In a piezoelectric actuator, a first driving signal amplified a predetermined sine wave is applied to a piezoelectric device in a direction parallel to a polarization direction thereof. One of the conductor elements is connected to a yielding structure that ensures the simultaneous contact of both conductor elements with the human body during the therapeutic treatment. The elevational step has an uppermost surface. The panel assembly further includes a first support member attached to the first platform and defining an initial state and a final state.