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Ion concentration effects on the zeta potential of bone

Stressed bone generates an electrical potential, the sign of which reverses in the presence of specific concentrations of sodium, potassium, or calcium ions. A study was designed to test the hypotheses, drawn from stress‐generated potential (SGP) studies, that the reversal in polarity of the zeta potential of bone is the cause of this SGP polarity reversal. The zeta potential of bone particles, prepared from fresh bovine metatarsals as homogeneous 5 μm dispersions, was measured by free‐fluid electrophoresis in different concentrations of sodium chloride. The zeta potential of bone particles reversed sign at a sodium ion concentration that was the same as that found to reverse measured SGPs, namely 0.74 molar. This finding is consistent with the hypothesis that streaming potentials are the cause of SGPs in fluid‐saturated bone. It was possible to obtain electrophoretic mobility histograms, and hence zeta‐potential histograms, of bone particles by using a Pen Kem, Inc. (Bedford Hills, NY, U.S.A.) “3000” Automated Electrokinetic Analyzer. At sodium ion concentrations at which bone particles were nearly neutral, the zeta‐potential histograms indicated a broad distribution of particle charge, with some particles being negative, some positive, and some neutral. In addition, it was found that particles prepared from Formalin‐fixed bone produced the same electrokinetic results as those prepared from fresh bone, and that the addition of MOPS buffer caused the zeta potential to invert sign from negative to positive values at ion concentrations exceeding 3.0 molar.