Towards an understanding and control of cavitation activity in 1 MHz ultrasound fields

Stimuli-Responsive Codelivery System-Embedded Polymeric Nanofibers with Synergistic Effects of Growth Factors and Low-Intensity Pulsed Ultrasound to Enhance Osteogenesis Properties

The present work aims to develop optimized scaffolds for bone repair by incorporating mesoporous nanoparticles into them, thereby combining bioactive factors for cell growth and preventing rapid release or loss of effectiveness. We synthesized biocompatible and biodegradable scaffolds designed for the controlled codelivery of curcumin (CUR) and recombinant human bone morphogenic protein-2 (rhBMP-2). Active agents in dendritic silica/titania mesoporous nanoparticles (DSTNs) were incorporated at different weight percentages (0, 2, 5, 7, 9, and 10 wt %) into a matrix of polycaprolactone (PCL) and polyethylene glycol (PEG) nanofibers, forming the CUR-BMP-2@DSTNs/PCL-PEG delivery system (S0, S2, S5, S7, S9, and S10, respectively, with the number showing the weight percentage). To enhance the formation process, the system was treated using low-intensity pulsed ultrasound (LIPUS). Different advanced methods were employed to assess the physical, chemical, and mechanical characteristics of the fabricated scaffolds, all confirming that incorporating the nanoparticles improves their mechanical and structural properties. Their hydrophilicity increased by approximately 25%, leading to ca. 53% enhancement in their water absorption capacity. Furthermore, we observed a sustained release of approximately 97% for CUR and 70% for BMP-2 for the S7 (scaffold with 7 wt % DSTNs) over 28 days, which was further enhanced using ultrasound. In vitro studies demonstrated accelerated scaffold biodegradation, with the highest level observed in S7 scaffolds, approximately three times higher than the control group. Moreover, the cell viability and proliferation on DSTNs-containing scaffolds increased when compared to the control group. Overall, our study presents a promising nanocomposite scaffold design with notable improvements in structural, mechanical, and biological properties compared to the control group, along with controlled and sustained drug release capabilities. This makes the scaffold a compelling candidate for advanced bone tissue engineering and regenerative therapies.

Origin of the broad-band noise in acoustic cavitation

The broad-band noise has been experimentally used to monitor the cavitation activity in a sonochemical reactor, an ultrasonic cleaning bath, a biological tissue, etc. However, the origin of the broad-band noise is still under debate. In the present review, two models for the mechanism of the broad-band noise are discussed. One is acoustic emissions from chaotically (non-periodically) pulsating bubbles. The other is acoustic emissions from bubbles with temporal fluctuation in the number of bubbles. It is suggested that the latter mechanism is sometimes dominant. Further studies are required on the role for bubble cluster dynamics as well as the bubble-bubble interaction in the broad-band noise especially at relatively low ultrasonic frequencies.

Sonoprocessing of wetting of SiC by liquid Al: A thermodynamic and kinetic study

The sonoprocessing of droplet spreading during the wetting process of molten aluminum droplets on SiC ceramic substrates at 700 °C is investigated in this paper. When wetting is assisted by a 20 kHz frequency ultrasonic field, the wettability of liquid metal gets enhanced, which has been determined by the variations in thermodynamic energy and wetting kinetics. Wetting kinetic characteristics are divided into two stages according to pinning and depinning states of substrate/droplet contact lines. The droplet is static when the contact line is pinning, while it is forced to move when the contact line is depinning. When analyzing the pinning stage, high-speed photography reveals the evidence of oxide films being rapidly crushed outside the aluminum droplet. In this work, atomic models of spherical Al core being wrapped by alumina shell are tentatively built, whose dioxide microstructures are being transformed from face-centered cubic into liquid at the atomic scale. At the same time, the wetting experiment reveals that the oxide films show changes in the period of sonoprocessing from 3rd to 5th second. During the ultrasonic spreading behavior in the late stage, there is a trend of evident expansion of the base contact area. The entire ultrasonic process lasts for no longer than 10 s. With the aid of ultrasonic sinusoidal waves, the wettability of metal Al gets a rapid improvement. Both molecular dynamic (MD) investigations and the experiments results reveal that the precursor film phenomenon is never found unless wetting is assisted by ultrasonic treatments. However, the precursor film appears near the triple line after using ultrasonics in the droplet wetting process, whose formation is driven by ultrasonic oscillations. Due to the precursor film, the ultrasonic wetting contact angle is lower than the non-ultrasonic contact angle. In addition, the time-variant effective ultrasonic energy has been quantitatively evaluated. The numerical expressions of thermodynamic variables are well verified by former ultrasonic spreading test results, which altogether provide an intrinsic explanation of the fast-decreasing contact angle of Al/SiC.

Modeling the Voltage Produced by Ultrasound in Seawater by Stochastic and Artificial Intelligence Methods

Experiments have proved that an electrical signal appears in the ultrasonic cavitation field; its properties are influenced by the ultrasound frequency, the liquid type, and liquid characteristics such as density, viscosity, and surface tension. Still, the features of the signals are not entirely known. Therefore, we present the results on modeling the voltage collected in seawater, in ultrasound cavitation produced by a 20 kHz frequency generator, working at 80 W. Comparisons of the Box–Jenkins approaches, with artificial intelligence methods (GRNN) and hybrid (Wavelet-ARIMA and Wavelet-ANN) are provided, using different goodness of fit indicators. It is shown that the last approach gave the best model.

Neuroprotective Effect of Low-Intensity Pulsed Ultrasound on the Mouse MPTP/MPP+ Model of Dopaminergic Neuron Injury

Ultrasound mediated neuromodulation has been demonstrated to a safe treatment strategy in the field of neuroscience. In this study, low-intensity pulsed ultrasound (LIPUS) was used to treat Parkinson's disease (PD) models induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP⁺) to explore the possibility of ultrasound neuroprotective effect on PD. The results demonstrated that LIPUS treatment can attenuate the central neurotoxicity of MPTP in mice, reduce the loss of tyrosine hydroxylase positive neurons in the substantia nigra pars compacta and decrease the apoptosis in the section of substantia nigra. The movement and balance dysfunctions in PD mice were improved with LIPUS treatment. In addition, we demonstrated that LIPUS can inhibit the decreased activity and increased apoptosis of dopaminergic neurons induced by MPP⁺, restrain the accumulation of reactive oxygen species (ROS) and decrease of mitochondrial membrane potential caused by MPP⁺. Moreover, LIPUS stimulation alone did not cause any cytotoxicity and tissue damage in our study. Taken together, the protective and regulatory effects of LIPUS on dopaminergic neurons make it possible as a new, safe and noninvasive treatment for PD.

Characterization of transient cavitation activity during sonochemical modification of magnesium particles

Investigation of the cavitation activity during ultrasonic treatment of magnesium particles during nanostructuring has been performed. Cavitation activity is recorded in the continuous mode after switching the ultrasound on with the use of ICA-5DM cavitometer. It has been demonstrated that this characteristic of the cavitation zone may be varied in a wide range of constant output parameters of the generator. The speed and nature of the cavitation activity alteration depended on the concentration of Mg particles in the suspension and the properties of the medium in which the sonochemical treatment has been performed. Three stages of the cavitation area evolution can be distinguished: 1 - the initial increase in cavitation activity, 2 - reaching a maximum with a subsequent decrease, and 3 - reaching the plateau (or the repeated cycles with feedback loops of enlargement/reduction of the cavitation activity). The ultrasonically treated magnesium particles have been characterized by scanning electron microscopy, X-ray diffraction analysis and thermal analysis. Depending on the nature of the dispersed medium the particles can be characterized by the presence of magnesium hydroxide (brucite) and magnesium hydride. It is possible to reach the incorporation of magnesium hydride in the magnesium hydroxide/magnesium matrix by varying the conditions of ultrasonic treatment (duration of treatment, amplitude, dispersed medium etc.). The influence of the magnesium reactivity is also confirmed by the measurements of cavitation activity in organic dispersed media (ethanol, ethylene glycol) and their aqueous mixtures.

The effect of dual-frequency ultrasound waves on B16F10 melanoma cells: Sonodynamic therapy using nanoliposomes containing methylene blue

BACKGROUND

We investigated the effect of dual-frequency sonication on the viability of B16F10 melanoma cells in the presence of methylene blue (MB) encapsulated in nanoliposomes.

METHODS

Treatment protocols were studied: sonication groups (40 kHz, 1 MHz and dual-frequency), the same sonication groups with nanoliposomes containing MB, MB free and nanoliposomes containing MB groups, and so sham and control groups. The nanoliposomes were prepared by the lipid film hydration method. The cell viability of the different treatment groups was evaluated by the MTT assay.

RESULTS

The dual-frequency protocols caused higher viability losses compared to the kHz and MHz sonications (P < .05). In presence of the nanoliposomes containing MB, dual frequency led to 6% and 3% viability for 600 and 1200 seconds, respectively, while the corresponding values were 10% and 4% for the 40 kHz protocols and 22% and 9% for the 1 MHz, as compared to the control group (100%). The result of KI dosimetry showed that the cavitation activity of the dual-frequency protocol was about 1.23, as compared to sonication at 40 kHz and 1 MHz.

CONCLUSION

Enhancement of inertial cavitation induction by dual-frequency sonication may be the primary effective mechanism, which causes increased sonochemical processes and drug release from nanocarriers.

Ultrasonic moulding: Current state of the technology

Ultrasonic moulding, also known as ultrasonic microinjection moulding, is a new technology that uses high-power ultrasound to melt and mould thermoplastic polymers to produce samples with mini- and micro-features. The main feature of this technology is the use of ultrasonic energy as the heating source instead of a conventional injection screw. Since 2002, several authors have experimentally and theoretically studied the ability of ultrasonic energy to mould polymers. However, different machine configurations and experimental design strategies have been used, which makes it very difficult to compare the results obtained from different articles. In this report, the authors have compiled experimental studies on the ultrasonic moulding process and analysed them along with providing a theoretical framework. An accurate description of the process and the machine configurations used in the literature is also presented. The results obtained from the analysis are summarized and discussed, and possible next steps to further the research in this field are suggested.

The in situ electrochemical detection of microbubble oscillations during motion through a channel

Bubble oscillation has many applications, from driving local fluid motion to cleaning. However, in order to exploit their action, a full understanding of this motion, particularly in confined spaces (such as crevices etc. which are important in ultrasonic decontamination) is important. To this end, here we show how a Coulter counter can be used to characterize microbubbles produced through the ultrasonication of electrolytes. These microbubbles are shown to exist in relatively high concentrations while bubble activity is driven by ultrasound. Detection of these microbubbles, and their oscillatory behaviour, is achieved via translocation through a cylindrical glass microchannel (GMC). The microbubbles oscillate within the 40 μm channel employed and this behaviour is observed to change over the translocation period. This is attributed to the acoustic environment present or changes to the physical conditions in the interior of the chamber compared to the exterior. High-speed imaging confirms the presence of microbubbles as they move or 'skate' across the surface of the structures present before translocating through the channel. The observations are useful as they show that microbubble oscillation occurs within small structures, is preceded by surface confined bubbles and could be enhanced through pressure driven flow through a structure.

Characterizing the cavitation development and acoustic spectrum in various liquids

A bespoke cavitometer that measures acoustic spectrum and is capable of operating in a range of temperatures (up to 750°C) was used to study the cavitation behaviour in three transparent liquids and in molten aluminium. To relate these acoustic measurements to cavitation development, the dynamics of the cavitation bubble structures was observed in three Newtonian, optically transparent liquids with significantly different physical properties: water, ethanol, and glycerine. Each liquid was treated at 20kHz with a piezoelectric ultrasonic transducer coupled to a titanium sonotrode with a tip diameter of 40mm. Two different transducer power levels were deployed: 50% and 100%, with the maximum power corresponding to a peak-to-peak amplitude of 17μm. The cavitation structures and the flow patterns were filmed with a digital camera. To investigate the effect of distance from the ultrasound source on the cavitation intensity, acoustic emissions were measured with the cavitometer at two points: below the sonotrode and near the edge of the experimental vessel. The behaviour of the three tested liquids was very different, implying that their physical parameters played a decisive role in the establishment of the cavitation regime. Non dimensional analysis revealed that water shares the closest cavitation behaviour with liquid aluminium and can therefore be used as its physical analogue in cavitation studies; this similarity was also confirmed when comparing the measured acoustic spectra of water and liquid aluminium.

Collapse of a lipid-coated nanobubble and subsequent liposome formation

We investigate the collapse of a lipid-coated nanobubble and subsequent formation of a lipid vesicle by coarse grained molecular dynamics simulations. A spherical nanobubble coated with a phospholipid monolayer in water is a model of an aqueous dispersion of phospholipids under negative pressure during sonication. When subjected to a positive pressure, the bubble shape deforms into an irregular spherical shape and the monolayer starts to buckle and fold locally. The local folds grow rapidly in multiple directions and forming a discoidal membrane with folds of various amplitudes. Folds of small amplitude disappear in due course and the membrane develops into a unilamellar vesicle via a bowl shape. Folds with large amplitude develop into a bowl shape and a multivesicular shape forms. The membrane shape due to bubble collapse can be an important factor governing the vesicular shape during sonication.

An activated fluid stream--New techniques for cold water cleaning

Electrochemical, acoustic and imaging techniques are used to characterise surface cleaning with particular emphasis on the understanding of the key phenomena relevant to surface cleaning. A range of novel techniques designed to enhance and monitor the effective cleaning of a solid/liquid interface is presented. Among the techniques presented, mass transfer of material to a sensor embedded in a surface is demonstrated to be useful in the further exploration of ultrasonic cleaning of high aspect ratio micropores. In addition the effect of micropore size on the cleaning efficacy is demonstrated. The design and performance of a new cleaning system reliant on the activation of bubbles within a free flowing stream is presented. This device utilised acoustic activation of bubbles within the stream and at a variety of substrates. Finally, a controlled bubble swarm is generated in the stream using electrolysis, and its effect on both acoustic output and cleaning performance are compared to the case when no bubbles are added. This will demonstrate the active role that the electrochemically generated bubble swarm can have in extending the spatial zone over which cleaning is achieved.

Sound field measurement in a double layer cavitation cluster by rugged miniature needle hydrophones

During multi-bubble cavitation the bubbles tend to organize themselves into clusters and thus the understanding of properties and dynamics of clustering is essential for controlling technical applications of cavitation. Sound field measurements are a potential technique to provide valuable experimental information about the status of cavitation clouds. Using purpose-made, rugged, wide band, and small-sized needle hydrophones, sound field measurements in bubble clusters were performed and time-dependent sound pressure waveforms were acquired and analyzed in the frequency domain up to 20 MHz. The cavitation clusters were synchronously observed by an electron multiplying charge-coupled device (EMCCD) camera and the relation between the sound field measurements and cluster behaviour was investigated. Depending on the driving power, three ranges could be identified and characteristic properties were assigned. At low power settings no transient and no or very low stable cavitation activity can be observed. The medium range is characterized by strong pressure peaks and various bubble cluster forms. At high power a stable double layer was observed which grew with further increasing power and became quite dynamic. The sound field was irregular and the fundamental at driving frequency decreased. Between the bubble clouds completely different sound field properties were found in comparison to those in the cloud where the cavitation activity is high. In between the sound field pressure amplitude was quite small and no collapses were detected.

Dependence of pulsed focused ultrasound induced thrombolysis on duty cycle and cavitation bubble size distribution

In this study, we investigated the relationship between the efficiency of pulsed, focused ultrasound (FUS)-induced thrombolysis, the duty cycle (2.3%, 9%, and 18%) and the size distribution of cavitation bubbles. The efficiency of thrombolysis was evaluated through the degree of mechanical fragmentation, namely the number, mass, and size of clot debris particles. First, we found that the total number and mass of clot debris particles were highest when a duty cycle of 9% was used and that the mean diameter of clot debris particles was smallest. Second, we found that the size distribution of cavitation bubbles was mainly centered around the linear resonance radius (2.5μm) of the emission frequency (1.2MHz) of the FUS transducer when a 9% duty cycle was used, while the majority of cavitation bubbles became smaller or larger than the linear resonance radius when a 2.3% or 18% duty cycle was used. In addition, the inertial cavitation dose from the treatment performed at 9% duty cycle was much higher than the dose obtained with the other two duty cycles. The data presented here suggest that there is an optimal duty cycle at which the thrombolysis efficiency and cavitation activity are strongest. They further indicate that using a pulsed FUS may help control the size distribution of cavitation nuclei within an active size range, which we found to be near the linear resonance radius of the emission frequency of the FUS transducer.

Ultrasonically triggered ignition at liquid surfaces

Ultrasound is considered to be an ignition source according to international standards, setting a threshold value of 1mW/mm(2) [1] which is based on theoretical estimations but which lacks experimental verification. Therefore, it is assumed that this threshold includes a large safety margin. At the same time, ultrasound is used in a variety of industrial applications where it can come into contact with explosive atmospheres. However, until now, no explosion accidents have been reported in connection with ultrasound, so it has been unclear if the current threshold value is reasonable. Within this paper, it is shown that focused ultrasound coupled into a liquid can in fact ignite explosive atmospheres if a specific target positioned at a liquid's surface converts the acoustic energy into a hot spot. Based on ignition tests, conditions could be derived that are necessary for an ultrasonically triggered explosion. These conditions show that the current threshold value can be significantly augmented.

Electrochemical ‘bubble swarm’ enhancement of ultrasonic surface cleaning

Control and excitation of electrochemical bubble swarms enhances surface cleaning within an ultrasonically activated stream.

Bubble size distribution in acoustic droplet vaporization via dissolution using an ultrasound wide-beam method

Performance and efficiency of numerous cavitation enhanced applications in a wide range of areas depend on the cavitation bubble size distribution. Therefore, cavitation bubble size estimation would be beneficial for biological and industrial applications that rely on cavitation. In this study, an acoustic method using a wide beam with low pressure is proposed to acquire the time intensity curve of the dissolution process for the cavitation bubble population and then determine the bubble size distribution. Dissolution of the cavitation bubbles in saline and in phase-shift nanodroplet emulsion diluted with undegassed or degassed saline was obtained to quantify the effects of pulse duration (PD) and acoustic power (AP) or peak negative pressure (PNP) of focused ultrasound on the size distribution of induced cavitation bubbles. It was found that an increase of PD will induce large bubbles while AP had only a little effect on the mean bubble size in saline. It was also recognized that longer PD and higher PNP increases the proportions of large and small bubbles, respectively, in suspensions of phase-shift nanodroplet emulsions. Moreover, degassing of the suspension tended to bring about smaller mean bubble size than the undegassed suspension. In addition, condensation of cavitation bubble produced in diluted suspension of phase-shift nanodroplet emulsion was involved in the calculation to discuss the effect of bubble condensation in the bubble size estimation in acoustic droplet vaporization. It was shown that calculation without considering the condensation might underestimate the mean bubble size and the calculation with considering the condensation might have more influence over the size distribution of small bubbles, but less effect on that of large bubbles. Without or with considering bubble condensation, the accessible minimum bubble radius was 0.4 or 1.7 μm and the step size was 0.3 μm. This acoustic technique provides an approach to estimate the size distribution of cavitation bubble population in opaque media and might be a promising tool for applications where it is desirable to tune the ultrasound parameters to control the size distribution of cavitation bubbles.

Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation

Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of 78 nm-diameter silica particles at a power density of 0.67 W/cm(2) is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3 μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.