Influence of Elastase and Collagenase: ex vivo study on the mechanical properties of porcine aorta by biaxial tensile tests.

The aorta is the largest artery in the mammal and it’s rather distensible due to its high degree of elasticity. It has a heterogeneous structure, where some of the key elements are the proteins collagen and elastin; which are essential for the mechanical properties of the aortic tissue. This thesis investigates the influence of elastase and collagenase on mechanical properties of porcine aorta. This was achieved by developing an appropriate biaxial tensile test method; which closely represents the in vivo conditions of the aortic tissue. To investigate the influence of collagen and elastin some samples were exposed to the enzymes collagenase and elastase to chemically manipulate a decomposition of the proteins in tissue. The exposed samples were tested in the developed biaxial test method and compared to native (untreated) samples. The wall samples exposed to enzymes were exposed for a varied time period of 1h, 3h and 6h. All experimental data from the mechanical tests were also used to identify model parameters of three different constitutive functions (Fung, HGO and Ogden), trying to define the differences in the results depending on type of enzyme exposure and exposure time. The results of the study show that collagenase exposed samples tends to be less distensible compared to the native samples in the circumferential direction. The longitudinal direction showed no differences between native and exposed samples, probably due to that the collagen fibers lay in the circumferential direction. The elastase exposed tissue samples tended to be more distensible than the native samples, especially for the circumferential direction. However, the results from the biaxial tests are complex and difficult to analyse. Further on additional test are necessary to achieve a deeper understanding of the results. The constitutive model results show that the anisotropic models (Fung and HGO) describe the experimental data more accurate than the isotropic model (Ogden), possibly due to that biological material is most often anisotropic. The parameter identification is not conclusive due to varied results and additional tests are desirable to reach a more statistical accuracy