Abstract
Macroscopic mechanical properties of human skin in vivo cannot be considered independent of adjacent subcutaneous white adipose tissue (sWAT). The layered system skin/sWAT appears as the hierarchical structural composite in which single layers behave as fiber-reinforced structures. Effective macroscopic mechanical properties of such composites are mainly determined either by the properties of the skin or by those of the sWAT, dependent on the conditions of mechanical loading. Mechanical interactions between the skin and the adjacent sWAT associated with a mismatch in the mechanical moduli of these two layers can lead to production of the skin wrinkles. Reinforcement of the composite skin/sWAT can take place in different ways. It can be provided through reorientation of collagen fibers under applied loading, through production of new bonds between existing collagen fibers and through induction of additional collagen structures. Effectiveness of this type of reinforcement is strongly dependent on the type of mechanical loading. Different physical interventions induce the reinforcement of at least one of these two layers, thus increasing the effective macroscopic stiffness of the total composite. At the same time, the standalone reinforcement of the skin appears to be less effective to achieve a delay or a reduction of the apparent signs of skin aging relative to the reinforcement of the sWAT.
Highlights
Macroscopic mechanical properties of human skin in vivo cannot be considered independent of adjacent subcutaneous white adipose tissue
We have proposed that such mechanical reinforcement of subcutaneous white adipose tissue (sWAT) together with androgen-dependent transition of adipocytes from this layer to myofibroblasts can be an important pathophysiological step in pattern baldness typical for androgenetic alopecia [42]
The skin/sWAT system in vivo demonstrate mechanical properties that are strongly dependent on the conditions of its mechanical loading
Summary
At low strain rates, Es should be considered to be much bigger than Ef. In this case, Eck % aEs, and the effective Young’s modulus of the composite is mainly determined by mechanical properties of the skin and not by those of the sWAT. Considering the skin as a fiber-reinforced composite with fibrous bundles having an orientation between the spatially uniform 3D structure and full alignment in the direction of loading, the effective Young’s modulus of the skin can be estimated as ð1 À bÞEsm þ 0:2gbEcol Esk ð1 À bÞEsm þ gbEcol ð13Þ
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