An electromechanochemical swelling model is developed to describe equilibrium and nonequilibrium swelling behavior of polyelectrolyte materials. A constitutive relation for the swelling pressure as a function of tissue deformation and external bath electrolyte concentration is developed, introducing the concept of the chemical stress. This constitutive relation is then used to describe the mechanical response of a tissue sample in uniaxial confined compression to a change in the salt concentration of the external bath. A coupled set of differential equations results. Expressions are derived that clearly show that when the surface displacement is held constant, the system response is at least as fast as the shortest of the mechanical and chemical diffusion time constants. When the stress is held constant, the creep deformation proceeds with the longer of the two time constants.

The material properties of bovine articular cartilage and corneal stroma are measured as a function of concentration and empirical constitutive relations describing the observed behavior are developed. These relations are then used in the model to numerically solve the coupled set of differential equations for chemically induced stress relaxation. The results of the numerical model compare favorably with experimental results obtained for cartilage and corneal stroma samples for reasonable values of the material parameters.

The phenomenological equations of nonequilibrium thermodynamics are introduced to describe electrokinetic interactions in charged porous materials. A first order electrokinetic micromodel is developed to describe the coupling coefficients in terms of microscopic structural and compositional parameters. A unit cell technique is used where the solid matrix of the tissue is modeled as an ordered array of electrically insulating cylinders supporting a fixed surface charge density. The double layer is modeled as a Helmholtz capacitor, with all of the mobile double layer charge located one Debye length from the surface of the charged cylinder. The coupling coefficients are derived for flow perpendicular and parallel to the axes of the charged cylinders. The coupling coefficients appropriate for a random assemblage of cylinders are obtained from a weighted average of the coefficients for parallel and perpendicular flow. Using appropriate parameter values, the electrokinetic micromodel predicts values for the coupling coefficients that are the same order of magnitude as coupling coefficients previously measured for cartilage, The micromodel also predicts an initial rise in the effective conductivity of cartilage which has been observed experimentally, and cannot be explained with traditional ion exchange macromodels.