In April, a team of university scientists reported evidence that the prey-snaring "capture silk" spun by orb-weaving spiders is made of a complex network of wet molecular springs [Nat. Mat., 2, 278 (2003)]. The study offers new insight into how spider silk functions, and maybe how it could be re-created in the lab for commercial uses.
Scientists have been studying spider silk for decades, attempting to understand how the nanometer-sized biodegradable threads can be stronger by weight than high-tensile steel and elastic enough to stretch up to 10 times their initial length. These superior fibers could potentially be used to create lightweight bullet-proof body armor, tough biodegradable bandages, and even artificial tendons.
"It's hard to make silk like spiders do," says Cheryl Y. Hayashi, an assistant professor of biology at the University of California, Riverside, and a member of the research team. "Lab efforts haven't created silk with quite the same tensile strength."
Unlike silk from silkworms, spiderwebs can't be harvested from large farms of cohabiting critters. Most spiders are territorial and cannibalistic and would kill each other before spinning enough silk to make marketable products.
What's more, Hayashi says, orbweaving spiders can spin up to seven types of silk. Although the silks have remarkably similar genetic structures, each type has unique variations in elasticity and other properties. With silks that have different properties all mixed up in a single web, harvesting industrial amounts of compatible silks would be very challenging.
"You can milk spiders for silk," says UC Santa Barbara biophysicist Helen G. Hansma, who led the research team. "If you anesthetize the spider and tickle its spinerettes with a dissecting needle, it can spin out yards and yards of silk." She cautions, however, that while milking works for creating study samples, it is still far too tedious to use on a commercial scale and is only good for generating one type of silk. Most lab efforts, therefore, focus on recombinant techniques--inserting silk-producing genes into hosts such as bacteria and having the hosts produce the material synthetically.
"Almost all spider silks are entirely protein," Hayashi says. "Spiders make a concentrated liquid stock of protein and spin it out of their bodies. By the time it exits, the protein is solid." The capture silk from orb-weavers--one of the most commonly studied spider groups--primarily contains flagelliform proteins, and the strands are extra-elastic because a hydroscopic, gluey coating keeps them wet. What remains unknown is the molecular structure that gives the solid protein strands their amazing properties.
With funding from the National Science Foundation, the research team used force spectroscopy to map how capture-silk molecules behave when they are pulled and relaxed. "We gathered strands of unstretched capture silk by pressing a glass slide against a newly spun web and trimming out a section," Hansma says. A region of the web was then covered with a drop of CaCl2 solution to minimize the meniscus forces in air.
Using a type of atomic force microscope called a molecular force probe, the tip of a needle on the end of a cantilever arm was pressed into the wet strands until it picked up silk molecules. The cantilever then stretched and relaxed the molecules for several consecutive pulls to plot curves of force against extension.
The resulting force spectra show a sawtooth pattern as the silk expands, indicating that as the proteins are pulled, they experience rupture events. These events break sacrificial hydrogen bonds to reduce tension and extend previously hidden lengths of polypeptides. The sawtooth pattern appears each time the same molecular sample is pulled, suggesting that the sacrificial bonds re-form when the silk relaxes.
The force spectra also revealed exponential force changes when the silk was stretched and relaxed. This behavior contrasts with the force increases seen in pulls of molecules, such as titin in human muscle, which fit a wormlike chain model of entropic molecular springs. The team proposes a model for capture silk with a cross-linked network of flagelliform -spiral springs joined by intervening spacer sequences of amino acids. Although the team admits that its model may not be perfect, it is a starting point to understanding this intriguing material.
"It's humbling to realize that a lot of very smart people are trying to replicate what the spiders in our basements can do naturally," Hayashi says.
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