Super strong artificial silk? That’s so metallic.
Giving a metallic coating to the renewed silk of the silkworm can make the strands strong and stiffscientists report Oct. 6 in Affair. Some strands were up to 70 percent stronger than silk spun by spiders, the team found.
The work is the latest in a decades-long quest to create fibers as strong, light and biodegradable as spider silk. If scientists could mass-produce such a material, the potential uses range from biomedical to athletic. Artificial sutures, ligaments and tendons, even sports equipment could get a spidery upgrade.
“If you have a climbing rope that’s half as heavy as it normally is and still has the same mechanical properties, then obviously you’re going to be a happy climber,” says Randy Lewis, a silk scientist at Utah State University in Logan, who did not participate in the study.
Collecting enough silky material to make these super strong products has been a huge hurdle. Silk from silkworms is easy to harvest, but not as strong. And spider silk, the gold standard for hand-spun strength and toughness, isn’t exactly easy to harvest. “Unlike silkworms, spiders cannot breed due to their territorial and aggressive nature,” study co-author Zhi Lin, a structural biologist at Tianjin University in China, and his colleagues write.
Scientists around the world have tried to spin strong threads in the laboratory using silkworm cocoons as a starting point. The first step is to remove the outer rubbery layer of silk. Scientists can do this by boiling the fibers in a chemical bath, but that can be like cutting through the silk proteins with an axe. If the proteins are damaged too much, it’s difficult for scientists to spin them back into high-quality strands, says Chris Holland, a materials scientist at the University of Sheffield in England who was not involved in the study.
Lin’s team tried gentler approaches, one of which used lower temperatures and a papaya enzyme to help dissolve the silk layer. That method of good manners seemed to work. “They don’t have bits of silk protein,” says Lewis. “That’s huge because the larger the proteins that are left, the stronger the fibers.”
After a few processing steps, the researchers forced the resulting silk sludge through a small tube, much like squeezing out toothpaste. They then bathed the extruded silk in a solution containing zinc and iron ions, eventually stretching the strands like taffy to make long, thin fibers. The metal plating could be the reason some of the yarns were so strong: Lin’s team detected zinc ions in the finished fibers. But Holland and Lewis aren’t so sure.
The team’s real innovation may be that “they’ve managed to fray silk in a less damaging way,” says Holland. Lewis agrees. “In my opinion,” he says, “that’s a big step forward.”