Abstract
A theoretical-experimental approach was taken to develop the constitutive relations of transport properties (i.e., hydraulic permeability and solute diffusivity) for cartilaginous tissues and to develop a mechano-electrochemical finite element model for analyzing physical signals and nutrient transport in such tissues. The motivation of these studies is to better understand the etiology of cartilaginous tissue degeneration. The identification of such mechanisms will provide tremendous potential for diagnosis, treatment, and prevention of pathological conditions, such as low back pain.A specialized triphasic theory for charged hydrated soft tissues was presented as a framework for the theoretical and experimental investigations. Based on this theory, the transport properties (e.g., hydraulic permeability and solute diffusivity) within the tissue were defined.New constitutive relationships between intrinsic permeability and volume fraction of water were obtained for both agarose gels and intervertebral disc (IVD) tissues using confined creep test. These new constitutive equations are capable of predicting deformation-dependent permeabilities. Analysis of the experimental data has shown that the water content plays a more important role in regulating tissue permeability than fixed charge density for normal tissues.New constitutive relations of relative solute diffusivity to tissue water content in agarose gels and IVD tissues were developed using an electrical conductivity method. Results indicate that the ratio of solute radius to effective pore radius is primarily responsible for the reduced solute diffusivity in such materials relative to the diffusivity in free solution. The anisotropic transport behavior in IVD was also investigated.A mixed finite element formulation of the specialized triphasic theory was developed by choosing the (electro)chemical potentials of ions, uncharged solute and fluid, and the solid displacement as the primary degree of freedom. This formulation was successfully employed to simulate 2D unconfined dynamic compression of the cartilage explant and 3D unconfined compression of the entire IVD. The analyses of the mechanical, electrical and chemical signals and nutrient transport within the tissue under mechanical loading were performed for these 2D and 3D cases.