10 research outputs found
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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.Engineering and Physical Sciences Research Counci
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Influence of overlapping pattern of multiple overlapping uncovered stents on the local mechanical environment: A patient-specific parameter study.
BACKGROUND: Multiple overlapping uncovered stents (MOUS) system has shown potentials in managing complex aortic aneurysms with side branches involvement. It promotes the development of thrombus by modulating local flow pattern that reduces the wall tension, while maintaining patency of side branches. However the modulation of local hemodynamic parameters depends on various factors that have not been assessed comprehensively. METHODS: Aneurysm 3D geometry was reconstructed based on CT images. One-way fluid-structure interaction analysis was performed to quantify structural stress concentration in the wall, and changes of blood velocity, wall shear stress (WSS), oscillatory shear index (OSI), relative residence time (RRT) and pressure in the sac due to the stent deployment. RESULTS: High structural stress concentration due to stent deployment was found in the landing zone and it increased linearly with the number of stents deployed. The wall tension in the sac was unaffected by the stent deployment. Stress within the wall was insensitive to the different overlapping pattern. After one stent was deployed, the mean flow velocity in the sac reduced by 36.4%. The deployment of the 2nd stent further reduced the mean sac velocity by 10%. WSS decreased while both OSI and RRT increased after stent deployment, however pressure in the sac remained nearly unchanged. Except for the cases with complete stents struts alignment, different overlapping pattern had little effect on flow parameters. CONCLUSIONS: Mechanical parameters modulated by the MOUS are insensitive to different overlapping pattern suggesting that endovascular procedure can be performed with less attention to the overlapping pattern.National Nature Science Foundation of China, Cambridge National Institute of Health Research Biomedical Research Centre, China Scholarship Counci
Study on the association of wall shear stress and vessel structural stress with atherosclerosis: An experimental animal study.
BACKGROUND AND AIMS: Artery is subject to wall shear stress (WSS) and vessel structural stress (VSS) simultaneously. This study is designed to explore the role of VSS in development of atherosclerosis. METHODS: Silastic collars were deployed on the carotid to create two constrictions on 13 rabbits for a distinct mechanical environment at the constriction. MRI was performed to visualize arteries' configuration. Animals with high fat (n = 9; Model-group) and normal diet (n = 4; Control-group) were sacrificed after 16 weeks. 3D fluid-structure interaction analysis was performed to quantify WSS and VSS simultaneously. RESULTS: Twenty plaques were found in Model-group and 3 in Control-group. In Model-group, 8 plaques located proximally to the first constriction (Region-1, close to the heart) and 7 distally to the second (Region-2, close to the head) and 5 plaques were found on the contralateral side of 3 rabbits. Plaques at Region-1 tended to be bigger than those at Region-2 and the macrophage density at these locations was comparable. Minimum time-averaged WSS (TAWSS) in Region-1 was significantly higher than that in Region-2, and both maximum oscillatory shear index (OSI) and particle relative residence time (RRT) were significantly lower. Peak and mean VSS in Region-1 were significantly higher than those in Region-2. Correlation analyses indicated that low TAWSS, high OSI and RRT were only associated with plaque in Region-2, while lesions in Region-1 were only associated with high VSS. Moreover, only VSS was associated with wall thickness of plaque-free regions in both regions. CONCLUSIONS: VSS might contribute to the initialization and development of atherosclerosis solely or in combination with WSS
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Bayesian Inference-Based Estimation of Normal Aortic, Aneurysmal and Atherosclerotic Tissue Mechanical Properties: From Material Testing, Modeling and Histology.
OBJECTIVE: Mechanical properties of healthy, aneurysmal, and atherosclerotic arterial tissues are essential for assessing the risk of lesion development and rupture. Strain energy density function (SEDF) has been widely used to describe these properties, where material constants of the SEDF are traditionally determined using the ordinary least square (OLS) method. However, the material constants derived using OLS are usually dependent on initial guesses. METHODS: To avoid such dependencies, Bayesian inference-based estimation was used to fit experimental stress-stretch curves of 312 tissue strips from 8 normal aortas, 19 aortic aneurysms, and 21 carotid atherosclerotic plaques to determine the constants, C1, D1, and D2 of the modified Mooney-Rivlin SEDF. RESULTS: Compared with OLS, material constants varied much less with prior in the Bayesian inference-based estimation. Moreover, fitted material constants differed amongst distinct tissue types. Atherosclerotic tissues associated with the biggest D2, an indicator of the rate of increase in stress during stretching, followed by aneurysmal tissues and those from normal aortas. Histological analyses showed that C1 and D2 were associated with elastin content and details of the collagen configuration, specifically, waviness and dispersion, in the structure. CONCLUSION: Bayesian inference-based estimation robustly determines material constants in the modified Mooney-Rivlin SEDF and these constants can reflect the inherent physiological and pathological features of the tissue structure. SIGNIFICANCE: This study suggested a robust procedure to determine the material constants in SEDF and demonstrated that the obtained constants can be used to characterize tissues from different types of lesions, while associating with their inherent microstructures.China Scholarship Council
NIHR Cambridge Biomedical Research Centr
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Association of Collagen, Elastin, Glycosaminoglycans, and Macrophages With Tissue Ultimate Material Strength and Stretch in Human Thoracic Aortic Aneurysms: A Uniaxial Tension Study.
Fiber structures and pathological features, e.g., inflammation and glycosaminoglycan (GAG) deposition, are the primary determinants of aortic mechanical properties which are associated with the development of an aneurysm. This study is designed to quantify the association of tissue ultimate strength and extensibility with the structural percentage of different components, in particular, GAG, and local fiber orientation. Thoracic aortic aneurysm (TAA) tissues from eight patients were collected. Ninety-six tissue strips of thickened intima, media, and adventitia were prepared for uni-extension tests and histopathological examination. Area ratios of collagen, elastin, macrophage and GAG, and collagen fiber dispersion were quantified. Collagen, elastin, and GAG were layer-dependent and the inflammatory burden in all layers was low. The local GAG ratio was negatively associated with the collagen ratio (r2 = 0.173, p < 0.05), but positively with elastin (r2 = 0.037, p < 0.05). Higher GAG deposition resulted in larger local collagen fiber dispersion in the media and adventitia, but not in the intima. The ultimate stretch in both axial and circumferential directions was exclusively associated with elastin ratio (axial: r2 = 0.186, p = 0.04; circumferential: r2 = 0.175, p = 0.04). Multivariate analysis showed that collagen and GAG contents were both associated with ultimate strength in the circumferential direction, but not with the axial direction (collagen: slope = 27.3, GAG: slope = -18.4, r2 = 0.438, p = 0.002). GAG may play important roles in TAA material strength. Their deposition was found to be associated positively with the local collagen fiber dispersion and negatively with ultimate strength in the circumferential direction.EPSRC (EP/P021654/1), NSERC (6799-427538-2012), NIHR Cambridge Biomedical Research Centre (BRC-1215-20014), EPSRC Doctoral Training Award at the University of Cambridge, China Scholarship Council