Chapter 7 Evolution of the Skin Microstructural Organization During a Mechanical Assay B. Lynch, S. Bancelin, C. Bonod-Bidaud, F. Ruggiero, M.-C. Schanne-Klein, and J.-M. Allain Abstract Skin is a complex multi-layered tissue, consisting of three main parts: the epidermis, the dermis and the hypodermis. The dermis is responsible for most of the complex mechanical properties of skin, such as viscoelasticity, non-linearity and anisotropy. At the microscopic level the dermis consists for the greater part of extracellular matrix, compounded mainly of collagen fibers forming an orderless network. The mechanical properties of skin have been studied in the past, but their exact link with the microscopic organization is still an open question. The goal of our study is to measure the evolution of the microstructure during a mechanical assay and to improve existing mechanical models of skin with relevant parameters identified at the microscopic level. We perform uniaxial tensile test on ex vivo mouse skin. The mechanical tests are performed in situ under a second harmonic generation microscope. This allows us to determine quantitatively and simultaneously the mechanical response and the microstructural reorganization of the tissue. This technique can be used to better understand the link between pathological alterations of collagen synthesis, fibers organization, and alteration of the biomechanical properties of skin, as in the Ehlers-Danlos syndrome (EDS). Keywords Biomechanics, Multiscale mechanics, Mice skin, Collagen, Multiphoton imaging 7.1 Introduction Connective tissues such as skin, tendon or cornea are mostly made of extracellular matrix proteins, with only few cells embedded. This matrix is in fact a mixture of different proteins, the most abundant proteins belonging to the collagen family. The ability of the certain types of collagen to self-organize into supramolecular cables—called fibrils—leads to the formation of a fibrillar network inside of the tissue, surrounded by a less organized medium of proteins and water. These fibers play a key role in the behavior of the connective tissues, by providing stiff structures and mechanical support to the residing cells. Alteration in the collagen structure or organization leads to degraded mechanical properties and poor regeneration, by affecting the cell interaction with the surrounding matrix. This has a strong impact on wound repair or ageing [1]. Macroscopic mechanical properties of skin are well known. They show an initial non-linear part, the heel region, followed by a linear part and a saturation, indicating the rupture of the tissue [2–4]. Similar responses are found in most connective tissues as aorta [5, 6], tendon [7–9] and cornea [10, 11]. However, the link with the microstructure has been less studied in disorganized tissues. In highly organized tendons, different methods have been used: polarized-light microscopy [12], OCT [13], multiphoton [9, 14] and confocal [8, 15] microscopy and X-ray diffractions [16, 17]. They all concluded that the heel region is associated with an uncrimping of the collagen fibers—which means that the initial macroscopic undulations of the fibers disappear under tension. In the linear part, the response of a tendon combines stretching of the fibers and sliding between adjacent fibers [16, 17]. Like for tendon, the mechanical response of skin has been interpreted as an initial alignment of the fibers in the heel region, followed by the stretching of the fibers in the linear part [18, 19]. This has led to a large set of theoretical works on the so-called microstructural models [20–23], which took as an entry the local B. Lynch • J.-M. Allain (*) LMS, Ecole Polytechnique, CNRS, Universite´ Paris-Saclay, Palaiseau, France e-mail: allain@lms.polytechnique.fr S. Bancelin • M.-C. Schanne-Klein LOB, Ecole Polytechnique, CNRS, INSERM, Universite´ Paris-Saclay, Palaiseau, France C. Bonod-Bidaud • F. Ruggiero Institut de Ge´nomique Fonctionnelle de Lyon, ENS-Lyon, CNRS, Universite´ Lyon 1, Lyon, France #The Society for Experimental Mechanics, Inc. 2017 C.S. Korach et al. (eds.), Mechanics of Biological Systems and Materials, Volume 6, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-41351-8_7 45
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