Artificial Spider Silk from Ion-Doped and Twisted Core-Sheath Hydrogel Fibres

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People have dreamed to have a rope that they can use to jump out of the window when there is emergency happening, for example, a fire accident. This rope must be strong enough to support people so that it does not break at high loading stress. Another important thing is that this rope can elongate to some extent so that people don’t feel too much force, because this elongation would decrease the impact force after it is straightened. Moreover, when this rope extends to the maximum length, it should not rebound like a rubber band. 
You may see such a rope in television programs, such as “Spider Man”. Actually, there is such a “rope” presenting in nature the spider silk. The spider silk has exceptional mechanical properties. They show exceptional high tensile strength (up to ~1.4 GPa), which is comparable to high-grade alloy steel (0.45 to 2 GPa), half as strong as Kevlar or Twaron (3000 MPa). Different from these rigid metal wires or aromatic fibres, which are almost non-extensile, the spider silk can be extended to ~40%, and sometimes can be extended to 5 times their initial length in a high moisture environment. 
The spider silk almost does not rebound after elongation. This makes this fibre material able to absorb a large amount of energy during elongation. The toughness averages 350 MJ/m3, which is several orders higher than steel or Kevlar fibres. Besides no rebounding, you almost never see a spider rotate when it is suspended from a silk fibre. This means that the spider silk can also absorb torsional rotational energy. Last but not least, the spider silk supercontracts to the initial length on exposure to moisture, after it is elongated. This makes it able to be automatically repaired and re-usable when it is elongated by incoming prey.
Now here comes a question, how to prepare the spider silk artificially? Although people have tried to make artificial spider silks using silk protein and get some success, it is still difficult to synthetically produce a fibre that can mimic the above properties of spider silk. In a recent article in Nature Communications, our research team, from Nankai University, China, has successfully prepared artificial spider silk from twisted hydrogel fibres using a very simple method. The hydrogel fibres are made from polyacrylic acid, which shows core-sheath structure, doped with divalent ions, and inserted with some twist. The fibres exhibit tensile strength of 895 MPa, stretchability of 44.3%, modulus of 28.7 GPa, high toughness of 370 MJ m3, and damping capacity of 95%, which display comparable mechanical properties to the natural spider silk.

Zunfeng Liu

Professor, Nankai University

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