Effects of enzyme-based removal of collagen and elastin constituents on the biaxial mechanical responses of porcine atrioventricular heart valve anterior leaflets

The leaflets of the atrioventricular heart valves (AHVs) regulate the one-directional flow of blood through a coordination of the extracellular matrix components, including the collagen fibers, elastin, and glycosaminoglycans. Dysfunction of the AHVs, such as those caused by unfavorable microstructural remodeling, lead to valvular heart diseases and improper blood flow, which can ultimately cause heart failure. In order to better understand the mechanics and remodeling of the AHV leaflets and how therapeutics can inadvertently cause adverse microstructural changes, a systematic characterization of the role of each constituent in the biomechanical properties is appropriate. Previous studies have quantified the contributions of the individual microstructural components to tissue-level behavior for the semilunar valve cusps, but not for the AHV leaflets. In this study, for the first time, we quantify the relationships between microstructure and mechanics of the AHV leaflet using a three-step experimental procedure: (i) biaxial tension and stress relaxation testing of control (untreated) porcine AHV anterior leaflet specimens; (ii) enzyme treatment to remove a portion of either the collagen or elastin constituent; and (iii) biaxial tensile and stress relaxation testing of the constituent-removed (treated) specimens. We have observed that the removal of ∼100% elastin resulted in a ∼10% decrease in the tissue extensibility with biaxial tension and a ∼10% increase in the overall stress reduction with stress relaxation. In contrast, removal of 46% of the collagen content insignificantly affected tissue extensibility with biaxial tension and significantly increased stress decay (10%) with stress relaxation. These findings provide an insight into the microstructure-mechanics relationship of the AHVs and will be beneficial for future developments and refinements of microstructurally informed constitutive models for the simulation of diseased and surgically intervened AHV function. Statement of significance: This study presents, for the first time, a thorough mechanical characterization of the atrioventricular heart valve leaflets before and after enzymatic removal of elastin and collagen. We found that the biaxial tensile properties of elastin-deficient tissues and collagen-deficient are stiffer. The fact of elastin supporting low-stress valve function and collagen as the main load-bearing component was evident in a decrease in the low-tension modulus for elastin-deficient tissues and in the high-tension modulus for collagen-deficient tissues. Our quantification and experimental technique could be useful in predicting the disease-related changes in heart valve mechanics. The information obtained from this work is valuable for refining the constitutive models that describe the essential microstructure-mechanics relationship.