Cardiovascular Engineering Research Laboratory (CERL), School of Mechanical and Design Engineering, University of Portsmouth, Anglesea Road, Portsmouth PO1 3DJ, United Kingdom.
National Heart and Lung Institute, Heart Science Centre, Imperial College London, Middlesex, United Kingdom.
Acta Biomater. 2019 Apr 1;88:120-130. doi: 10.1016/j.actbio.2019.02.008. Epub 2019 Feb 10.
This paper presents an experimental investigation and evidence of rate-dependency in the planar mechanical behaviour of semilunar heart valves. Samples of porcine aortic and pulmonary valves were subjected to biaxial deformations across 1000-fold stretch rate, ranging from λ̇=0.001 to 1 s. The experimental campaign encompassed protocols covering (i) tests on samples without preconditioning, (ii) preconditioning immediately followed by tensile tests; and (iii) tensile tests at different rates performed on the same preconditioned specimen. Our results indicate that under all employed loading protocols, heart valve samples exhibit a marked rate-dependency in their deformation behaviour. This rate-dependency is reflected in stress-stretch curves and the calculated ensuing gradients, where samples typically show stiffening with increased rate. These results underpin one conclusive outcome: the in-plane mechanical behaviour of semilunar valves is rate-dependent (p<0.05 for Cauchy stress levels ≥50 kPa). This outcome implies that the rate of deformation for characterising the mechanical behaviour of semilunar heart valves may not be chosen arbitrarily low, and models that incorporate rate-effects may be more appropriate for better capturing the mechanical behaviour of heart valves. STATEMENT OF SIGNIFICANCE: This study presents for the first time a comprehensive set of results and evidence of rate-dependency in the mechanical behaviour of semilunar heart valves under biaxial deformation. Our results challenge the widely-applied assumption in the bulk of the existing literature, where an implicit rate-independency is assumed in both experimental and modelling propositions related to the biomechanics of the aortic and pulmonary valves. This study therefore creates a solid platform for future research in heart valve biomechanics with two important implications. First, experimental campaigns have to be carried out at high stretch rates; ideally as close to the physiological rate as possible. Second, new continuum/computational models are required to address the rate-dependent mechanical behaviour of the semilunar valves.
本文提出了对半新月形心脏瓣膜平面力学行为的速率相关性的实验研究和证据。猪主动脉瓣和肺动脉瓣样本在拉伸率为 1000 倍的双轴变形下进行测试，拉伸率范围为 λ̇=0.001 至 1 s。实验方案包括涵盖以下内容的协议：（i）未经预处理的样品测试，（ii）立即进行预处理然后进行拉伸测试；以及（iii）对同一预处理样本在不同速率下进行拉伸测试。我们的结果表明，在所采用的所有加载方案下，心脏瓣膜样本在其变形行为中表现出明显的速率相关性。这种速率相关性反映在应力-应变曲线和计算得出的梯度中，其中样本通常表现出随着速率增加而变硬。这些结果得出一个结论：半新月形瓣膜的平面力学行为是速率相关的（在 50kPa 以上的考西应力水平下，p<0.05）。这一结果意味着，为了描述半新月形心脏瓣膜的力学行为而选择的变形速率不能任意选择得过低，并且包含速率效应的模型可能更适合更好地捕捉心脏瓣膜的力学行为。
本研究首次提出了对半新月形心脏瓣膜在双轴变形下的力学行为的速率相关性的全面研究结果和证据。我们的结果挑战了现有文献中广泛应用的假设，即在与主动脉瓣和肺动脉瓣生物力学相关的实验和建模假设中，都存在着隐含的速率独立性。因此，这项研究为心脏瓣膜生物力学的未来研究创造了一个坚实的平台，具有两个重要的意义。首先，实验方案必须在高拉伸率下进行；理想情况下，尽可能接近生理速率。其次，需要新的连续体/计算模型来解决半新月形瓣膜的速率相关力学行为。
