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It seems that spiders have passed the age of nanomaterials by about 300 million years, turning nanoparticles into solid, expandable wires, far beyond our modern manufacturing capabilities. But now, we are trying to catch up, studying their secrets using our own advanced technology.
Spider silk begins as a creamy mixture somewhere between a liquid and a gel that arachnids reduce to very hard yarns through a complex recipe for pressure, acidity and additional chemicals. Researchers know what's going on in the liquid and what the finished product looks like, but they still do not understand how spiders make one of nature's most solid materials at room temperature and without sophisticated machines.
With the help of an over-cooled electron microscope, a motley crew of biochemists and material scientists are more deeply interested than ever in the black widow's silk glands. Their work, which appeared Monday in the Proceedings of the National Academy of Sciences, confirms that the spider's proteins gather in balls at the nanoscale and reveal for the first time the flake-like structures inside these balls. The researchers hope their contribution will help those who are trying to make spider silk as we are currently mass manufacturing. "Practical applications for a material like this are virtually limitless," Nathan Gianneschi, nanomaterials chemist at Northwestern University and one of the team's leaders, said in a statement.
A problem for silk weavers is that no one knows exactly what "spidroin" proteins do inside the silk glands of the spider, that they pass from molecules to threads. For more than a decade, researchers had assumed that the strands would wind into spheres called "micelles" because they had parts attracted by water and others that repelled them. But there was little direct evidence of the theory and no one knew what specific form the micelles would take.
Gregory Holland, a biochemist at San Diego State University, was the first concrete evidence of the correlation between spiders. Based on how they went through the fluids in various circumstances, he concluded that the particles measured a few hundred billion meters in diameter – too big to be solitary proteins. Holland felt the results were promising, but it was only when he met Gianneschi at a conference that he realized they could go further. "Nathan started talking and I said, 'My God, if we could get pictures of what they look like,' remembers Holland.
Gianneschi specializes in cryo-transmission electron microscopes, devices for creating images of objects too small for optical microscopes by sending electrons through a thin sample at extremely cold temperatures. Others had already tried this with spider fluid, but no one had found a way to prepare the sample without destroying the delicate protein structures. "If you breathe on it, it starts to change," says Holland. "Paste a needle inside and you can remove a fiber right out."
The strengths of the usual pipette techniques reduce spidroins to threaded fibers. The team therefore carefully dissected the black widow spiders and prepared drops of silk fluid slowly and gently using various customized tools. Finally, they managed to obtain samples that, as much as possible, preserved the natural structure of the molecules.
The team then reconstructed the sample images, layer by layer, to give a first glimpse of the actual composition of the silk protein bundles. Far from mere spheres, they discovered that the micelles were filled with hundreds of smaller bundles, which Mr. Gianneschi describes as "flakes" or "discs," each with its own interweaving of spidroin fibers. When they are properly pressurized, researchers suspect these flakes to expand into the fibers that then become a silk thread. They hope that this new model will eventually allow those who try to cook synthetic spider silk to troubleshoot their recipes, verifying that their spiders are tangling in the desired way.
Anna Rising and Jan Johansson of the Swedish University of Agricultural Sciences, Uppsala, Sweden, are two of these researchers. They lead an international team that has described the most resistant synthetic silk ever produced at the beginning of last year. They praised the new study for expanding the basic theory of silk proteins, but pointed out that many details were still missing. "Ideally, we would like to have an image of how spideros are arranged at atomic resolution," they wrote in an email. Previous research has investigated the ends of the protein, but the intermediate chain remains unexplored.
They also said that cryo-electronic microscopy would be a valuable tool to fill some of these virgin areas of the silk protein map, a result that also excites Gianneschi and Holland. The black widow's silk is one of the strongest in the arachnid class and she plans to consider other species to see if their different types of silk combine with different forms of protein bundles.
"Now that you know how to look," said Gianneschi, "you can go watch.
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