Modelling of and Mixed Finite Element Methods for Gels in Biomedical Applications

A set of equilibrium equations for a biphasic polymer gel are considered with the end purpose of studying stress and deformation in confinement problems encountered in connection with biomedical implants. The existence of minimizers for the gel energy is established first. Further, the small-strain equations are derived and related to the linear elasticity equations with parameters dependent on the elasticity of the polymer and the mixing of the polymer and solvent. Two numerical methods are considered, namely a two-field displacement-pressure formulation and a three-field stressdisplacement-rotation formulation with weakly imposed symmetry. The symmetry of the stress tensor is affected by the residual stress induced by the polymer-solvent mixing. A novel variation of the stress-displacement formulation of linear elasticity with weak symmetry is therefore proposed and analyzed. Finally, the numerical methods are used to simulate the stresses arising in a confined gel implant.