Humidity-Driven Supercontraction and Twist in Spider Silk

Spider silk is a protein material that exhibits extraordinary and nontrivial properties such as the ability to soften, decrease in length (i.e., supercontract), and twist upon exposure to high humidity. 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. In this Letter we propose four length scales that govern the mechanical response of the silk during this transition. In addition, we develop a model that describes the microstructural evolution of the spider silk thread and explains the response due to the diffusion of water molecules. The merit of the model is demonstrated through an excellent agreement to experimental findings. The insights from this Letter can be used as a microstructural design guide to enable the development of new materials with unique spiderlike properties.

Figure 1

Structure model of spider dragline silk; four helical fibrillar strands are highlighted in color for clarity. (a) Glassy state of natural dry fiber; (b) transient states during water absorption showing a contracted and a twisted thread (clockwise torsion); (c) fully rubbery state.

Figure 2

Comparison of the model predictions (continuous lines) to experimental findings (circle and square marks). (a) Supercontraction [33] and (b) supercontraction stress [39] that develop in N. Clavipes. Twist of (c) N. pilipes and (d) A. versicolor [29].