Scientists hoping to produce super-tough, bio-inspired fibers are a step closer with a new model for the molecular arrangement of spider silk, proposed by Cornell University researchers in the Jan. 5 issue of the journal Science.
Alexandra H. Simmons, Carl A. Michal and Lynn W. Jelinski reported their findings in the article, "Molecular Orientation and Two-component Nature of the Crystalline Fraction of Spider Dragline Silk."
Focusing NMR (nuclear magnetic resonance) studies on one of nature's most remarkable materials, the dragline silk from the golden orb-weaver spider -- and on one crystalline amino acid in particular, alanine -- the Cornell scientists found a surprising blend: highly oriented segments, amorphous material and barely oriented segments, all working together to make a fiber that is stronger than steel and much more elastic.
"Developing an understanding of the molecular origins of silk's excellent mechanical properties takes on a new urgency," said Jelinski, director of Cornell's Center for Biotechnology and professor of engineering. "Now the tools of biotechnology make it possible to produce designer materials. We envision an era when bacteria or plants, rather than oil wells and petroleum refineries, will produce high-performance, bio-inspired materials." So far, no metal or synthetic fiber can match the properties of the orb-weaver's dragline silk, which is spun first to make the spokes of the web and support the arachnid's weight as it hangs from branches. Dragline silk is stronger, per cross-sectional area, than steel, yet it can stretch to, and rebound from, 15 percent of its original length.
Led by Jelinski, physics graduate student Michal and postdoctoral researcher Simmons, now a staff scientist at DuPont Canada, examined alanine, which is known to reside in the crystalline regions of spider silk. They fed the spiders a special diet that included deuterated (or "heavy") alanine and collected the deuterium-labeled silk on a spindle with their homemade silking machine.
"It's like painting all of the alanines red," explained Jelinski. "We then query the 'red' parts using nuclear magnetic resonance," she said, noting that the same technology used in medical MRI scans in this case gives biophysicists information about orientation and motion of molecules.
"The exciting part," said Michal, who built one-of-a-kind hardware for collecting the data, "was that we found two types of crystalline alanines." Using computer simulations, Michal found that 40 percent of the alanines are as highly oriented as molecules in the synthetic fiber Kevlar -- an unexpected finding for a biological fiber. The other 60 percent are far less oriented, but are crystalline nevertheless.
"The way we think it works," Michal said, "is that the poorly oriented crystalline segments are like fingers, reaching out to make a good coupling between the highly oriented and the amorphous domains."
Describing their new model in Science, the Cornell researchers wrote: "These poorly oriented crystallites may be important in effectively coupling the highly oriented domains and the amorphous regions, producing a biomaterial with exceptional toughness."
As close at the scientists have come to unraveling the spiders' secret, genetically engineered silk plants or bacteria-filled bioreactors are still several steps away, said Simmons. "Our model is attractive in that it predicts several hypotheses that can be tested," she observed. "The model lets us predict that all of the glycine-serine amino acid pairs form loops, and it is the slight irregularity in their placement which will have to be duplicated in synthetic genes to achieve the same mechanical properties." Silk studies by the interdisciplinary team were supported by the National Science Foundation and the National Science and Engineering Research Council of Canada. Working on another piece of the super-fiber puzzle is Cornell Professor of Materials Science David T. Grubb, who is examining spider fiber with X-ray diffraction at CHESS, the Cornell High Energy Synchrotron Source.
"This multidisciplinary team highlights one of the real strength's of Cornell's biophysics and bioengineering programs," said John E. Hopcroft, dean of the Cornell College of Engineering. "Where else can you bring together engineering, materials science, instrumentation, physics and biology in such a natural and high-impact way?"