Research led by Imperial College London, University of Michigan Engineering and Tufts University demonstrates the way silk threads can be turned into clear, plastic-like materials that bend terahertz frequencies of light. The discovery of silk into strong plastic-like materials with 6G potential could lead to parts of it being made from recycled silk.
The advent of silk into strong plastic-like materials with 6G potential is also lightweight but more potent than many metal alloys and traditional plastics made from fossil fuels. Their mechanical characteristics could make them suitable for sports equipment, shipping containers and certain specialised packaging. In ballistic testing, the new materials were resilient to puncture, as carbon-fibre-reinforced polymers are utilised in the bodies of aeroplanes and automobile chassis. And the material degrades gradually when implanted in mice, which makes it potentially useful for temporary medical implants.
The scientists are especially excited about the way the material can twist, or polarise, terahertz frequencies of light. The 6G band extends to terahertz frequencies and can carry data much faster than 5G networks, hence making it especially appealing for high-speed internet in rural areas. Another way to encode the data is with a standard polarisation, and it might open up more channels, but the elliptical polarisations observed with the silk material are by no means usually so simple. By altering the temperature and pressure at which they pushed the silk into the plastic-like material, the team could optimise the amount of twist.
Says Nick Kotov, the Irving Langmuir Distinguished University Professor of Chemical Sciences and Engineering at U-M and a co-corresponding author of the study published in Nature Sustainability, “It’s difficult to engineer a material with terahertz optical activity that can rotate light while also being nearly transparent. This composite is unique in that it can do it for the frequencies that are essential for multiple future technologies. Typically, such bio-derived materials absorb terahertz light very strongly, so you get very little light out.”
Keeping the best elements of silk
It is well to be noted that the properties of the materials arise from the chemical structure of the silk. There are alternate areas of order and disorder. The fibers consist of long chains of a variety of amino acids. In some parts of the chain the amino acid sequence is random and goes on to form a distorted tangle. The amino acids in other parts of the chain also link in a pattern that repeats, and the chain folds neatly into twisted, crystalline sheets. Silk is durable and flexible, a blend of crystalline and unruly features.
According to Chunmei Li, a research assistant professor in biomedical engineering at Tufts University and a co-corresponding author of the study, “It’s surprisingly strong for something so flexible. By processing it, we can go beyond the capabilities of many other biomaterials.”
Heating the fibers to between 257 and 419 degrees Fahrenheit and 1900 and 9800 atmospheres of pressure goes on to evaporate water from the silk and fuses the tangled regions together in a single sheet without destroying the clean folds that occur inside the fibers.
Opines associate professor in multifunctional and sustainable polymer composites at Imperial College London and a lead author of the study, Emiliano Bilotti, silk has very interesting properties due to its hierarchical microstructure, with crystalline domains embedded in a complex multiscale architecture. We wanted to save as many of the pristine fibers as possible.”
In contrast to that, earlier attempts to make plastic-like materials derived from silk required dissolving the silk in a chemical solvent and processing it into a powder. Heating and pressing the powder generates materials more durable than traditional plastics, but much of the crystal structure is eliminated.
Minimizing waste in chemical and textile industries
One of the reasons the team is working on this is to help cut waste within the fashion and textile industry.
“If you can retrieve very long threads, you can weave again, but when the fibers get shorter and shorter, there is no other way to recycle them than to dissolve them into a powder,” says Bilotti. He adds that “I never believed that was a sustainable solution.”
The new method does not need huge quantities of chemical solvents, salt and water, but just the boiling of the silk to eliminate the natural protein, known as sericin, that binds the fibers into threads. Even small fibers may be pressed into plates.
Says Li, “It can be a very simple, one-step process.”
The team is now looking into how to grow their production procedure to larger, more intricate shapes and lifecycle evaluations in order to measure the full environmental advantages. The researchers are also looking at how to include the fused silk into sensors and various other applications, while they are also seeking industrial and commercial partners in order to assist in scaling up the process and bringing the materials to market.
This research was backed by the U-M Center for Complex Particle Systems – COMPASS, a National Science Foundation Science and Technology Center, as well as the Air Force Office of Scientific Research, the Engineering and Physical Sciences Research Council and also Tufts Launchpad Accelerator funding.