Secret of strong silk.

How spiders and silkworms manage to produce strong fibers without clogging their silk-producing glands has puzzled scientists for years. While trying to mimic the process in the lab, researchers at Tufts University in Medford, Mass., stumbled across the answer.

David Kaplan and Hyoung-Joon Jin took natural silk from a silkworm, extracted the silk proteins known as fibroins, and dissolved them in water. The researchers then added increasing amounts of polyethylene oxide, a polymer that gradually removed water from the solution. As water volume decreased and the concentration of fibroin increased, the proteins folded in on themselves, forming round structures called micelles. Measuring between 100 and 200 nanometers in diameter, each micelle had a hydrophobic (water-avoiding) interior and a hydrophilic (water-seeking) surface.

In the Aug. 28 Nature, Kaplan and Jin explain that this micelle structure enables the fibroins to remain soluble in water. That prevents the proteins from crystallizing prematurely and gumming up the bugs' silk-producing glands. As the polyethylene oxide drew more water out of the fibroin solution, the micelles aggregated into microscopic globules. When Kaplan and Jin analyzed the glands of silkworms and their fibers, they found similar structures.

The combination of the tightly bound micelle nanostructures and the microscale globules is what gives the silk its strength, says Kaplan. Although many companies have produced silk fibers in the lab, the synthetic strands don't match the strength of natural fibers. Kaplan says he hopes his work will help industry move closer to that goal.--A.G.