Identification of the mechanical properties of the skin by electromechanical impedance analysis of resonant piezoelectric actuator

This paper is devoted to the analysis and verification of developed piezoelectric sensor/actuator for measuring the mechanical properties of soft tissues, especially human skin. The key element of the measurement structure is an electromechanical system that uses both the reverse piezoelectric effect (vibration generation - stimulation of the tissues in the appropriate frequency range) and direct piezoelectric effect (system response registration). By analyzing the response of the piezoelectric sensor/actuator - an electromechanical signature of the tested tissue, it is possible to identify a series of mechanical quantities describing the tested object, such as stiffness, viscosity, Young´s modulus. An analytical approach is shown in order to establish a link between the electromechanical equivalent circuit obtained for a resonance frequency of the actuator and the calculations based on the Hertz´s contact mechanics theory. A finite element method analysis is then presented in order to assess the performance of the chosen topology of piezoelectric sensor/actuator. Finally, the proposed method of impedance characterization is verified experimentally by a prototype of piezoelectric bimorph sensor/actuator.