  A multiphasic constitutive model of the skin that implicitly accounts for the
process of intrinsic (i.e.\ chronological) ageing via variation of the
constitutive parameters is proposed. The structurally-motivated constitutive
formulation features distinct mechanical contributions from collagen and
elastin fibres. The central hypothesis underpinning this study is that the
effects of ageing on the mechanical properties of the tissue are directly
linked to alterations in the microstructural characteristics of the collagen
and elastin networks. Constitutive parameters in the model, corresponding to
different ages, are identified from published experimental data on bulge tests
of human skin. The identification procedure is based on an inverse finite
element method. The numerical results demonstrate that degradation of the
elastin meshwork and variations in anisotropy of the collagen network are
plausible mechanisms to explain ageing in terms of macroscopic tissue
stiffening. Whereas alterations in elastin affect the low-modulus region of the
skin stress-strain curve, those related to collagen have an impact on the
linear region.
