How super is supercontraction? Persistent versus cyclic responses to humidity in spider dragline silk

Abstract

SUMMARYSpider dragline silk has enormous potential for the development of biomimetic fibers that combine strength and elasticity in low density polymers. These applications necessitate understanding how silk reacts to different environmental conditions. For instance, spider dragline silk 'supercontracts' in high humidity. During supercontraction, unrestrained dragline silk contracts up to 50% of its original length and restrained fibers generate substantial stress. Here we characterize the response of dragline silk to changes in humidity before, during and after supercontraction. Our findings demonstrate that dragline silk exhibits two qualitatively different responses to humidity. First, silk undergoes a previously unknown cyclic relaxation-contraction response to wetting and drying. The direction and magnitude of this cyclic response is identical both before and after supercontraction. By contrast, supercontraction is a 'permanent' tensioning of restrained silk in response to high humidity. Here, water induces stress, rather than relaxation and the uptake of water molecules results in a permanent change in molecular composition of the silk, as demonstrated by thermogravimetric analysis (TGA). Even after drying, silk mass increased by ~1% after supercontraction. By contrast, the cyclic response to humidity involves a reversible uptake of water. Dried, post-supercontraction silk also differs mechanically from virgin silk. Post-supercontraction silk exhibits reduced stiffness and stress at yield, as well as changes in dynamic energy storage and dissipation. In addition to advancing understanding supercontraction, our findings open up new applications for synthetic silk analogs. For example, dragline silk emerges as a model for a biomimetic muscle, the contraction of which is precisely controlled by humidity alone.

Spider silk as a novel high performance biomimetic muscle driven by humidity

“…However, cyclic contraction is clearly a phenomenon distinct from supercontraction (Blackledge et al, 2009). It is repeatable both before and after supercontraction, and infiltration of water causes the relaxation phase.…”

Environmental response and adaptation of glycoprotein glue within the droplets of viscous prey capture threads from araneoid spider orb-webs

“…This phenomenon of supercontraction is well documented for dry dragline threads Blackledge et al, 2009;Boutry and Blackledge, 2010;Work, 1981), but has not been studied in viscous threads. Although it might appear that the already water-covered axial lines of viscous threads would not further supercontract, when immersed in water they do (Gosline et al, 1999) and become more taut.…”

The role of mechanics in biological and bio-inspired systems

“…Systems are further classified by colour in terms of the mechanism of their activation and include chemical (red) and electrical (blue) active mechanisms, as well as hygroscopic (green) and thermal (yellow) passive mechanisms. The specific systems included in this figure (abbreviations used in the figure are shown in parentheses) are the sea cucumber dermis 9 , venus fly trap (VFT) 69 , human muscle contraction 59 , pH-activated polymers (pH) 59 , liquid crystal elastomers (LCE) 99 , shape-memory alloys (SMAs) 63 , mimosa plant 64 , legume forisomes 62 , shape-memory polymers (SMPs) 67 , electroactive polymers (EAPs) 59 , tree branch movement 57 , seed opening 110 , carbon nanotube (CNT) muscles 65 , electrically activiated hydrogels 61,66 , coiled nylon and polyethylene (PE) muscles 60 , cardiomyocytes 68 and spider silk 58 .…”