In a new study co-led by researchers at Tufts University, Imperial College London, and the University of Michigan, scientists have developed a significantly improved way of transforming silk into solid materials—one that preserves and enhances the natural strength of silk fibers. The materials were made by fusing the silk fibers under carefully controlled conditions of temperature and pressure without synthetic additives, and have been shown to outperform bone and wood and come remarkably close to Kevlar in tensile toughness. The study has been published in the journal Nature Sustainability .
Silk has long been admired for its versatility and strength, and humans have used it for textiles and clothing for thousands of years. In nature, insects including spiders and moths use silk to form ropes, nets, glue, armor, and sensors. More recently, engineers have adapted silk for biomedical implants, electronic devices, and other advanced applications by dissolving the fibers and rebuilding them into new forms.
However, that approach comes at a cost involving large amounts of water, chemicals, energy, and time. "The process breaks down the natural fibers into the individual silk fibroin proteins before processing them into new shapes, so we lose a lot of the inherent strength of the original fibers," said Chunmei Li, research assistant professor in the Tufts School of Engineering. "With this new method, there's no need to dissolve the silk—we simply align the fibers and apply heat and pressure, and they fuse together in one step."
This seemingly simple change makes a profound difference. By preserving the original structure of the silk fibers, the new process retains features that give the resulting material exceptional strength.
How fused silk is produced
The fused silk material was produced in the laboratory of Emiliano Bilotti, associate professor in multifunctional and sustainable polymer composites at Imperial College London, in collaboration with Li and David Kaplan, Stern Family Endowed Professor of Engineering at Tufts. Li and Kaplan have had extensive experience in thermoplastic molding of silk products designed for medical implants and other purposes, and they explored the potential utility of the new material for medical applications.
The new method begins with reels of commercially available silk moth cocoon fibers from textile manufacturing supplies, which are then treated with a mild sodium carbonate solution to remove sericin—the sticky adhesive covering the fibers that helps the insects build the cocoon structure. The fibers are pulled from the bath to help align them and then subjected to hot-pressing. During the heating, parts of the silk's molecular structure become mobile, allowing neighboring fibers to bond together.
"The silk is like a composite," Kaplan explained. "There is a more mobile, amorphous phase of the fiber proteins, and there is the part of the protein chain that folds to form sheet-like surfaces that stack up into crystalline structures. Together they give silk fibers their strength, toughness and flexibility. But it's the mobile part that allows the fibers to fuse together under heat and pressure."
The degree of fusion between fiber bundles depends on the level of heat and pressure applied. Lower temperatures and pressures lead to a looser structure, while higher temperatures and pressures result in a denser, generally stronger material. Too high temperatures result in a breakdown of the fibers and brittleness.
Within an optimal processing window – temperatures between 257 and 419 degrees Fahrenheit and pressures between 1,900 and 9,800 atmospheres -- you end up with a remarkably strong material that retains most of the molecular organization of the original silk along with a macro structure of bundled and fused fibers.
In its new form, fused silk takes on a hierarchical structure which has some properties similar to wood. In both materials, fiber bundles are aligned in a common direction and bound together with either lignin (in wood) or the fusing of amorphous protein regions (in silk). The bonding between fiber bundles helps transfer stress between them, creating enormous strength throughout the structure.
Fused silk is a surprisingly tough, solid material that can compete with some of the best man-made plastics, glass fiber and carbon fiber composites, while also matching the natural strength of wood. Its ability to withstand ballistic impact exceeds that of high-performance materials like carbon fiber reinforced polymer composites.
Researchers in the laboratory of Nicholas Kotov, professor of chemical sciences and engineering at the University of Michigan, explored the material's remarkable optical properties. It is transparent to visible light, but they also found that fused silk has a unique ability to polarize terahertz radiation, which sits between infrared and microwave wavelengths. Terahertz radiation is used in applications such as airport body scanners, medical imaging, and the identification of chemicals, drugs, and explosives. Kotov and his team are particularly interested in applications to 6G communications, which could transmit data up to hundreds of times faster than 5G networks. Polarization could also increase the density of encoded information.
Tunable for medicine
Li and Kaplan evaluated how the fused silk behaves inside the body. They found that the material is both stable and biocompatible—two properties that are often difficult to achieve together.
In animal studies, implanted samples produced only mild immune responses that diminished over time. Importantly, the interaction between the material and surrounding tissue could be adjusted by changing how the silk was processed. Less densely fused materials allowed cells to gradually infiltrate and integrate, while more densely fused versions remained stable and resisted breakdown.
"We can control how fast the material degrades depending on the conditions we use," Li said. This tunability may broaden its potential medical applications. Fused silk could be useful in regenerative settings, where gradual cellular infiltration and material degradation is beneficial, or as a durable implant for long-term support. Li noted that one promising direction could be orthopedic uses: "Because of its strength, it could potentially be used for fixation devices like plates, pins, and screws as supports for bone fractures."
