Liao Jun, Yang Lin, Grashow Jonathan, Sacks Michael S
Engineered Tissue Mechanics Laboratory, Department of Bioengineering, and the McGowan Institute for Regenerative Medicine, University of Pittsburgh, Pittsburgh, PA 15219, USA.
J Biomech Eng. 2007 Feb;129(1):78-87. doi: 10.1115/1.2401186.
We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315-325; Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509-1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (epsilonD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90 N/m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, epsilonD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using epsilonD, the tangent modulus of collagen fibrils was estimated to be 95.5+/-25.5 MPa, which was approximately 27 times higher than the tissue tensile tangent modulus of 3.58+/-1.83 MPa. In creep tests performed at 90 N/m equibiaxial tension for 60 min, both tissue strain and epsilonD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90 min epsilonD was found to rapidly decrease in the first 10 min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3 min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9 min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a "load-locking" behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function.
我们最近证实，二尖瓣前叶（MVAL）表现出极小的滞后现象、无应变率敏感性、应力松弛但无蠕变（Grashow等人，2006年，《生物医学工程年鉴》，34(2)，第315 - 325页；Grashow等人，2006年，《生物医学工程年鉴》，34(10)，第1509 - 1518页）。然而，这种独特的准弹性力学行为的潜在结构基础目前尚不清楚。由于胶原蛋白是MVAL的主要结构成分，我们使用小角X射线散射研究了双轴加载下MVAL中胶原纤维运动学（旋转和拉伸）与组织水平力学性能之间的关系。开发并利用了一种新型装置，以在平面双轴加载状态下同时测量组织水平的力和应变。在等双轴拉伸、蠕变以及应力松弛至90 N/m的峰值张力下，测量了胶原纤维D周期应变（εD）和纤维角分布。结果表明，在等双轴拉伸下，直到组织水平应力 - 应变曲线的非线性区域结束时，胶原纤维应变才开始。在较高的组织张力水平下，εD随张力增加呈线性增加。胶原纤维角分布的变化主要发生在组织趾区。使用εD，估计胶原纤维的切线模量为95.5±25.5 MPa，约为组织拉伸切线模量3.58±1.83 MPa的27倍。在90 N/m等双轴张力下进行60分钟的蠕变试验中，组织应变和εD均保持恒定，在试验长度内无明显变化。相反，在进行90分钟的应力松弛试验中，发现εD在最初10分钟内迅速下降，随后在试验剩余时间内下降速率较慢。使用单指数模型，胶原纤维应变降低的时间常数为8.3分钟，小于组织水平在周向和径向的应力松弛时间常数，分别为22.0分钟和16.9分钟。此外，在蠕变和应力松弛试验期间，纤维角分布均无变化。我们的结果表明：（1）MVAL胶原纤维不表现出内在的粘弹性行为；（2）组织松弛是由于纤维上应力的去除，可能是通过由非胶原蛋白成分（如蛋白聚糖）调节的滑动机制；（3）缺乏蠕变但存在应力松弛表明在维持加载条件下存在“负载锁定”行为。这些独特的力学特性可能是正常瓣膜功能所必需的。
