Skin, the outermost layer of the body, fulfills a broad range of functions, protecting internal organs from damage and infection, while regulating the body’s temperature and water content via the exchange of heat and fluids. It must be able to withstand and recover from extensive deformation and damage that can occur during growth, movement, and potential injuries. A detailed investigation of the evolution of the collagen architecture of the dermis as a function of deformation is conducted, which reveals new aspects that help us to understand the mechanical response of skin. Juvenile pig is used as a model material because of its similarity to human skin. The dermis is found to have a tridimensional woven structure of collagen fibers, which evolves with deformation. After failure, we observe that the fibers have straightened and aligned in the direction of tension. The effects of strain-rate change, cyclic loading, stress relaxation, and orientation are quantitatively established. Digital image correlation techniques were implemented to quantify skin’s anisotropy; measurements of the Poisson ratio are reported. This is coupled with transmission electron microscopy which enables obtaining quantitative strain parameters evaluated through the orientation and curvature of the collagen fibers and their changes, for the first time in all three dimensions of the tissue. A model experiment using braided human hair in tension exhibits a similar J-curve response to skin, and we propose that this fiber configuration is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain. The obtained results are intended to serve as a basis for structurally-based models of skin.

Statement of Significance

Our study reveals a new aspect of the dermis: it is comprised of a tridimensional woven structure of collagen fibers, which evolves with deformation. This is enabled by primarily two techniques, transmission electron microscopy on three perpendicular planes and confocal images with second harmonic generation fluorescence of collagen, captured at different intervals of depth. After failure, the fibers have straightened and aligned in the direction of tension. Digital image correlation techniques were implemented to quantify skin’s anisotropy; measurements of the Poisson ratio are reported. A model experiment using braided human hair in tension exhibits a similar J-curve response to skin, and we propose that this fiber configuration is at least partially responsible for the monotonic increase of the tangent modulus of skin with strain.

中文翻译：

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猪真皮的拉伸行为和结构表征

皮肤是人体的最外层，具有广泛的功能，可以保护内部器官免受损害和感染，同时可以通过热量和液体的交换来调节人体的温度和水分含量。它必须能够承受生长，移动和潜在伤害期间可能发生的大范围变形和损坏并从中恢复。进行了真皮胶原蛋白结构作为变形的函数的详细研究，揭示了有助于我们了解皮肤机械反应的新方面。幼猪由于与人类皮肤的相似性而被用作模型材料。发现真皮具有胶原纤维的三维编织结构，该结构随着变形而演化。失败后 我们观察到，纤维在拉紧方向上已经拉直并对齐。定量确定了应变率变化，循环载荷，应力松弛和取向的影响。实施了数字图像相关技术以量化皮肤的各向异性。报告了泊松比的测量结果。这与透射电子显微镜相结合，这使得能够首次获得在组织的所有三个维度上通过胶原纤维的取向和曲率及其变化评估的定量应变参数。使用编织的人发在张力下进行的模型实验显示出对皮肤的相似的J曲线响应，我们提出，这种纤维构型至少部分负责随应变使皮肤的切线模量单调增加。

重要声明

我们的研究揭示了真皮的一个新方面：它由胶原纤维的三维编织结构组成，该结构随着变形而演化。这主要是通过两种技术实现的：在三个垂直平面上的透射电子显微镜以及在不同深度间隔捕获的具有胶原的二次谐波荧光的共聚焦图像。破坏后，纤维已经拉直并在张力方向上对齐。实施了数字图像相关技术以量化皮肤的各向异性。报告了泊松比的测量结果。使用编织的人发在张力下进行的模型实验显示出对皮肤的相似的J曲线响应，并且我们提出，这种纤维构型至少部分负责应变引起的皮肤切线模量的单调增加。