TRANSPORT OF FLUID AND IONS THROUGH A POROUS-PERMEABLE CHARGED-HYDRATED TISSUE, AND STREAMING POTENTIAL DATA ON NORMAL BOVINE ARTICULAR-CARTILAGE

Using the triphasic mechano-electrochemical theory [Lai et al., J, bio mech. Engng 113, 245-258 (1991)], we analyzed the transport of water a nd ions through a finite-thickness layer of charged, hydrated soft tis sue (e.g. articular cartilage) in a one-dimensional steady permeation experiment. For this problem, we obtained numerically the concentratio ns of the ions, the strain field and the fluid and ion velocities insi de when the specimen is subject to an applied mechanical pressure and/ or osmotic pressure across the layer. The relationships giving the dep endence of streaming potential and permeability on the negative fixed charge density (FCD) of thee tissue were derived analytically for the linear case, and calculated for the nonlinear case. Among the results obtained were: (1) at a fluid pressure difference of 0.1 MPa across th e specimen layer, there is a 10% flow-induced compaction at the downst ream boundary; (2) the flow-induced compaction causes the FCD to incre ased and the neutral salt concentration to decrease in the downstream direction; (3) while both ions move downstream, relative to the solven t (water), the anions (Cl-) move with the flow whereas cations (Na+) m ove against the flow. The difference in ion velocities depends on the FCD, and this difference attained a maximum at a physiological FCD of around 0.2 meq ml-1; (4) the apparent permeability decreases nonlinear ly with FCD, and the apparent stiffness of the tissue increases with F CD; and (5) the streaming potential is not a monotonic function of the FCD but rather it has a maximum value within the physiological range of FCD for articular cartilage. Finally, experimental data on streamin g potential were obtained from bovine femoral cartilage. These data su pport the triphasic theoretical prediction of non-monotonicity of stre aming potential as a function of the FCD.