SPIDER silk, one of nature's wonder materials, is slowly yielding its secrets - to the delight of scientists trying to duplicate the strongest fiber known.
One of the most daunting tasks has been to unravel the source of the silk's strength and elasticity. Now, a team of researchers at Cornell University in Ithaca, N.Y., appears to have found the answer by looking at how the silk's various types of molecules are organized.
The work not only advances efforts to understand spider silk itself, according to David Terrill, with the Department of Polymer Science and Engineering at the University of Massachusetts at Amherst. The Cornell team's research also represents a "substantial step forward" for using nature as a blueprint for developing new synthetic materials.
The appeal of nature becomes clear by comparing spider silk and Kevlar, a synthetic fiber used in bullet-proof vests, says Alexandra Simmons, research scientist at Du Pont Canada and member of the Cornell team. "Spiders spin their silk using water as a solvent and at room temperatures. Kevlar also comes from a spinning process. But it's made at high temperatures from molten material or by using quite strong solvents."
The research team, led by Lynn Jelinksi, a professor of engineering and director of the university's Center for Biotechnology, examined drag-line silk from the orb-weaver spider, the species responsible for the complicated circular webs found among tree branches or between plant stems.
Previous studies of spider-silk composition showed that roughly a third of the fiber is made up of crystals, mostly of the amino acid alanine. These crystals were the focus of the Cornell team's efforts.
As part of their diet, the spiders were given food containing alanine that had been tagged with a form of hydrogen known as deuterium. The team then used a technique known as nuclear magnetic resonance imaging to detect the deuterium and determine the structure of the fibers.
Instead of a single type of alanine crystal, the researchers found two types. The known type, which makes up 40 percent of the crystals, formed in sheets that lay roughly parallel to the axis of the fiber. But the team discovered another type, with a folded shape, that was less dense and more randomly oriented. The poorly oriented crystals "are like fingers, reaching out to make a good coupling" between the highly oriented crystals and the rest of the silk material, says Carl Michal, a member of the team.
But even if the team's explanation is verified, formidable challenges to duplicating spider silk still exist. One approach would be through genetic engineering. Last fall, a molecular biologist at the University of Wyoming spliced the gene responsible for spider-silk production into bacteria, which then produced the silk protein. But Simmons points out that making bulk quantities this way will be demanding. The gene that codes for the repetitious molecules is not stable in other organisms.
To manufacture the silk synthetically, she adds, researchers would have to find ways to shape the molecules that form the weakly oriented crystals.
Indeed, this is the broader message to the material science community as they try to mimic nature's engineers, according to the report of the team's findings in the current issue of the journal Science. The research suggests that it isn't enough to make the right molecules for a biologically inspired material and connect them in the right order, the team writes. Such molecules also must be given the right shape in order to duplicate the desired properties.