Department of Mechanical Engineering, Israel Institute of Technology, Haifa 32000, Israel.
Laboratory for Research in Reproductive Sciences, Department of Obstetrics and Gynecology, Emek Medical Center, Afula 18100, Israel.
Acta Biomater. 2016 Jan;30:299-310. doi: 10.1016/j.actbio.2015.11.011. Epub 2015 Nov 11.
We perform bulge tests on live fetal membrane (FM) tissues that simulate the mechanical conditions prior to contractions. Experimental results reveal an irreversible mechanical behavior that appears during loading and is significantly different than the mechanical behavior that appears during unloading or in subsequent loading cycles. The irreversible behavior results in a residual strain that does not recover upon unloading and remains the same for at least 1h after the FM is unloaded. Surprisingly, the irreversible behavior demonstrates a linear stress-strain relation. We introduce a new model for the mechanical response of collagen tissues, which accounts for the irreversible deformation and provides predictions in agreement with our experimental results. The basic assumption of the model is that the constitutive stress-strain relationship of individual elements that compose the collagen fibers has a plateau segment during which an irreversible transformation/deformation occurs. Fittings of calculated and measured stress-strain curves reveal a well-defined single-value property of collagenous tissues, which is related to the threshold strain εth for irreversible transformation. Further discussion of several physio-mechanical processes that can induce irreversible behavior indicate that the most probable process, which is in agreement with our results for εth, is a phase transformation of collagen molecules from an α-helix to a β-sheet structure. A phase transformation is a manifestation of a significant change in the molecular structure of the collagen tissues that can alter connections with surrounding molecules and may lead to critical biological changes, e.g., an initiation of labor.
This study is driven by the hypothesis that pre-contraction mechanical stretch of the fetal membrane (FM) can lead to a change in the microstructure of the FM, which in turn induces a critical biological (hormonal) change that leads to the initiation of labor. We present mechanical characterizations of live FM tissues that reveal a significant irreversible process and a new model for the mechanical response of collagen tissues, which accounts for this process. Fittings of calculated and measured results reveal a well-defined single-value property of collagenous tissues, which is related to the threshold strain for irreversible transformation. Further discussion indicates that the irreversible deformation is induced by a phase transformation of collagen molecules that can lead to critical biological changes.
我们对活胎膜（FM）组织进行隆起测试，模拟收缩前的机械条件。实验结果显示出一种不可逆的机械行为，这种行为在加载过程中出现，与卸载或随后的加载循环中出现的机械行为明显不同。这种不可逆行为导致残余应变在卸载时不会恢复，并且在 FM 卸载后至少 1 小时内保持不变。令人惊讶的是，不可逆行为表现出线性应力-应变关系。我们引入了一种新的胶原组织力学响应模型，该模型考虑了不可逆变形，并提供了与实验结果一致的预测。该模型的基本假设是，组成胶原纤维的单个元素的本构应力-应变关系在发生不可逆转化/变形的平台段具有一个平台段。计算和测量的应力-应变曲线的拟合揭示了胶原组织的一种明确定义的单值特性，该特性与不可逆转化的阈值应变ε th 有关。对几种可能导致不可逆行为的生理力学过程的进一步讨论表明，最可能的过程与我们对ε th 的结果一致，即胶原分子从α-螺旋结构向β-折叠结构的相转变。相转变是胶原组织分子结构发生显著变化的表现，这种变化可能改变与周围分子的连接，并可能导致关键的生物学变化，例如分娩的开始。
本研究的假设是，FM 的预收缩机械拉伸会导致 FM 微观结构发生变化，进而引发关键的生物学（激素）变化，从而导致分娩开始。我们对活 FM 组织进行了机械特性表征，结果显示出一种显著的不可逆过程和一种新的胶原组织力学响应模型，该模型考虑了这一过程。计算和测量结果的拟合揭示了胶原组织的一种明确定义的单值特性，该特性与不可逆转化的阈值应变有关。进一步的讨论表明，不可逆变形是由胶原分子的相转变引起的，这种相转变可能导致关键的生物学变化。
