Recluse spider silk could hold the key to space-age materials

Tamara Dietrich
W&M study shows recluse spider silk could hold the key to space-age materials

Picture it: A giant, carbon fiber spiderweb in space netting dangerous floating debris; battlefield armor far tougher than Kevlar; and bicycles, automobiles, aircraft and even spacecraft that are lighter yet stronger than ever.

Materials scientist Hannes Schniepp can picture this and more — all because of the tiny recluse spider.

Schniepp and his colleague Sean Koebley have been studying recluse spider silk for years in the Department of Applied Science at the College of William and Mary in Williamsburg.

They've sought answers to why recluse silk is so freakishly strong — gram for gram, five times tougher than steel.

Now they believe they have the answer: Recluse spiders spin their silk using a "micro-loop" technique that appears to be unique to the species. And it helps makes the silk more flexible, less prone to rupture and incredibly strong.

"Spider silk is already one of the toughest materials we know," Schniepp said in an interview Monday. "It's several times tougher than Kevlar, and Kevlar is one of the best materials we currently use. ... And these micro-loops, they seem to be just yet another trick that nature invented to go even further. We found that, even with relatively modest amount of loops, you can actually improve the toughness of the material by tens of percent."

The scientists believe their discovery could lead one day to engineering novel, extra-tough fibers suitable for a wide range of uses.

Carbon fibers, for instance, are the best lightweight, high-strength material available, Schniepp said. They're used in areas such as aeronautics and aerospace, the automobile industry, microelectrodes and textiles. But they're also very brittle. Adding loops might make them tougher, thus avoiding catastrophic failures.

The recluse spider is already special among silk spinners — its silk is a flat ribbon rather than a rounded strand. And so thin, you'd have to stack 1,000 of those ribbons on top of each other to reach the thickness of a human hair.

It's that flatness — along with microscopic bumps, or nanopapillae, on its surface — that appear to give the ribbon a natural clingy property, like plastic wrap.

That flatness also appears to be what makes it so successful at micro-looping.

"Imagine you take a piece of sticky tape and you can fold it onto itself and it makes a loop," said Schniepp. "That's what the spider does with this built-in spinning mechanism, except it does this many times per second. That actually works especially well with that ribbon, because with that ribbon you have good contact area."

It's been known that knots can add strength to fibers, Schniepp said, but loops in synthetic filaments usually end in fiber failure. The key to success could be using a flat, ribbon-type fiber, instead.

Schniepp and Koebley are the first to observe this micro-loop technique, and they essentially stumbled upon it.

For years, they've been "milking" spiders in the lab, typically the brown recluse and its cousin, the Chilean recluse. They would anesthetize the arachnids, harmlessly strap them down on their backs until they revived, then gently tease out the silk from their spinnerets with a needle.

But they began to notice a tiny bundle of silk, like a tiny cotton ball, and a strange movement when the spinnerets began to work. Most spiders, said Koebley, show no obvious spinneret movement when they extrude silk.

"That's what led me to take a closer look," Koebley said.

He placed a spider under a microscope and saw the spinneret oscillating, but their lab camera wasn't fast enough to capture the motion. So they got access to a high-speed camera at NASA Langley Research Center in Hampton and tried again.

"With that camera, we were able to pick out exactly what was happening," Koebley said. "This really intricate motion of the six spinnerets performing this kind of dual weaving action. Kind of an oscillation and clamp motion that was forming this looped silk."

Schniepp likened it to a "microscopic sewing machine."

"And, really," said Schniepp, "the entire system is so small that the diameter of each of these loops is a few hundred microns — maybe the diameter of five humans hairs — and it makes loop after loop after loop."

That was a couple of years ago. Since then, the two have been working to figure out what was happening and why. They designed experiments, then a computational model and finally a proof-of-concept system using tape.

Now their findings are being published Wednesday in "Materials Horizons," a high-impact materials science journal of the Royal Society of Chemistry in Great Britain. The study is co-authored by Fritz Vollrath, a colleague at Oxford University's Department of Zoology.

The recluse's micro-loops don't last, though. If the silk is tugged on — as the scientists were doing routinely in the lab when milking the spiders — the loops unravel.

"That's kind of one of the limitations of this system right now," said Koebley. "Once these loops break open, they don't re-form."

Even if you compress the loops back to their natural state, he said, they simply don't re-form in the same way.

"But that doesn't mean it's not possible," Koebley said. "We think there are ways we could work with a system like that, now that we have this inspiration. We could possibly imagine a system that is able to re-form bonds, if you work on the technology in the right way."

They haven't measured an increase in comparative toughness in recluse spider silk compared to other spiders, but Koebley suggested that could be the result of limitations in their testing.

"It's hard to exactly replicate what the spider's doing and get that set up in a way that we can actually test that accurately," Koebley said. "We think that if we could actually test the stuff in its natural state, then, yeah, it could be the strongest silk."

Schniepp said they have a patent pending on the technology. If approved, he said they plan to reach out and see who might be good customers.

Dietrich can be reached by phone at 757-247-7892.

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