Stella John A, Liao Jun, Sacks Michael S
Engineered Tissue Mechanics Laboratory, Department of Bioengineering and the McGowan Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA, USA.
J Biomech. 2007;40(14):3169-77. doi: 10.1016/j.jbiomech.2007.04.001. Epub 2007 Jun 13.
Despite continued progress in the treatment of aortic valve (AV) disease, current treatments continue to be challenged to consistently restore AV function for extended durations. Improved approaches for AV repair and replacement rests upon our ability to more fully comprehend and simulate AV function. While the elastic behavior the AV leaflet (AVL) has been previously investigated, time-dependent behaviors under physiological biaxial loading states have yet to be quantified. In the current study, we performed strain rate, creep, and stress-relaxation experiments using porcine AVL under planar biaxial stretch and loaded to physiological levels (60 N/m equi-biaxial tension), with strain rates ranging from quasi-static to physiologic. The resulting stress-strain responses were found to be independent of strain rate, as was the observed low level of hysteresis ( approximately 17%). Stress relaxation and creep results indicated that while the AVL exhibited significant stress relaxation, it exhibited negligible creep over the 3h test duration. These results are all in accordance with our previous findings for the mitral valve anterior leaflet (MVAL) [Grashow, J.S., Sacks, M.S., Liao, J., Yoganathan, A.P., 2006a. Planar biaxial creep and stress relaxatin of the mitral valve anterior leaflet. Annals of Biomedical Engineering 34 (10), 1509-1518; Grashow, J.S., Yoganathan, A.P., Sacks, M.S., 2006b. Biaxial stress-stretch behavior of the mitral valve anterior leaflet at physiologic strain rates. Annals of Biomedical Engineering 34 (2), 315-325], and support our observations that valvular tissues are functionally anisotropic, quasi-elastic biological materials. These results appear to be unique to valvular tissues, and indicate an ability to withstand loading without time-dependent effects under physiologic loading conditions. Based on a recent study that suggested valvular collagen fibrils are not intrinsically viscoelastic [Liao, J., Yang, L., Grashow, J., Sacks, M.S., 2007. The relation between collagen fibril kinematics and mechanical properties in the mitral valve anterior leaflet. Journal of Biomechanical Engineering 129 (1), 78-87], we speculate that the mechanisms underlying this quasi-elastic behavior may be attributed to inter-fibrillar structures unique to valvular tissues. These mechanisms are an important functional aspect of native valvular tissues, and are likely critical to improve our understanding of valvular disease and help guide the development of valvular tissue engineering and surgical repair.
尽管在主动脉瓣（AV）疾病的治疗方面不断取得进展，但目前的治疗方法在持续长时间恢复AV功能方面仍面临挑战。AV修复和置换的改进方法取决于我们更全面理解和模拟AV功能的能力。虽然之前已经研究了AV瓣叶（AVL）的弹性行为，但生理双轴加载状态下的时间依赖性行为尚未得到量化。在当前研究中，我们使用猪AVL在平面双轴拉伸下进行了应变率、蠕变和应力松弛实验，并加载到生理水平（60 N/m等双轴张力），应变率范围为准静态到生理状态。结果发现应力-应变响应与应变率无关，观察到的滞后水平较低（约17%）也是如此。应力松弛和蠕变结果表明，虽然AVL表现出显著的应力松弛，但在3小时的测试持续时间内蠕变可忽略不计。这些结果与我们之前对二尖瓣前叶（MVAL）的研究结果一致[Grashow, J.S., Sacks, M.S., Liao, J., Yoganathan, A.P., 2006a. Planar biaxial creep and stress relaxatin of the mitral valve anterior leaflet. Annals of Biomedical Engineering 34 (10), 1509 - 1518; Grashow, J.S., Yoganathan, A.P., Sacks, M.S., 2006b. Biaxial stress - stretch behavior of the mitral valve anterior leaflet at physiologic strain rates. Annals of Biomedical Engineering 34 (2), 315 - 325]，并支持我们的观察结果，即瓣膜组织是功能各向异性的准弹性生物材料。这些结果似乎是瓣膜组织所特有的，表明在生理加载条件下能够承受加载而无时间依赖性影响。基于最近一项表明瓣膜胶原纤维本质上不是粘弹性的研究[Liao, J., Yang, L., Grashow, J., Sacks, M.S., 2007. The relation between collagen fibril kinematics and mechanical properties in the mitral valve anterior leaflet. Journal of Biomechanical Engineering 129 (1), 78 - 87]，我们推测这种准弹性行为的潜在机制可能归因于瓣膜组织特有的纤维间结构。这些机制是天然瓣膜组织的一个重要功能方面，可能对于增进我们对瓣膜疾病的理解以及帮助指导瓣膜组织工程和手术修复的发展至关重要。
