VARIATION OF THE LOCAL MATERIAL PROPERTIES OF AORTA

Abstract

Understanding the aortic wall deformation and failure during traumatic aortic rupture (TAR), which is a leading cause of fatality in motor vehicle accidents is of great concern. The specific objective of the present study is to develop a material model that can describe the multi layer nature of the aortic wall. Fundamentally, the aortic wall is composed mainly of three layers, tunica intima, media and adventitia, and they are known to have different structures. Understanding the material properties of these layers is essential in order to study the local mechanisms of deformation, force transmission, and failure. The hypothesis of this study is that the tissue's instantaneous shear modulus grows along the radial direction while moving from the intima toward the adventitia. The higher compliance of the tissue near the intima, which is partly due to the concentration of the smooth muscle cells and partly due to the arrangement of collagen and elastin fibers, can explain the nature of aorta failure which is primarily generated from the inside towards the outer layers. A combination of micro- and nano-indentation tests were used to measure the local material properties of porcine aorta at the length scales of 160 µm and 40 µm respectively. The material properties of aorta were investigated in the lateral (left) region in several longitudinal locations of the descending aorta and the observed viscoelastic behavior was summarized in the form of instantaneous shear moduli and reduced relaxation functions. The instantaneous shear modulus was found to generally increase along the radial direction to about 0.6 normalized radial distance and then became almost constant but with higher variability. The reduced relaxation functions were generally independent of the location and test method. Comparing the mechanical results with the histological results obtained through Van-Guisen staining showed that the material properties are partly related to the distribution of smooth muscle cells. The results of this study can be used to explain the mechanisms of failure in aorta and contribute to improve the computational modeling of aorta's deformation which is valuable in a variety of applications including automotive accidents, endovascular grafts, and angioplasty.