Experimental study and computational modelling of the biomechanical properties of arteries

• Main Author: Ooi, Chen Yen
• Published: University of Cambridge 2014
• Subjects: 612.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.659255

Biomechanics of artery has been shown to play an important role in the progression of cardiovascular diseases. Thus studies of arterial biomechanics are essential to find the causes and preventions of these diseases. The thesis first introduces work demonstrating the use of the optical flow estimation technique to track the deformation of biological tissue in biomechanical experiments. The study shows that the optical flow technique provides a good estimation of the strains for the constructed deformation images by comparing the results with those of the finite element (FE) models. The optical flow technique was then used on the analysis of the images obtained from the experiments of human coronary arteries with atherosclerosis. Atherosclerosis is a cardiovascular disease in which the rupture of atherosclerotic plaque leads to stroke or myocardial infarction, where the risk of the rupture has been associated with mechanical factors. An approach was developed combining the tensile ring tests, optical flow analysis, and FE modelling to obtain the heterogeneous mechanical properties of atherosclerotic arteries. This approach provides a sensible estimation of the hyperelastic properties of atherosclerotic plaques by matching the forces and strains of the experiments and FE models using an error minimisation approach. The study on the biomechanics of atherosclerotic arteries was then extended to the muscle active properties of the diseased vessels. Previous drug response experiments have shown that human atherosclerotic arteries maintained active forces of more than 70% of those in healthy arteries in response to endothelin-1 (ET-1) despite an extensive thinning of the media smooth muscle layer. FE analyses have been performed to model the passive tensile ring tests and active ET-1 response tests, which demonstrate the potential elevated contractile strains of diseased smooth muscle cells in response to ET-1. The results suggest that adaptation mechanisms occur with atherosclerosis to maintain the distensibility of the diseased artery in physiological condition with the presence of cell activation agents. In addition, a project investigating the role of hyaluronan in systemic inflammation-induced aortic stiffening has been carried out. Systemic inflammation occurs in several diseases such as rheumatoid arthritis which increases aortic stiffness and causes altered distribution and function of arterial hyaluronan. Tensile ring tests were performed on rat aortas that have hyaluronan digested by hyaluronidase and control samples. The results show no significant difference between the elastic modulus of the two groups despite successful digestion of the hyaluronan content in the treated samples in biochemistry assays. A potential mechanism of this contradiction is that the hyaluronan accumulation-induced aortic stiffening is a long term effect in patients and cannot be shown with the methodology of this study. In order to solve the non-physiological loading limitation of the tensile ring tests in the studies above, an inflation test system has been designed for future experiments. This system inflates an artery ring with a balloon tube with the cross-sectional deformation captured using a camera in an inverted position. The system has been tested with a pig aortic ring with the deformation analysed using the optical flow technique. The estimated elastic modulus of the artery was compared to that obtained from a tensile ring test, which shows a good matching between the two values. Overall, the thesis demonstrates experimental and computational approaches to study the biomechanical properties of arteries. The results also provide information on the hyperelastic and active properties of atherosclerotic arteries as well as the effects of hyaluronan digestion on the stiffness of rat aortas.