NANOTECH: WEAVING A SAFER WEB
How tough is spider silk? Tough enough to stop a bullet? Yes, that's how tough spider silk can be, tougher even than the Kevlar used to make bullet-proof vests. Gareth McKinley, a professor of mechanical engineering at the Massachusetts Institute of Technology has been studying this aspect of silk. He and Shawna Liff, one of his graduate students, are trying to develop a synthetic material with some characteristics of spider silk. If they succeed, this new material could lessen the bulk of soldiers gear, and improve their mobility.
From a mechanical engineer's perspective a material's toughness is measured by how much energy it can absorb before breaking. A spider's dragline silk - the strands that radiate from the center to the edge of a web - are about 10 times tougher than Kevlar. McKinley and Liff want to figure out how to replicate that toughness in a synthetic polymer. "Of natural and synthetic materials," says Liff, "spider silk is the king of toughness."
Spider's silk gets its toughness from its nanoscale structure. The dragline silk proteins are composed of amino-acid sequences that are rich in alanine and glycine. Some of the alanine-rich segments aggregate to make highly ordered, plate-like structures - called b-sheets - with all dimensions under 10 nanometers, but other alanine-rich segments are less oriented. The ordered regions are immersed in the soft, flexible glycine-rich matrix. "People have been trying for a long time to synthetically develop a polymer similar to spider silk," says Liff. McKinley's lab keeps getting closer.
To start with something stretchy, Liff and her colleagues thought of athletic clothing, and they picked polyurethane, a material with a nanostructure similar to spider-dragline silk, but with inferior mechanical properties. Then, they needed something to increase the stiffness, strength, and toughness, like the small alanine-rich b-sheets do in spider silk. They finally chose a "nanoclay," consisting of stiff, ceramic-like disks that are 1 nm thick and 25 nm in diameter. These are often used in hair products and lotions to increase viscosity. Working with Nitin Kumar, then a postdoc in the McKinley lab, Liff used a series of solvents that dispersed the nanoclay and dissolved the polyurethane. The resulting composite is up to 25 times as stiff as the original material and almost twice as tough as polyurethane alone, but only a bit more than half as tough as a spider's dragline silk. "We have not reached the spider-silk properties yet," says Liff, "but we are slowly moving in that direction."
This work takes place at MITs Institute for Soldier Nanotechnologies. Liff imagines this synthetic spider silk one day making a soldier's uniform lighter and tougher. "Maybe it could be blended with cotton to make lightweight uniforms that don't tear when running through the jungle," she says. Of course, it might also be woven into lightweight ropes.
