Mechanical Behavior of Octopus Egg Tethers Composed of Topologically Constrained, Tandemly Repeated EGF Domains.

Whether and how intramolecular crosslinks in polymeric materials contribute to mechanical properties is debated in both experimental and theoretical arenas. The tethering threads of Octopus bimaculoides egg cases provide a rare window to investigate this question in a biomaterial. The only detectable component of the load-bearing fibers in octopus threads is a 135 kDa protein, octovafibrin, comprising 29 tandem repeats of epidermal growth factor (EGF) each of which contains 3 intramolecular disulfide linkages. The N- and C-terminal C-type lectins mediate linear end-to-end octovafibrin self-assembly. Mechanical testing of threads shows that the regularly spaced disulfide linkages result in improved stiffness, toughness, and energy dissipation. In response to applied loads, molecular dynamics and X-ray scattering show that EGF-like domains deform by recruiting two hidden length ß-sheet structures nested between the disulfides. The results of this study further the understanding of intramolecular crosslinking in polymers and provide a foundation for the mechanical contributions of EGF domains to the extracellular matrix.

Viscoelastic analysis of mussel threads reveals energy dissipative mechanisms.

Mussels use byssal threads to secure themselves to rocks and as shock absorbers during cyclic loading from wave motion. Byssal threads combine high strength and toughness with extensibility of nearly 200%. Researchers attribute tensile properties of byssal threads to their elaborate multi-domain collagenous protein cores. Because the elastic properties have been previously scrutinized, we instead examined byssal thread viscoelastic behaviour, which is essential for withstanding cyclic loading. By targeting protein domains in the collagenous core via chemical treatments, stress relaxation experiments provided insights on domain contributions and were coupled with in situ small-angle X-ray scattering to investigate relaxation-specific molecular reorganizations. Results show that when silk-like domains in the core were disrupted, the stress relaxation of the threads decreased by nearly 50% and lateral molecular spacing also decreased, suggesting that these domains are essential for energy dissipation and assume a compressed molecular rearrangement when disrupted. A generalized Maxwell model was developed to describe the stress relaxation response. The model predicts that maximal damping (energy dissipation) occurs at around 0.1 Hz which closely resembles the wave frequency along the California coast and implies that these materials may be well adapted to the cyclic loading of the ambient conditions.