The most serious side effect observed during the destruction of gallstones by shock waves in dogs was lung bleeding. To determine the conditions leading to lung damage, pressure probes were implanted into dogs between the lung and the diaphragm. The distance between the lung and the focal point of the pressure field was determined at which 1000 shock waves caused no more lung hemorrhage. On the long axis it is greater than 15 cm and perpendicular to the long axis it is 4 cm. Shock wave pressures over 2 MPa could be administered safely, whereas a pressure of 10 MPa caused bleedings in beagles, but probably not in boxers.

Biological effects of shock waves: lung hemorrhage by shock waves in dogs--pressure dependence.

The piezoelectricity observed in poly(vinylidene fluoride) (PVDF) and its copolymers involves three components that are associated with the presence of at least two phases (crystalline and amorphous) in the polymer structure. The main contributions to the phenomenon observed are made by the size effect and electrostriction, which are related to each other. These contributions manifest themselves through the mechanism of strain-induced reversible transformations of a number of domains of the anisotropic amorphous phase into the crystalline state under the action of mechanical or electrical fields. With due regard for different packings of chains in the amorphous and crystalline phases, this mechanism accounts for the large Poisson ratios μ31 obtained for textured films of flexible-chain crystallizing polymers. The dependence of the piezoelectric coefficient d
32 on the static stress in textured films is governed by the change in the fraction of the crystalline phase due to strong anisotropy of the elastic constants in the film plane. It is shown that the shear deformations of polymers are characterized by a strong piezoelectric response. The specific features revealed in the piezoelectric effect under bending deformations are described for films with an inhomogeneous distribution of polarization over the cross section. The general regularities of the electrostriction in the polymers and inorganic relaxor ferroelectrics studied are considered.

Piezoelectricity in crystallizing ferroelectric polymers: Poly(vinylidene fluoride) and its copolymers (A review)

Development of calibration techniques for ultrasonic hydrophone probes in the frequency range from 1 to 100 MHz.

The pressure fields of two different high intensity focused ultrasound (HIFU) transducers operated in burst mode were measured at acoustical power levels of 25 and 50 W (continuous wave equivalent) with three different hydrophones: A fiber-optic displacement sensor, a commercial HIFU needle hydrophone, and a prototype of a membrane hydrophone with a protective coating against cavitation effects. Additionally, the fields were modeled using a freely available simulations software package. The measured waveforms, the peak pressure profiles, as well as the spatial-peak temporal-average intensities from the different devices and from the modeling are compared and possible reasons for differences are discussed. The results clearly show that reliable pressure measurements in HIFU fields remain a difficult task concerning both the reliability of the measured values and the robustness of the sensors used: Only the fiber-optic hydrophone survived all four exposure regimes and the measured spatial-peak temporal-aver...

A comparative evaluation of three hydrophones and a numerical model in high intensity focused ultrasound fields.

Focused fields of finite-amplitude ultrasonic pulses occurring in lithotripsy can be measured with hydrophones made of the piezoelectric polymer polyvinylidene fluoride. Extreme requirements for the hydrophones-peak pressure of more than 20 MPa with a risetime of less than 50 ns (shock pulses)-lead to a special construction of the hydrophone where sensitive area and metal electrodes are well separated spatially. The signal from the sensitive area is capacitively coupled to the metal electrodes by a dielectric liquid medium. With a thin, 25- mu m PVDF foil, and an unmetalized sensitive area of 3 mm diameter, a hydrophone of 2*10/sup -2/ V/MPa sensitivity with a bandwidth near 20 MHz is achieved. More than 10/sup 5/ focused shock pulses of approximately=20 MPa were measured without a significant decrease in sensitivity of the hydrophone. >

PVDF hydrophone for the measurement of shock waves (lithotripsy)

The effects of hydrostatic pressure and pressure cycling on the piezoelectric properties of polyvinylidene fluoride (PVF2) have been experimentally investigated by using an acoustic reciprocity technique. The hydrostatic piezoelectric constants and the relative dielectric constant were measured as a function of pressure and pressure cycling for both voided and nonvoided PVF2 samples. The dynamic response of these materials to high‐amplitude pressure pulses having a rise time of 1–3 ms was also determined. The results showed that the microvoid structures in PVF2 improve the material’s piezoelectric properties but at the same time increase its pressure dependence and introduce a hysteresis effect with changing hydrostatic pressure. Nonvoided PVF2 films were also shown to have a good dynamic response to pressure pulses and, therefore, are promising for applications such as underwater shock sensors.

Effects of static and dynamic stress on the piezoelectric and dielectric properties of PVF2

A thick‐film polyvinylidene fluoride (PVF2) sample was used to evaluate this material for underwater shock‐sensor applications. The free‐field voltage sensitivity of the sensor showed a flat frequency response with a bandwidth of 2 MHz. The response of the PVF2 sample to a 1‐ms pressure pulse was linear to within 0.6 dB at a peak pressure of 69 MPa. The PVF2 shock sensor also showed reduced high‐frequency ringing in its response to a shock wave when compared with a similar device using a PZT element.

Robert Y. Ting

Papers

Effects of static and dynamic stress on the piezoelectric and dielectric properties of PVF2

The evaluation of PVF2 for underwater shock‐wave sensor application