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(2023)ThesisBiodegradable implant materials are more appropriate for temporary support applications compared with their inert counterparts since the former requires no removal surgery because they naturally degrade and eventually dissolve completely during healing. Iron and its alloys are a possible substitute for the commercial magnesium biodegradable implants because of their superior mechanical properties and slower corrosion rates. The addition of manganese and silicon in iron imparts another interesting property to the material–the shape memory effect. There is copious research on the structure and properties of the biodegradable face centred cubic (FCC) Fe-30Mn-6Si shape memory alloy (SMA) that exhibits the reversible FCC austenite to hexagonal close packed (HCP) ε-martensite transformation. However, recent advances in additive manufacturing of metals, brought by the development of the laser powder bed fusion (LPBF) technique, warrant the need for an investigation on the adaptability of the technique in fabricating this alloy composition. The LPBF technique is limited by the need for specialty raw material powder, and this thesis extends the application of the technique in fabricating the Fe-30Mn-6Si shape memory alloy (SMA) from homogenised powder precursors. More so, LPBF processing of Fe-30Mn-6Si alloy from either pre-alloyed powder or blended powder has not been reported. To successfully fabricate a Fe-30Mn-6Si LPBF product, the influence of key LPBF processing parameters on product quality was identified as a major challenge. This was addressed by investigating the influence of laser power, laser scan speed, laser re-scanning, and their equivalent input energy on the relative density and defect formation. A relative density of over 99% with few processing defects was achieved using the optimised parameters of 175 W laser power, 400 mm/s scan speed, and no re-scanning. The influence of these parameters on the solidification microstructure was also investigated using key techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Further, the simulated thermal profile of the melt pool region as a function of process parameters via single scan track experiments was calculated using the finite element method (FEM). These data were used to explain the key microstructural features observed in the as-solidified microstructure of the LPBF alloy as a function of the processing parameters. The mechanical properties of the LPBF alloy were then assessed by hardness and tensile testing and then compared with a reference alloy produced by arc melting. The hardness of the LPBF as-built alloy was ∼20% higher than the reference alloy. To identify the factors affecting the increased hardness of the former, the influence of grain size and morphology, crystallographic texture, phase constituents (mainly austenite and martensite), and residual strain were investigated. The hardness of the reference alloy was affected mainly by the grain size and residual strain, but for the LPBF-built alloy, the relative volume fractions of austenite and martensite strongly influenced the hardness. Meanwhile, the tensile properties of the LPBF alloy, such as the yield stress, ultimate tensile stress, and ductility, were adversely affected by the internal defects present, such that high temperature homogenisation and hot isostatic pressing (HIP) post-process treatments were investigated to improve these properties. The homogenisation and HIP treatments increased both the tensile strength and ductility of the LPBF-built alloy. Homogenisation altered the grain morphology by promoting recrystallisation and grain growth, and this increased the tensile strength by ∼80%. The hardness, however, decreased due to a reduction in the volume fraction of HCP martensite in the FCC austenitic microstructure. HIP retained some of the columnar microstructure generated by the LPBF process, marginally increased the density, and increased the tensile strength by ∼65%. The improvement in tensile properties through these post-process treatments allowed for the measurement of LPBF alloy’s shape memory behaviour, whereby a tensile recovery strain of 2% was achieved for the HIP-treated alloy. Finally, the biocorrosion behaviour of the LPBF-processed and HIP-treated alloy was investigated, whereby the in vitro corrosion potential and current density of the alloy were determined to be -769 mV and 5.6 μA/cm2, respectively, indicating a reasonable corrosion rate for this material. Overall, this thesis enabled the first demonstration of the shape memory effect in an LPBF-built Fe-based alloy fabricated from homogenised powder, an alloy which also exhibits biodegradable properties.
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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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(2023)ThesisThe high cooling rates in metal additive manufacturing (AM) of high-entropy alloys (HEAs) can not only prohibit the formation of intermetallic compounds and detrimental elemental segregation but also facilitate microstructural refinement, thereby providing improved mechanical properties. However, there are still many challenges, including the presence of AM fabrication defects and residual stresses as well as anisotropic properties. Thus, further understanding is needed regarding applying post heat treatment to release residual stresses in as-built HEAs, and the techniques that can be employed to reduce or minimize the property anisotropy in as-built HEAs. In this thesis, a model Cantor alloy was fabricated and subject to a series of post heat treatment conditions. The results showed that annealing at a temperature lower than 900 °C gave rise to the formation of M23C6 carbides while recrystallization took place at temperatures over 900 °C. Interestingly, three distinct types of microstructures were exhibited at the annealing temperature of 900 °C. This is primarily attributable to the different thermal histories undergone through the LPBF process among these areas where heterogeneity in chemical elements and microstructure was revealed. The second group of samples were fabricated to develop a machine learning coupled microstructure mapping approach for tailoring mechanical property anisotropy. Three distinct types of microstructures were generated by varying the laser power and scanning speed, namely, herringbone, bimodal, and columnar microstructures. Epitaxial grain growth and side-branching in the melt pool center and overlapped regions are the main reasons responsible for the different microstructures. The underlying mechanisms were discussed in terms of effective grain size, heterogeneous microstructure, and the twinning induced plasticity (TWIP) effect. Subsequently, pre-deforamtion was applied before post heat treatment to study the influence of grain boundary engineering (GBE) on the recrystallization and resultant deformation behavior. Twinning-assisted recrystallization nucleation accelerated the formation of a nearly a fully recrystallized microstructure in the heavily deformed sample. As a result, the present strategy of GBE can provide further insight regarding the manipulation of the post heat treatment in as-built alloys particularly for those precipitation hardenable alloys where precipitation kinetics can be varied.
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(2023)ThesisTissue engineering aims to create functional tissues by cultivating cells in a laboratory setting. A primary area of focus to achieve that objective is the development of scaffolds capable of providing a suitable environment for cellular adhesion, growth, and the execution of fundamental cellular functions to establish tissue-scale properties. However, scaffold systems in the laboratory do not benefit from the dynamic forces that are exerted on tissues in an organism. In pursuit of this aim, the overall objective of this thesis was to develop a tissue engineering scaffold system mimicking the natural tissue-like environment, with in-built capabilities for external control of dynamic mechanical properties to modulate cell differentiation. We first developed a magnetic nanoparticle-loaded hydrogel system, where the modulus of the hydrogels can be reversibly altered by applying a magnetic field. To demonstrate versatility, we have used two popular hydrogel systems broadly used in tissue engineering: poly (ethylene glycol dimethacrylate) and gelatine methacryloyl. We analysed the effects of the field-induced change in stiffness on cell behaviour upon the attenuation of a magnetic field. Our studies demonstrate that adipose-derived stem cells (ADSC) and embryonic muscle cells (C2C12 cell line) can perceive these stiffness changes and differentiate towards myofibroblast and myoblast, respectively. We then developed a composite hydrogel system to segregate the magnetic particles within gelatin fibres, which simultaneously provides nanotopography to the adherent cells. We used electrospinning to synthesize magnetic gelatin nanofibers containing 5 wt/v% iron oxide nanoparticles. This concentration was selected to ensure maintenance of fiber morphology while simultaneously ensuring magnetic response. To stabilize the nanofiber structure, we used a crosslinking method involving citric acid and high temperature to stabilize the gelatin via amide bonds between strands. Introducing a magnetic nanofiber mat at the interface of the hydrogel system provides remote actuation of the nanotopography through an external magnetic field. We found that the nanotopography alone directed adipogenesis, while mechanical actuation of the interface drove osteogenesis in adherent ADSCs. The adhesion characteristics suggest that the field influences the nanofiber structure, greatly enhancing focal adhesion. The field induced actuation was also found to stimulate the formation of aligned multinucleated myotubes and markers associated with maturation in adherent C2C12. Finally, we integrated the magnetic nanofiber into hydrogels as a modular system that closely resembles the fibrous network in the natural extracellular matrix. These hydrogels can be reversibly stiffened in response to external magnetic fields within cell-laden 3D constructs. Including a small fraction of short nanofibers (<3%) can significantly influence ADSC and C2C12 differentiation. As before, nanotopography was beneficial to adipogenesis while stiffening promoted enhanced osteogenesis and myogenesis. Together, this body of work provides a modular platform with broad versatility in format to study the effects of nano-topography and dynamic mechanics in cell systems. Moreover, these hydrogels and the magnetic components are cytocompatible with scope for inclusion in tissue bioreactors as a means for dynamic stimulation of cell differentiation for tissue engineering.
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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.