Stretching spider silk makes it stronger

Stronger than steel, tougher than Kevlar

Researchers long have 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 coauthor 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 effects 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 stretch 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.”

Notes

The study was published in the journal Science Advances and supported by the National Science Foundation.