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Friday, March 31, 2023

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Spiders, Doomed Flies and Tougher Textiles — Just Add Water

A DoD grant will help SDSU’s Gregory Holland further explore spider silk and its possible application toward incredibly tough biomaterials.
By Jeff Ristine

“This project is fun because it’s a new frontier.”

The silk a common variety spider spins to trap its prey is the focus of new San Diego State University research, being supported by the Department of Defense for its potential breakthroughs in producing bioinspired materials U.S. troops may use someday.

Gregory Holland, associate professor of analytical chemistry and an authority on the super-strong properties of spider silk,  is studying a specialized silk spun by a spider in the orb weaver family. The species is common to San Diego, found along the trails of Torrey Pines and the Silver Strand, among other areas.

“Lucky us,” said Holland. “They’re in our backyard.”

The U.S. Army Research Office has awarded Holland a $365,000, three-year grant to pursue the work.

Holland and collaborators previously peered into the molecular structure of the silk fibers Black Widow spiders spin, focusing on the gland that kicks off the process of converting a protein-rich liquid into a thread. The fibers are stronger than the material in bulletproof vests.

The new project turns to orb weavers, who spin intricate, circular webs up to three feet across. But it’s not that regular dragline silk Holland is investigating, it’s the much different prey-wrapping silk the spider uses to keep flies, crickets and other tasty morsels in an escape-proof cocoon until feeding time.

“The silk itself has to be very flexible because the spider is moving all around and rotating the prey as it wraps it in silk,” Holland said. “The fly is not dead, it’s moving, so he needs something that’s malleable, not like a steel cable.” Indeed, the initial material extends, stretching up to 60-70% of its original length.

That all changes when the spider vomits onto the silk and prey, “and it’s the primary linchpin of this grant,” Holland said. When the digestive liquid dries, “the silk fibers fuse together and the material becomes really rigid. The silk protein actually changes its structure at a molecular level — going from this flexible, extensible material to a rigid, fixed, matted material. The silk hardens like a straightjacket.”

Spiders rely upon digestive enzymes to trigger the transformation. But Holland's lab found water has the same effect.

What makes the silk so strong and rigid when the material dehydrates? “That’s what we’re going to learn.”

Different silks are made from proteins with different sequences, Holland explained. So just like the Black Widow dragline silk, the research will look at the atomic and molecular structure of the silk protein starting material, in the gland, as well as the final fiber.

Holland’s lab will use nuclear magnetic resonance spectroscopy, the same technology involved in a hospital MRI to image soft tissue. An additional technique, cryogenic transmission electron microscopy, flash-freezes the nanometer-sized protein particles to obtain an image.
“This project is fun because it’s a new frontier,” said Dillan Stengel, a Ph.D. student in Holland’s lab. “We have seen silks contract when they come into contact with water, but never cross-link, or stiffen like this. The evolutionary adaptations we learn from spider silk continue to garner interest from the DoD community, which is rewarding.”

Stephanie McElhinny, program manager for the Army Research Office, said a better understanding of the molecular structure, dynamics and assembly process of the orb weaver’s silk “will provide a fundamental basis for developing a new class of biomaterials that have physical and mechanical properties that mimic the silk.”

The ability to transition from a flexible to stiff state “could enable future materials that could be used for a variety of Army applications including advanced textiles for protective clothing, tents, parachutes and the repair of such materials,” said McElhinny.

Holland has an even broader mindset on applications for the material, one that encompasses such issues as ocean pollution and climate change. What if bio-based materials could be created to replace oil-based, nonbiodegradable plastics?

Single-use water bottles have become a detested source of ocean pollution, Holland noted. “If you could start making everything in the future out of protein-based materials, it goes back into the earth.”

“I wouldn’t say that the Department of Defense cares about that but certainly sustainability people do,” he added.

“This is 100% green.”