Spiders are well known for making the strongest natural silks on Earth, but, understandably, harvesting it from them isn’t commercially viable. Spiders need a lot of space to make their webs, don’t produce large quantities of silk, and are too aggressive and territorial to be farmed together.
Now, scientists might have a new solution to this dilemma. They have developed a new strategy for re-spinning silkworm silk fibres, creating artificial silk that’s 70% stronger than spider silks.
According to a new study published in Matter, it took a combination of removing the sticky outer layer of the silk and solidifying the fibres in special baths of chemicals.
“Our finding reverses the previous perception that silkworm silk cannot compete with spider silks on mechanical performance,” says senior author Dr Zhi Lin, a biochemist at Tianjin University, China.
To the authors’ knowledge, this is the first report ever on the spinning of artificial silk that significantly surpasses natural spider silks in strength and stiffness.
When you think of silk, the first thing that likely comes to mind is a luxurious piece of clothing or fabric.
But today, silk-based materials can be found in a multitude of applications – from biomedicine as a material for stitches and surgical mesh, to silk patches to monitor health, and even protective coatings for growing seeds.
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This is because silk has incredible mechanical, biocompatible, and biodegradable properties.
The most common method to procure silk is through farming silkworms, but these silks are less durable and weaker than those spun by spiders – specifically, spider dragline silks which perform really well under high tension.
“Dragline silk is the main structural silk of a spider web. It is also used as a lifeline for a spider to fall from trees,” says Lin.
Other research in improving silkworm silk has involved genetically engineering silkworms with spider genes to produce silk with more desirable properties.
Lin’s group wanted to use commercialised mulberry silkworms (Bombyx mori) because they’re more accessible and easily managed, but natural silkworm fibre is made of a core wrapped by a silk glue which interferes with the spinning of the fibres for commercial purposes.
The team determined the best method to dissolve the glue, while also minimising the degradation of the silk proteins, by boiling it in a special bath of chemicals. Then, to solidify the silk back into continuous fibres, they tested the silk proteins in baths containing different metals and sugars to find the method that produced the best rate of fibre formation.
The best coagulation bath contained two metal ions – Zn2+ and Fe3+ – with 6% sucrose at room temperature.
After the silks were manually spun and drawn, the diameter of the silk was reduced from 15 micrometres (μm) down to about 4-6μm – nearly the same diameter as spider dragline silk (3-5μm).
Testing the mechanical properties of this silk revealed a tensile strength – the maximum stress that a material can withstand while being stretched or pulled before breaking – of 2000 MPa, which is more than 70% higher than that of spider dragline silks.
(Steel ranges from about 295 – 450MPa but depending on treatment can rise to 2400 MPa.)
The silk is also extraordinarily stiff, significantly more so than any known natural silks.
The authors say that the key to its excellent mechanical properties lies in its high crystallinity and the small nanocrystals that are formed in the artificial silk fibre. They also suggest that incorporating zinc metal ions (Zn2+) into the fibre may also be contributing to its strength.
“We hope that this work opens up a promising way to produce profitable high-performance artificial silks,” Lin says.