Piezoelectric and inversely piezoelectric responses of bone tissue plates in the MHz range

Piezoelectricity in bone is thought to be a mechanism by which ultrasound promotes the healing of bone fractures. However, few studies have been conducted in the more clinically relevant MHz range. To understand the piezoelectricity in bone, we fabricated ultrasound transducers using bone samples as piezoelectric materials and identified the longitudinal ultrasound radiation and reception in the MHz range. The maximum transmitting sensitivity of the bone transducer was 140 mPa/V, which was nearly 1/1000 of a polyvinylidene difluoride (PVDF) transducer that has better electrical properties and piezoelectricity. The resonance frequencies of the transducer depend on the plate thickness and angle between the bone axis (alignment direction of the hydroxyapatite crystallites) and ultrasound propagation direction, reflecting the anisotropic character of the bone. The reception and transmission sensitivities of the bone transducers also depend on the plate thickness and angle, showing maximum values at off-axis angles. These results indicate the existence of both piezoelectricity and inverse piezoelectricity in bone, which may be key factors in understanding the bone healing by lowintensity biophysical (electrical or mechanical) stimulation.