Many scientists are eager to understand the extraordinary ability of spiders to spin silk threads that are extremely strong, heavy, and flexible. In fact, spider silk is stronger than steel and tougher than Kevlar. However, no one has yet been able to replicate the spider's work.

Biophysicist Irina Iachina of the University of Southern Denmark holds silk fibers produced by the golden orb-web spider. Image credit: AndersBoe/University of Southern Denmark

If we could develop synthetic materials with these properties, a whole new world would be possible: artificial spider silk could replace materials like Kevlar, polyester and carbon fiber in industry, for example in making lightweight, flexible bulletproof vests.

Irina Iachina, a postdoctoral fellow and biophysicist at the Department of Biochemistry and Molecular Biology at the University of Southern Denmark (SDU), is involved in the race to find the formula for super spider silk. She became fascinated with spider silk while working on her master's degree at the University of Southern Denmark, and is currently researching the topic at MIT in Boston with support from the Villum Foundation.

Biophysicist Irina Iachina of the University of Southern Denmark studies spider silk on a computer. Image credit: AndersBoe/University of Southern Denmark

As part of her research, she is collaborating with biophysicist Jonathan Brewer, an associate professor at the University of Southern Denmark, who is an expert in using various microscopes to peer into biological structures.

Now, for the first time together, they have used light microscopy to study the internal structure of spider silk without having to cut or open it in any way. The findings have now been published in the journals Scientific Reports and Scan.

"We used several advanced microscopy techniques and also developed a new optical microscope that allows us to see all the way inside the fibers," explains Jonathan Brewer.

Golden-orbited spiders produce silk from the rear. Image credit: AndersBoe/University of Southern Denmark

To date, spider silk has been analyzed using a variety of techniques, all of which have provided new insights. However, as Jonathan Brewer points out, these techniques also have drawbacks, as they often require cutting the threads (also called fibers) into sections to obtain cross-sections for microscopy, or freezing the samples, which can change the structure of the silk fibers.

"We wanted to study pure fibers that had not been cut, frozen or otherwise manipulated," says Irina Iachina. To do this, the team used less invasive techniques such as coherent anti-Stokes Raman scattering, confocal microscopy, super-resolution confocal reflected fluorescence depletion microscopy, scanning helium ion microscopy and helium ion sputtering.

Different studies have shown that spider silk fibers are composed of at least two outer layers of lipids, or fats. Behind them, i.e. inside the fibers, there are many so-called filaments, which are arranged in straight lines and closely side by side (see illustration). The diameter of the fibers is between 100 and 150, which is below the measurement limit of ordinary optical microscopes.

Illustration from the Scientific Reports paper: Schematic (not to scale) of the proposed structure of the spider silk fibers discovered in this study. (A) Fiber side view, (B) Fiber cross section. The outer layer is a non-conductive lipid-rich layer (green) with a thickness between 0.6 and 1 micron, and the inner layer is two conductive autofluorescent protein layers: one with a strong affinity for FITC (blue) and the other with a strong affinity for rhodamine B (orange). The inner protein core consists of crystalline fibers aligned parallel to the long axis of the fiber, surrounded by amorphous protein domains. Image credit: University of Southern Denmark Ichner/Brühl.

"They are not twisted as much as people thought, so we now know that there is no need to twist them when trying to make synthetic spider silk," Iachina said.

The spider silk fibers used by Ichina and Brewer come from the Madagascar golden-orbited spider (Nephila Madagascariensis). This spider produces two different types of spider silk: one is called MAS (Major Ampullate Silkfibers), which is used to build spider webs and is also the silk used by spiders to hang, which can be regarded as the lifeline of spiders; it is very strong and has a diameter of about 10 microns.

The other is called MiS (Minor Ampullate Silk fiber), which is an auxiliary material for construction. It is more elastic and typically has a diameter of 5 microns. According to the pair's analysis, MAS silk contains fibers with a diameter of about 145 nanometers. The diameter of MiS is about 116 nanometers. Each fiber is made of protein, and there are many different proteins involved. These proteins are produced by spiders when they make silk fibers.

Understanding how they create such strong fibers is important, but making them is also challenging. Therefore, researchers in the field often rely on spiders to produce silk for them.

Alternatively, they can turn to computational methods, which is what Irina-Ichna is currently doing at MIT: "Right now, I'm doing computer simulations of how proteins are converted into silk. Of course, the goal is to learn how to produce artificial spider silk, but I'm also interested in helping people better understand the world around us."