Articular cartilage contains a high fixed charge density under physiological conditions associated primarily with the ionized proteoglycan molecules of the extracellular matrix. Oscillatory compression of cartilage using physiological loads produces electrical potentials that have been shown previously to be the result of an electrokinetic (streaming) transduction mechanism. Two additional electromechanical phenomena not previously seen in cartilage or other soft tissues have now been demonstrated:​ "streaming current" and "current-generated stress.” Sinusoidal mechanical compression of cartilage specimens induced a sinusoidal streaming current density through cartilage disks when the Ag/AgCl electrodes that were used to compress the cartilage were shorted together externally. Conversely, a sinusoidal current density applied to the tissue generated a sinusoidal mechanical stress within the tissue. Both these phenomena were found to be consistent with the same electrokinetic transduction mechanism responsible for the streaming potential.

Changes in the measured streaming potential response that resulted from modification of bath ionic strength and pH have provided additional insights into the molecular origins of electrokinetic transduction. Significant changes in the p tential response of cartilage were also observed when the extracellular matrix was chemically modified by extraction of proteoglycan and glycosaminoglycan moities using chondroitinase-ABC and trypsin. The streaming potential was found to be a sensitive index of the degradative loss of these matrix constituents and of the kinetics of the enzymatic degradative process.

A continuum model has been formulated for linear electrokinetic transduction in cartilage. Expressions are derived for the streaming potential and streaming current induced by oscillatory, uniaxial confined compression of the tissue, as well as the mechanical stress generated by a current or potential difference applied to the tissue. The experimentally observed streaming potential and current-generated stress response, measured on the same specimens, are compared with the predictions of the theory over a wide frequency range. The theory compares well with the data for reasonable values of cartilage intrinsic mechanical parameters and electrokinetic coupling coefficients. Experiments also show a linear relationship between the stimulus amplitude and the transduction response amplitude, consistent with the predictions of the linear theory, To examine variations in material properties that evolve in space and time, the continuum model is extended by splicing together the transfer relations that describe the properties of a set of piecewise uniform layers which approximate the continuous properties of the tissue.

Electrokinetic transduction and electrostatic swelling stress are described by a macroscopic model based on the assumption of uniform space charge throughout the fluid phase of the tissue. This model relates phenomenological coefficients to physical constants such as the hydraulic permeability, charge density, and fluid conductivity of the ECM. Using an equilibrium titration model to self-consistently find the charge density and internal ionic concentrations, the model appears to successfully predict the observed streaming potential variation with pH and ionic strength.