Damping, amplitude, aging tests of stacked transducers for shock wave generation

New clinical concepts in lithotripsy demand small shock heads. Reducing the size of piezoelectric shock heads will be possible only if the pressure generated at the surface of each transducer can be increased so that the total pressure at the focus remains the same. To solve this problem, different solutions were proposed. For example, it has been demonstrated that piezocomposite material, as opposed to piezoceramic material, allows the generation of a higher surface pressure before breaking, mainly because radial modes are dramatically reduced. In addition, in a previous paper, we showed the feasibility of generating high-pressure pulse waves without increasing the transducer voltage by using sandwiched transducers, which are a stack of two or more transducers. Some discrepancies appeared, however, between the pressure measured at the surface of the front transducer and the arithmetic sum of the pressures generated by each transducer constituting the stack. In fact, development of such stacked transducers capable of generating surface pressures in the range of 2 to 5 MPa is very complex, which may explain why no aging tests have been reported in the literature thus far. In the first part of this paper, we theoretically determine the importance of the electroacoustical coupling between the two transducers on the generated surface pressure. We show that pressure losses due to these electroacoustical couplings are less than 5%. Experimental measurements done on a stacked transducer assembled and tightened in a castor oil-filled tank are in excellent accordance with the theoretical measurements. Using this assembly technique, it was possible to obtain, on average, out of four elements, a pressure of 7.5 MPa for the duration of 4 million shocks, which would allow the treatment of approximately 1000 patients.