Combination of Traction Assays and Multiphoton Imaging to Quantify Skin Biomechanics

International audienceAn important issue in tissue biomechanics is to decipher the relationship between the mechanical behavior at macroscopic scale and the organization of the collagen fiber network at microscopic scale. Here, we present a protocol to combine traction assays with multiphoton microscopy in ex vivo murine skin. This multiscale approach provides simultaneously the stress/stretch response of a skin biopsy and the collagen reorganization in the dermis by use of second harmonic generation (SHG) signals and appropriate image processing

Multiscale characterization of skin mechanics through in situ imaging

International audienceThe complex mechanical properties of skin have been studied intensively over the past decades. They are intrinsically linked to the structure of the skin at several length scales, from the macroscopic layers (epidermis, dermis and hypodermis) down to the microstructural organization at the molecular level. Understanding the link between this microscopic organization and the mechanical properties is of significant interest in the cosmetic and medical fields. Nevertheless, it only recently became possible to directly visualize the skin’s microstructure during mechanical assays, carried out on the whole tissue or on isolated layers. These recent observations have provided novel information on the role of structural components of the skin in its mechanical properties, mainly the collagen fibers in the dermis, while the contribution of others, such as elastin fibers, remains elusive. In this chapter we present current methods used to observe skin’s microstructure during a mechanical assay, along with their strengths and limitations, and we review the unique information they provide on the link between structure and function of the skin

Experimental Characterization of Adventitial Collagen Fiber Kinematics Using Second-Harmonic Generation Imaging Microscopy: Similarities and Differences Across Arteries, Species and Testing Conditions

International audienceFibrous collagen networks are well known to play a central role in the passive biomechanical response of soft connective tissues to applied loads. In the current chapter we focus on vascular tissues and share our extensive experience in coupling mechanical loading and multi-photon imaging to investigate, across arteries, species and testing conditions, how collagen fibers move in response to mechanical loading. More specifically, we assess the deformations of collagen networks in rabbit, porcine or human arteries under different loading scenarios: uniaxial tension on flat samples, tension-inflation on tubular samples, bulge inflation on flat samples. We always observe that collagen fibers exhibit a wavy or crimped shape in load-free conditions, and tend to uncrimp when loads are applied, engaging sequentially to become the main load-bearing component. This sequential engagement, which is responsible for the nonlinear mechanical behaviour, is essential for an artery to function normally and appears to be less pronounced for arteries in elderly and aneurysmal patients. Although uncrimping of collagen fibers is a universal mechanism, we also observe large fiber rotations specific to tensile loading, with significant realignment along the loading axis. A unified approach is proposed to compare observations and quantitative analyses as the type of image processing may affect significantly the estimation of collagen fiber deformations. In summary, this chapter makes an important review of the basic roles of arterial microstructure and its deformations on the global mechanical response. Eventually, directions for future studies combining mechanical loading and multi-photon imaging are suggested, with the aim of addressing open questions related to tissue adaptation and rupture