Piezoelectric properties of bone as functions of moisture content

FUKADA and Yasuda¹ showed that dry bone is piezoelectric in the classic sense, that is, mechanical stress produces polarisation (direct effect) and application of an electric field produces strain (converse effect)². The possibility that the mechanism for osteogenesis is electrical³ has aroused interest in an investigation of such effects in the more nearly physiological condition represented by wet bone. Recent developments in electrical stimulation of fracture healing^4,5 have further emphasised the importance of characterising the piezoelectric properties of wet bone. As the piezoelectricity of wet collagen⁶ decreases to zero at a moisture content equal to 45% of the dry weight (which corresponds to almost 100% humidity)⁷, there is some doubt as to whether wet bone could be piezoelectric. Observations of voltages in wet bone under stress, have been made under conditions giving such ambiguous results that they suggested alternative concepts to that of classic piezoelectricity^8–11. In view of the confusion about the nature of the voltage developed when wet bone is stressed, we have used a simple longitudinal stress system with sinusoidally varying drive, to measure both converse and direct piezoelectric coefficients of bone as functions of humidity (and ultimately of moisture content). The results show unambiguously that wet bone behaves as a piezoelectric material.