A multiphasic constitutive model of the skin that implicitly accounts for theprocess of intrinsic (i.e.\ chronological) ageing via variation of theconstitutive parameters is proposed. The structurally-motivated constitutiveformulation features distinct mechanical contributions from collagen andelastin fibres. The central hypothesis underpinning this study is that theeffects of ageing on the mechanical properties of the tissue are directlylinked to alterations in the microstructural characteristics of the collagenand elastin networks. Constitutive parameters in the model, corresponding todifferent ages, are identified from published experimental data on bulge testsof human skin. The identification procedure is based on an inverse finiteelement method. The numerical results demonstrate that degradation of theelastin meshwork and variations in anisotropy of the collagen network areplausible mechanisms to explain ageing in terms of macroscopic tissuestiffening. Whereas alterations in elastin affect the low-modulus region of theskin stress-strain curve, those related to collagen have an impact on thelinear region.
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