Abstract
The underwater silk of an aquatic casemaking caddisfly larvae (Hesperophylax occidentalis) is viscoelastic, and displays distinct yield behaviour, large strain cycle hysteresis and near complete recovery of its initial strength and stiffness when unloaded. Yield followed by a stress plateau has been attributed to sequential rupture of serial Ca2+-cross-linked phosphoserine (pS) β-domains. Spontaneous recovery has been attributed to refolding of the Ca2+/pS domains powered by an elastic network. In this study, native Ca2+ ions were exchanged with other metal ions, followed by combined mechanical and FTIR analysis to probe the contribution of pS/metal ion complexes to silk mechanical properties. After exchange of Ca2+ with Na+, the fibres are soft elastomers and the infrared spectra are consistent with Cv3 symmetry of the – groups. Multivalent metal ions decreased the – symmetry and the symmetric stretching modes (vs) split in a manner characteristic of ordered phosphate compounds, such as phosphate minerals and lamellar bilayers of phosphatidic acid lipids. Integrated intensities of the vs bands, indicative of the metal ion's effect on transition dipole moment of the P–O bonds, and thereby the strength of the phosphate metal complex, increased in the order: Na+ < Mg2+ < Sr2+ < Ba2+ < Ca2+ < Eu3+ < La3+ < Zn2+ < Fe2+. With a subset of the metal ion series, the initial stiffness and yield stress of metal ion-exchanged fibres increased in the same order: establishing the link between phosphate transition dipole moments and silk fibre strength.
Highlights
Aquatic caddisfly larvae use sticky silk fibres like adhesive tape to construct protective shelters with species-specific designs using species-specific materials [1]
The work of elongation to fracture of approximately 17.3 + 6.2 MJ m23 is impressive for a fully hydrated biphasic material; the silk is tougher than tendon collagen [3], articular cartilage and the best synthetic double network hydrogels [4,5]
The abrupt softening at low strains is transient; silk fibres cyclically strained between 0% and 20% elongation fully and repeatedly recover their initial stiffness and yield stress within 120 min after unloading [6]
Summary
Aquatic caddisfly larvae (order Trichoptera) use sticky silk fibres like adhesive tape to construct protective shelters with species-specific designs using species-specific materials [1]. Casemakers, such as the species used in this study (Hesperophylax occidentalis), tape together adventitiously gathered stones into transportable body armour. The fibrous core is stiff (80–200 MPa) at low strains but dramatically softens, or yields, at 2–4% elongation, after which the stress plateaus. The abrupt softening at low strains is transient; silk fibres cyclically strained between 0% and 20% elongation fully and repeatedly recover their initial stiffness and yield stress within 120 min after unloading [6]. Significant for the caddisworm’s lifestyle, the mechanical yield and stress plateau of the fibres shields the adhesive joints between fibre and substrate from irreversible damage, and repeated dissipation of strain energy without degradation of the silk’s mechanical properties maintains the integrity of the composite case in its turbulent mountain stream environment
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