Heterogeneous Distribution and Corresponding Mechanical Significance of The Mineral Phase in Fish Scales

Fish scales can be considered as a laminated composite based on collagen fibrils arranged in a cross-plywood structure. This collagen-based composite is often partially mineralized (primarily hydroxyapatite) in the scale exterior in order to resist penetration and hence to enhance protection. Together with the overlapping assembly, the fish scales offer an excellent model system for developing fiber composite materials and flexible armor systems. The primary objective of this thesis is to characterize the distribution of the mineral phase within individual scale and to investigate the corresponding mechanical consequences of the scale as a whole and its different fields through experimental and computational approaches. In this thesis, we chose the scales from the black drum (Pogonias cromis) fish as a model system. First of all, the exterior surface morphology of individual scales was systematically studied, from which several distinct structural regions are identified, including focus field (central), lateral field (dorsal and ventral), rostral field (anterior), and caudal field (posterior). In the focus field, the classic two-layer design, i.e., mineralized exterior layer and collagen-based interior layer, was observed, and nanoindentation results revealed that the high mineral exterior layer results in a much higher hardness (800 vs 450 MPa). Moreover, macroscopic tensile tests indicate that the mechanical removal of mineralized layer did not lead to reduction in strength values, whereas acid-treated demineralized scales showed reduced mechanical properties. Finally, we identified a previously unreported mineral distribution pattern in the rostral field, in which the mineral phase is segregated into long strips along the anterior-posterior direction (width, ~300 μm). In addition, towards the interior of the scale, it appears that the mineral deposition is highly correlated with the collagen orientation, resulting a unique mineralized-unmineralized collagen-based composite structure. We built finite element models to compare this unique structure to two other mineral phases in different fields at the individual scale. This unique structure demonstrates a larger deformation displacement when load was applied, indicating that it provides further flexibility in anterior end of an individual scale. The mineralized phases and structures of various fields within a single scale provide different mechanical characteristics and properties. The structural and mechanical analysis of the various regions of the fish scale can further investigate the flexibility and protective capacity of the individual scale.