Liquid crystal bridges silk-spinning gap.

Liquid crystal bridges silk-spinning gap

It's a silken storyline with several threads still missing. When protein-rich droplets squeeze through tiny pores on the abdomen of a silkworm or orb-weaving spider, they have nearly the consistency of water. But somehow, the creature manages to spin these silk-glad secretions into fibers strong enough to support sevral times more weight than an equivalent strand of steel.

Scientists have now begun to unravel the mystery behind this material metamorphosis.

New studies with polarized light reveal that the protein-rich droplets pass through a brief interim phase, taking on a semi-ordered structure known as a nematic liquid crystal, before they develop into silk fibers. The rob-like alighment of molecules in the liquid crystal -- a state halfway between the strict ordering of solids and the chaos of normal fluids -- gives the droplets sufficient backbone to form fibers when stretched by the spider or silkworm, says Christopher Viney, a biomaterials engineer at the University of Washington in Seattle.

Viney likens the protein molecules to matchsticks embedded in Jell-O. If the matchsticks have a random orientation, they will resist forming a sturdy framework wor the material. "But if you have [already] alighed the matchsticks with the direction of the applied force, you can create a strong, stiff structure," he says.

Viney and graduate student Keven Kerkam, together with collaborators from the Army Research, Development and Engineering Center in Natick, Mass., report their research findings in the Fed. 14 NATURE.

The discovery confirms earlier speculation that the droplets undergo a structural change that allows rapid formation of dragline silk at normal pressures and temperatures, Viney says. The work rules out other proposed explanations for fiber formation, including the suggestion that adjacent protein molecules develop cross-linking structures as a way to establish a more orderly alignment, he adds.

The researchers extracted silk-glad fluids from the orb-weaving spider Nephila clavipes and the silkworm Bombyx mori, positioning each type of fluid between rotating disks of light-polarizing material. Observations under a microscope revealed that the freshly isoloted secretions transmitted light in the pattern characteristic of an ordinary liquid. But as the watery fluid began to evaporate, leaving behind a concentrated solution of protein molecules, the lifht pattern shifted to that of a nematic liquid crystal. This, says Viney, indicates that the secretions retain a normal fluid structure while inside the silk glands, but begin to change into a liquid crystal as they exit.

He adds that the new finding lends credence to an emerging technique for manufacturing durable fibers by using liquid crystals as the source material, although the artificial process requires harsh solvents or unnaturally high temperatures and pressures. Nature's silk-spinning success under milder conditons suggests a major research challenge, he suggests.

"Where the spiders beat us handsdown is that they [make their fibers] under very reasonable conditons," Viney says. "We want to find out how on Earth they can do this -- not in sulfuric acid, but in water; not at a temperature of 200 [degrees] C and a pressure of a4 atmospheres, but at whatever the conditons happen to be outside today."