Scientists have now observed a remarkable mechanism by which polymer materials are formed. Under the influence of shear forces, nanoparticles from the secretion of velvet worms form polymer fibers that can be recycled in water.
Nature is an excellent teacher – even for material scientists. Researchers, including scientists at the Max Planck Institute of Colloids and Interfaces, have now observed a remarkable mechanism by which polymer materials are formed. In order to capture prey, velvet worms shoot out a sticky secretion that stiffens into strong threads under the action of force. The extraordinary thing about these threads is that they can be dissolved and then reformed again. The fact that reversible polymer fibers can be drawn from the previously liquid secretion is a very interesting concept for researchers. It is quite possible that one day it will be possible to synthesize novel recyclable materials based on the principle of velvet worms.
Some animals produce amazing materials. Spider silk, for example, is stronger than steel. Mussels secrete byssus threads, which they use to cling tightly to stones under water. The material secreted by velvet worms is no less impressive. These small worm-like animals, which look like a cross between an earthworm and a caterpillar, spray a sticky liquid to ward off enemies or catch prey that is particularly deadly for prey such as woodlice, crickets and spiders: As soon as they try to wriggle out of the slimy threads, their struggles cause the threads to harden, leaving no hope of escape.
“The shear forces generated by the prey’s struggles cause the slime to harden into stiff filaments,” explains Alexander Bär, a doctoral student at the University of Kassel, who is studying under the velvet-worm expert Georg Mayer. In order to investigate the slime of an Australian velvet worm species, the biologist worked closely with researchers from the Max Planck Institute of Colloids and Interfaces in Potsdam. The chemist Stephan Schmidt, for example, now a junior professor at Heinrich Heine University in Düsseldorf, helped to elucidate the nanostructure of the slime. A research group headed by biochemist Matt Harrington in the Biomaterials Department of the Potsdam Institute focused on other questions concerning the chemical composition and molecular processing. The interdisciplinary group of scientists was particularly interested in how the composition and structure of the secretion changes during thread formation.
Slimy mix of proteins and fatty acids
“We had already known that the slime consists mainly of large protein molecules and fatty acids,” Alexander Bär says. At the Max Planck Institute in Potsdam, the researchers discovered that the proteins and lipids combine to form tiny globules. “Velvet worms produce the protein and fat molecules as well as other components separately”, Bär explains. “Outside the gland cells, the nanoglobules then form independently to create the thread-forming and adhesive properties.” The globules are formed with remarkable precision in that they are uniform in shape and always around 75 nanometers in diameter.
Velvet worms store their liquid weapon until it is needed. They then shoot the slime at their prey or foe through two glands located on either side of their head by means of muscular contractions. “At first the sticky consistency does not change,” Bär says. “However, as soon as the prey begins to struggle, shear forces act on the slime to rupture the nanoglobules.” Vibrational spectroscopy studies in Potsdam showed that proteins and fatty acids separate in the process. “Whereas the proteins form long fibers in the interior of the slime, the lipid and water molecules are displaced to the outside and form a kind of sheath,” Bär explains. The researchers also found that the protein strand inside has a tensile stiffness similar to that of Nylon. This explains the remarkable performance of the filaments.