Nanofibrils study successfully measures strength of spider web 'super fibers'

How small is "small?" Spider silk nanofibrils are just a few molecular layers thick, equivalent to approximately one ten-thousandth the diameter of a human hair. They're invisible to the naked eye and cannot be seen under an ordinary microscope.

The findings are published in the journal Advanced Functional Materials.

"We humans think we're so great and we can invent things, but if you just take a step outside, you find so many things that are more exciting."

By weight, spider silk is approximately five times stronger than steel, but what makes the material even more impressive is its extensibility. The combination of strength and stretchiness allows it to absorb large amounts of energy. If humans find a way to replicate the structure of spider silk, it could be manufactured for use in practical applications.

We could make a super bungee cord from it "Or a shield around a structure where we have something incoming at high velocity and we need to absorb a lot of energy. Things like that."

Spider web strands may seem simple to the casual observer, but closer inspection reveals that they're complex structures of interwoven components. It is explained that there are approximately 50,000 different species of spiders, and each produces its own unique silk. The silk of the southern house spider was used for this experiment due to the nanofibril mesh found in its structure.

The spinning apparatus of the southern house spider consists of hundreds of nozzles that—with the help of the spider's back legs—construct a complicated, three-dimensional strand. The core of the strand is composed of two distinct warps that form helical loops around a central foundation fiber. The tiniest fibers, nanofibrils, are simultaneously spun into a mesh that surrounds those supporting structures.

While webs of some spiders capture prey by way of glue drops spaced regularly along strands, cribellate strands like those spun by the southern house spider ensnare prey in the mesh by way of entanglement, van der Waals forces and capillary adhesion.

To find and measure the strength and stretchability of nanofibrils within the mesh of the southern house spider's silk, they used atomic force microscopy (AFM), a powerful technique that can be used to study the physical properties of extremely tiny objects.

The exfoliated nanofibrils from the silk onto a thumb tip-sized silicon disk containing approximately two million holes, each 200 nanometers in diameter. He then probed for fibers stretched over the holes and measured their strength and stretchability.

"Basically you use a needle that's super, super small—just a few nanometers wide at the end of the needle—and we tap it across the sample,". "That builds a topographic map pixel by pixel that gives you an image."

They explained that the tip of the needle is so sharp that at the end it's only a few atoms thick.

"We would not see the end of it in the best optical microscope,". "It will just disappear because it's so small that you can't even see it. It's probably one of the highest developed technologies on the planet."

The technology is so sensitive that the basement lab is constructed on top of a concrete slab supported by steel springs and the microscope platform is suspended on bungee cords during measurements. These structural modifications help to isolate the microscope from vibrations.

Because the AFM needle is so small, the process is slow-moving. In an hour, Silliman said, he might be able to scan 30 to 40 of the miniscule holes.

"You just have to keep taking scans over and over and hope that you find a nanofibril that's placed over a hole in such a way that you can actually do that test," he said. "After enough iterations of that, we finally got the chance to do tiny mechanical tests with that same needle. You push on the fiber, and then you can calculate the strength of that fiber from those measurements and other material properties."

Silliman and Schniepp found that the nanofibrils could stretch 11 times their original length, more than twice the amount of any spider silk previously tested.

"As amazing as spider silk as a whole is," said Schniepp, "looking at these tiny fibrils, they are even stretchier. Learning about these structures could lead to production of a material that can absorb a lot of energy, just because it's so, so extremely stretchy. We want to get deeper and deeper into really figuring out what makes spider silk so special, and I think there are more surprises waiting for us."