Three-dimensional Characterization of Mechanical Properties and Microstructures of Human Dermal Skin

The intact and healthy skin forms a barrier to the outside world and protects the body from mechanicalimpact. The skin is a complex structure with unique mechano-elastic properties. To better direct the design of biomimetic materials and induce skin regeneration in wounds with optimal outcome, more insight is required in how the mechano-elastic properties emerge from skin’s main constituents, collagen and elastin fibers. Here, we employed two-photon auto excited fluorescence (2PEF) and secondharmonic generation (SHG) microscopy to characterize collagen and elastin fibers in 3D in 24 humandermis skin samples. We performed uniaxial stretching experiments to determine mechanical tissueparameters from the resulting strain-stress curves. We also monitored changes in collagen and elastin alignment during uniaxial stretching in real-time. The strain-stress curves show a large variation, with an average Young’s modules in the heel and linear elastic regions of 0.1 MPa and 21 MPa. We performed a comprehensive analysis of the correlation between key mechanical properties, Young’smodulus, maximal strain and maximal stress to micro-structural parameters, fiber density, diameterand orientation, and age. To capture non-linear dependencies between the four characteristics and the micro-structural properties, we calculated the so-called distance correlation (DC) between the variousvariables. Age was found to correlate negatively with Youngs modulus and collagen density. Elastin fibers aligned significantly in both the heel and linear regions, the collagen bundles engaged andoriented mainly in the linear region. This research advances our understanding of skin biomechanicsand yields input for future first principles full modeling of skin tissue.