Nanoindentation of Bio- and Geo-mineralized Composites: Contribution of Microstructure and Composition

摘要

Bio-mineralized composite tissues, such as bone and teeth, are heterogeneous in both mineral composition and crystallinity. These tissues are altered throughout life by aging, wear, microfracture, and various disease states (e.g., osteoporosis), and are then further altered geologically after death by fossilization to create geo-mineralized tissues. Bone and enamel exhibit a wide range of mechanical responses at nanometer-length scales, where large-scale porosity and macro-structural variation are not factors. Some variability seen in mineralized tissues can be attributed to the amount of mineral; where a general increase in mechanical properties occurs with increasing mineral volume fraction. However, a large range of modulus values for bone is observed at a constant mineral content indicating that both the composition and microstructure play a vital role in the nanomechanical response. This dissertation is aimed at understanding the nanomechanical properties of these heterogeneous mineralized composites in order to elucidate the interplay between composition, microstructure, and tissue mechanical behavior.A combined approach using nanoindentation testing and complimentary techniques, such as X-ray diffraction, Fourier transform infrared spectroscopy, and quantitative backscattered electron microscopy (qBSE) are used to investigate the effect of variations in crystallography, microstructure, and mineral composition on the nanomechanical properties of these materials. Further, development of novel qBSE glass standards allow for site-matched measurement of mineral volume fraction and nanomechanical properties. Additionally, a finite element analysis (FEA) allows for isolation of individual parameters and their contribution to the nanomechanical properties. The relative contribution of the composition and microstructure are explored through two experimental model systems of bone fossilization and lemur enamel.In fossilization, or diagenesis, composition is altered over geologic time as minerals are incorporated into pore spaces within the bone. Fossilized bone samples demonstrate a larger range of mineral composition, mineral volume fraction, and crystallinity than is found in modern samples. Nanoindentation revealed that anisotropy of modern bone can be preserved in fossil bones going back at least to the early Eocene (approximately 50 million years). Further, both increased crystallinity and density correlated with increased modulus values, suggesting that size of bioapatite crystals contribute to the mechanical properties. Nanoindentation is useful in investigating tissue-level diagenesis in bone, and can provide insight into the functional significance of mineralized tissues even after diagenesis has occurred.Variations in microstructure and mineralization were examined in the enamel of three lemur species Lemur catta, Lepilemur leucopus, Propithecus verreauxi, and Homo sapiens. Nanoindentation revealed a natural gradation of mechanical properties where a 2-12% increase in modulus and hardness correlated to increased mineral content (p u3c 0.001) measured by qBSE. Enamel microcracking in Lemur catta resulted in a 49% reduction in nanomechcanical properties at the occlusal (or chewing) surface of the tooth. FEA modeling demonstrated a similar decrease in modulus values for indentation within 20 microns of a crack. Variations in enamel microstructure and microcracking in lemur species enables study of the interplay between tissue microstructure and nanomechanical properties, and further explores variations with diet.The investigation of nanomechanical property dependence on microstructure and mineral composition in two experimental model systems combined with FEA is used to understand the fundamental mechanical behavior of biological heterogeneous composite materials. Understanding the interplay between material structure and function in biomineralized composites will help to elucidate the relative contributions of various factors to nanomechanical behavior and will ultimately lead to improved development of biomimetic materials.