Silk has always been recognized for its impressive strength. Spiders utilize it to construct webs capable of withstanding strong winds and the impact of flying insects. Silkworms envelop themselves in silk for protection. Humans began employing it for clothing fabrication millennia ago due to its lightness, flexibility, and durability.
Now, researchers have devised a method to transform silk into a substance far more robust than fabric. The findings of this new study have been published in the journal Nature Sustainability.
In essence, the novel material rivals some cutting-edge industrial composites and approaches the strength of Kevlar. Furthermore, silk demonstrates superior resistance to ballistic impacts compared to carbon fiber-reinforced plastic. This elevates silk to a category entirely distinct from scarves and dresses.
The creation of this new material does not involve complex synthetic chemicals or energy-intensive manufacturing processes. Instead, scientists have discovered a simpler approach to bonding natural silk fibers while largely preserving their inherent strength.
The research was spearheaded by investigators from Tufts University, Imperial College London, and the University of Michigan.
For years, engineers have explored the potential of silk in medical and electronic applications. Silk proteins are biocompatible, meaning the body generally tolerates them well. This characteristic makes silk attractive for implants, tissue regeneration, and even flexible electronic devices.
However, a challenge existed. Most current processing methods involve dissolving silk fibers into their constituent proteins and then reconstructing them. This procedure demands significant quantities of water, chemicals, energy, and time. Crucially, it also compromises the material’s strength.
“In the processing, the natural fibers are broken down into individual silk fibroin proteins and then reformed into a new shape, so we lose a significant portion of the inherent strength of the original fibers,” explained Professor Chunhai Li from Tufts University. “With this new method, there’s no need to dissolve the silk—we simply align the fibers, apply heat and pressure, and they fuse together in a single step.”
This single alteration has ultimately proven to be immensely significant. The process commences with commercially available silk fibers derived from moth cocoons. Initially, researchers remove sericin, the sticky outer coating that aids insects in constructing cocoons. Subsequently, the fibers are aligned and compressed using heat and pressure.
Within silk fibers lie two distinct protein regions. One region possesses a high degree of order and crystallinity, which imparts strength to the silk. The other region is more flexible and mobile.
“Silk is a composite material,” stated Professor David Kaplan of Tufts University. “There’s a more mobile, amorphous phase of the protein fibers, and then there’s a part of the protein chain that folds up to form sheet-like surfaces, which in turn stack into crystalline structures. Together, these give silk fibers their strength, stiffness, and flexibility. But it’s the mobile part that allows the fibers to fuse together under heat and pressure.”
The researchers identified an optimal temperature range for processing, spanning from 125 to 215 degrees Celsius and pressures from 1900 to 9800 atmospheres. At lower temperatures, the material became too porous. Excessive heat rendered the silk brittle. The final product retains much of silk’s original molecular structure, with the fibers bonded into a dense, solid material.
The resulting fused silk exhibits a layered internal structure, bearing some resemblance to wood. In both materials, bundles of fibers are oriented in the same direction, effectively distributing stress throughout the structure. This architecture endows the material with remarkable strength. The researchers reported that the fused silk surpassed both wood and bone in tensile strength.
It also withstood stress tests applied to some of the most robust structural plastics and composites in use today. Its ballistic impact performance was equally impressive.
Carbon fiber composites are extensively used in aviation, racing cars, and sporting equipment owing to their lightweight and strong properties. The silk-based material outperformed them in impact resistance. This combination of strength and low weight could pave the way for industries seeking alternatives to petroleum-based materials.
Unlike many synthetic composite materials, silk is a renewable and biodegradable resource. This material has also demonstrated intriguing optical properties.
Scientists at the University of Michigan discovered that fused silk can polarize terahertz radiation, which occupies the spectrum between infrared light and microwaves.
Terahertz waves are already employed in airport scanners and certain medical imaging systems. Researchers are also exploring their applications in future wireless communication networks.
The research team anticipates that fused silk could eventually play a significant role in 6G technology, which scientists predict will enable data transmission speeds far exceeding current 5G networks.
Polarization may also facilitate the encoding of more information within wireless signals—a capability few would associate with silk.
Researchers also investigated the behavior of fused silk when implanted in animals. The material elicited only a mild immune response, which diminished over time. Its behavior can also be modulated based on the density of the fused fibers.
Less dense variants permit cell ingress and gradual degradation. Denser variants maintain their integrity for extended periods.
“We can control the degradation rate of the material based on the application,” stated Li.
Due to its adaptability, fused silk holds potential in medicine. Temporary materials that degrade slowly are valuable in tissue engineering and regenerative medicine. More durable versions could provide support for compromised bones or joints.
“Given its strength, it could potentially be used to fix bone fractures with plates, pins, and screws,” noted Li.
Silk has existed for millennia, serving as one of nature’s most useful creations. This new iteration may unlock an entirely different future for it.
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