Cavitation Nuclei Research Articles

Using a confined space to boost the driving amplitude of pulsating bubbles to facilitate jetting

Abstract A bubble collapsing near an interface may result in the formation of a liquid jet protruding from the distal bubble side, through the bubble, towards the interface. Ultrasound assisted jetting has been observed when subjecting, by approximation, infinite fluids to acoustic amplitudes above the inertial cavitation threshold, limiting the possibility of ultrasoundguided, bubble-assisted drug or gene delivery. However, the vascular system can be regarded as a finite fluid. The purpose of this study was to investigate the feasibility of lowamplitude jetting for fluid containing biocompatible cavitation nuclei, by placing the region of interest in a confined space to ensure a standing wave field. Droplets of QuantisonTMultrasound contrast agent were pipetted into a Perspex cylindrical compartment of 8-mm diameter and 2-mm height, which was part of an imaging system. The contrast agent was subjected to 3-cycle ultrasound pulses with a centre frequency of 1MHz whilst being observed at a frame rate of ten million frames per second. Jetting was observed to occur with microbubbles nucleated from the contrast agent in an acoustic regime whose free-field mechanical index was 0.6. Empirical curve matching showed a pulse amplification by a factor of six owing to the chosen geometry. Visible jet lengths of twice the bubble radius on the verge of collapse were measured. Owing to the confined space, the local acoustic amplitude was amplified to surpass the cavitation threshold. This finding is of interest for medical ultrasonic applications where the local environment comprises reflectors.

Localized Oxidative Catalytic Reactions Triggered by Cavitation Bubbles Confinement on Copper Oxide Microstructured Particles.

Efficient energy transfer management in catalytic processes is crucial for overcoming activation energy barriers while minimizing costs and CO2 emissions. We exploit here a concept of CuO particle design with multiple gas-stabilizing sites, engineered to function as cavitation nuclei and catalysts. This concept facilitates the selective and efficient acoustic energy transfer directly to the catalyst surface, avoiding the undesired dissipation of acoustic energy into the bulk solution while demonstrating superior cavitation properties at lower acoustic pressure amplitudes. Utilizing a chemical thermometric approach, we demonstrate that the local temperature on the surface of our CuO particles during cavitation bubble implosions can create an effective equivalent temperature of about 360 °C. This temperature effect facilitates the efficient catalysis of oxidative reactions using an organic pollutant probe molecule. Density functional theory (DFT) calculations were used to assess the decomposition of H2O2 and of pollutant probe molecule on CuO (111). Our work represents a significant advance in sonocatalytic systems, promising efficient energy use in catalytic reactions.

Localized Oxidative Catalytic Reactions Triggered by Cavitation Bubbles Confinement on Copper Oxide Microstructured Particles.

AbstractEfficient energy transfer management in catalytic processes is crucial for overcoming activation energy barriers while minimizing costs and CO2 emissions. We exploit here a concept of CuO particle design with multiple gas‐stabilizing sites, engineered to function as cavitation nuclei and catalysts. This concept facilitates the selective and efficient acoustic energy transfer directly to the catalyst surface, avoiding the undesired dissipation of acoustic energy into the bulk solution while demonstrating superior cavitation properties at lower acoustic pressure amplitudes. Utilizing a chemical thermometric approach, we demonstrate that the local temperature on the surface of our CuO particles during cavitation bubble implosions can create an effective equivalent temperature of about 360 °C. This temperature effect facilitates the efficient catalysis of oxidative reactions using an organic pollutant probe molecule. Density functional theory (DFT) calculations were used to assess the decomposition of H2O2 and of pollutant probe molecule on CuO (111). Our work represents a significant advance in sonocatalytic systems, promising efficient energy use in catalytic reactions.

Analysis of multi-frequency ultrasound effect on acoustic microcavitations in [formula omitted]-dimensional fluid

Acoustic microcavitation and associated mechanical effects, such as stress and strain, are significant in decreasing cavitation thresholds and improving cavitation effects at multiple (i.e., single, dual, and triple) frequencies. Ultrasonic excitation and the introduction of microbubbles as cavitation nuclei are both extremely successful techniques. Besides, the enhancing effects brought on by a dual and triple frequency approach, the cavitation dynamics of microbubbles in viscoelastic tissues under the influence of dual and triple frequency excitation are poorly known. This work deals with the modelling and numerical investigation of acoustic cavitation in N-dimensional fluid based on the multifrequency of ultrasound fields. The mathematical model of ultrasonic microcavitation bubble dynamics is formulated by continuity and Navier-Stokes equations in N-dimensions and the equation of pressure due to acoustic multifield. The proposed model is solved and investigated numerically based on the hybrid-B-Spline method. The numerical results illustrate that the behaviour of the microcavitation bubble dynamics increases with a decrease of the value of the N-dimensional fluid. Moreover, the results of the numerical simulation show the importance of the effect of different multifrequency on the behaviour of the microbubble dynamics, as the results obtained showed that in the case of a single frequency, the radius of the microbubbles is higher than in the cases of dual and triple frequency. Additionally, at different initial radii values for cavitation microbubbles, the growth and departure rates for microcavitation dynamics were examined. Finally, in the proposed model, numerical solutions are studied, their stability is verified, and they are compared with previous theoretical works.

Quantitative evaluation of anti-biofilm cavitation activity seeded from microbubbles or protein cavitation nuclei by passive acoustic mapping

Bacterial biofilms represent a major challenge for effective antibiotic therapy as they confer physical and functional changes that protect bacteria from their surrounding environment. In this work, focused ultrasound in combination with cavitation nuclei was used to disrupt biofilms of Staphylococcus aureus and Pseudomonas aeruginosa, both of which are on the World Health Organization's priority list for new antimicrobial research. 
Approach: Single species biofilms were exposed to ultrasound (0.5 MHz centre frequency, 0.5-1.5 MPa peak rarefactional pressure, 200 cycle pulses, 5 Hz repetition frequency, 30 s duration), in the presence of two different types of cavitation nuclei. Quantitative passive acoustic mapping (PAM) was used to monitor cavitation emissions during treatment using a calibrated linear array. 
Main Results: It was observed that the cumulative energy of acoustic emissions during treatment was positively correlated with biofilm disruption, with differences between bacterial species attributed to differences in biofilm morphology. PCaN provided increased biofilm reduction compared to microbubbles due in large part to their persistence over the duration of ultrasound exposure. There was also good correlation between the spatial distribution of cavitation as characterized by PAM and the extent of biofilm disruption observed with microscopy. 
Significance: Collectively, the results from this work indicate the potential broad applicability of cavitation for eliminating biofilms of priority pathogens and the opportunity presented by Passive Acoustic Mapping for real-time monitoring of antimicrobial processes.

Characterisation of Gas Vesicles as Cavitation Nuclei for Ultrasound Therapy using Passive Acoustic Mapping.

Genetically encodable gas filled particles known as gas vesicles (GVs) have shown promise as a biomolecular contrast agent for ultrasound imaging and have the potential to be used as cavitation nuclei for ultrasound therapy. In this study, we used passive acoustic mapping techniques to characterize GV-seeded cavitation, utilizing 0.5 and 1.6 MHz ultrasound over peak rarefactional pressures ranging from 100 to 2200 kPa. We found that GVs produce cavitation for the duration of the first applied pulse, up to at least 5000 cycles, but that bubble activity diminishes rapidly over subsequent pulses. At 0.5 MHz the frequency content of cavitation emissions was predominantly broadband in nature, whilst at 1.6 MHz narrowband content at harmonics of the main excitation frequency dominated. Simulations and high-speed camera imaging suggest that the received cavitation emissions come not from individual GVs but instead from the coalescence of GV-released gas into larger bubbles during the applied ultrasound pulse. These results will aid the future development of GVs as cavitation nuclei in ultrasound therapy.

Removal of surface-attached micro- and nanobubbles by ultrasonic cavitation in microfluidics

Surface-attached micro- and nanobubbles are known for their resistance to external forces. This study experimentally and theoretically investigates their response to strong ultrasonic fields. Surface-attached micro- and nanobubbles with contact radii from 2 μm to 20 μm are generated in a microchannel and exposed to ultrasound through a vibrating glass substrate. At a driving frequency over 200 kHz up to 2 MHz tested, no significant response from the micro- and nanobubbles is observed. By contrast, at 100 kHz–200 kHz, ultrasonic cavitation bubbles appear in the microchannel and migrate toward the surface micro- and nanobubbles. Then the surface micro- and nanobubbles merge with the ultrasonic cavitation bubbles, detach from the substrate, and become free gaseous nuclei susceptible to further cavitation. Notably, the removal process leaves no observable residue. Theoretical analysis suggests that the directional migration of cavitation bubbles is driven by mutual acoustic radiation forces. This work demonstrates that ultrasonic fields can effectively remove surface micro- and nanobubbles, transforming them into free gaseous cavitation nuclei.

Turning Waste into Wealth: A Potent Sono-Immune Strategy Based on Microcystis.

Currently, sonodynamic therapy (SDT) has limited therapeutic outcomes and immune responses, highlighting the urgent need for enhanced strategies that can stimulate robust and long-lasting antitumor effects. Microcystis, a notorious microalga, reveals the possibility of mediating SDT owing to the presence of gas vesicles (GVs) and phycocyanin (PC). Herein, a nontoxic strain of Microcystis elabens (labeled Me) is developed as a novel agent for SDT because it generates O2 under red light (RL) illumination, while GVs and PC act as cavitation nuclei and sonosensitizers, respectively. Moreover, algal debris is released after ultrasound (US) irradiation, which primes the Toll-like receptor pathway to initiate a cascade of immune responses. This sono-immune strategy inhibits CT26 colon tumor growth largely by promoting dendritic cell (DC) maturation and cytotoxic T-cell activation. After combination with the immune checkpoint blockade (ICB), the therapeutic outcome is further amplified, accompanied by satisfactory abscopal and immune memory effects; the similar potency is proven in the "cold" 4T1 triple-negative breast tumor. In addition, Me exhibits good biosafety without significant acute or chronic toxicity. Briefly, this study turns waste into wealth by introducing sono-immunotherapy based on Microcystis that achieved encouraging therapeutic effects on cancer, which is expected to be translated into the clinic.

Nanoparticle-Mediated Histotripsy Using Dual-Frequency Pulsing Methods

ObjectiveNanoparticle-mediated histotripsy (NMH) is a novel ablation method that combines nanoparticles as artificial cavitation nuclei with focused ultrasound pulsing to achieve targeted, non-invasive, and cell-selective tumor ablation. The study described here examined the effect of dual-frequency histotripsy pulsing on the cavitation threshold, bubble cloud characteristics, and ablative efficiency in NMH. High-speed optical imaging was used to analyze bubble cloud characteristics and to measure ablation efficiency for NMH inside agarose tissue phantoms containing perfluorohexane-filled nanocone clusters, which were previously developed to reduce the histotripsy cavitation threshold for NMH. MethodsDual-frequency histotripsy pulsing was applied at a 1:1 pressure ratio using a modular 500 kHz and 3 MHz dual-frequency array transducer. Optical imaging results revealed predictable, well-defined bubble clouds generated for all tested cases with similar reductions in the cavitation thresholds observed for single-frequency and dual-frequency pulsing. ResultsDual-frequency pulsing was seen to nucleate small, dense clouds in agarose phantoms, intermediate in size of their component frequencies but closer in area to that of the higher component frequency. Red blood cell experiments revealed complete ablations were generated by dual-frequency NMH in all phantoms in <1500 pulses. This result was a significant increase in ablation efficiency compared with the ∼4000 pulses required in prior single-frequency NMH studies. ConclusionOverall, this study indicates the potential for using dual-frequency histotripsy methods to increase the ablation efficacy of NMH.

Thermodynamic effects in cavitating flow

Cavitation is a phenomenon characterized by vapor generation and condensation in liquid flows, at approximately constant temperature. Cavitation is usually triggered by a local pressure drop in the vicinity of a cavitation nucleus. As the bubble grows, latent heat is supplied from the surrounding liquid to the interface, creating a thermal boundary layer. This creates a thermal boundary layer. The result is a local drop in the liquid temperature, which leads to a drop in vapor pressure. This delays the further development of the bubble, as a greater pressure drop is required to maintain the process. This phenomenon is known as thermal delay or thermodynamic effect and plays a moderating role in the development of cavitation. Thermodynamic effects can usually be neglected for liquids whose critical temperature is much higher than the operating temperature. However, this is not the case for liquids whose operating temperature is close to the critical temperature, such as in cryogenics.The understanding of the thermodynamic effects of cavitating flow is crucial for applications like turbopumps for liquid hydrogen LH2 and oxygen LOx in space launcher engines. Experimental studies of this phenomenon are rare as most of them were performed in the 1960s and 1970s. Due to the extreme difficulties of experimental investigation, predicting of thermodynamic effects in cavitation often bases on data in liquids other than cryogenics. Most often used surrogate liquids are hot water or certain refrigerants, which are selected by a single fluid property, most commonly by the thermodynamic parameter ∑.

Application of high intensity focused ultrasound combined with nanomaterials in anti-tumor therapy

High intensity focused ultrasound (HIFU) has demonstrated its safety, efficacy and noninvasiveness in the ablation of solid tumor. However, its further application is limited by its inherent deficiencies, such as postoperative recurrence caused by incomplete ablation and excessive intensity affecting surrounding healthy tissues. Recent research has indicated that the integration of nanomaterials with HIFU exhibits a promising synergistic effect in tumor ablation. The concurrent utilization of nanomaterials with HIFU can help overcome the limitations of HIFU by improving targeting and ablation efficiency, expanding operation area, increasing operation accuracy, enhancing stability and bio-safety during the process. It also provides a platform for multi-therapy and multi-mode imaging guidance. The present review comprehensively expounds upon the synergistic mechanism between nanomaterials and HIFU, summarizes the research progress of nanomaterials as cavitation nuclei and drug carriers in combination with HIFU for tumor ablation. Furthermore, this review highlights the potential for further exploration in the development of novel nanomaterials that enhance the synergistic effect with HIFU on tumor ablation.

Research of bubble motion and correction of drag coefficient in viscous oil at low Reynolds number

As the core fundamental components in the field of fluid transmission and control, viscous oil fluid components determine the technological level of fluid machinery and high-end equipment. However, bubbles as effective cavitation nuclei widely exist in the interior of viscous oil fluid components, directly affecting their performance and technological development. In order to reveal the bubble motion characteristics in viscous oil and establish a high-precision prediction method, this paper focuses on No. 110 technical white oil and develops a visualized observation system for studying the motion characteristics and morphological features of bubbles in viscous oil. The bubble motion trajectory, rising rate, and force state are analyzed, and a prediction model for the drag coefficient is proposed and corrected. The research results indicate that during the rising process, the bubble morphologies in viscous oil show an elliptical shape. The oil temperature has a significant influence on the bubble motion characteristics, with the occurrence of oscillations during the rise process after reaching 80 °C. Additionally, a new prediction model for the drag coefficient of bubble is fitted, where the relative error of bubble motion rate can be controlled within 5.5% for 0.3 &amp;lt; Re &amp;lt; 50. This research provides significant theoretical significance and engineering application value.

Native bubble nuclei for acoustic cavitation in 3D cell cultures

The safety of biomedical ultrasound largely relies on controlling cavitation bubbles in vivo; however, bubble nuclei in biological tissues remain unexplored compared to water. No previous studies have observed where bubble nuclei form in tissues at a microscopic level or how the variation in tissue biomechanical properties can impact the presence of bubble nuclei for acoustic cavitation. In this study, we evaluated whether bubble nuclei form intracellularly or extracellularly in rat epithelial hepatoma (McA-RH7777) and musculoskeletal myoblast (L6) cell lines (n = 5 each), cultured in 3D using Matrigel™ scaffolds. A 3.68 MHz focused ultrasound transducer with f# = 1 was used to induce low-density cavitation using varying pulse lengths (10 to 100 μs) with pressures ranging up to p+ = 29 and p− = 12 MPa. The spatial location of the bubbles was monitored microscopically using high-speed photography at 20 000 fps. Despite significant radiation force on the cell scaffold, preliminary results show that acoustic cavitation bubbles (r = 20 μm approx.) preferentially form extracellularly near the cell membranes. This result suggests that the hydrophobicity in the amphipathic cell membrane may contribute to bubble nuclei formation. Future work includes investigating the distribution of bubble nuclei in healthy versus cancerous cell cultures. [Work supported by NSF CAREER 1943937.]

Investigation of the ultrasound-mediated toxicity mechanisms of IR780 iodide

IR780 iodide is a lipophilic cation heptamethine dye that has emerged as a potential fluorescent probe for in vivo tumor imaging. Previous work has shown it to be a sono- and photo-sensitizer (sound- and light-activated molecule) suitable for use in sonodynamic therapy (SDT) due to its proposed ability to produce ROS when ‘activated' by ultrasound. This study evaluated IR780 iodide as an SDT agent in vitro using A549s, HeLas, and HeLa S3 cell lines, a broad range of ultrasound and light parameters, multiple ultrasound and light systems, and with and without cavitation nuclei. Through temporal uncoupling of ultrasound application and compound administration in vitro and in vivo, evaluation of cavitation-only related cell death, assessment of the dark toxicity of IR780 iodide, and development of positive controls for cell permeabilization (sonoporation), this study shows that dark toxicity and cavitation play an important role in IR780 SDT-induced cell death. This potentially explains the high levels of cell death observed for comparatively low concentrations of ROS. Additionally, this study proposes a standard set of controls for SDT mechanistic studies, which were used to further help study the mechanisms of other SDT drugs including Rose Bengal, 5-aminolevulinic acid, and indocyanine green.

Microcavitation dynamics of vaporized perfluorocarbon nanodroplets captured with an acoustical camera

Perfluorocarbon nanodroplets are on-demand cavitation nuclei for numerous biomedical applications. These nanodroplets remain in a metastable state until an ultrasound pulse of sufficient negative pressure initiates conversion from a liquid droplet into a gaseous micro- or nano-bubble in a process termed acoustic droplet vaporization (ADV). Extensive research has gone into understanding their interaction with ultrasound at varying acoustic and ambient conditions. Recent studies have detected intra- and post-excitation collapse emissions indicative of inertial cavitation. This study aims to further understand the post-ADV dynamics of perfluorocarbon nanodroplets with an acoustical camera technique. Nanodroplets were activated with a low-frequency ultrasound pulse (5 MHz) while being insonified with a high-frequency (35 MHz) probing wave. The side scattered emissions from the probing wave were captured with another matched high-frequency transducer. The amplitude modulation of these emissions is proportional to the radial strain; thus, a relative radius-time curve can be extracted from the scattered signal after filtering and enveloping. Results show that this technique can capture the radial growth and collapse curves of these newly formed bubbles and aligns well with intra- and post-excitation emissions indicative of inertial cavitation. This approach could be useful to understand post-ADV dynamics of various droplet formulations.

Real time cavitation monitoring during ultrasound-mediated drug delivery in normothermically perfused human tumour-bearing livers

In the fight against a broad spectrum of human diseases, cavitation techniques show great promise for overcoming physical barriers that lead to suboptimal uptake of passively administered therapeutics. However, small animal testing of candidate therapies remains a poor predictor of clinical success. Here we demonstrate ultrasound-mediated drug delivery in normal and tumour-bearing human livers infused with protein-based cavitation nuclei (PCaN). Whole and partial human livers were obtained immediately from hepatectomy surgeries and were normothermically sustained using a clinically approved perfusion system (OrganOx Metra). Ultrasound was applied using a 0.5 MHz focused source (Sonic Concepts H107) and was monitored with a calibrated linear array (ATS L7-4) for real time structural and cavitational imaging implemented on an array controller (Verasonics Vantage 256). Specifically, the therapy process was monitored using passive acoustic mapping (PAM) of broadband cavitation emissions, employing a non-adaptive beamformer that deconvolves the array point spread function. Levels of fluorescently labelled drugs incorporated in and co-administered with the PCaN were quantified in blood and tissue samples collected during and following treatment, respectively. This presentation highlights PAM observations of broadband cavitation persistence and drug delivery in untargeted and targeted tissues, including the first ever experiments in tumour-bearing human livers.

Effect of gas content on cavitation nuclei

Cavitation inception originates from nuclei in a liquid. This paper proposes a Gibbs free energy approach that provides a smooth transition from homogeneous to heterogeneous nucleation when gas is present. The impact of gas content on nucleation is explored. It is found that the gas content stabilises nuclei, a phenomenon not present in pure liquid–vapour systems. This reduces the energy barrier over that required to nucleate a vapour bubble. Different gas saturation levels are studied. Gas content can significantly reduce the energy barrier required for nucleation, and under certain circumstances eliminate it. An analytic solution for the critical radius and activation energy is obtained that accounts for gas content. The classical Blake radius is recovered as a limiting case. The hysteresis between incipience and desinence is explained using the asymmetry observed in the critical radii. The solution is used to obtain the initial bubble radius, given a critical pressure condition in cavitation susceptibility meter experiments. The relationship between initial bubble diameter and critical pressure is described by an analytic solution that accounts for gas content. A model for the derivative of the cumulative nuclei histogram with respect to bubble diameter is proposed. An analytic expression is obtained that shows good agreement with decades worth of experimental data compiled by Khoo et al. (Exp. Fluids, vol. 61, issue 2, 2020, pp. 1–20) from ocean to water tunnels. The expression recovers the $-4$ power law that is observed experimentally.

Enhanced microbubble-mediated cavitation by using acoustic droplet vaporization

Acoustic cavitation serves various therapeutic purposes, with both microbubbles and phase-transit droplets serving as potential cavitation nuclei. This project focused on investigating the intricate interaction between microbubbles and droplets and the resultant impact on acoustic cavitation.To facilitate this investigation, microbubbles with a negative potential of −51.58 mV were prepared, while droplets were endowed with a positive potential of +4.22 mV, facilitating the formation of microbubble-droplet clusters. Microscopic analysis was employed to observe the morphology of these clusters, revealing an initial mean size of 4.85 µm before ultrasound activation and subsequent transformation into larger bubbles with a mean size of 12.54 µm after ultrasound activation. The experimental approach involved the use of varying driven voltages to generate ultrasound waves with different intensities. A passive cavitation detector (PCD) was then employed to capture cavitation signals. Results demonstrated that, when maintaining a microbubble ratio of 50% and varying droplet ratios from 0% to 50%, the driven voltage at which the cavitation peak appeared decreased from 180 mV to 140 mV. Additionally, the mean cavitation intensity exhibited an initial increase from 0.58 to 0.78, followed by a subsequent decrease from 0.78 to 0.63 after normalization.This study provided valuable insights into the dynamic interactions between microbubbles and droplets. Specifically, it showed that microbubble cavitation transferred energy to droplets, leading to the generation of larger bubbles and higher cavitation intensity. Nevertheless, the observed cavitation intensity and threshold were contingent upon the ratios of microbubbles and droplets. Notably, to achieve an ideal cavitation intensity, the energy generated by microbubble cavitation must be substantial enough to activate all droplets, inducing acoustic droplet vaporization (ADV) and cavitation.

Ultrasound Activated Nanobowls with Deep Penetration for Enhancing Sonodynamic Therapy of Orthotopic Liver Cancer.

Owing to the high penetration ability and the safety of ultrasound (US) of sonodynamic therapy (SDT), it has gained significant attention in tumor treatment. However, its therapeutic efficiency depends on the performance of the sonosensitizers. The hypoxic microenvironment and abnormal stromal matrix restrict the full potential of sonosensitizers. In this study, a US-activated bowl-shaped nanobomb (APBN) is designed as a novel sonosensitizer to enhance the SDT effect through various means. This enhancement strategy combines three major characteristics: relieving tumor hypoxia, amplifying bubble cavitation damage, and US-movement-enhanced permeation. The unique bowl-shaped structure of APBN provides more favorable attachment sites for the generated oxygen gas bubbles. Thus, when catalase-like APBN catalyzes endogenous hydrogen peroxide to produce oxygen, bubbles accumulate at the groove, preventing the dissipation of oxygen and increasing the number of cavitation nuclei to improve the acoustic cavitation effect. This approach differs from traditional SDT strategies because it couples the sonodynamic effect with reactive oxygen species generation and bubble cavitation damage rather than a single action. Additionally, the asymmetric bowl-shaped structure generates a driving force under the US field, improving the distribution of sonosensitizers in the tumors. Using US and photoacoustic imaging for dual localization, these sonosensitizers can improve the accuracy of orthotopic liver tumor treatment, which presents a promising avenue for the treatment of deep tumors.

High-speed photography of gas release from bioactive glass

Bioactive glass has been of interest for applications in bone regeneration. Floating bioactive glass particles were observed to sink in ultrasound. The purpose of this study was to qualify and quantify bubble formation from floating bioactive glass particles. Water droplets containing borosilicate glass 13-93B20 particles, where 20% of the SiO2 was replaced with B2O3, of dimensions <38 μm were subjected to pulsed ultrasound, whilst being video-recorded at high speed. Measured radial expansions >20 μm corresponded to cavitation nuclei of initial radius 0.6 μm. This study provides experimental evidence that gas trapped inside bioactive glass may be released using high-amplitude ultrasound pulses.