Biomineralized structural materials with functional optical properties

Abstract

Many biological structural materials exhibit "mechanical property amplification" through their intricate hierarchical composite designs. In the past several decades, significant progress has been achieved in elucidating the structure/mechanical property relationships of these materials. However, the design strategies of structural biomaterials with additional functional roles are still largely unexplored. This thesis, by selecting three unique mollusk shell model systems, explores the fundamental design strategies of multifunctional biomineralized materials with dual mechanical and optical functions: transparency, photonic coloration, and lens-mediated vision. The model systems are the bivalve Placuna placenta, the limpet Patella pellucida, and the chiton Acanthopleura granulata, respectively. By investigating the relationships between the mechanical and optical properties and the structural characteristics, this thesis uncovers novel design strategies used to integrate optical functions into mechanically-robust material systems. The high transmission property of the P. placenta shells (~99 wt% calcite), for example, is elucidated through experimental and theoretical analysis based on a light scattering model. This armor utilizes deformation twinning and additional mechanisms at the nanoscale to enhance the energy dissipation efficiency by almost an order of magnitude relative to abiotic calcite. 3D quantitative analysis of the damage zone resulting from high load indentations was performed via synchrotron X-ray micro-computed tomography, revealing the formation of a complex network of microcracks. A unique structural motif, screw dislocation-like connection centers, is identified to enable a high density of crack deflection and bridging. This thesis also leads to the discovery of a unique biomineralized photonic structure in the shell of the blue-rayed limpet P. pellucida. The photonic system consists of a calcite multilayer and underlying particles, which provide selective light reflection through constructive interference and contrast enhancement through light absorption, respectively. Lastly, this thesis presents a detailed investigation of the biomineralized lenses embedded in the armor plates of the chiton A. granulata. The image formation capability of these lenses is experimentally demonstrated for the first time. The optical performance of the eyes is studied via comprehensive ray-trace simulations that take into account the experimentally measured geometry and crystallography of the lens. Mechanical studies illustrate that trade-offs between protection and sensation are present in the plates.