Effect of microstructure on mechanical behaviour of arterial tissue

The total number of annual deaths caused by cardiovascular diseases (CVDs), aortic aneurysms (AAs) and aortic dissections (ADs) is more than from any other cause. Most CVDs are a result of atherosclerosis, in which a fibrous cap covered lipid-rich plaque is formed in the arteries. The fibrous cap can rupture, blocking the artery and limiting the flow of blood to vital organs. AAs and ADs manifest as localised dilatations of the aorta, while simultaneously weakening the aortic wall. In atherosclerosis, AAs and ADs disruptions of the fibrous network and incorporation of microstructural defects make the artery vulnerable to rupture. This dissertation is aimed at describing the relationship between microstructure and mechanical, including fracture behaviour of arterial tissue. Unnotched and notched mechanical tests were performed on tissue samples from aneurysm-affected aortas. Mechanical parameters of ultimate material strength and extreme extensibility were measured, showing arterial layer and direction dependent differences in both unnotched and notched test. Similar differences were observed in fracture parameters of J-integral and crack tip curvature at failure. Interestingly, aneurysmal tissues were found to be notch-insensitive as the notched samples did not fail at a low stretch level. Histopathological analysis was performed on the mechanically tested tissues to investigate the influence of collagen, elastin, macrophage and glycosaminoglycan (GAG) contents, as well as collagen fibre dispersion on the mechanical and fracture behaviours. Both collagen and GAGs were associated with tissue strength, while higher GAG deposition was found to result in larger local collagen fibre dispersion in media and adventitia layers, but not in the intima. Stress-stretch behaviour of healthy, aneurysmal and atherosclerotic tissues were characterised with the modified Mooney-Rivlin constitutive material model. Curve fitting was used to compute the corresponding values of the model constants for each tissue type. Fitted material constants were found to differ amongst distinct types of tissue, while histological analyses showed that material constants were associated with waviness and dispersion of collagen fibres. To study the stretch driven microstructural reorganisations of the fibrous network, tensile tests with unnotched and notched tissue strips from porcine carotid arteries were performed while imaging with multiphoton microscopy. In general, fibres rotated towards the direction of stretch, where in notched samples, fibres rearranged themselves to redistribute loads away from the notch tip. Finally, a complete microstructural component based finite element model of the arterial tissue was developed, which consisted of the collagen fibres and the other structural entities such as elastin and GAGs. The collagen fibres were modelled as discrete entities within the model, where all the other structural entities were modelled in the form of ground matrix. The developed model was used to study the influence of distinct microstructural parameters on the mechanical behaviour of arterial tissues.