Mechanical characterization of human aortas from pressurization testing and a paradigm shift for circumferential residual stress.

Material properties needed for accurate stress analysis of the human aorta are still incompletely known, especially as many reports have ignored the presence of residual stresses in the aortic wall. To contribute new material regarding these issues, we carried out measurements and pressurization testing on ascending, thoracic and abdominal aortic samples from 24 human subjects aged 38-77 years, and evaluated the opening angle describing the circumferential residual stress level present in the aorta. We determined material constants for the aorta by gender, anatomic location and age group, according to a simple phenomenological constitutive model. The unpressurized aortic radius positively correlated with age, and the circumferential and longitudinal stretch ratios under systemic pressure negatively correlated with age, confirming the known enlargement and stiffening of the aorta with aging. The opening angle was measured to range from a minimum of 89° to above 360° for extreme cases. For given aortic dimensions and material properties, analysis of the in vivo circumferential and longitudinal mural stress distributions indicated a profound influence of the opening angle. For instance, in the thoracic aorta of males aged 38-66, opening angles in the range of 0° to 80° (resp. 60°) may equalize the gradient of in vivo circumferential (resp. longitudinal) stress between the inner and outer layers of the aorta, as commonly expected; however, opening angles above 160° (resp. 120°) may cause the gradient of circumferential (resp. longitudinal) stress to reverse and increase compared to the case without residual stress, putting the maximum stresses toward the adventitia instead of the intima. Even though the analysis of the aortic wall excluded possible longitudinal residual stresses as well as material inhomogeneities, such as constitutive differences between the intimal, medial and adventitial layers, the experimental data reported herein are important to aortic modeling at large and the better understanding of aortic pathophysiology in particular.