The Role of Anisotropy in Nonlinear Elastic Models of Skin
The skin is an inherently anisotropic material, a direct result of its fibre-reinforced structure, meaning its mechanical behaviour varies with direction. We investigate the significance of this anisotropy when modelling the skin within a nonlinear elastic framework and determine the minimum level of anisotropy required within a constitutive model to accurately predict the experimentally observed data. We begin with a discussion of the relevant biological features of the skin, focusing our attention on its collagenous architecture. We then present the necessary prerequisites from nonlinear elasticity theory before examining three possible models of the skin: the neo-Hookean, transversely istoropic Holzapfel-Gasser-Ogden and orthotropic Holzapfel-Gasser-Ogden, comparing each to experimental data and, finally, we conclude by showing that orthotropy can be considered the minimum level of anisotropy required for an accurate constitutive model of the skin.