How a spider's spinning duct turns protein solutions into threads as strong as steel is still largely a mystery. But German scientists have now designed a microfluidic device (shown) to serve as an artificial spinning duct (Proc. Natl. Acad. Sci. USA 2008, 105, 6590). The device allows precise control over conditions such as ion concentration and pH that are believed to be critical for silk formation. Led by Thomas Scheibel at the University of Bayreuth and Andreas R. Bausch at the Technical University of Munich, the researchers worked with two components of the dragline silk used to make a spiderweb's outer rim and spokes. They found that one protein, eADF4, would not make fibers on its own and instead aggregated into solid protein spheres. The other protein, eADF3, formed fibers, but only at pH 6 and high phosphate concentration and with accelerated flow through the device. A mixture of the two proteins also yielded fibers. The results point toward a model for silk formation in which the proteins form colloidal intermediates before assembling into fibers, the authors say.