Stretching spider silk makes it stronger by aligning protein chains

When spiders spin their webs, they use their hind legs to pull silk threads from their spinnerets. This pulling action doesn’t just help the spider release the silk, it’s also a crucial step in strengthening the silk fibers for a more durable web.

In a new study, Northwestern University researchers have discovered why the role of stretching is so important. By simulating spider silk in a computational model, the team discovered the stretching process aligns the protein chains within the fibers and increases the number of bonds between those chains. Both factors lead to stronger, tougher fibers.

The team then validated these computational predictions through laboratory experiments using engineered spider silk. These insights could help researchers design engineered silk-inspired proteins and spinning processes for various applications, including strong, biodegradable sutures and tough, high-performance, blast-proof body armor.

The study appears in Science Advances.

“Researchers already knew this stretching, or drawing, is necessary for making really strong fibers,” said Northwestern’s Sinan Keten, the study’s senior author. “But no one necessarily knew why. With our computational method, we were able to probe what’s happening at the nanoscale to gain insights that cannot be seen experimentally. We could examine how drawing relates to the silk’s mechanical properties.”

“Spiders perform the drawing process naturally,” said Northwestern researcher Jacob Graham, the study’s first author. “When they spin silk out of their silk gland, spiders use their hind legs to grab the fiber and pull it out. That stretches the fiber as it’s being formed. It makes the fiber very strong and very elastic. We found that you can modify the fiber’s mechanical properties simply through modifying the amount of stretching.”

An expert in bioinspired materials, Keten is the Jerome B. Cohen Professor of Engineering, professor and associate chair of mechanical engineering and professor of civil and environmental engineering at Northwestern’s McCormick School of Engineering. Graham is a Ph.D. student in Keten’s research group.

Stronger than steel, tougher than Kevlar

Researchers have long been interested in spider silk because of its remarkable properties. It’s stronger than steel, tougher than Kevlar and stretchy like rubber. But farming spiders for their natural silk is expensive, energy-intensive and difficult. So, scientists instead want to recreate silk-like materials in the lab.

“Spider silk is the strongest organic fiber,” Graham said. “It also has the advantage of being biodegradable. So, it’s an ideal material for medical applications. It could be used for surgical sutures and adhesive gels for wound-closure because it would naturally, harmlessly degrade in the body.”

Study co-author Fuzhong Zhang, the Francis F. Ahmann Professor at Washington University (WashU) in St. Louis, has been engineering microbes to produce spider-silk materials for several years. By extruding engineered spider silk proteins and then stretching them by hand, the team has developed artificial fibers similar to threads from the golden silk orb weaver, a large spider with a spectacularly strong web.

Simulating stretchiness

Despite developing this “recipe” for spider silk, researchers still don’t fully understand how the spinning process changes fiber structure and strength. To tackle this open-ended question, Keten and Graham developed a computational model to simulate the molecular dynamics within Zhang’s artificial silk.

Through these simulations, the Northwestern team explored how stretching affects the proteins’ arrangement within the fibers. Specifically, they looked at how stretching changes the order of proteins, the connection of proteins to one another and the movement of molecules within the fibers.

Keten and Graham found that stretching caused the proteins to line up, which increased the fiber’s overall strength. They also found that stretching increased the number of hydrogen bonds, which act like bridges between the protein chains to make up the fiber. The increase in hydrogen bonds contributes to the fiber’s overall strength, toughness and elasticity, the researchers found.

“Once a fiber is extruded, its mechanical properties are actually quite weak,” Graham said. “But when it’s stretched up to six times its initial length, it becomes very strong.”

Experimental validation

To validate their computational findings, the team used spectroscopy techniques to examine how the protein chains stretched and aligned in real fibers from the WashU team. They also used tensile testing to see how much stretching the fibers could tolerate before breaking. The experimental results agreed with the simulation’s predictions.

“If you don’t stretch the material, you have these spherical globs of proteins,” Graham said. “But stretching turns these globs into more of an interconnected network. The protein chains stack on top of one another, and the network becomes more and more interconnected. Bundled proteins have more potential to unravel and extend further before the fiber breaks, but initially extended proteins make for less extensible fibers that require more force to break.”

Although Graham used to think spiders were just creepy-crawlies, he now sees their potential to help solve real problems. He notes that engineered spider silk provides a stronger, biodegradable alternative to other synthetic materials, which are mostly petroleum-derived plastics.

“I definitely look at spiders in a new light,” Graham said. “I used to think they were nuisances. Now, I see them as a source of fascination.”