Spider silk can absorb water and still become shorter. As humidity rises, water diffuses into the protein fiber, where it can both swell the material and disrupt bonds that help keep its molecular chains aligned. If the resulting contraction outweighs swelling, a free fiber shrinks—a humidity-driven response called supercontraction.
What happens when spider silk absorbs water?
“Soaking up water” here means water diffusing into the silk’s protein network, not just droplets resting on its surface. The added water changes more than the fiber’s mass: depending on conditions and whether the fiber can move, it can swell, soften, shorten, or twist.
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That combination can seem paradoxical. Swelling reflects water entering and taking up space in the material; shortening reflects changes to the fiber’s internal structure and the way its molecular chains are arranged. The two effects compete rather than canceling out in one predictable way.
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A proposed molecular explanation is that humidity-driven water disrupts some intermolecular hydrogen bonds. With fewer of these interactions holding the protein chains in an orderly arrangement, parts of the fiber can lose alignment and behave more like a rubber. At the same time, the fiber swells as it accommodates water. If the chain rearrangement and resulting contraction dominate, an unrestrained strand gets shorter.
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This is a supported model, not a complete explanation for every kind of spider silk. A 2022 paper by Noy Cohen and Claus D. Eisenbach describes a humidity-driven glassy-to-rubbery transition as water diffuses through the fiber, and models structural responses that include twisting. Their abstract summarizes the proposed connection: “These behaviors stem from a unique microstructure in combination with a transition from glassy to rubbery as a result of humidity-driven diffusion of water.” Read the 2022 paper in Physical Review Letters.
Composition may also matter. An experimental study of defined biomimetic fibers found that tyrosine in amorphous regions played an important role, alongside proline. Because the fibers were biomimetic and made with defined proteins, that finding should not be treated as a complete account of the chemistry in native spider silk. See the study of biomimetic fibers.
What is supercontraction?
Supercontraction is the substantial shortening of an unconstrained spider silk fiber as humidity increases. “Unconstrained” is crucial: if the fiber is free to move, it can shorten. If it is held at a fixed length, it cannot contract in the same way, so the response appears as stress in the fiber instead.
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The reported maximum shortening is large, but it is not an everyday or universal result. Cohen and coauthors’ 2021 paper describes spider silk as capable of decreasing in length by up to approximately 60% under particular conditions. Blackledge and coauthors’ 2009 study describes unrestrained dragline silk shortening by up to 50% of its original length. These are study-specific upper values, not a prediction for any strand at any humidity.
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In the restrained-fiber experiments reported by Blackledge and coauthors in 2009, supercontraction stress ranged from 10 to 140 MPa across tested conditions. These stress values describe fibers prevented from shortening; they are not interchangeable with percentage changes measured in free fibers. Read the 2009 study in Macromolecules.
Why do humidity and hydration rate matter?
There is no single humidity threshold established for all spider silk. In one 2009 experiment, substantial stress was reported above approximately 70% relative humidity, but the same study found that stress varied with how quickly humidity increased. Rapid increases typically produced higher stress than slower hydration.
Hydration history therefore matters alongside the final humidity. A strand held at a fixed length and exposed to a rapid humidity rise may develop a different stress response from one hydrated more gradually. Differences in silk type, whether the fiber is free or restrained, the humidity level, and the speed of humidification all affect what a study measures.
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Not necessarily. Some ordinary humidity cycles of wetting and drying can be reversible, but supercontraction can leave persistent changes. Blackledge and coauthors reported an approximately 1% residual mass increase after supercontraction and drying in their study. That result is specific to their measurements; it does not mean every silk fiber retains the same amount of water or change after drying.
To compare results, distinguish measurements taken during a wet/dry cycle from those taken after supercontraction and drying. Also check whether the fiber was free to shorten or held at length, and how quickly it was humidified: those conditions describe different responses, not conflicting measurements of one fixed behavior.
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