Streaming potential and the electromechanical response of physiologically-moist bone

Two competing mechanisms--piezoelectricity and the streaming potential--have been proposed for the origin of electrical signals produced by the bending of physiologically-moist bone. We focus here on the streaming potential and present new experimental data in an attempt to identify the dominant mechanism. We note, however, that a clear choice cannot be made solely from qualitative observations of the bending of wet bone specimens, as both mechanisms depend in the same way on the gradient of the strain from the compression to tension faces of the specimen, where the electrodes are located, and both mechanisms produce a current proportional to the strain rate. The relation between this current and the measured voltage is shown to be influenced by specimen mechanical relaxation, which has a much longer time constant than the RC decay of the equivalent circuit. To develop more information on the streaming potential in bone, direct measurements were performed: eletrolytes of various ionic concentrations and viscosities were forced through a flat disc of bone and the potential difference across the specimen measured for corresponding fluid pressure differences. Bending tests for bone specimens soaked in the same solution used in the direct streaming potential study suggest that streaming potentials dominate piezoelectricity in wet bone bending.