Abstract. We derive a model of the coupled mechanical and electrochemical effects of polyelectrolyte gels. We assume that the gel, which is immersed in a fluid domain, is an immiscible and incompressible mixture of a solid polymeric component and the fluid. As the gel swells and de-swells, the gel-fluid interface can move. Our model consists of a system of partial differential equations for mass and linear momentum balance of the polymer and fluid components of the gel, the NavierStokes equations in the surrounding fluid domain, and the Poisson-Nernst-Planck equations for the ionic concentrations on the whole domain. These are supplemented by a novel and general class of boundary conditions expressing mass and linear momentum balance across the moving gel-fluid interface. Our boundary conditions include the permeability boundary conditions proposed in earlier studies. A salient feature of our model is that it satisfies a free energy dissipation identity, in accordance with the second law of thermodynamics. We also show, using boundary layer analysis, that the well-established Donnan condition for equilibrium arises naturally as a consequence of taking the electroneutral limit in our model.

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.