On the electric potentials inside a charged soft hydrated biological tissue: Streaming potential versus diffusion potential

The main objective of this study is to determine the nature of electric fie lds inside articular cartilage while accounting for the effects of both str eaming potential and diffusion potential. Specifically, we solve two tissue mechano-electrochemical problems using the triphasic theories developed by Lai et al. (1991, ASME J. Biomech Eng., 113, pp. 245-258) and Gu et al. (1 998, ASME J. Biomech. Eng., 120, pp. 169-180) (1) the steady one-dimensiona l permeation problem; and (2) the transient one-dimensional ramped-displace ment, confined-compression, stress-relaxation problem (both in an open circ uit condition) so as to be able to calculate the compressive strain, the el ectric potential, and the fixed charged density (FCD) inside cartilage. Our calculations show that in these two technically important problems, the di ffusion potential effects compete against the flow-induced kinetic effects (streaming potential) for dominance of the electric potential inside the ti ssue. For softer tissues of similar FCD (i.e., lower aggregate modulus), th e diffusion potential effects are enhanced when the tissue is being compres sed (i.e., increasing its FCD in a nonuniform manner) either by direct comp ression or by drag-induced compaction; indeed the diffusion potential effec t may dominate over the streaming potential effect. The polarity of the the electric potential field is in the same direction of interstitial fluid fl ow when streaming potential dominates, and in the apposite direction of flu id flow when diffusion potential dominates. For physiologically realistic a rticular cartilage material parameters, the polarity of electric potential across the tissue on the outside (surface to surface) may be opposite to th e polarity across the tissue on the inside (surface to surface). Since the electromechanical signals that chodrocytes perceive in situ are the stresse s, strains, pressures and the electric field generated inside the extracell ular matrix when the tissue is deformed, the results from this study offer new challenges for the understanding of possible mechanisms that control ch ondrocyte biosyntheses. [S0148-0731(00)00604-X].