Nanostructure and nanomechanics of stomatopod cuticle

Abstract

Crustacean cuticle has attracted extensive attention for biomimetic purposes due to its outstanding mechanical properties including high toughness and stiffness. The mantis shrimp (stomatopod) telson is an extreme example, structurally optimized for dynamic loading at high impact forces. Alpha-chitin fibrillar building blocks play a key role in determining the overall mechanical properties due to its hierarchical design across all length scales. Synchrotron X-ray diffraction combined with in-situ mechanical testing was employed to investigate the structural and mechanical responses in telson cuticle at different hierarchical levels. The stomatopod tergite was used as a reference to learn how the structural and mechanical design will be correlated to the morphological changes in telson. In the thesis, we developed a novel three dimensional fibre orientation reconstruction method using mathematical models. The method was used to determine the orientation and intensity distributions of both the in-plane and out-of-plane mineralised chitin fibres from the two dimensional synchrotron X-ray diffraction scans collected from telson and tergite samples. Subsequently, in-situ tensile tests were performed on tergite cuticle to study the deformation and reorientation mechanisms of chitin fibres in the Bouligand layers comprising the bulk of the tergite, by tracing the variation of lattice spacing of the (002) peaks (along fibril direction) and (110) peaks (perpendicular to the fibril) found in the X-ray diffraction spectrum of α-Chitin. Finally, in-situ four-point bending tests were conducted to study how morphological changes contribute to the nanomechanical properties of telson across the entire tissue 4 cross-section. Pre-strains were identified in the cuticle in static states, and vary according to the morphological position. Under loading, fibrils near physiological impact site deformed more than those in adjacent region, demonstrating different roles in energy dissipation in telson. These above findings provide useful guidelines for designing synthetic composite materials that resist repetitive impact loading.