Arterial mechanics considering the structural, mechanical, and biochemical contributions of elastin
Elastin provides many tissues with remarkable resilience and longevity. In elastic arteries such as aorta, elasticity is crucial for energy storage and transmission of the pulsatile blood flow. Human aorta is comprised of approximately 47% elastic fibers and undergoes several billion stretch cycles in the course of one's lifetime. Elastin is remarkably long lived, and it can suffer from cumulative effects of exposure to biochemical damages. Non-enzymatic glycation is one of the main mechanisms of aging and its effect is magnified in diabetic patients. The overall goal of this research is to advance the current understanding of the structural and mechanical roles of elastin in arterial mechanics with the effects of immediate biochemical environments using a coupled experimental and modeling approach. Such knowledge is integral to understanding the performance of elastin in living biological systems. Our study shows that there exists an intrinsic mechanical interaction among extracellular matrix (ECM) constituents that determines the mechanics of arteries and carries important implications to vascular mechanobiology. Considering the organization and engagement behavior of different ECM constituents in the arterial wall, we proposed a new constitutive model of ECM mechanics that considers the distinct structural and mechanical contributions of medial elastin, medial collagen, and adventitial collagen, to incorporate the constituent-specific fiber orientation and the sequential fiber engagement in arterial mechanics. Our study also reveals several interesting and important changes associated with non-enzymatic elastin glycation. Specifically, with in vitro glucose treatment, the stiffness of elastin increases significantly, and elastin exhibits a large hysteresis in the stress-stretch curves and an increase stress relaxation. Analysis of the relaxation time distribution spectra suggests that hydrogen bonding plays a major role in the relaxation behavior after glucose exposure. A multi-exponential model was developed to describe the relaxation behavior with material parameters obtained directly from continuous relaxation time distribution spectra. Elastin at different hydration levels was also studied to further understand the effects of immediate biochemical environments on the biomechanical behavior of elastin, and the close association of extra- and intrafibrillar water with the mechanical behavior of elastin.