Measurement of the piezoelectric effect in bone using quasi-heterodyne holographic interferometry

The piezoelectric properties in bone have been recognized for nearly three decades. Most investigations concerning this effect have focused on strain induced potentials. In our research, a quasi-heterodyne holographic system was used to measure the piezoelectric effect in bone. Data are acquired at video rates using four phase shifts and the phase is reconstructed at discrete locations over the surface of the bone. Phase changes can be measured to a precision of at least 4 degrees. The interference fringes are observed in real-time by producing a hologram of dry, bovine tibia held in a vice. A voltage is applied to electrodes on the surface of the bone and a single component of surface displacement is measured. Because the piezoelectric coefficients in bone are extremely small, approximately a picometer per volt, it is useful to improve the signal-to-noise of the technique. The method we have pursued is to average over repeatedly acquired data sets. The presence of air turbulence, however, requires that compensation for spurious phase changes be applied to the data before averaging. We have developed a technique which compensates for low spatial frequency phase changes, allowing averaging over temporally separated data sets. The basis of the technique is to surround the object by a stationary control. The phase over this control is used to correct for the phase over the object. The corrected phase images are averaged together to improve the background signal-to-noise. The concept was evaluated on an aluminum blade and was shown to improve the phase estimate. Preliminary results on bone indicate cantilever bending due to an applied voltage with a piezoelectric constant of 0.6 pm/V.