Failure Mechanics of Nonlinear, Heterogeneous, Anisotropic Cardiovascular Tissues: Implications for Ascending Thoracic Aortic Aneurysms

University of Minnesota Ph.D. dissertation. June 2019. Major: Biomedical Engineering. Advisor: Victor Barocas. 1 computer file (PDF); xi, 235 pages.Characterizing the mechanical response and failure mechanisms of cardiovascular tissues is critically important, as these tissues play a vital role in the native functioning of the body. In the case of pathological events, such as aortic aneurysms or myocardial infarctions, mechanical behavior can be altered due to adverse remodeling, and thus affect the integrity of the tissue. Ascending thoracic aortic aneurysms (ATAAs) occur when the aorta enlarges beyond its normal diameter, and dilation is typically accompanied by disorganization of the underlying aortic fibrous structure. Current diagnostic methods depend solely on measuring aneurysm diameter, neglecting considerations of mechanical strength, which results in an inefficient risk assessment. To better understand the failure mechanism of ATAAs, the work presented here used a combination of experimental testing and computational modeling to characterize failure in human ATAA tissue. Experimental testing showed that ATAA tissue exhibited significantly lower mechanical strength when compared to healthy porcine tissue in multiple loading configurations. Furthermore, experimental tests highlighted the large disparity between uniaxial and shear strength in ATAA tissue, where the tissue was substantially weaker in shear loading conditions. A custom multiscale finite-element model was then used to interrogate fiber failure more closely in both experimental loading conditions, and inflation of a patient-specific ATAA geometry. Modeling results showed that fibers between the lamellar layers of the aortic wall failed significantly more than fibers within the planar layers in shear loading conditions, as well as during inflation of the patient-specific geometry. Taken together, these results suggest that intramural shear could be an important contributor to the failure or dissection of ATAAs