This pulling action doesn’t just help the spider release the silk, it’s also a crucial step in strengthening the silk fibres for a more durable web. Spider silk is known for its remarkable properties, such as being stronger than steel, tougher than Kevlar, and stretchy like rubber. Understanding why the stretching process is so vital in enhancing these properties is the focus of recent research by Northwestern University.
How Stretching Affects the Strength of Spider Silk
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 fibres and increases the number of bonds between those chains. Both factors lead to stronger, tougher fibres.
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 armour.
Understanding Spider Silk’s Unique Properties
“Researchers already knew this stretching, or drawing, is necessary for making really strong fibres,” 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’s Jacob Graham, the study’s first author. “When they spin silk out of their silk gland, spiders use their hind legs to grab the fibre and pull it out. That stretches the fibre as it’s being formed. It makes the fibre very strong and very elastic. We found that you can modify the fibre’s mechanical properties simply through modifying the amount of stretching.”
Applications of Spider Silk in Engineering
Researchers long have been interested in spider silk because of its strength and stretchability. 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 fibre,” 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.”
Simulating the Stretching Process of Spider Silk
Despite developing this “recipe” for spider silk, researchers still don’t fully understand how the spinning process changes fibre 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 fibres. 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 fibres.
Keten and Graham found that stretching caused the proteins to “line up,” which increased the fibre’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 fibre. The increase in hydrogen bonds contributes to the fibre’s overall strength, toughness, and elasticity, the researchers found.
Experimental Validation of Spider Silk’s Properties
To validate their computational findings, the team used spectroscopy techniques to examine how the protein chains stretched and aligned in real fibres from the WashU team. They also used tensile testing to see how much stretching the fibres 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 fibre breaks, but initially extended proteins make for less extensible fibres that require more force to break.”
Revolutionising Materials with Spider Silk-Inspired Solutions
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.”
