Spider silk is one of the most fascinating natural materials known to humans. It is more flexible than rubber yet tougher than bulletproof Kevlar and five times stronger than steel on a weight per weight basis. Scientists are still learning about the many factors that give spider silk its intriguing properties. Now, a new study sheds light on one key factor that makes spider silk so astonishingly strong.

At the microscopic level, spider silk is a mix of proteins forming long connected chains. These proteins are produced in special glands inside the spider’s body and then spun into silk through tiny nozzles called spinnerets. When spiders make a web, they pull silk threads from the spinnerets using their hind legs.

What’s interesting here is that the silk starts as a liquid but solidifies as it is pulled out, transforming into the strong, lightweight fibers spiders use to build webs, catch prey, and even glide through the air (just like Spiderman).

According to the new study, it is the pulling or stretching action that gives spider silk the remarkable strength it is known for. 

“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,” Jacob Graham, first author of the study, and a PhD candidate at Northwestern University, said.

The science of silk stretching

Scientists previously knew that the stretching action plays a role in strengthening spider silk, but they didn’t exactly understand the nanoscale dynamics that were behind it. The study authors developed a computational model and used it to simulate the stretching action of an engineered spider silk with properties similar to the real one.

The simulation allowed them to take a look at the molecular level changes occurring in the spider silk proteins when the material is pulled. They came across several interesting findings. For instance, before stretching, the material exists as a compact, globe-like cluster of proteins. However, once stretched, this structure transforms into an interconnected network with protein chains layered on top of one another.

While explaining the reason behind this, the study authors note, “We found that stretching caused the proteins to line up. We 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.”

They suggest that when the silk first oozes out of the spinnerets, it is weak. However, when it is stretched about six times its original length during the pulling out process, it becomes super strong due to the new hydrogen bonds and realignment of the protein chains. 

“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 authors added.

Spiders won’t give you enough silk

The study authors also performed an experiment to check whether the results from their computational model were valid. They examined the protein molecules from real spider silk using spectroscopy and made them undergo stretching. 

The results from this experiment were in alignment with the computer simulations. This means that by manipulating the stretching of silk fiber, one can introduce changes in its mechanical properties and produce different types of silk for different purposes. 

You can imagine all sorts of useful applications. However, there’s a catch. Farming spiders and harvesting silk from them isn’t as practical as rearing silkworms. This is because most spiders prefer to live alone in the wild, with many being territorial or even cannibalistic, making large-scale farming nearly impossible. Additionally, a spider produces only a small amount of silk; collecting it would be a tricky and labor-intensive process. 

This is why scientists have been trying to create spider-silk-like materials in the lab. They already know the ingredients (the proteins that make up spider silk), and now they need to figure out the right recipe. Previous efforts and even well-funded startups have failed to scale up synthetic spider silk. The current study brings them one step closer to figuring out the right formula.  

The study is published in the journal Science Advances.