Ex vivo experimental characterizations for understanding the interrelationship between tissue mechanics and collagen...

Unidirectional blood flow in the left side of the heart is regulated by the mitral valve. To better understand the mitral valve function, researchers have examined the structural and mechanical properties of the mitral valve leaflets; however, limitations of the previous studies include the use of mechanics- and structure-altering tissue modifications (e.g., optical clearing) that limit the ability to quantify the unique load-dependent reorientation and realignment of the collagen fibers as well as their interrelation with the valve tissue mechanics. Herein, we aimed to circumvent these limitations by utilizing an integrated polarized-light imaging and biaxial testing system for understanding the mechanics-microstructure interrelationship for porcine mitral valve leaflets. We further performed constitutive modeling and evaluated the accuracy of the affine fiber kinematics theory. From the tissue mechanics perspective, the posterior leaflet was more extensible in the radial direction than the anterior leaflet (14.2% difference in radial tissue stretch), while exhibiting smaller collagen and elastin moduli based on the determined constitutive model parameters. From the collagen microstructure's standpoint, the posterior leaflet had smaller increases in optical anisotropy (closely related to the degree of fiber alignment) than the anterior leaflet (32.8±7.7% vs. 50.0±19.7%). Further, the leaflets were found to possess two distinct fiber families - one family oriented along the circumferential tissue direction, and another more disperse family with a 30°-40° offset from the first fiber family. Finally, affine fiber kinematics consistently underpredicted the collagen fiber reorientations Overall, this study improved our understanding of the mitral valve leaflets that is essential for facilitating tissue-emulated valve replacement and cardiac valve modeling frameworks.