Evaluation and development of the instrumentation used in the measurement of bone piezoelectricity

The current amplifier, which is introduced in this study for the measurement of bone piezoelectricity, was theoretically analyzed along with voltage and charge amplifiers similar to those used previously to determine the limitations of each device when measuring electrical signals from a high impedence source like bone. Experimental prototypes of voltage and current amplifers were then constructed and used to experimentally measure the electrical response obtained from small rectangular bars of dried human femur subjected to cantilever bending.

From both the theoretical and experimental analysis it was found that the speed of response of the voltage amplifier was severely limited by a high impedence source with a finite value of capacitance whereas the speed of response of the current amplifier was essentially independent of the value of source resistance. (The calculated value of rise time for the voltage amplifer was 4 milliseconds compared with 0.0171 milliseconds for the current amplifier.) From the experimental analysis it was also found that the magnitude of current obtained from stressed bone samples which had been dried using different techniques varied considerably (3 to 35 picoamps), whereas the magnitude of voltage remained constant (30 millivolts), thus indicating that the current waveform may contain valuable information which had essentially been "integrated out" by the voltage amplifier.

Therefore, future studies of bone piezoelectricity should take into account the limitations of the transduction system and must provide for the measurement of both voltage and current to obtain a more complete understanding of this phenomena.