Scientists have discovered a previously unknown property of spider silk, and used it to create a remarkable new “hybrid” material. The new bio-inspired thread, which acts like both a solid and a liquid, could lead to a host of new materials and technologies.

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Spider silk is one of the most extraordinary materials found in nature, featuring tensile strengths comparable to steel, and elasticity commensurate with rubber (at least on a weight-to-weight basis). Together, these two properties make it two to three times tougher that some of the strongest synthetic materials, including Kevlar and nylon. What’s more, spider silk is sticky (to catch unsuspecting prey), antimicrobial, hypoallergenic, and biodegradable. So scientists and engineers would like to understand as much about it as possible to develop similarly robust synthetic materials.

In this video, the thread can be seen contracting in a process that’s very liquid-like. (Credit: Hervé Elettro et al., 2016)

New research conducted by scientists at the University of Oxford and the Université Pierre et Marie Curie, Paris, France, has uncovered yet another remarkable attribute. The capture silk produced by orb spiders (a common garden spider) is always taut, even after it has been stretched to many times its original length. What’s more, it contracts in a way that’s quite liquid-like.

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When spider silk is compressed, its filaments shorten by spooling inside tiny droplets of watery glue that cling to the threads, allowing the web to remain under tension. The process is reversible, allowing the thread to be extended. These findings now appear in the Proceedings of the National Academy of Sciences.

“The thousands of tiny droplets of glue that cover the capture spiral of the spider’s orb web do much more than make the silk sticky and catch the fly,” said study co-author and Oxford scientist Fritz Vollrath in a statement. “Surprisingly, each drop packs enough punch in its watery skins to reel in loose bits of thread. And this winching behaviour is used to excellent effect to keep the threads tight at all times, as we can all observe and test in the webs in our gardens.”

Vollrath and his team were inspired by this “liquid wire” to create their own composite fibers in the lab. They tapped into the delicate and subtle balance that exists between fiber elasticity and droplet surface tensions. The resulting artificial spider silk worked just like the spider’s natural winch silk; the spools of filament reeled and unreeled inside the oil droplets as the thread expanded and contracted. The material is considered a hybrid because it extends like a solid and compresses like a liquid. (The process is similar to what’s observed when water droplets come into contact with one another.)

A thin polyurethane fiber instantly turns into a liquid wire when coated with oil droplets. (Credit: Hervé Elettro et al., 2016)

Eventually, these hybrid threads could lead to advancements in materials science, engineering, and medicine.

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“Our bio-inspired hybrid threads could be manufactured from virtually any components,” said first author Hervé Elettro. “These new insights could lead to a wide range of applications, such as microfabrication of complex structures, reversible micro-motors, or self-tensioned stretchable systems.”

[PNAS]