Layered piezoelectric structures with arbitrary acoustic termination impedances.

Multilayer piezoelectric transducers and resonators are widely used for generating propagating and standing acoustic waves as well as for sensor devices. More recently, layered piezoelectric structures based on thin film technology became increasingly important for electromechanical filters used in mobile phones. As a consequence, analytical mathematical modeling of such structures is of high interest. In this paper, a general rigorous transfer matrix description for one-dimensional (1D) layered structures consisting of piezoelectric, visco-elastic, and dielectric layers of arbitrary number is introduced to characterize the electrical and mechanical behavior of a general piezoelectric transducer or resonator with two electrodes and arbitrary acoustic termination impedances (Rig-1D-model). This model is the most general 1D analytical description of layered piezoelectric structures available and can be used for the characterization of various composite transducer and resonant sensor applications. Considered in detail are layered structures with the technically important cases of only one electromechanically coupled mode, and the structure at one or both outer surfaces is in contact with semi-infinite media. For such devices, it is shown how the frequency dependence of the total electrical admittance and spatial dependence of the displacements can be calculated.

Can a commercial diagnostic ultrasound device accelerate thrombolysis? An in vitro skull model.

BACKGROUND AND PURPOSE: Recently, 3 clinical trials revealed encouraging results in recanalization and clinical outcome in acute stroke patients when 2-MHz transcranial Doppler monitoring was applied. This study investigated whether a 1.8-MHz commercial diagnostic ultrasound device has the potential to facilitate thrombolysis using an in vitro stroke model. METHODS: Duplex-Doppler, continuous wave-Doppler, and pulsed wave (PW)-Doppler were compared on their impact on recombinant tissue plasminogen activator (rtPA)-mediated thrombolysis. Blood clots were transtemporally sonicated in a human stroke model. Furthermore, ultrasound attenuation of 5 temporal bones of different thickness was determined. RESULTS: In comparison, only PW-Doppler accelerated rtPA-mediated thrombolysis significantly. Without temporal bone, PW-Doppler plus rtPA showed a significant enhancement in relative clot weight loss of 23.7% when compared with clots treated with rtPA only (33.9+/-5.5% versus 27.4+/-5.2%; P<0.0005). Ultrasound attenuation measurements revealed decreases of the output intensity of 86.8% (8.8 dB) up to 99.2% (21.2 dB), depending on temporal bone thickness (1.91 to 5.01 mm). CONCLUSIONS: Without temporal bone, PW-Doppler significantly enhanced thrombolysis. However, because of a high attenuation of ultrasound by temporal bone, no thrombolytic effect was observed in our in vitro model, although Doppler imaging through the same temporal bone was still possible.

Ultrasound affects distribution of plasminogen and tissue-type plasminogen activator in whole blood clots in vitro.

Ultrasound of 2 MHz frequency and 1.2 W/cm(2) acoustic intensity was applied to examine the effect of sonication on recombinant tissue-type plasminogen activator (rt-PA)-induced thrombolysis as well as on the distribution of plasminogen and t-PA within whole blood clots in vitro. Thrombolysis was evaluated quantitatively by measuring clot weight reduction and the level of fibrin degradation product D-dimer (FDP-DD) in the supernatant. Weight reduction in the group of clots treated both with ultrasound and rt-PA was 35.2% +/-6.9% which is significantly higher (p<0.0001) than in the group of clots treated with rt-PA only (19.9% +/-4.3%). FDP-DD level in the supernatants of the group treated with ultrasound and rt-PA increased sevenfold compared to the group treated with rt-PA alone, (14895 +/-2513 ng/ml vs. 2364 +/-725 ng/ml). Localization of fibrinolytic components within the clots was accomplished by using gel-entrapping technique and immunohistochemistry. Spatial distributions of t-PA and plasminogen showed clearly that ultrasound promoted the penetration of rt-PA into thrombi significantly (p<0.0001), and broadened the zone of lysis from 8.9 +/-2.6 microm to 21.2 +/-7.2 microm. We speculate that ultrasound enhances thrombolysis by affecting the distribution of rt-PA within the clot.

A capacitance ultrasonic transducer for high-temperature applications.

This paper introduces a novel ultrasonic capacitance transducer for operation at elevated gas temperatures of several hundred degrees Celsius. The transducer design is based on a metallic membrane foil and a backplate made of an electrically conducting substrate coated with an insulation layer. Guidelines are given for selecting suitable materials for the membrane foil, the backplate substrate, and the coating. Manufacturing techniques applied for fabrication of different transducer types are described in detail. Transducers tested were composed of titanium substrates with insulation layers of silicon nitride or of silicon substrates with silicon oxide coatings. Experimental setup and test procedures are described. Results of transducer characterization and performance tests at elevated temperatures are presented and discussed. Transducer functionality is proven for air temperatures of up to 500 degrees C.

2MHz ultrasound enhances t-PA-mediated thrombolysis: comparison of continuous versus pulsed ultrasound and standing versus travelling acoustic waves.

In addition to fibrinolytic enzymes, ultrasound has the potential to enhance thrombolysis. High frequency ultrasound has the advantage that a combination of diagnostic and therapeutic ultrasound with only one device is possible. Therefore, we investigated the optimal high frequency (2 MHz) ultrasound field characteristics and application mode in vitro. Continuous ultrasound significantly enhanced rt-PA mediated thrombolysis: in a travelling wave field thrombolysis was augmented by 49.0 +/- 14.7% and in a standing wave field by 34.8 +/- 7.3%. In an intermittent application mode (1Hz, 10Hz, 100Hz, 1kHz) most efficient results were obtained for both wave fields using 1 Hz (46.4 +/- 10.7% and 39.1 +/- 6.6%, respectively). Referring to a possible in vivo application our in vitro data suggests that an intermittent application of a 2 MHz high frequency ultrasound using a travelling wave field would be the most potent application for lysing blood clots.

Quantification of a novel h-shaped ultrasonic resonator for separation of biomaterials under terrestrial gravity and microgravity conditions.

A novel, h-shaped ultrasonic resonator was used to separate biological particulates. The effectiveness of the resonator was demonstrated using suspensions of the cyanobacterium, Spirulina platensis. The key advantages of this approach were improved acoustic field homogeneity, flow characteristics, and overall separation efficiency (sigma = 1 - ratio of concentration in cleared phase to input), monitored using a turbidity sensor. The novel separation concept was also effective under microgravity conditions; gravitational forces influenced overall efficiency. Separation of Spirulina at cleared flow rates of 14 to 58 L/day, as assessed by remote video recording, was evaluated under both microgravity (

In vitro thrombolysis enhanced by standing and travelling ultrasound wave fields.

Success of thrombolytic therapy depends on penetration of recombinant tissue plasminogen activator (rt-PA) into clots. Ultrasound (US) of therapeutic quality accelerates thrombolysis in vitro. As yet, only the effects of travelling acoustic waves on thrombolysis have been investigated, and the impact of standing acoustic waves has been neglected. In the present study, we examined the effects of standing and travelling US wave fields applied continuously for 1 h (frequency 2 MHz, acoustic intensity 1.2 W/cm(2)) on thrombolysis enhancement by measuring clot weight reduction and concentration of fibrin degradation product D-dimer (FDP-DD) produced from clots subjected to rt-PA. The level of FDP-DD was 1.8 times greater in travelling than in standing acoustic waves. Thrombolysis enhancement was 46.0 +/- 20.8% in standing and 116.8 +/- 23.1% in travelling acoustic waves. Travelling waves enhanced thrombolysis significantly more (p < 0.0001) than did standing waves.

Single half-wavelength ultrasonic particle filter: predictions of the transfer matrix multilayer resonator model and experimental filtration results.

The quantitative performance of a "single half-wavelength" acoustic resonator operated at frequencies around 3 MHz as a continuous flow microparticle filter has been investigated. Standing wave acoustic radiation pressure on suspended particles (5-microm latex) drives them towards the center of the half-wavelength separation channel. Clarified suspending phase from the region closest to the filter wall is drawn away through a downstream outlet. The filtration efficiency of the device was established from continuous turbidity measurements at the filter outlet. The frequency dependence of the acoustic energy density in the aqueous particle suspension layer of the filter system was obtained by application of the transfer matrix model [H. Nowotny and E. Benes, J. Acoust. Soc. Am. 82, 513-521 (1987)]. Both the measured clearances and the calculated energy density distributions showed a maximum at the fundamental of the piezoceramic transducer and a second, significantly larger, maximum at another system's resonance not coinciding with any of the transducer or empty chamber resonances. The calculated frequency of this principal energy density maximum was in excellent agreement with the optimal clearance frequency for the four tested channel widths. The high-resolution measurements of filter performance provide, for the first time, direct verification of the matrix model predictions of the frequency dependence of acoustic energy density in the water layer.