Spider silk already has a reputation for strength. Now researchers say a new way of processing silkworm silk can produce fibers with tensile toughness nearly as high as Kevlar, without relying on heavy chemical processing.
According to a recent Nature Sustainability study, heat and pressure fused silk fibers into a dense, transparent material with tensile toughness greater than bone and nearly as high as Kevlar fibers. The paper said the fiber also degrades more easily than artificial materials, making it a possible component in sustainable technologies.
Chunmei Li, a study co-author and biomedical engineer at Tufts University, told Gizmodo the “fused silk” is transparent in the visible range and has optical properties relevant for next-generation wireless and imaging technologies.
The paper said humans began extracting silk from silkworms as early as 8,500 years ago. It also said silk’s chemical organization has attracted a “resurgence of interest” in recent years for high-tech materials in biomedical engineering, energy generation, food preservation and sensors.
“The initial question stemmed from a long-standing problem in processing natural biopolymers,” Li said.
Natural silk has “impressive mechanical and functional properties,” Li added, but processing silk has required a “slow, chemically intensive” process that “can destroy the hierarchical structure that gives silk many of its useful properties.”
Li said the new method does not need extensive chemical processing because researchers “simply align the fibers and apply heat and pressure, and they fuse together in one step,” according to a Tufts statement.
For the study, Li and colleagues treated commercially available silk fibers with sodium carbonate to remove the sticky covering produced by silk moths.
“The goal was not only to see whether the fibers could be fused directly, but also to understand what was happening during the process, how the fibers came together, how the structure changed, and why the final material performed well,” Li told Gizmodo.
The researchers said the key was finding the right hot-pressing range. If the temperature and pressure were too low, the silk became too limp. If they were too high, the silk became brittle or broke down.
The team settled on a range between 125 and 215 degrees Celsius and between 1,900 and 9,800 atmospheres of pressure. Under those conditions, the fibers bundled and fused into a new form that a statement said resembled wood in structure.
According to the statement, strong inter-fiber bonds help distribute stress through the material, producing a solid material that keeps the best properties of natural silk.
Researchers also carried out ballistics tests and confirmed the material was “as puncture-resistant as carbon-fiber-reinforced polymers” used in airplanes and cars, according to a University of Michigan statement on the findings.
The team also implanted some of the material in mice and found it slowly degraded, suggesting it could be useful for temporary medical implants.
The researchers said they now want to make the material more scalable and capable of supporting complex shapes. They are also working on follow-up investigations and want industrial and commercial partners to test the fused silk in sensors and other technologies.
“Sustainable materials do not have to be weak or only symbolic replacements for plastics,” Li told Gizmodo. “Some natural materials are highly engineered by nature, and silk is one of them. Sustainability can come from better design, better processing, and a deeper understanding of materials that already exist in nature.”
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