A model of the human cornea as a hydrated, fluid-saturated medium

We introduce a new model of the human corneal stroma, regarded as a fluid-saturated continuum, able to describe surface flattening and thickness thinning observed in several pathological conditions. In contrast with more common approaches that describe the human cornea as a quasi-incompressible hyperelastic medium, eventually including micro-structured anisotropy and heterogeneity, here we focus on the multi-phase nature of the tissue, where the content of water reaches about 78% in weight. The study is motivated by the fact that, although purely mechanical continuum models have been demonstrated to be satisfactory and accurate at predicting physiological behaviors, they have not been able to capture the geometrical features of tissue degeneration clearly associated to a reduction of the fluid content in the stroma, such thinning and loss of curvature. Here, we model the cornea as a fully saturated mixture of a solid phase and a fluid phase, in principle without restricting the formulation to specific assumptions on the actual inhomogeneous nature of both phases. The focus of the study is to understand whether a multiphysics model is capable of explaining, in terms of fluid flux imbalance, such as ectasia and keratoconus. As a first attempt in this direction, we make simple isotropic constitutive assumptions for both phases.