Layer- and region-specific material characterization of ascending thoracic aortic aneurysms by microstructure-based models.

Material characterization of ascending thoracic aortic aneurysms is indispensable for the determination of stress distributions across wall thickness and the different aneurysm regions that may be responsible for their catastrophic rupture or dissection, but only few studies have addressed this issue hitherto. In this article, we are presenting our findings of implementing microstructure-based formulations for characterizing layer- and region-specific variations in wall properties, which is a reasonable consensus today. Together, we performed image-based analysis to derive collagen-fiber orientation angles that may serve as validation of the preferred candidate for a fiber-reinforced constitutive descriptor. We considered a four-fiber model with dispersions of fiber angles about the main directions, based on our histological observations, demonstrating a wide distribution of fiber orientations spanning circumferential to longitudinal directions, and its successful implementation to our biomechanical data from tensile testing. However, an in-depth parametric analysis showed that a condensed model without longitudinal-fiber family described the data just as well and did not omit essential histological organization of collagen fibers, while reserving a smaller number of parameters, which makes it advantageous for computational applications. A major aberration from almost all existing models in the literature is the hypothesis made that fibers can support compressive stresses. Such a hypothesis needs further examination but it has the benefits of allowing improved fits to the vanishing transverse stresses under uniaxial test conditions and of properly reflecting the exponential nature of the compressive stress-strain response of aortic tissue, being consistent with observations of collagen being under compression in the unloaded wall.