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(2022)ThesisA three-dimensional multi-scale finite element analysis is performed to ascertain the effects of geometrical variations at multiple structural scales on the mechanical properties, including the stiffness, strength and onset of damage, of cortical bone. Finite element models are developed, with reference to experimental and numerical data from existing literature, to account for cortical bone’s anisotropy and viscoelastic behaviour from the most fundamental level of cortical bone consisting of mineralised collagen fibrils, up to the macroscopic bone consisting of osteons and Haversian canals. A user-defined material subroutine is developed to account for the viscoelastic and anisotropic properties of cortical bone in a three-dimensional setting, at multiple length scales. Further, the Taguchi-ANOVA statistical approach is applied to perform sensitivity analyses on the effects of geometrical parameters on the effective material properties, including stiffness and strength, of cortical bone at each structural scale. A cohesive zone based finite element model is further incorporated to examine the effects of geometrical variations on the damage onset and strength properties of cortical bone at multiple structural scales. Numerical results indicate that there is a positive correlation between the mineral volume fraction and the effective stiffness constants, as well as tensile and shear strength, at each length scale. Variations in the effective geometrical parameters at each structural scale also contributed to changes in the damage initiation sites and damage mechanisms, particularly at the lower length scales. Further, numerical results indicate that cortical bone exhibits a two-phase stress relaxation process: a fast and a slow response relaxation process, which can be mathematically represented by the Generalised Maxwell Model. Numerical results also indicate that the anisotropic and hierarchical structure of cortical bone contribute to significant changes in the stress relaxation behaviour, damage onset, and strength properties of cortical bone at each structural scale.
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(2024)ThesisDespite their wide application range owing to the high biocompatibility, conventional single network (SN) hydrogels always suffer from brittleness and weakness. To address this issue, double network (DN) gels consisting of two different polymer networks have been developed to achieve high mechanical performance. Stimulus responsiveness is another potential target for hydrogel bioapplications. Accordingly, the main aim of this thesis is to uncover some fundamental principles for tailoring the properties of tough and mechanochemically active hybrid DN hydrogels via structural control that are suitable for biomedical applications. Poly(ethylene glycol) (PEG) linked by different bonds and ionically linked sodium alginate were selected as covalent and physical networks, respectively. To understand the structure-property relationships of DN hydrogels with strong covalent bonds, PEG (meth)acrylate hydrogels with varying monomer molecular weights (MW) and architectures (linear vs. 4-arm) with and without alginate were used. Compression testing results showed that while PEG SN hydrogels behaved similarly with varied MW and stronger using 4-arm monomers, alginate reinforced DN gels were stronger and tougher when the PEG network was looser with larger MW and/or linear monomers. When using weak dynamic disulfide bonds, alginate reinforced disulfide networks using 4-arm PEG thiol (PEG4SH) with varied MW and mass fractions were investigated with the goal of achieving tough and stretchable DN hydrogels with a capacity for mechanochemical reactions. Tensile testing results demonstrated that the fracture strain and stress of DN gels benefited from looser PEG networks with lower mass concentrations and larger MW of PEG4SH monomers, while stiffness increased with a higher density of disulfide bonds. Considering the mechanochemical response, thiols produced by disulfide bond rupture were sensed by reaction with fluorophores. DN gels showed increased integrated fluorescence intensities upon stretching, demonstrating the activated response of disulfide bond rupture despite alginate reinforcement. Higher mechanochemical reaction rates were obtained from the most stretchable DN gels with looser PEG networks and less alginate reinforcement. In summary, this thesis presents a comprehensive study on how to design tough and mechanochemically active hydrogels using alginate reinforced covalent networks. These results are expected to aid the development of mechanoresponsive DN hydrogels with controllable properties for biomedical applications.