Design of a multifunctional biomineralized armor system : the shell of chitons

Abstract

Nature provides many examples of flexible armor systems which may serve as a source of inspiration for materials scientists and engineers. This thesis explores multiscale material and morphological design principles of the shells of chitons (Mollusca: Polyplacophora). The chiton shell consists of eight plates encircled by a structure known as a girdle, which is often covered by scales. The shell provides protection while permitting the flexibility needed to conform to rough substrata, as well as to roll defensively into ball-like conformation to cover its soft ventral side. In typical flat conformations, X-ray micro-computed tomography revealed that the shape and imbrication of the plates results in an overall continuous curvature and constant armor thickness. However, in defensive postures, vulnerable regions exist between the plates due to decreases in plate overlap. In the peripheral scale armor, gradients in the size and overlap of the scales control local levels of flexibility and protection. Scale armor prototypes inspired by the girdle scales were fabricated via multi-material 3D printing. Bending tests demonstrated that the stiffness of the bio-inspired scale armor is highly anisotropic. Remarkably, in certain species, a visual system is integrated within the shell plates. The system contains hundreds of lens eyes, which were found to be capable to forming images. Ray-trace simulations of individual eyes determined that they have a resolution of ~9°, which is consistent with prior behavioral experiments. Unlike the protein-based lenses of most animal eyes, the lenses of chitons, like their shells, are principally composed of aragonite. Chitons are able to tailor the local shape, crystallography, and interfaces of aragonite to achieve a multifunctional armor. However, the integration of lens eyes was found to locally decrease penetration resistance, suggesting a materials-level trade-off between protection and sensation.