that allowed us to look at that and understand how spinning, and drawing translates to forces at the molecular level, how this changes the microstructure of spider silk and how that ties to the connectivity of crystallites and the overall strength and toughness of the material.”

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Their simulations demonstrated that stretching the silk as the spider releases it from the spinnerets does more than just extrude the fiber, it also changes the order of proteins, helping them to “line up” in a way that strengthens the material without compromising toughness. In addition, stretching also increased the number of hydrogen bonds between the proteins, which also expanded the material’s strength, toughness, and elasticity.
 
After running the simulations, the group then validated their findings by using spectroscopy and tensile testing to further examine the benefits of stretching in fibers from Zhang’s laboratory. The results were nicely lined up with those produced by the computational model.

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“Most materials have a balance between strength and toughness. You can make strong materials like carbon fiber or glass, but they usually take little strain to break. You can have stretchy materials like elastomers, but they are relatively easy to break,” he said. “We learned that this molecular stretching induced by the drawing process changes the morphology of the material, ultimately giving you a way to improve strength while maintaining a high degree of toughness.”

The research was published in the journal Science Advances.
 
When asked how the laboratory will follow this study, Keten said he and his colleagues are following several different threads. First, they are exploring new machine learning techniques to optimize spinning and solvent conditions to make synthetic bio-inspired materials. They are also studying how the protein sequence and process conditions interplay to improve the tenacity of the fibers formed. That combined understanding could help support a future where synthetic, bio-inspired spider silk can be produced at scale, replacing petroleum-derived products that, while cheap, come at great cost to the environment. 
 
“As we learn more about what aspects of these spinning sequences are important, which ones need to be preserved, and which ones could be improved to fit into an engineered process, we can get closer to being able to manufacture these kinds of materials because the way a spider spins their web is very different from how we would produce yarn via biomanufacturing,” Keten said. 
 
Kayt Sukel is a technology and business writer in Houston.