dc.description.abstract
Bone mechanobiology comprises all the processes by which bones 'sense' and 'react on' mechanical loading, through the corresponding activity of biological cells and biochemical factors. In this context, the transfer of mechanical loads from the macroscopic scale down to the cellular level is governed by the hierarchical interaction of bone, as well as its mechanical properties; thereby, elasticity and porosity play a particularly eminent role. The latter two quantities, shortly reviewed in Chapter 1, as well as the interdependencies of these properties and their relationship with bone mechanobiology are investigated in the present thesis, by means of experiments and computer simulations. Notably, both approaches are guided by the concept of multiscale continuum (poro)micromechanics, an essential theoretical framework when dealing with a multiscale, hierarchically structured material such as bone. In Chapter 2, a multiscale mathematical model for simulation of bone remodeling is presented, describing the porosity-specific processes and relationships between bone cells, biochemical factors, and mechanical loads occuring at the level of the vascular and lacunar pores. Particularly, the mechanical stimuli acting on the bone cells involved in bone remodeling are quantified in terms of hydrostatic pore pressures, estimated from the macroscopic loading by means of a continuum (poro)micromechanics representation of bone. The model is then validated quantitatively and qualitatively with experimental data from literature, showing the infuence of different mechanical loading conditions on bone adaptation for various animal species. Chapters 3 and 4 deal with determination of the elastic modulus of bone by means of a new method which, based on the concept of statistical nanoindentation and an evolutionary algorithm, can distinguish between damaged and undamaged material phases - or, more generally, between indents where the elastic half space theory applies, or not (e.g., due to the presence or initiation of microcracks). More precisely, in Chapter 3, the elastic modulus of undamaged, cortical bone, at the scale of the extracellular matrix, is determined throughout different plane sections through the midshaft of a human femur, and the differences in stiffness between endosteal and periosteal regions, as well as between loaded and not loaded areas are investigated. In Chapter 4, Young-s modulus of intact bovine extracellular femur bone is investigated. In both chapters, the hypothesis that nanoindentation testing may also deliver elasticity values related to damaged material is checked, by imaging microcracks with a Scanning Electron Microscope (SEM). Finally, in Chapters 5 and 6, experimental methods are employed for determination of the mechanical properties of ceramic materials for bone tissue engineering scaffold production, namely baghdadite (Ca3ZrSi2O9) and Bioglass®. Ideally, such scaffolds should reproduce the properties of bone as closely as possible. In the case of baghdadite, scaffolds seeded with bone cells have shown good biological properties in vivo, but research on their mechanical properties are scarce. In Chapter 5, by means of statistical nanoindentation combined with ultrasonic tests, the elasticity of porous baghdadite is characterized across a wide range of material porosities. In the case of Bioglass® scaffolds, mechanical properties have been measured before, and require improvement in order to come close to those of trabecular bone. The study in Chapter 6 investigates, by means of multiscale ultrasound-nanoindentation measurements, the possibilities of enhancing the stiffness of these scaffolds by coating them with various types of polymers.
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