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Acoustofluidics inside the microchannel has already found its wide applications recently. Acoustic streaming and radiation force are two underlying mechanisms that determine the trajectory of microparticles and cells in the manipulation. Critical particle size of viscous effects is found to be about 1.6 µm in the conventional rectangular microchannel (W × H = 380 m × 160 m) at the frequency of 2 MHz, below which the acoustic streaming dominants, and is independent of the driving voltage. In order to effectively adjust such a critical size, a approach is proposed and evaluated numerically to enhance the acoustic streaming by adding some protrusions (i.e., in the shape of a wedge, rod, half-ellipse) to the middle of the top or bottom wall. It is found that the resonant frequency and acoustic pressure will decrease and the acoustic streaming velocity will increase significantly, respectively, with the increase of protrusion height (up to 30 µm while keeping the width the same as 8 µm). Subsequently, trajectory motion patterns of microparticles have apparent changes in comparison to those inside the rectangular microchannel, and acoustic streaming can even dominate the motion of large microparticles (i.e., 10 µm). As a result, the critical particle size could be increased up to 72.5 µm. Furthermore, different protrusion shapes (i.e., wedge, rod, half-ellipse) on the top wall were compared. The sharpness of protrusion at its tip seems to determine the acoustic streaming velocity. The wedge attached to the bottom wall had higher resonant frequency and lower acoustic streaming velocity compared with the top wedge in the same dimension. The patterns of acoustic streaming and microparticle trajectory motion in the microchannel with dual wedges on the top and bottom walls are not the superposition of those of the top and bottom wedge individually. In summary, the geometry of the microchannel has a significant effect on the induced acoustofluidics by the bulk acoustic waves. A much larger acoustic streaming velocity is produced at the tip of the protrusion to change the critical size of microparticles between acoustic streaming and radiation force. It suggests that more applications of acoustofluidics (i.e., mixing and sonoporation) to microparticles and cells in various sizes are feasible by designing an appropriate geometry of the microchannel., (© 2021. The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.)

The acoustic power of high-intensity focused ultrasound (HIFU) is an important parameter that should be measured prior to each treatment to guarantee effective and safe outcomes. A new calibration technique was developed that involves estimating the pressure distribution, calculating the acoustic power using an underwater pressure blast sensor, and compensating the contribution of harmonics to the acoustic power. The output of a clinical extracorporeal HIFU system (center frequency of ~1 MHz, p+ = 2.5-57.2 MPa, p(-) = -1.8 to -13.9 MPa, I(SPPA) = 513-22,940 W/cm(2), -6 dB size of 1.6 × 10 mm: lateral × axial) was measured using this approach and then compared with that obtained using a radiation force balance. Similarities were found between each method at acoustic power ranging from 18.2 W to 912 W with an electrical-to-acoustic conversion efficiency of ~42%. The proposed method has advantages of low weight, smaller size, high sensitivity, quick response, high signal-to-noise ratio (especially at low power output), robust performance, and easy operation of HIFU exposimetry measurement., (Copyright © 2015 IPEM. Published by Elsevier Ltd. All rights reserved.)

Purpose: A new method was devised to suppress the bubble cavitation in the lithotripter focal zone to reduce the propensity of shockwave-induced renal injury., Materials and Methods: An edge extender was designed and fabricated to fit on the outside of the ellipsoidal reflector of an electrohydraulic lithotripter to disturb the generation of diffraction wave at the aperture, but with little effect on the acoustic field inside the reflector., Results: Although the peak negative pressures at the lithotripter focus using the edge extender at 20 kV were similar to that of the original configuration (-11.1 ± 0.9 vs -10.6 ± 0.7 MPa), the duration of the tensile wave was shortened significantly (3.2 ± 0.54 vs 5.83 ± 0.56 μs, P<0.01). There is no difference, however, in both the amplitude and duration of the compressive shockwaves between these two configurations as well as the -6 dB beam width in the focal plane. The significant suppression effect of bubble cavitation was confirmed by the measured bubble collapse time using passive cavitation detection. At the lithotripter focus, while only about 30 shocks were needed to rupture a blood vessel phantom using the original HM-3 reflector at 20 kV, no damage could be produced after 300 shocks using the edge extender. Meanwhile, the original HM-3 lithotripter at 20 kV can achieve a stone comminution efficiency of 50.4 ± 2.0% on plaster-of-Paris stone phantom after 200 shocks, which is comparable to that of using the edge extender (46.8 ± 4.1%, P=0.005)., Conclusions: Modifying the diffraction wave at the lithotripter aperture can suppress the shockwave-induced bubble cavitation with significant reduced damage potential on the vessel phantom but satisfactory stone comminution ability.

This study investigated the characteristics of the secondary bubble cluster produced by an electrohydraulic lithotripter using high-speed imaging and passive cavitation detection techniques. The results showed that (i) the discrepancy of the collapse time between near a flat rigid boundary and in a free field of the secondary bubble cluster was not as significant as that by the primary one; (ii) the secondary bubble clusters were small but in a high bubble density and nonuniform in distribution, and they did not expand and aggregate significantly near a rigid boundary; and (iii) the corresponding bubble collapse was weaker with few microjet formation and bubble rebound. By applying a strong suction flow near the electrode tip, the production of the secondary shock wave (SW) and induced bubble cluster could be disturbed significantly, but without influence on the primary ones. Consequently, stone fragmentation efficiency was reduced from 41.2 ± 7.1% to 32.2 ± 3.5% after 250 shocks (p < 0.05). Altogether, these observations suggest that the secondary bubble cluster produced by an electrohydraulic lithotripter may contribute to its ability for effective stone fragmentation., (Copyright © 2012 World Federation for Ultrasound in Medicine & Biology. Published by Elsevier Inc. All rights reserved.)

The focusing of laser-generated shock waves by a truncated ellipsoidal reflector was experimentally and numerically investigated. Pressure waveform and distribution around the first (F(1)) and second foci (F(2)) of the ellipsoidal reflector were measured. A neodymium doped yttrium aluminum garnet laser of 1046 nm wavelength and 5 ns pulse duration was used to create an optical breakdown at F(1), which generates a spherically diverging shock wave with a peak pressure of 2.1-5.9 MPa at 1.1 mm stand-off distance and a pulse width at half maximum of 36-65 ns. Upon reflection, a converging shock wave is produced which, upon arriving at F(2), has a leading compressive wave with a peak pressure of 26 MPa and a zero-crossing pulse duration of 0.1 mus, followed by a trailing tensile wave of -3.3 MPa peak pressure and 0.2 mus pulse duration. The -6 dB beam size of the focused shock wave field is 1.6 x 0.2 mm(2) along and transverse to the shock wave propagation direction. Formation of elongated plasmas at high laser energy levels limits the increase in the peak pressure at F(2). General features in the waveform profile of the converging shock wave are in qualitative agreement with numerical simulations based on the Hamilton model.

The acoustic fields of a high intensity focused ultrasound (HIFU) transducer operating either at its fundamental (1.1 MHz) or third harmonic (3.3 MHz) frequency were measured by a fiber optic probe hydrophone (FOPH). At 1.1 MHz when the electric power applied to the transducer was increased from 1.6 to 125 W, the peak positive/negative pressures at the focus were measured to be p(+) = 1.7-23.3 MPa and p(-) = -1.2(-) -10.0 MPa. The corresponding spatial-peak pulse-average (I(SPPA)) and spatial-average pulse-average (I(SAPA)) intensities were I(SPPA) =77-6000 W/cm2 and I(SAPA) = 35-4365 W/cm2. Nonlinear propagation with harmonics generation was dominant at high intensities, leading to a reduced -6 dB beam size (L x W) of the compressional wave (11.5 x 1.8-8.8 1.04 mm) but an increased beam size of the rarefactional wave (12.5 x 1.6-13.2 x 2.0 mm). Enhancement ratio of absorbed power density in water increased from 1.0 to 3.0. In comparison, the HIFU transducer working at 3.3 MHz produced higher peak pressures (p(+) = 3.0-35.1 MPa and p(-) = -2.5(-) - 13.8 MPa) with smaller beam size (0.5 x 4 mm). Overall, FOPH was found to be a convenient and reliable tool for HIFU exposimetry measurement.

High-intensity focused ultrasound (HIFU) has been used successfully in clinics for the treatment of a variety of cancerous and benign tumors. Characterization of the focused acoustic field is of importance in the planning of the ablation procedure and further development of HIFU technology. Quantitative estimation of acoustic intensity is feasible using the infrared (IR) thermography on an absorber. However, the current approach is limited to low power output. In this study, a theoretical model was established to describe the acoustic field and absorbed energy in the absorber with the presence of harmonics in the HIFU pressure waveform at the focus and then to calculate the temperature elevations during the HIFU heating, from which the acoustic intensities could be derived. The absolute difference between the derived and incident acoustic intensities in a 2-mm absorber at the varied acoustic power output (up to 80 W) and attenuation of the absorber (up to 346 Np/m/MHz) is <6%. In addition, the proposed approach was found to also work well for the absorber with a varied thickness because of the monotonic decrease of the temperature elevation. The effects of pulse duration and duty cycle of pulsed HIFU ablation on the estimation accuracy were also investigated. It is found that the estimation accuracy is good for short pulse duration using the equivalent acoustic intensity. Overall, the proposed approach could estimate the acoustic intensity and harmonic distribution of HIFU at the high power output using IR thermography for a large parametric range of absorber and HIFU output in the numerical investigation. [ABSTRACT FROM AUTHOR]

Various modulation approaches, such as amplitude and frequency modulations, have been applied widely to modify the acoustic field and improve the performance of ultrasound imaging and therapy. However, phase modulation (PM) has not been investigated extensively in the ultrasound applications, especially at a long-pulse duration. In this study, the effects of PM on the acoustic field were investigated. The radiated acoustic pressure waveforms produced using different PM strategies (i.e., sequential phase inversion every cycle, every two cycles, and random phase inversion) were explored, and the distributions of acoustic pressure and average acoustic intensity along and transverse to the transducer axis were compared with those of a sinusoidal wave excitation in both measurement and simulation. It is found that the phase inversion between the modulated signals is not clearly seen in the radiated waveform because of the limited fractional bandwidth of the therapeutic ultrasound transducer. As a result, the radiated waveform has a higher oscillating frequency, and the pressure at the focus and the −6-dB beam size are decreased. Both simulation and measurement show similar trends. Furthermore, produced acoustic fields of the phased array using these PM strategies were also simulated at the varied lateral and axial focus shifting distances. The magnitude and beam size of both the main lobe and grating lobe are found between them, especially at the large focus shifting. In summary, the acoustic field is dependent on the PM, and the appropriate excitation scheme could improve the ultrasound application. [ABSTRACT FROM AUTHOR]

High-intensity focused ultrasound (HIFU) has emerged as an effective and noninvasive therapeutic modality for cancer and solid tumor. Despite its promising clinical outcomes and the approval of the Food and Drug Administration of many countries, the ablation time of a large target is long, so enhancement of the lesion production is highly desired. In this study, dual-frequency (or amplitude modulation) excitation was evaluated both numerically and experimentally, and its performance was compared with that using single-frequency excitation at the same power output. The nonlinear wave propagation model was used to simulate the acoustic field of HIFU exposure, the Gilmore model was used to determine the induced bubble dynamics, and then absorbed acoustic energy and bubble-enhanced heating were put into the BioHeat equation to calculate the temperature elevation. HIFU-produced lesion in the bovine serum albumin-embedded polyacrylamide was recorded photographically. It is found that dual-frequency excitation (3.16 + 3.20MHz) can increase the lesion area by 35%–65% compared to single-frequency excitation (3.18 MHz) at the same power output. The lesion enhancement increases with the pulse repetition frequency, duty cycle, and modulation depth and decreases with the frequency difference. In summary, dual-frequency excitation can increase the bubble cavitation and the associated heating for HIFU ablation for large lesion production. [ABSTRACT FROM AUTHOR]

High-intensity focused ultrasound (HIFU) has been emerging as an effective and noninvasive modality in cancer treatment with very promising clinical results. However, a small vessel in the focal region could be ruptured, which is an important concern for the safety of HIFU ablation. In this study, lesion formation in the polyacrylamide gel phantom embedded with different tubing (inner diameters of 0.76 mm and 3 mm) at varied flow speeds (17–339 cm/s) by HIFU ablation was photographically recorded. Produced lesions have decreased length (~30%) but slightly increased width (~6%) in comparison to that without the embedded tubing. Meanwhile, bubble activities during the exposures were measured by passive cavitation detection (PCD) at the varied pulse repetition frequency (PRF, 10–30 Hz) and duty cycle (DC, 10%-20%) of the HIFU bursts. High DC and low flow speed were found to produce stronger bubble cavitation whereas no significant influence of the PRF. In addition, high-speed photography illustrated that the rupture of tubing was produced consistently after the first HIFU burst within 20 ms and then multiple bubbles would penetrate into the intraluminal space of tubing through the rupture site by the acoustic radiation force. Alignment of HIFU focus to the anterior surface, middle, and posterior surface of tubing led to different characteristics of vessel rupture and bubble introduction. In summary, HIFU-induced vessel rupture is possible as shown in this phantom study; produced lesion sizes and shapes are dependent on the focus alignment to the tubing, flow speed, and tubing properties; and bubble cavitation and the formation liquid jet may be one of the major mechanisms of tubing rupture as shown in the high-speed photography. [ABSTRACT FROM AUTHOR]

A new piezoelectric micromachined ultrasonic transducer (pMUT) with an isolation trench between cells was proposed to improve the output pressure. A 2-D finite-element model was utilized to evaluate and compare the performance of the conventional design with fully clamped boundary and the trench design. It shows that the trench design can improve the membrane displacement or output pressure of pMUTs without a significant change in the resonant frequency; $8\times 8$ aluminum nitride (AlN)-based pMUTs arrays with fully clamped boundary design and the isolation trench were fabricated and characterized. An impulse response of the pMUTs array was first employed to determine the resonant frequency. A 200-cycle burst at the resonant frequency was then delivered to pMUTs and the acoustic output pressure was measured by a hydrophone. The trench design could increase the output pressure by $\sim 76$ % with a shift of its center frequency by only 0.03 MHz. The nonlinear relationship between pressure output and applied high voltage still exists in the trench design. The presence of residual stress in the membrane, and substrate during fabrication was found to have little impact on the displacement and resonant frequency of pMUT. In summary, the presence of isolation trench can reduce the deflection-induced tensile stress on the edge of the membrane and subsequently improve the performance of pMUTs. [2016-0001] [ABSTRACT FROM PUBLISHER]

Platelet-rich plasma (PRP) is a volume of autologous plasma that has a higher platelet concentration above baseline. It has already been approved as a new therapeutic modality and investigated in clinics, such as bone repair and regeneration, and oral surgery, with low cost-effectiveness ratio. At present, PRP is mostly prepared using a centrifuge. However, this method has several shortcomings, such as long preparation time (30 min), complexity in operation, and contamination of red blood cells (RBCs). In this paper, a new PRP preparation approach was proposed and tested. Ultrasound waves (4.5 MHz) generated from piezoelectric ceramics can establish standing waves inside a syringe filled with the whole blood. Subsequently, RBCs would accumulate at the locations of pressure nodes in response to acoustic radiation force, and the formed clusters would have a high speed of sedimentation. It is found that the PRP prepared by the proposed device can achieve higher platelet concentration and less RBCs contamination than a commercial centrifugal device, but similar growth factor (i.e., PDGF-ββ). In addition, the sedimentation process under centrifugation and sonication was simulated using the Mason–Weaver equation and compared with each other to illustrate the differences between these two technologies and to optimize the design in the future. Altogether, ultrasound method is an effective method of PRP preparation with comparable outcomes as the commercially available centrifugal products. [ABSTRACT FROM PUBLISHER]