Ultrasound-induced Cavitation Research Articles

Effect of system parameters on the size distributions of hollow nickel microspheres produced by an ultrasound-aided electrical discharge machining process

Ultrasound-aided electric discharge machining (EDM) is an emerging technology for producing hollow nickel microspheres. This technology combines traditional EDM with the cavitation and vibration effects of ultrasound to produce hollow microspheres. In this paper, ultrasound-aided EDM was carried out in a kerosene medium (the working solution). The effects of various parameters on the sizes of microspheres were investigated using scanning electronic microscopy (SEM). Smileview software was used to measure the sizes of the microspheres. Originpro software was used for statistical analysis to determine the size distributions of the microspheres. To study the effects of the system parameters on the sizes of the microspheres, we first investigated the necessity of using an ultrasonic wave with EDM. After comparing the experimental results with and without the ultrasonic field, we found that ultrasound-induced cavitation and vibration effects reduced the diameters of the microspheres. We then studied the effects of several electrical parameters, including the arc current, pulse width, and gap voltage, on the sizes of the microspheres at an ultrasound frequency of 40kHz. Smaller microspheres could be obtained by lowering the arc current, pulse width, and gap voltage.

Enhanced Tumor Uptake and Penetration of Virotherapy Using Polymer Stealthing and Focused Ultrasound

BackgroundOncolytic viruses are among the most powerful and selective cancer therapeutics under development and are showing robust activity in clinical trials, particularly when administered directly into tumor nodules. However, their intravenous administration to treat metastatic disease has been stymied by unfavorable pharmacokinetics and inefficient accumulation in and penetration through tumors.MethodsAdenovirus (Ad) was “stealthed” with a new N-(2-hydroxypropyl)methacrylamide polymer, and circulation kinetics were characterized in Balb/C SCID mice (n = 8 per group) bearing human ZR-75-1 xenograft tumors. Then, to noninvasively increase extravasation of the circulating polymer-coated Ad into the tumor, it was coinjected with gas microbubbles and the tumor was exposed to 0.5 MHz focused ultrasound at peak rarefactional pressure of 1.2MPa. These ultrasound exposure conditions were designed to trigger inertial cavitation, an acoustic phenomenon that produces shock waves and can be remotely monitored in real-time. Groups were compared with Student t test or one-way analysis of variance with Tukey correction where groups were greater than two. All statistical tests were two-sided.ResultsPolymer-coating of Ad reduced hepatic sequestration, infection (>8000-fold; P < .001), and toxicity and improved circulation half-life (>50-fold; P = .001). Combination of polymer-coated Ad, gas bubbles, and focused ultrasound enhanced tumor infection >30-fold; (4×106 photons/sec/cm2; standard deviation = 3×106 with ultrasound vs 1.3×105; standard deviation = 1×105 without ultrasound; P = .03) and penetration, enabling kill of cells more than 100 microns from the nearest blood vessel. This led to substantial and statistically significant retardation of tumor growth and increased survival.ConclusionsCombining drug stealthing and ultrasound-induced cavitation may ultimately enhance the efficacy of a range of powerful therapeutics, thereby improving the treatment of metastatic cancer.

Histological and Ultrastructural Effects of Ultrasound-induced Cavitation on Human Skin Adipose Tissue.

Background:In aesthetic medicine, the most promising techniques for noninvasive body sculpturing purposes are based on ultrasound-induced fat cavitation. Liporeductive ultrasound devices afford clinically relevant subcutaneous fat pad reduction without significant adverse reactions. This study aims at evaluating the histological and ultrastructural changes induced by ultrasound cavitation on the different cell components of human skin.Methods:Control and ultrasound-treated ex vivo abdominal full-thickness skin samples and skin biopsies from patients pretreated with or without ultrasound cavitation were studied histologically, morphometrically, and ultrastructurally to evaluate possible changes in adipocyte size and morphology. Adipocyte apoptosis and triglyceride release were also assayed. Clinical evaluation of the effects of 4 weekly ultrasound vs sham treatments was performed by plicometry.Results:Compared with the sham-treated control samples, ultrasound cavitation induced a statistically significant reduction in the size of the adipocytes (P < 0.001), the appearance of micropores and triglyceride leakage and release in the conditioned medium (P < 0.05 at 15 min), or adipose tissue interstitium, without appreciable changes in microvascular, stromal, and epidermal components and in the number of apoptotic adipocytes. Clinically, the ultrasound treatment caused a significant reduction of abdominal fat.Conclusions:This study further strengthens the current notion that noninvasive transcutaneous ultrasound cavitation is a promising and safe technology for localized reduction of fat and provides experimental evidence for its specific mechanism of action on the adipocytes.

Inertial cavitation at the nanoscale

Our group has recently developed novel nano-sized drug carriers that spatially target a tumor and release their payload in the presence of ultrasound-induced inertial cavitation. To maximize drug release and distribution within the tumor, co-localization of the drug carrier and cavitation nuclei is necessary. We have recently demonstrated that rough-patterned silica nanoparticles can reduce inertial cavitation thresholds to clinically relevant levels, and will extravasate in tumors alongside the liposomes by virtue of their size. We now report on the underlying mechanisms that these nanoparticles, which are orders of magnitude smaller than the acoustic wavelength, can instigate inertial cavitation. The rough surface of the nanoparticle is modelled as a plane with a crevasse that traps a nanobubble. Using this model, we predict the motion of a gas bubble as it emerges from the cavity in response to the compressional and rarefactional ultrasonic pressures. We show that cavitation occurs when the nanobubble breaks free from the surface, growing unstably before collapsing during the compressional half cycle of the acoustic wave. Calculations show that a nanoscaled cavity greatly reduces the cavitation threshold across all frequencies and geometries studied. In addition, cavitation thresholds nonlinearly decrease with increasing cavity size.

Enhancement effect of ultrasound-induced microbubble cavitation on branched polyethylenimine-mediated vascular endothelial growth factor 165 (VEGF165) transfection

Angiogenesis is a complex process that is mediated by growth factor. One isoform of the vascular endothelial growth factor, VEGF165, has been reported to be a dominant mediator and regulator of angiogenic process. Branched polyethylenimine (bPEI) has been widely used as a non-viral delivery vector for gene therapy. HEK 293T cells, mixed with bPEI:VEGF165 complexes with different N/P ratios, were exposed to 1-MHz ultrasound (US) pulses. The enhancement effect of microbubble inertial cavitation (IC) on bPEI-mediated VEGF165 transfection was systemically investigated, in an effort to optimize transfection efficiency using low nitrogen:DNA phosphate (N/P) ratios. The results show that: (1) Microbubble IC activity can be quantified as an IC “dose” (ICD) and will be affected by US parameters; (2) DNA transfection efficiency initially increases with the increasing ICD, then tends to saturate instead of achieving a maximum value while ICD keeps going up; (3) the measured ICD, sonopration pore size, and cell viability exhibit high correlation among each other; and (4) microbubble IC activity has less cytotoxicity than bPEI, although a combinatorial effect of IC activity and bPEI can be observed on cell viability. All the results indicated that ICD could be used as an effective tool to monitor and control US-mediated gene/drug delivery effect, and it is possible to optimize bPEI-mediated VEGF transfection efficiency with relatively low N/P ratios by employing appropriate US parameters.

Enhancement Effect of Ultrasound-Induced Microbubble Cavitation on Branched Polyethylenimine-Mediated VEGF165 Transfection With Varied N/P Ratio

One isoform of the vascular endothelial growth factor, VEGF165, has been reported to be a dominant mediator and regulator of angiogenic process, which plays an important role in treating cardiovascular diseases and chronically ischemic wounds. Branched polyethylenimine (bPEI) has been widely used as a non-viral delivery vector for gene therapy. Although bPEI-mediated DNA transfection efficiency can be raised by increasing the PEI nitrogen:DNA phosphate (N/P) ratio, cytotoxicity increases as well. In this study, the enhancement effect of microbubble inertial cavitation (IC) on bPEI-mediated VEGF165 transfection was investigated, in an effort to optimize transfection efficiency using low N/P ratios. HEK 293T cells, mixed with bPEI:VEGF165 complexes, were exposed to 1-MHz ultrasound pulses. The results show that: (1) IC activity induced by microbubble destruction can be quantified as an IC “dose” (ICD) and will increase with increasing acoustic driving pressure; (2) larger sonoporation pores can be generated by increasing ICD; (3) the transfection efficiency can be enhanced by increasing ICD until reaching a saturation level; and (4) microbubble IC activity has less cytotoxicity than bPEI, although a combinatorial effect of microbubble IC activity and bPEI could be observed on cell viability. The results suggest that, with appropriate ultrasound parameters, it is possible to optimize bPEI-mediated VEGF transfection efficiency using relatively low N/P ratios by employing ultrasound-induced microbubble inertial cavitation.

Fabrication and characterization of CdSe/ZnS quantum dots-doped polystyrene microspheres prepared by self-assembly

Abstract

Cavitation mechanisms in ultrasound-enhanced permeability of ex vivo porcine skin

Ultrasound-induced cavitation is known to enhance transdermal transport of drugs for local and systemic delivery. However, the specific cavitation mechanisms responsible are not well understood, and the physical location of permeability-enhancing cavitation is also unknown. The experiments reported here investigated the role of stable and inertial cavitation, both within the skin and at the dorsal skin surface, in ultrasound enhancement of skin permeability. Full-thickness porcine skin was hydrated with either air-saturated phosphate buffered saline (PBS) or vacuum-degassed PBS to localize cavitation activity within or outside the skin, respectively. Skin samples were sonicated for 30 minutes over a range of frequencies (0.41 and 2.0 MHz) and peak rarefactional pressure amplitudes (0-750 kPa) with a 20% duty cycle (1 s on, 4 s off). Cavitation activity was monitored using a 1.0 MHz unfocused, wideband passive cavitation detector (PCD). Changes in skin permeability were quantified by measuring the electrical resistance of skin every 10 seconds during insonation. Subharmonic acoustic emissions revealed a strong correlation with decreasing electrical resistance of skin when cavitation was isolated within the tissue, suggesting that stable cavitation within the skin plays a primary role in ultrasound-enhanced permeability over the frequencies investigated.

An introduction to the Biomedical Acoustics Technical Committee

An overview of the activities of the Biomedical Acoustics Technical Committee will be presented, along with a brief survey of some of the ongoing research projects in Biomedical Acoustics. For example, the Biomedical Acoustics Technical Committee organizes a student poster competition every year, and special sessions are arranged at each meeting to highlight some of the latest research in the diagnostic and therapeutic applications of acoustics. Some of the topics include ultrasound imaging, high intensity focused ultrasound, ultrasound mediated drug delivery, ultrasound contrast agents, ultrasound-induced cavitation, and ultrasound elastography, among several others. In this presentation, selected examples of active research topics in Biomedical Acoustics will also be described in greater detail.

A Simple Road for the Transformation of Few-Layer Graphene into MWNTs

We report the direct formation of multiwalled carbon nanotubes (MWNT) by ultrasonication of graphite in dimethylformamide (DMF) upon addition of ferrocene aldehyde (Fc-CHO). The tubular structures appear exclusively at the edges of graphene layers and contain Fe clusters. Fc in conjunction with benzyl aldehyde, or other Fc derivatives, does not induce formation of NT. Higher amounts of Fc-CHO added to the dispersion do not increase significantly MWNT formation. Increasing the temperature reduces the amount of formation of MWNTs and shows the key role of ultrasound-induced cavitation energy. It is concluded that Fc-CHO first reduces the concentration of radical reactive species that slice graphene into small moieties, localizes itself at the edges of graphene, templates the rolling up of a sheet to form a nanoscroll, where it remains trapped, and finally accepts and donates unpaired electron to the graphene edges and converts the less stable scroll into a MWNT. This new methodology matches the long held notion that CNTs are rolled up graphene layers. The proposed mechanism is general and will lead to control the production of carbon nanostructures by simple ultrasonication treatments.

A High-Speed Imaging and Modeling Study of Dendrite Fragmentation Caused by Ultrasonic Cavitation

The dynamic behavior of ultrasound-induced cavitation bubbles and their effect on the fragmentation of dendritic grains of a solidifying succinonitrile 1 wt pct camphor organic transparent alloy have been studied experimentally using high-speed digital imaging and complementary numerical analysis of sound wave propagation, cavitation dynamics, and the velocity field in the vicinity of an imploding cavitation bubble. Real-time imaging and analysis revealed that the violent implosion of bubbles created local shock waves that could shatter dendrites nearby into small pieces in a few tens of milliseconds. These catastrophic events were effective in breaking up growing dendritic grains and creating abundant fragmented crystals that may act as embryonic grains; therefore, these events play an important role in grain refinement of metallurgical alloys.

A New Look at Cavitation and the Applications of Its Liquid-Phase Effects in the Processing of Food and Fuel

Ultrasound-induced acoustic cavitation has been studied in detail due to its chemical effects (sonochemistry) and light emission (sonoluminescence). However, the physical effects such as shear, shockwaves, etc., can also be used for useful applications despite their harmful effects, such as cavitation erosion. It has been suggested that the physical forces generated during cavitation may alter the 3-dimensional network of water molecules and hence a better hydration of macromolecules can be achieved using the cavitation process. In recent times, the physical effects of acoustic cavitation are used in food processing applications. The possibility of using hydrodynamic cavitation for food processing as an alternative approach to ultrasonic processing has been discussed in this manuscript. A special rotary disintegrator, developed by Dr. Hint in the last century is used for the generation of efficient hydrodynamic cavitation. A mathematical model has been developed for the optimization of rotary disintegrators. The suitability of the model for evaluating the process efficiency has also been tested using experimental data obtained for the production of emulsions used as fuels. The development and testing of a new mathematical model for optimizing rotary disintegrators pave new pathways to the use of hydrodynamic cavitation for processing large volumes of liquid ingredients making them suitable for fuel and food industries.

Effect of ultrasonic frequency on the structure and sonophotocatalytic property of CdS/TiO2 nanocomposite

Visible-light-induced nano CdS/TiO2 sonophotocatalysts with different morphologies and particle sizes were prepared under multi-bubble sonoluminescence condition with different ultrasonic frequencies. The characterization and property studies were performed by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, UV–vis diffuse reflectance spectroscopy and surface area measurement, etc. In addition, we discussed the mechanical effects of shockwave formed from ultrasound-induced cavitation and investigated the degradation of methyl orange under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis, in the presence of the CdS/TiO2 nanoparticles. The results suggest that the compactness, crystallinity and grain size of CdS/TiO2 nanocomposites produced are governed by the applied frequency, which in turn influence the sonophotocatalytic activity.

Ultrasound-induced cavitation enhances the delivery and therapeutic efficacy of an oncolytic virus in an in vitro model

We investigated whether ultrasound-induced cavitation at 0.5MHz could improve the extravasation and distribution of a potent breast cancer-selective oncolytic adenovirus, AdEHE2F-Luc, to tumour regions that are remote from blood vessels. We developed a novel tumour-mimicking model consisting of a gel matrix containing human breast cancer cells traversed by a fluid channel simulating a tumour blood vessel, through which the virus and microbubbles could be made to flow. Ultrasonic pressures were chosen to maximize either broadband emissions, associated with inertial cavitation, or ultraharmonic emissions, associated with stable cavitation, while varying duty cycle to keep the total acoustic energy delivered constant for comparison across exposures. None of the exposure conditions tested affected cell viability in the absence of the adenovirus. When AdEHE2F-Luc was delivered via the vessel, inertial cavitation increased transgene expression in tumour cells by up to 200 times. This increase was not observed in the absence of Coxsackie and Adenovirus Receptor cell expression, discounting sonoporation as the mechanism of action. In the presence of inertial cavitation, AdEHE2F-Luc distribution was greatly improved in the matrix surrounding the vessel, particularly in the direction of the ultrasound beam; this enabled AdEHE2F-Luc to kill up to 80% of cancer cells within the ultrasound focal volume in the gel 24 hours after delivery, compared to 0% in the absence of cavitation.

Cavitation-Enhanced Extravasation for Drug Delivery

A flow-through tissue-mimicking phantom composed of a biocompatible hydro-gel with embedded tumour cells was used to assess and optimize the role of ultrasound-induced cavitation on the extravasation of a macromolecular compound from a channel mimicking vessel in the gel, namely a non-replicating luciferase-expressing adenovirus (Ad-Luc). Using a 500 KHz therapeutic ultrasound transducer confocally aligned with a focussed passive cavitation detector, different exposure conditions and burst mode timings were selected by performing time and frequency domain analysis of passively recorded acoustic emissions, in the absence and in the presence of ultrasound contrast agents acting as cavitation nuclei. In the presence of Sonovue, maximum ultraharmonic emissions were detected for peak rarefactional pressures of 360 kPa, and maximum broadband emissions occurred at 1250 kPa. The energy of the recorded acoustic emissions was used to optimise the pulse repetition frequency and duty cycle in order to maximize either ultraharmonic or broadband emissions while keeping the acoustic energy delivered to the focus constant. Cell viability measurements indicated that none of the insonation conditions investigated induces cell death in the absence of a therapeutic agent (i.e. virus). Phase contrast images of the tissue-mimicking phantom showed that short range vessel disruption can occur when ultra-harmonic emissions (nf0/2) are maximised whereas formation of a micro-channel perpendicular to the flow can be obtained in the presence of broadband acoustic emissions. Following Ad-Luc delivery, luciferase expression measurements showed that a 60-fold increase in its bioavailability can be achieved when broadband noise emissions are present during insonation, even for modest contrast agent concentrations. The findings of the present study suggest that drug delivery systems based on acoustic cavitation may help enhance the extravasation of anticancer agents, thus increasing their penetration distance to hypoxic regions and poorly vascularised tumour regions.

Acoustic emission spectra and sonochemical activity in a 36 kHz sonoreactor

During ultrasound-induced cavitation in liquids, acoustic emissions at fundamental and harmonic frequencies can be detected. The effect of acoustic emissions at harmonic frequencies on the sonochemical and sonophysical activities has not been explored, especially in large-scale sonoreactors. In this study, the acoustic emissions in the range, 0–250 kHz in a 36 kHz sonoreactor with varying liquid heights were studied and compared with the sonochemical activities. The acoustic pressures at both fundamental and harmonics decreased drastically as the liquid height was increased due to the attenuation of sound energy. It was observed that the increase in input power resulted in only an increase in the acoustic emissions at derivative frequencies such as, harmonics and subharmonics. The sonochemical activity, evaluated in terms of sonochemiluminescence and H 2O 2 yield, was not significantly enhanced at higher input power levels. This suggests that at higher power levels, the “extra” acoustic energy is not effectively used to generate primary cavitation activity; rather it is converted to generate acoustic emissions at harmonic and subharmonic frequencies. This is an important observation for the design of energy efficiency large-scale sonochemical reactors.

A Tissue Phantom for Visualization and Measurement of Ultrasound-Induced Cavitation Damage

Many ultrasound studies involve the use of tissue-mimicking materials to research phenomena in vitro and predict in vivo bioeffects. We have developed a tissue phantom to study cavitation-induced damage to tissue. The phantom consists of red blood cells suspended in an agarose hydrogel. The acoustic and mechanical properties of the gel phantom were found to be similar to soft tissue properties. The phantom’s response to cavitation was evaluated using histotripsy. Histotripsy causes breakdown of tissue structures by the generation of controlled cavitation using short, focused, high-intensity ultrasound pulses. Histotripsy lesions were generated in the phantom and kidney tissue using a spherically focused 1-MHz transducer generating 15 cycle pulses, at a pulse repetition frequency of 100 Hz with a peak negative pressure of 14 MPa. Damage appeared clearly as increased optical transparency of the phantom due to rupture of individual red blood cells. The morphology of lesions generated in the phantom was very similar to that generated in kidney tissue at both macroscopic and cellular levels. Additionally, lesions in the phantom could be visualized as hypoechoic regions on a B-mode ultrasound image, similar to histotripsy lesions in tissue. High-speed imaging of the optically transparent phantom was used to show that damage coincides with the presence of cavitation. These results indicate that the phantom can accurately mimic the response of soft tissue to cavitation and provide a useful tool for studying damage induced by acoustic cavitation. (E-mail: adamdm@umich.edu)

Ultrasonic spectral analysis of cavitation bubbles in vegetable oils.

Ultrasound-induced cavitation in vegetable oils can be used to control the crystallization and resulting texture of food products such as shortenings. Cavitation in oils has not been explored as extensively as in aqueous systems, however. To more fully understand cavitation in vegetable oils, high-intensity ultrasound (HIU) and broadband 1.0-MHz transducers were used to generate and measure cavitation in soybean oil experimentally. To interpret the results, multipole-based models were used to simulate ultrasonic scattering from the cavitation bubbles in the 0.4–4.0 MHz range. Ultrasonic reflection spectra, transmission spectra, and velocities were simulated for spherical bubble clouds centered between two transducers. The size, concentration, and random configuration of the bubbles in the clouds were varied to find diagnostic features to correlate with experimental data. The models indicated that the configuration of the bubbles have unexpectedly large effects on the ultrasonic spectra even at dilute concentrations (0.2–5.0%). The simulated spectra additionally showed low-frequency (&amp;lt;2 MHz) and high-frequency (&amp;gt;2 MHz) features that correlated with bubble concentration and bubble size, respectively. The experimental results included spectral features that may be associated with turbulence in the oil and the persistence of low-frequency backscatter after stopping the HIU, indicating a slow decay in bubble concentration.

Evaluation of the Temporal Window for Drug Delivery Following Ultrasound-Mediated Membrane Permeability Enhancement

Ultrasound-induced cavitation facilitates cellular uptake of drugs via increased membrane permeability. Here, the purpose was to evaluate the duration of enhanced membrane permeability following ultrasound treatment in cell culture. Optical chromophores with fluorescence intensity increasing 100-1,000-fold upon intercalation with nucleic acids served as smart agents for reporting cellular uptake. Opticell chambers with a monolayer of C6 cells were subjected to ultrasound in the presence of microbubbles followed by varying delays between 0 and 24h before addition of Sytox Green optical contrast agent. Micro- and macroscopic fluorescence were used for qualitative and quantitative analysis. Up to 25% of viable cells showed uptake of contrast agent with a half time of 8h, with cellular uptake persisting even at 24h. Only cells exposed to ultrasound showed the effect. The temporal window of increased membrane permeability is much longer in these studies than previously suggested. This may have important repercussions for in vivo studies in which membrane permeability may be temporally separated from drug delivery.

Effects of Ultrasound-Induced Inertial Cavitation on Enzymatic Thrombolysis

Cavitation induced by ultrasound enhances enzymatic fibrinolysis by increasing the transport of reactants. However, the effects of cavitation need to be fully understood before sonothrombolysis can be applied clinically. In order to understand the underlying mechanisms, we examined the effects of combining ultrasound, microbubbles and thrombolytic enzymes on thrombolysis. First, we evaluated the relations between inertial cavitation and the reduction in the weight of a blood clot. Inertial cavitation was varied by changing the amplitude and duration of the transmitted acoustic wave as well as the concentration of microbubbles used to induce cavitation. Second, we studied the combined effects of streptokinase and inertial cavitation on thrombolysis. The results show that inertial cavitation increases the weight reduction of a blood clot by up to 33.9%. With linear regression fitting, the measured differential inertial cavitation dose and the weight reduction had a correlation coefficient of 0.66. Microscopically, enzymatic thrombolysis effects manifest as multiple large cavities within the clot that are uniformly distributed on the side exposed to ultrasound. This suggests that inertial cavitation plays an important role in producing cavities, while microjetting of the microbubbles induces pits on the clot surface. These observations preliminarily demonstrate the clinical potential of sonothrombolysis. The use of the differential inertial cavitation dose as an indicator of blood clot weight loss for controlled sonothrombolysis is also possible and will be further explored.