Electromechanical effect in bone

Abstract

Although bone is piezoelectric, the signal from bent bone cannot be described by the piezoelectric effect if the samples are assumed to be homogeneous. To account for the symmetry properties and z-dependence of the signal from dry bone in cantilever bending, a theory postulating a polarization proportional to the gradient of the stress was proposed. The present work is concerned with a further examination of the applicability of the stress gradient theory to both dry and wet bone. Our measurements on dry bone samples cut from a bovine tibia indicate that the signal is not inversely proportional to the thickness squared, contrary to the prediction of the stress gradient theory for a homogeneous sample. Our measurements show that the moduli responsible for the signal in bent bone vary with position, a finding consistent with some earlier reports that the piezoelectric moduli determined in uniform stress measurements vary with position. These results imply that bone cannot be considered homogeneous. We then show that variations in the piezoelectric moduli can also account for the z-dependence and symmetry properties of the signal observed in bent bone. Since bone is not homogeneous, it is not necessary to introduce the phenomenological stress gradient theory to account for the otherwise anomalous electromechanical effect in bent bone, and we conclude that it is the variations in the piezoelectric moduli that produce the electromechanical effect in bent bone.