Mechanical properties of load-bearing biological materials

Bone, a typical load-bearing biological material, composed of ordinary base materials such as organic protein and inorganic mineral arranged in a hierarchical architecture, exhibits extraordinary mechanical properties. Up to now, most of previous studies focused on its mechanical properties under static loading. However, failure of the bone occurs often under dynamical loading. One of the key functions of load-bearing biological materials, is to protect their inside fragile organs by effectively damping dynamic impact. How those materials achieve this remarkable function remains largely unknown. An interesting question is: Are the structural sizes and layouts of the bone related or even adapted to the functionalities demanded by its dynamic performance? In this thesis, systematic finite element analysis was performed on the dynamic response of nanoscale bone structures under dynamic loading. It was found that for a fixed mineral volume fraction and unit cell area, there exists a nanoscale staggered structure at some specific feature size and layout that exhibits the fastest attenuation of stress waves. Remarkably, these specific feature sizes and layouts are in excellent agreement with those at experimentally observed in the bone at the same scale, indicating that the structural size and layout of the bone at the nanoscale are evolutionarily adapted to its dynamic behavior. Furthermore, This study shows that the nanostructure of bone very well synergizes the two mechanisms to achieve the fast stress wave attenuation which are stress wave scattering due to the protein-mineral interfaces and kinetic energy dissipation due to the viscosity of biopolymer components. Moreover, considering a self-similar hierarchical model, a theoretical approach was established to investigate the damping behavior of load-bearing biological materials in relation to the biopolymer viscous characteristics, the loading frequency, and the geometrical parameters of mineral inclusions as well as the hierarchy number. It was found that the damping properties of biological materials are greatly tuned and enhanced by the staggered and hierarchical organization of the organic and inorganic constituents. Finally, a theoretical framework was developed to establish the elastic bounds for the storage and loss moduli of various bioinspired staggered composites such as regular staggering, regular staggering with an offset, stairwise, herringbone and random staggering architectures. In a recursive way, the elastic bounds were further extended for bioinspired composites with multiple levels of structural hierarchy, and the effect of structural hierarchy was investigated; a theoretical approach was developed to estimate the damping properties of the hierarchical biocomposite at different length scales. It was found that, in comparison with other structural architectures, stairwise staggering structure generally gives higher loss viscoelasticity. The present work points out the importance of dynamic effect on the biological evolution of load-bearing biological materials.Doctor of Philosoph

On the relationship between the dynamic behavior and nanoscale staggered structure of the bone

Bone, a typical load-bearing biological material, composed of ordinary base materials such as organic protein and inorganic mineral arranged in a hierarchical architecture, exhibits extraordinary mechanical properties. Up to now, most of previous studies focused on its mechanical properties under static loading. However, failure of the bone occurs often under dynamic loading. An interesting question is: Are the structural sizes and layouts of the bone related or even adapted to the functionalities demanded by its dynamic performance? In the present work, systematic finite element analysis was performed on the dynamic response of nanoscale bone structures under dynamic loading. It was found that for a fixed mineral volume fraction and unit cell area, there exists a nanoscale staggered structure at some specific feature size and layout which exhibits the fastest attenuation of stress waves. Remarkably, these specific feature sizes and layouts are in excellent agreement with those experimentally observed in the bone at the same scale, indicating that the structural size and layout of the bone at the nanoscale are evolutionarily adapted to its dynamic behavior. The present work points out the importance of dynamic effect on the biological evolution of load-bearing biological materials.ASTAR (Agency for Sci., Tech. and Research, S’pore

Hierarchical structure enhances and tunes the damping behavior of load-bearing biological materials

One of the most crucial functionalities of load-bearing biological materials such as shell and bone is to protect their interior organs from damage and fracture arising from external dynamic impacts. However, how this class of materials effectively damp stress waves traveling through their structure is still largely unknown. With a self-similar hierarchical model, a theoretical approach was established to investigate the damping properties of load-bearing biological materials in relation to the biopolymer viscous characteristics, the loading frequency, the geometrical parameters of reinforcements, as well as the hierarchy number. It was found that the damping behavior originates from the viscous characteristics of the organic (biopolymer) constituents and is greatly tuned and enhanced by the staggered and hierarchical organization of the organic and inorganic constituents. For verification purpose, numerical experiments via finite-element method (FEM) have also been conducted and shown results consistent with the theoretical predictions. Furthermore, the results suggest that for the self-similar hierarchical design, there is an optimal aspect ratio of reinforcements for a specific loading frequency and a peak loading frequency for a specific aspect ratio of reinforcements, at which the damping capacity of the composite is maximized. Our findings not only add valuable insights into the stress wave damping mechanisms of load-bearing biological materials, but also provide useful guidelines for designing bioinspired synthetic composites for protective applications.ASTAR (Agency for Sci., Tech. and Research, S’pore

Numerical study of crack initiation and growth in human cortical bone: Effect of micro-morphology

In this study, crack initiation and growth in four different groups of human cortical bones, i.e., young, aged, diseased (osteoporosis) and treated are investigated numerically with a zero-thickness Cohesive Element Method, employing statistical realisations of randomly distributed microstructural constituents. The obtained simulation results demonstrated distinct crack paths in bones with varying microstructures, based on analysis of initiation, propagation and branching of multiple cracks, with supporting fracture toughening mechanisms. It is shown that superior mechanical properties and fracture resistance in the young and treated groups originated from both the qualitative and quantitative features of microstructural constituents

Collagen Fiber Orientation Is Coupled with Specific Nano-Compositional Patterns in Dark and Bright Osteons Modulating Their Biomechanical Properties

Bone continuously adapts to its mechanical environment by structural reorganization to maintain mechanical strength. As the adaptive capabilities of bone are portrayed in its nano- and microstructure, the existence of dark and bright osteons with contrasting preferential collagen fiber orientation (longitudinal and oblique-angled, respectively) points at a required tissue heterogeneity that contributes to the excellent fracture resistance mechanisms in bone. Dark and bright osteons provide an exceptional opportunity to deepen our understanding of how nanoscale tissue properties influence and guide fracture mechanisms at larger length scales. To this end, a comprehensive structural, compositional, and mechanical assessment is performed using circularly polarized light microscopy, synchrotron nanocomputed tomography, focused ion beam/scanning electron microscopy, quantitative backscattered electron imaging, Fourier transform infrared spectroscopy, and nanoindentation testing. To predict how the mechanical behavior of osteons is affected by shifts in collagen fiber orientation, finite element models are generated. Fundamental disparities between both osteon types are observed: dark osteons are characterized by a higher degree of mineralization along with a higher ratio of inorganic to organic matrix components that lead to higher stiffness and the ability to resist plastic deformation under compression. On the contrary, bright osteons contain a higher fraction of collagen and provide enhanced ductility and energy dissipation due to lower stiffness and hardness