Arterial mechanics considering structural inhomogeneity in the extracellular matrix
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Elastin and collagen fibers are the major extracellular matrix (ECM) constituents of the arterial wall. Elastic fibers in the medial layer form concentric layers of elastic lamella, together with smooth muscles cells and collagen fibers, organizing into a lamellar unit that is considered as a functional unit of the arterial wall. The lamellar unit is designed to support and evenly distribute the mechanical loading in the arterial wall. The ECM fiber networks in the arterial wall are highly inhomogeneous in structure with varying fiber diameters, density, orientation distribution, etc. The objective of this work is to advance the current understanding of the multi-scale ECM mechanics and the role of structural inhomogeneity in the arterial wall using a coupled experimental and modeling approach that integrates mechanical characterization, advanced optical imaging, and computational modeling. Our study on the micromechanics of elastic lamellae shows that structural inhomogeneity is important in maintaining tissue homeostasis. The higher lamellae unfolding in the inner lamellae layer compensates the larger strain experienced at the inner surface of the arterial wall, and plays an important role in maintaining a more evenly distributed stretching/stress in the lamellar layers. Studies on elastin fiber organization reveal that there is a transmural variation in the orientation distribution of elastin fibers through the arterial wall, which is closely correlated with the anisotropic behavior of elastin network. The study of power-law behavior in the arterial wall revealed the structural inhomogeneity in the inter-lamellar ECM network in the form of a nonuniform spatial distribution of interlamellar fibers, in terms of the fiber density along the axial direction, as well as their orientation with respect to lamellar layers. We found that this structural inhomogeneity is the underlying mechanism of the avalanche behavior in the propagation of aortic dissection. In the study of a discrete fiber network model, we proposed a finite element based framework considering the interfiber crosslinking properties of ECM network that successfully predicts the mechanical behavior of arterial elastin network. Our results suggest that rotational stiffness of the crosslinks plays more important role in local fiber-level than in tissue-level responses in the ECM network.