Scientists Discover Technique for Producing Silk Fibers Stiffer than Natural Silk

While attempting to combine the complex mixture of molecules that form the fibers of natural silk, nature is always ahead of human engineering. In spite of best attempts to produce the material, artificial fibers cannot match the strength of the natural fiber.

A photograph shows regenerated helical silk fibers colored by Rhodamine dyes, under UV light. CREDIT: Courtesy of the researchers.

However, researchers started with silk obtained from silkworms, disintegrated it chemically, and subsequently reassembled it only to discover that a material with stiffness more than twice as that of natural silk can be produced and shaped into complex structures (e.g. lattices and meshes).

According to the researchers, the new material is known as regenerated silk fiber, or RSF, and can be used in different biomedical and commercial settings. The outcomes of the study have been published in the Nature Communications journal in a paper authored by Markus Buehler, McAfee Professor of Engineering; Shengjie Ling, postdoc; Zhao Qin, research scientist; and three other researchers from the Tufts University.

Certain types of silk made by spiders are known to be one of the stiffest materials on earth. However, in contrast to silkworms, it is almost impossible to rear spiders to produce the fibers in usable quantities. Rather, several scientists—including Buehler and his colleagues—have endeavored to make entirely synthetic silk. However, the attempts were not useful in producing fibers with strengths comparable to that of the natural fibers.

In contrast, the researchers have formulated a technique for getting the best qualities of natural silk made by silkworms, while treating it such that it gets stronger and paves the way for a broad range of innovative shapes and structures that cannot be synthesized using natural silk.

According to the researchers, the clue is to disintegrate the natural silk, but only to a certain level. In other words, the cocoons that the silkworms build are dissolved only until an intermediate form including microfibrils is formed, and not to the extent that the molecular structure of the material is broken down. The microfibrils are thread-like, tiny assemblies conserving certain significant hierarchical structures from where silk derives its strength.

Buehler, head of the Department of Civil and Environmental Engineering, compares recycling of materials to breaking a house brick by brick. In contrast to pulling down the house into a rubble pile, each individual brick is cautiously separated and used to construct a new structure. According to Buehler “nature is still better at making the microstructures” that—as shown in some of his prior studies—render silk distinctively stretchy and stiff. “In this case, we take advantage of what nature provides.”

Ling explained that despite the fact that silk thread and fabric are very costly, the cost of the material is mainly due to the labor-intensive technique of removing the thread from the cocoon and weaving it, but not due to the original production of the silkworms and their cocoons, which are really cheap. He further stated that when procured in bulk, the cost of unprocessed silkworm cocoons is approximately just $5/kg (where 1 kg = 2.2 pounds).

The team discovered that when the silk was disintegrated and then extruded it through a tiny opening, they were able to synthesize a fiber with stiffness twice that of traditional silk and close to the stiffness of spider drag-line silk. This technique can pave the way for a range of potential new applications. As silk is naturally biocompatible and does not create any adverse reactions in the human body the new material may be perfect for use in medical sutures, or as scaffolding for the regeneration of new skin or other biomaterials.

The technique also enables the scientists to shape the material such that it is not duplicated by natural silk. For instance, it can be formed into tubes, coils, meshes, sheets, fibers with thickness greater than natural silk, and other forms. “We’re not satisfied with what [the silkworms] make,” stated Buehler. “We want to make our own new materials.”

According to Qin, forms such as these can be developed by adopting the reconstituted material in a type of 3-D printing system tailor-made for silk solution. Moreover, one benefit of the innovative method is that it can be performed by adopting traditional manufacturing technique; therefore, using it to produce commercial amount will be simple. The particular characteristics of the fiber, such as its toughness and stiffness, can be regulated as per requirements by simply altering the pace of the extrusion process.

The reconstituted fibers were also found to be very reactive to different humidity levels, and they can be made electrically conductive by coating a thin layer of another material (e.g. carbon nanotubes). This can allow the fibers to be used in different sensing devices in which a surface coated with a mesh or layer of these fibers can react to a fingertip, or to variations in the ambient conditions.

According to Buehler, one prospective application may be a bedsheet weaved from these fibers. This bedsheet can be used in nursing care facilities to assist in preventing bedsores by monitoring pressure and automatically giving warnings to caregivers if a patient lies in the same position for a long time with pressure concentrating in a specific area of the human body. He stated that applications such as these can be rendered practical very easily because there are to real-time difficulties in producing the material appropriate for such applications.

This is neat research that draws on a powerful blend of the interdisciplinary strengths of the MIT and Tufts labs, the technology has the potential to lay the foundation for new types of woven materials and functional composites—these could be for a whole range of uses, such as a new generation of textiles and biosensors.

Anthony Weiss, professor of biochemistry and molecular biotechnology at the University of Sydney in Australia

Postdocs Chunmei Li and Wenwen Huang, and David Kaplan, professor and chair of biomedical engineering at Tufts University, were part of the team. The National Institutes of Health and the Department of Defense supported the study.

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