Development of a compact self-focusing piezoelectric generator using electrical pre-strain piezocomposite material.

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will only be possible if the pressure generated at the surface of each transducer can be increased so that, the total pressure at the focus remains very high. So, we propose a new method allowing the generation of large surface pressures. It is well known that the piezoelectric rods in piezocomposite material are more fragile in the extension mode than in the compression mode. For this reason, actuators are mechanically pre-stressed between two flasks. This method cannot be used for transducers working at high frequencies, about 0.5 MHz. For this reason, we proposed to electrically pre-strain the piezoelectric material by applying high electric field in the opposite direction of the polarisation. In a first mode we proposed to pre-strain in continuous mode the transducer. Unfortunately we noticed a rapid de-poling and re-poling in the inverse direction. In a second mode to reduce depolarisation, this field was applied only during a short time just before the generation of the pulse which generate the compressive wave and in a third mode, the transducer was re-poled between two successive electrical pulses. Using this last method, it was possible to increase the maximum pressure at the surface of a 20 mm diameter plane piston to 20% and reach 4 MPa. According to this idea a very compact shock wave generator was designed. The generator made of a 1-3 piezocomposite material has a diameter of 120 mm and focused at 120 mm. The maximum pressure and the width of the compressive wave at the focus were, respectively, 60 MPa and 1.5 micros. The focal zone measured at -3 dB is an ellipsoid 6 mm high in the propagating axis and 3 mm width in the perpendicular direction. The efficacy of this generator was measured as the number of shocks necessary to totally disintegrate plaster balls 15 mm in diameter mimicking the kidney stones. At full power the number of shocks was only 150 which is rather the same number as the one obtained using electrohydraulic machine generally considered as the gold standard. This results show that piezoelectric material may be advantageously used for the manufacturing of shock wave generators.