WEB WEAVER: A new microfluidics tool spins short silk strands out of proteins derived from the European garden spider, Araneus diadematus, shown here. Image: CC: Luc Viatour
A new study should delight fans of the comic book superhero Spider-Man. Researchers from the Munich Technical University (M.T.U.) in Germany have built a microfluidics (miniature plumbing) system for spinning short strands of artificial spider silk. The new strands are less than half a millimeter, or 0.02 inch long, and their strength and elasticity are still untested.
Other research groups have already created strands yards or more in length that come close to the properties of real spider silk. Such strands have long been desired for weaving into lighter bulletproof vests or artificial tendons and ligaments, but even relatively short strands could find niche applications in medical procedures such as brain surgery, says M.T.U. biophysicist Andreas Bausch, a co-author of the new study.
Devotees of Spider-Man comics will recall that—contrary to cinematic revisionism—Spidey slung webs not from glands in his wrists but by using a special web fluid of his own design, shot from a pair of wrist-mounted sprayers. Similarly, researchers have built their own versions of web shooters in the past out of syringes. Forcing silk proteins with the right additives through a narrow opening fuses the proteins into strands.
The Munich researchers took the technique a step farther. They studied artificial dragline silk, which spiders use to hang from ceilings and outline their webs, based on that of Araneus diadematus, the European garden spider [see image]. One of the team members had previously inserted a pair of dragline silk genes (dubbed eADF3 and eADF4) into bacteria for easy production. Without such tricks, "you would have to milk spiders to do these experiments," Bausch says.
Bausch and his colleagues used a trident-shaped set of channels [see image] to first mix their dissolved silk proteins with potassium phosphate, which causes the silk proteins to clump into microscopic spheres measuring a mew microns in diameter (a micron is one millionth of a meter). They then lowered the pH of the mixture from 8.0 to 6.0 (to make it slightly more acidic) and forced it through a constriction measuring roughly 10 microns wide.
The scientists report in Proceedings of the National Academy of Sciences USA that they created a series of short strands by ratcheting up the speed past 600 microliters per hour (equivalent to less than 10 ounces per minute from a garden hose).
Bausch and company believe that the pH drop causes the spheres to change shape after bonding together via chemically sticky ends, and the narrow constriction mashes the silk globs together once they are flowing fast enough. They note that real spiders can spin their silk at a more leisurely pace because they start with a more viscous silk solution.
Making fibers is a small first step, says Randy Lewis, a molecular biologist at the University of Wyoming in Laramie. He says his team has spun strands up to a yard long that stretch as much as 50 percent and have roughly half the tensile strength of real spider silk. "Without mechanical testing data," he says, "it's really hard to analyze whether the kind of system they have is going to be particularly useful" for applications.
But he says the new system may help researchers improve artificial silks, which vary from strand to strand in unpredictable ways. Spider-Man's science-whiz alter ego Peter Parker would surely approve.