MMaterialsgateNEWS 2017/11/13

How to build better silk

Reconstituted silk can be several times stronger than the natural fiber and made in different forms.

When it comes to concocting the complex mix of molecules that makes up fibers of natural silk, nature beats human engineering hands down. Despite efforts to synthesize the material, artificial varieties still cannot match the natural fiber’s strength.

But by starting with silk produced by silkworms, breaking it down chemically, and then reassembling it, engineers have found they can make a material that is more than twice as stiff as its natural counterpart and can be shaped into complex structures such as meshes and lattices.

The new material is dubbed regenerated silk fiber (RSF) and could find a host of applications in commercial and biomedical settings, the researchers say. The findings are reported in the journal Nature Communications, in a paper by McAfee Professor of Engineering Markus Buehler, postdoc Shengjie Ling, research scientist Zhao Qin, and three others at Tufts University.

Some kinds of silk produced by spiders are among the strongest materials known, pound for pound. But unlike silkworms, spiders cannot be bred to produce the fibers in useful amounts. Various researchers, including Buehler and his collaborators, have attempted to make purely synthetic silk instead, but those efforts have not yet yielded fibers that can match the strength of the natural versions.

Instead, the team has now developed a way to harness the best qualities of natural silk produced by silkworms, while processing it in a way that makes it stronger and opens up a wide variety of new shapes and structures that could never be formed from natural silk.

The key is to break down the natural silk, but not too much, the team says. That is, they dissolve the cocoons built by silkworms, not to the point that the material’s molecular structure breaks down but rather into an intermediate form composed of microfibrils. These tiny, thread-like assemblies preserve some of the important hierarchical structures that give the silk its strength.

Buehler, who is the head of the Department of Civil and Environmental Engineering, compares this recycling of materials to tearing down an old brick house. Instead of just knocking the house down into a pile of rubble, however, the individual bricks are carefully separated and then used to build a new structure. “Nature is still better at making the microstructures” that, as demonstrated in some of his earlier research, are responsible for silk’s unique stiff, stretchy properties, he says. “In this case, we take advantage of what nature provides.”

Though silk thread and fabric are expensive, the material’s cost comes mainly from the labor-intensive process of unraveling the thread from the cocoon and weaving it, not from the actual production of the silkworms and their cocoons, which are quite inexpensive, explains Ling. In bulk, unprocessed silkworm cocoons cost only about $5 per kilogram (2.2 pounds), he says.

By breaking down the silk and then extruding it through a tiny opening, the researchers found they could produce a fiber twice as stiff as conventional silk and approaching the stiffness of spider drag-line silk. This process could open up a variety of possibilities for new uses. For example, silk is a naturally biocompatible substance that does not produce any adverse reactions in the body, so the new material could be ideal for applications such as medical sutures, or scaffolding for the growth of new skin or other biomaterials.

The method also allows the researchers to shape the material in ways that could never be duplicated by natural silk. It could be formed, for example, into meshes, tubes, fibers much thicker than natural silk, coils, sheets and other forms. “We’re not satisfied with what [the silkworms] make,” Buehler says. “We want to make our own new materials.”

Such forms can be created by using the reconstituted material in a kind of 3-D printing system customized for silk solution, Qin says. And one advantage of the new process is that it can be carried out using conventional manufacturing technologies, so scaling it up to commercial quantities should not be difficult. The specific properties of the fiber, including its stiffness and toughness, can be controlled as needed simply by varying the speed of the extrusion process.

These reconstituted fibers are also very sensitive to different levels of humidity, and they can be made electrically conductive by adding a thin coating of another material such as a layer of carbon nanotubes. This could enable their use in a variety of sensing devices, where a surface covered with a layer or mesh of such fibers could respond to the press of a fingertip, or to changes in the ambient conditions.

One possible application, for example, might be a bedsheet made from such fibers, Buehler says. Such a sheet could be used in nursing care facilities to help avoid bedsores by monitoring pressure and automatically warning caregivers when a patient has been lying in the same position too long with pressure in a particular area of the body. Such applications could be made practical very quickly, he says, as no real obstacles remain to producing material suitable for such uses.

"This is neat research that draws on a powerful blend of the interdisciplinary strengths of the MIT and Tufts labs," says Anthony Weiss, a professor of biochemistry and molecular biotechnology at the University of Sydney in Australia, who was not involved in this work. "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," he says.

Source: Massachusetts Institute of Technology – 09.11.2017.

Investigated and edited by:

Dr.-Ing. Christoph Konetschny, Inhaber und Gründer von Materialsgate
Büro für Material- und Technologieberatung
The investigation and editing of this document was performed with best care and attention.
For the accuracy, validity, availability and applicability of the given information, we take no liability.
Please discuss the suitability concerning your specific application with the experts of the named company or organization.

You want additional material or technology investigations concerning this subject?

Materialsgate is leading in material consulting and material investigation.
Feel free to use our established consulting services

MMore on this topic

The silk of the Ornithoctonus Huwena spider demonstrates impressive weight-lifting abilities with efficient, water-driven actuation

Our muscles are amazing structures. With the trigger of a thought, muscle filaments slide past each other and bundles of contracting fibers pull on the bones moving our bodies. The triggered stretching behavior of muscle is inherently based in geometry, characterized by a decrease in length and increase in volume (or vice versa) in response to a change in the local environment, such as humidity or heat. Variations of this dynamic geometry appear elsewhere in nature, exhibiting a variety of mechanisms and structures and inspiring development in artificial muscle technology. Spider silk, specifically Ornithoctonus Huwena spider silk, now offers the newest such inspiration thanks to research... more read more

An international scientific team in which UPM researchers are involved has developed a bioinspired method that for the first time will allow researchers to spin artificial silk fibers as spiders do and to efficiently produce kilometers of silk

Researchers from the Centre for Biomedical Technology at Universidad Politécnica de Madrid were involved in this research project that was recently published in the journal Nature Chemical Biology. The results of the study show the first procedure to produce artificial spider silk by imitating the natural procedure of spiders. This imitation was obtained by developing recombinant proteins with the same water solubility that the natural silk and a spinning system based on, as it is occurs in the spider glands, aqueous solutions, stress generated during the spinning and pH reduction. The production of fibers that equal or improve the excellent properties of spider silk is a breakthrough... more read more

Tufts University engineers have created a new format of solids made from silk protein that can be preprogrammed with biological, chemical, or optical functions, such as mechanical components that change color with strain, deliver drugs, or respond to light, according to a paper published online this week in Proceedings of the National Academy of Sciences (PNAS).

Using a water-based fabrication method based on protein self-assembly, the researchers generated three-dimensional bulk materials out of silk fibroin, the protein that gives silk its durability. Then they manipulated the bulk materials with water-soluble molecules to create multiple solid forms, from the nano- to the micro-scale, that have embedded, pre-designed functions. For example, the researchers created a surgical pin that changes color as it nears its mechanical limits and is about to fail, functional screws that can be heated on demand in response to infrared light, and a biocompatible component that enables the sustained release of bioactive agents, such as enzymes. Although more... more read more

Why doesn't a spider's web sag in the wind or catapult flies back out like a trampoline? The answer, according to new research by an international team of scientists, lies in the physics behind a 'hybrid' material produced by spiders for their webs.

Pulling on a sticky thread in a garden spider's orb web and letting it snap back reveals that the thread never sags but always stays taut – even when stretched to many times its original length. This is because any loose thread is immediately spooled inside the tiny droplets of watery glue that coat and surround the core gossamer fibres of the web's capture spiral. This phenomenon is described in the journal PNAS by scientists from the University of Oxford, UK and the Université Pierre et Marie Curie, Paris, France. The researchers studied the details of this 'liquid wire' technique in spiders' webs and used it to create composite fibres in the laboratory which... more read more


Partner of the Week

Search in MaterialsgateNEWS