Fibrous microstructure and biomechanics of healthy and diseased aortic tissues

Typical cardiovascular diseases are mostly asymptomatic until fatal consequences are caused. Patient-specific simulations have the potential to guide clinical diagnosis but are impeded by the missing patient-specific material properties. To close this gap, this dissertation explored the role of the two most significant loading components, collagen and elastin, in defining mechanical responses of artery walls. These associations are crucial for understanding the mechanical remodelling of diseased tissues and the potential failure mechanisms implicated. These aims are achieved through experiments and numerical approaches. Firstly, collagen and elastin fibre parameters of human aortic dissection flaps were extracted from histology slides. Their material properties and relationships with fibrous parameters were also studied. Secondly, a computational framework based on the unsupervised deep learning UNet model was proposed to characterise the heterogeneity of vessels. Lastly, a simulation framework was developed and used for parameter studies to estimate artery material performances according to a few critical collagen fibre parameters. The results demonstrated that fibre dispersion and waviness in the aortic dissection flap changed with patient age and clinical presentations, and these changes can be captured by the material constants in the strain energy density function. Additionally, high material heterogeneity was characterised by the localised strain maps and simulations. Strain maps of axial strips demonstrated a zebra pattern with vertical high strain concentrations due to fibre configurations when subjected to uniaxial tensile tests. Furthermore, based on the fibrous parameters obtained in the preceding study, the proposed simulation method predicted strain-stress curves that fit well with experiments. Parameter studies using this methodology proved that the fibre recruitment efficiency dictates tissue mechanical performances and largely depends on the primary fibre orientation regarding the loading direction. In addition, collagen fibre waviness determines the starting point of nonlinearity in the strain-stress responses of arteries. In summary, the remodelling of collagen and elastin fibres within artery walls can explain the clinical observations and is mechanically significant in predicting the adjustments of material performances.