Behavior Of Microbubbles Research Articles

Resonance behaviors of encapsulated microbubbles oscillating nonlinearly with ultrasonic excitation

The resonance behaviors of a few lipid-coated microbubbles acoustically activated in viscoelastic media were comprehensively examined via radius response analysis. The size polydispersity and random spatial distribution of the interacting microbubbles, the rheological properties of the lipid shell and the viscoelasticity of the surrounding medium were considered simultaneously. The obtained radius response curves present a successive occurrence of linear resonances, nonlinear harmonic and sub-harmonic resonances with the acoustic pressure increasing. The microbubble resonance is radius-, pressure- and frequency-dependent. Specifically, the maximum bubble expansion ratio at the main resonance peak increases but the resonant radius decreases as the ultrasound pressure increases, while both of them decrease with the ultrasound frequency increasing. Moreover, compared to an isolated microbubble case, it is found that large microbubbles in close proximity prominently suppress the resonant oscillations while slightly increase the resonant radii for both harmonic and subharmonic resonances, even leading to the disappearance of the subharmonic resonance with the influences increasing to a certain degree. In addition, the results also suggest that both the encapsulating shell and surrounding medium can substantially dampen the harmonic and subharmonic resonances while increase the resonant radii, which seem to be affected by the medium viscoelasticity to a greater degree rather than the shell properties. This work offers valuable insights into the resonance behaviors of microbubbles oscillating in viscoelastic biological media, greatly contributing to further optimizing their biomedical applications.

An optical and acoustic investigation of microbubble cavitation in small channels under therapeutic ultrasound conditions.

Therapeutic focused ultrasound in combination with encapsulated microbubbles is being widely investigated for its ability to elicit bioeffects in the microvasculature, such as transient permeabilization for drug delivery or at higher pressures to achieve 'antivascular' effects. While it is well established that the behaviors of microbubbles are altered when they are situated within sufficiently small vessels, there is a paucity of data examining how the bubble population dynamics and emissions change as a function of channel (vessel) diameter over a size range relevant to therapeutic ultrasound, particularly at pressures relevant to antivascular ultrasound. Here we use acoustic emissions detection and high-speed microscopy (10 kframes/s) to examine the behavior of a polydisperse clinically employed agent (Definity®) in wall-less channels as their diameters are scaled from 1200 to 15µm. Pressures are varied from 0.1 to 3MPa using either a 5ms pulse or a sequence of 0.1ms pulses spaced at 1ms, both of which have been previously employed in an in vivo context. With increasing pressure, the 1200µm channel - on the order of small arteries and veins - exhibited inertial cavitation, 1/2 subharmonics and 3/2 ultraharmonics, consistent with numerous previous reports. The 200 and 100µm channels - in the size range of larger microvessels less affected by therapeutic focused ultrasound - exhibited a distinctly different behavior, having muted development of 1/2 subharmonics and 3/2 ultraharmonics and reduced persistence. These were associated with radiation forces displacing bubbles to the distal wall and inducing clusters that then rapidly dissipated along with emissions. As the diameter transitioned to 50 and then 15µm - a size regime that is most relevant to therapeutic focused ultrasound - there was a higher threshold for the onset of inertial cavitation as well as subharmonics and ultraharmonics, which importantly had more complex orders that are not normally reported. Clusters also occurred in these channels (e.g. at 3MPa, the mean lateral and axial sizes were 23 and 72µm in the 15µm channel; 50 and 90µm in the 50µm channel), however in this case they occupied the entire lumens and displaced the wall boundaries. Damage to the 15µm channel was observed for both pulse types, but at a lower pressure for the long pulse. Experiments conducted with a 'nanobubble' (<0.45µm) subpopulation of Definity followed broadly similar features to 'native' Definity, albeit at a higher pressure threshold for inertial cavitation. These results provide new insights into the behavior of microbubbles in small vessels at higher pressures and have implications for therapeutic focused ultrasound cavitation monitoring and control.

Abstract 15305: The Dynamic Behavior of Polymer Microbubbles During Long Ultrasound Tone-Burst Excitation and Its Application for Sonoreperfusion Therapy

Objective: Ultrasound (US)-targeted microbubble (MB) cavitation (UTMC) mediated therapies have been shown to restore perfusion and enhance drug/gene delivery. Due to the potentially longer circulation time and relative ease of storage and reconstitution of polymer shelled MBs compared with lipid MBs, we investigated the dynamic behavior of polymer MBs and its therapeutic potential for sonoreperfusion (SRP) therapy. Methods: We explored the fate of polymer MBs during a single long tone-burst exposure (1 MHz, 5 ms) at various acoustic pressures (0.25-1.5 MPa) and MB concentrations (2х10 6 -2х10 8 MB/mL) via direct high-speed microscopy observation and passive cavitation detection (PCD) and compared with lipid MBs. SRP efficacy of these MBs was investigated in an in vitro constant flow system at various pulse durations (1 MHz, 1-5 ms every 3 sec for 20 min) and acoustic pressures (0.6-1.5 MPa). The pressure differential across a porous mesh partially occluded with microthrombi was recorded as a function of time during treatment and used as a measure of thrombus burden. The terminal pressure drop and the initial rate of pressure drop were derived and compared. Results: Both polymer and lipid MBs first underwent stable or inertial cavitation depending on the acoustic pressure, and then formed gas-filled clusters that continued to oscillate, fragment, and form new gas-filled clusters during the single US burst. PCD confirmed continued acoustic activity throughout the 5-ms US excitation. SRP efficacy with polymer MBs increased with the pulse duration and acoustic pressure ( p &lt;0.05, ANOVA), similarly to that of lipid MBs, but no significant differences were found between polymer and lipid MBs ( Figure 1 ). Conclusion: Under the acoustic conditions tested, both polymer and lipid MBs demonstrate similar thrombolytic effects in vitro suggesting that persistent cavitation activity from polymer MBs during long tone-burst US confers excellent reperfusion efficacy.

Material Properties, Dissolution and Time Evolution of PEGylated Lipid-Shelled Microbubbles: Effects of the Polyethylene Glycol Hydrophilic Chain Configurations.

Polyethylene glycol (PEG) is often added to the lipid coating of a contrast microbubble to prevent coalescence and improve circulation. At high surface density, PEG chains are known to undergo a transition from a mushroom configuration to an extended brush configuration. We investigated the effects of PEG chain configuration on attenuation and dissolution of microbubbles by varying the molar ratio of the PEGylated lipid in the shell with three (0%, 2% and 5%) in the mushroom configuration and two (10% and 20%) in the brush configuration. We measured attenuation through the bubble suspensions and used it to obtain the characteristic rheological properties of their shells according to two interfacial rheological models. The interfacial elasticity was found to be significantly lower in the brush regime (∼0.6 N/m) than in the mushroom regime (∼1.3 N/m), but similar in value within each regime. The dissolution behavior of microbubbles under acoustic excitation inside an air-saturated medium was studied by measuring the time-dependent attenuation. Total attenuation recorded a transient increase because of growth resulting from air influx and an eventual decrease caused by dissolution. Microbubble shell composition with varying PEG concentrations had significant effects on dissolution dynamics.

Dynamical In Situ Study of Morphological Variations on Stainless Steel in a Sour Corrosive Environment

It is still unclear how environmentally assisted cracking, especially sulfide stress cracking (SSC), is caused by corrosion and fracture behavior. In this study, a new in situ technique for an SSC visualization challenge was established and applied for the visualization of microcrack propagation resulting from a single pitting on the steel surface of 13% Cr martensitic stainless steel during four-point bending SSC tests and discussed by combining ex situ x-ray computed tomography (XCT) and electron backscatter diffraction (EBSD). The dynamic behavior of microcracks was visualized successfully, and the microbubbles behavior with the corrosion reaction was also captured. Regarding the relationship between internal structure and microstructure for cracking, it was found that a cavity was formed in the interior of prior to the cracks seen on the surface by XCT, and cracks by EBSD were propagated along prior austenite grain boundaries. On the basis of in situ observations and complementary ex situ analyses, the actual situation of crack on the steel surface was revealed and discussed with both corrosion and fracture over time. This approach provides an opportunity to clarify the SSC occurrence process based on actual SSC tests and contributes to the determination of phenomenological criteria. Furthermore, by combining this in situ technique with commonly known electrochemical measurements such as polarization curves, open-circuit potentials, and electrochemical impedance spectroscopy, it is possible to directly discuss various electrochemical data corresponding to corrosion and fracture behavior, including dissolution and gas evolution.

Investigating the resonance response of a system of two ultrasound-driven lipid encapsulated microbubbles

Ultrasound-stimulated microbubbles are clinical imaging agents and being developed for therapeutic applications. Here, we aim to understand the behaviour of two individual phospholipid-encapsulated microbubbles in close proximity to each other, typically the case given clinical doses. We developed a finite element model to study the radial resonance response of each microbubble within a two-microbubble system from 1–8 MHz with bubble diameters ranging from 2 to 4 μm, bubble center-to-center distances h = 8–24 μm, and peak-negative pressures of 30–45 kPa. For two identical microbubbles, our results show the frequency of maximum response (fMR) decreases (7%–10%) and the amplitude of maximum response (AMR) increases (9%–11%) as the microbubbles approach one another. For a two-bubble system of different microbubble sizes, the larger microbubble shows no change in fMR and a slight shift of AMR (2–3%). However, the smaller microbubble exhibits an increase in fMR (7–11%) and a significant decrease of AMR (38–52%). Furthermore, when in very close proximity (h = 8 μm), smaller bubbles exhibit a secondary resonance peak corresponding to the fMR of the larger bubble with amplitudes comparable to its primary resonance peak. Our work suggests that microbubble resonance behaviour is greatly affected by the presence of nearby bubbles, which has implications in imaging and therapy.

The Impact of Surface Drug Distribution on the Acoustic Behavior of DOX-Loaded Microbubbles.

Previous studies have reported substantial improvement of microbubble (MB)-mediated drug delivery with ultrasound when drugs are loaded onto the MB shell compared with a physical mixture. However, drug loading may affect shell properties that determine the acoustic responsiveness of MBs, producing unpredictable outcomes. The aim of this study is to reveal how the surface loaded drug (doxorubicin, DOX) affects the acoustic properties of MBs. A suitable formulation of MBs for DOX loading was first identified by regulating the proportion of two lipid materials (1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and 1,2-distearoyl-sn-glycero-3-phospho-rac-glycerol sodium salt (DSPG)) with distinct electrostatic properties. We found that the DOX loading capacity of MBs was determined by the proportion of DSPG, since there was an electrostatic interaction with DOX. The DOX payload reduced the lipid fluidity of MBs, although this effect was dependent on the spatial uniformity of DOX on the MB shell surface. Loading DOX onto MBs enhanced acoustic stability 1.5-fold, decreased the resonance frequency from 12–14 MHz to 5–7 MHz, and reduced stable cavitation dose by 1.5-fold, but did not affect the stable cavitation threshold (300 kPa). Our study demonstrated that the DOX reduces lipid fluidity and decreases the elasticity of the MB shell, thereby influencing the acoustic properties of MBs.

Quantitative evaluation of the mortality rate of HeLa cells induced by microbubble vibration and collapse under pulsed ultrasound irradiation

Drug delivery systems (DDS) using ultrasound and microbubbles have been proposed to enhance drug efficacy and reduce undesirable side effects. While several researchers have reported the vibrational characteristics of lipid-coated microbubbles under ultrasound irradiation and the effects on cell damage, it is necessary to quantitatively evaluate the effects to establish the safety criteria of ultrasound DDS. In this paper, the effects of the microbubble behavior on adhesive HeLa cells under ultrasound irradiation were evaluated. Microbubbles coated with a phospholipid shell were fabricated and two size distributions of bubbles were prepared. The HeLa cells were exposed to pulsed ultrasound at 1 MHz with microbubbles in a water tank, and the cell mortality rate was measured quantitatively under fluorescent observation. The collapse of the microbubbles with a resonance size of approximately 2 μm enhanced the cell mortality rate. Greater sound pressure amplitude produced a higher collapsing rate of the microbubble and cell mortality rate, implying the destruction of microbubbles with resonance size generated a shock wave or microjet, inducing cell damage. While the cell mortality rate decreased exponentially with the increase in distance between cells and microbubbles, a smaller distance induced a higher cell mortality rate: 84% of the HeLa cells died within 2.5 μm of microbubbles produced by a pulsed ultrasound with 1.0 MPa at 1 MHz.

The behavior of magnetic microbubble in acoustic-magnetic field

A novel dynamic model of magnetic microbubble subjected to acoustic-magnetic field is developed by Lagrange method. Numerical works are effectively replaced by a non-magnetic microbubble immersed in highly magnetic fluid. Simulation shows that bubble’s pulsation and translation can be controlled by an applied uniform magnetic field. Depending on the coupled time scales between relaxation parameters, the oscillation of bubble can be improved or reduced by magnetic field. Besides, magnetic field significantly reduces bubble natural frequency due to the enhanced viscosity. Also, the properties of the magnetic fluid may affect bubble’s responses, such as viscosity and magnetic susceptibility. In the case of strong magnetic susceptibility and low viscosity, magnetic field has a remarkable effect on bubble manipulation. Finally, the larger bubble may have a better response to magnetic field. These results may be beneficial to the manipulation of magnetic microbubbles by a suitable magneto-acoustic field.

Multi-timescale Microscopy Methods for the Characterization of Fluorescently-labeled Microbubbles for Ultrasound-Triggered Drug Release.

Microbubble contrast agents hold great promise for drug delivery applications with ultrasound. Encapsulating drugs in nanoparticles reduces systemic toxicity and increases circulation time of the drugs. In a novel approach to microbubble-assisted drug delivery, nanoparticles are incorporated in or on microbubble shells, enabling local and triggered release of the nanoparticle payload with ultrasound. A thorough understanding of the release mechanisms within the vast ultrasound parameter space is crucial for efficient and controlled release. This set of presented protocols is applicable to microbubbles with a shell containing a fluorescent label. Here, the focus is on microbubbles loaded with poly(2-ethyl-butyl cyanoacrylate) polymeric nanoparticles, doped with a modified Nile Red dye. The particles are fixed within a denatured casein shell. The microbubbles are produced by vigorous stirring, forming a dispersion of perfluoropropane gas in the liquid phase containing casein and nanoparticles, after which the microbubble shell self-assembles. A variety of microscopy techniques are needed to characterize the nanoparticle-stabilized microbubbles at all relevant timescales of the nanoparticle release process. Fluorescence of the nanoparticles enables confocal imaging of single microbubbles, revealing the particle distribution within the shell. In vitro ultra-high-speed imaging using bright-field microscopy at 10 million frames per second provides insight into the bubble dynamics in response to ultrasound insonation. Finally, nanoparticle release from the bubble shell is best visualized by means of fluorescence microscopy, performed at 500,000 frames per second. To characterize drug delivery in vivo, the triggered release of nanoparticles within the vasculature and their extravasation beyond the endothelial layer is studied using intravital microscopy in tumors implanted in dorsal skinfold window chambers, over a timescale of several minutes. The combination of these complementary characterization techniques provides unique insight into the behavior of microbubbles and their payload release at a range of time and length scales, both in vitro and in vivo.

Multi-timescale Microscopy Methods for the Characterization of Fluorescently-labeled Microbubbles for Ultrasound-Triggered Drug Release

Microbubble contrast agents hold great promise for drug delivery applications with ultrasound. Encapsulating drugs in nanoparticles reduces systemic toxicity and increases circulation time of the drugs. In a novel approach to microbubble-assisted drug delivery, nanoparticles are incorporated in or on microbubble shells, enabling local and triggered release of the nanoparticle payload with ultrasound. A thorough understanding of the release mechanisms within the vast ultrasound parameter space is crucial for efficient and controlled release. This set of presented protocols is applicable to microbubbles with a shell containing a fluorescent label. Here, the focus is on microbubbles loaded with poly(2-ethyl-butyl cyanoacrylate) polymeric nanoparticles, doped with a modified Nile Red dye. The particles are fixed within a denatured casein shell. The microbubbles are produced by vigorous stirring, forming a dispersion of perfluoropropane gas in the liquid phase containing casein and nanoparticles, after which the microbubble shell self-assembles. A variety of microscopy techniques are needed to characterize the nanoparticle-stabilized microbubbles at all relevant timescales of the nanoparticle release process. Fluorescence of the nanoparticles enables confocal imaging of single microbubbles, revealing the particle distribution within the shell. In vitro ultra-high-speed imaging using bright-field microscopy at 10 million frames per second provides insight into the bubble dynamics in response to ultrasound insonation. Finally, nanoparticle release from the bubble shell is best visualized by means of fluorescence microscopy, performed at 500,000 frames per second. To characterize drug delivery in vivo, the triggered release of nanoparticles within the vasculature and their extravasation beyond the endothelial layer is studied using intravital microscopy in tumors implanted in dorsal skinfold window chambers, over a timescale of several minutes. The combination of these complementary characterization techniques provides unique insight into the behavior of microbubbles and their payload release at a range of time and length scales, both in vitro and in vivo.

Oscillatory behavior of microbubbles impacts efficacy of cellular drug delivery

Drug-loaded microbubbles have been proven to be an effective strategy for non-invasive and local drug delivery when combined with ultrasound excitation for targeted drug release. Inertial cavitation is speculated to be a major mechanism for releasing drugs from drug-loaded microbubbles, but it results in lethal cellular pore damage that greatly limits its application. Thus, we investigated the cellular vesicle attachment and uptake to evaluate the efficiency of drug delivery by modulating the behaviors of targeted microbubble oscillation. The efficiency of vesicle attachment on the targeted cell membrane was 36.5 ± 15.9% and 3.8 ± 2.3% under stable and inertial cavitation, respectively. Further, stable cavitation enhanced cell permeability (26.8 ± 3.2%), maintained cell viability (90.8 ± 2.1%), and showed 7.9 ± 1.9-fold enhancement of in vivo vesicle release on tumor vessels. Therefore, our results reveal the ability to improve drug delivery via stable cavitation induced by targeted microbubbles. We propose that this strategy might be suitable for tissue repair or neuromodulation.

The Impact of Lipid Handling and Phase Distribution on the Acoustic Behavior of Microbubbles.

Phospholipid-coated microbubbles are ultrasound contrast agents that can be employed for ultrasound molecular imaging and drug delivery. For safe and effective implementation, microbubbles must respond uniformly and predictably to ultrasound. Therefore, we investigated how lipid handling and phase distribution affected the variability in the acoustic behavior of microbubbles. Cholesterol was used to modify the lateral molecular packing of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)-based microbubbles. To assess the effect of lipid handling, microbubbles were produced by a direct method, i.e., lipids directly dispersed in an aqueous medium or indirect method, i.e., lipids first dissolved in an organic solvent. The lipid phase and ligand distribution in the microbubble coating were investigated using confocal microscopy, and the acoustic response was recorded with the Brandaris 128 ultra-high-speed camera. In microbubbles with 12 mol% cholesterol, the lipids were miscible and all in the same phase, which resulted in more buckle formation, lower shell elasticity and higher shell viscosity. Indirect DSPC microbubbles had a more uniform response to ultrasound than direct DSPC and indirect DSPC-cholesterol microbubbles. The difference in lipid handling between direct and indirect DSPC microbubbles significantly affected the acoustic behavior. Indirect DSPC microbubbles are the most promising candidate for ultrasound molecular imaging and drug delivery applications.

Ultrasound Contrast: Gas Microbubbles in the Vasculature.

Gas-filled microbubbles are currently in clinical use as blood pool contrast agents for ultrasound imaging. The goal of this review is to discuss the trends and issues related to these relatively unusual intravascular materials, which are not small molecules per se, not polymers, not even nanoparticles, but larger micrometer size structures, compressible, flexible, elastic, and deformable. The intent is to connect current research and initial studies from 2 to 3 decades ago, tied to gas exchange between the bubbles and surrounding biological medium, in the following areas of focus: (1) parameters of microbubble movement in relation to vasculature specifics; (2) gas uptake and loss from the bubbles in the vasculature; (3) adhesion of microbubbles to target receptors in the vasculature; and (4) microbubble interaction with the surrounding vessels and tissues during insonation.Microbubbles are generally safe and require orders of magnitude lower material doses than x-ray and magnetic resonance imaging contrast agents. Application of microbubbles will soon extend beyond blood pool contrast and tissue perfusion imaging. Microbubbles can probe molecular and cellular biomarkers of disease by targeted contrast ultrasound imaging. This approach is now in clinical trials, for example, with the aim to detect and delineate tumor nodes in prostate, breast, and ovarian cancer. Imaging of inflammation, ischemia-reperfusion injury, and ischemic memory is also feasible. More importantly, intravascular microbubbles can be used for local deposition of focused ultrasound energy to enhance drug and gene delivery to cells and tissues, across endothelial barrier, especially blood-brain barrier.Overall, microbubble behavior, stability and in vivo lifetime, bioeffects upon the action of ultrasound and resulting enhancement of drug and gene delivery, as well as targeted imaging are critically dependent on the events of gas exchange between the bubbles and surrounding media, as outlined in this review.

放電誘起水中衝撃波の干渉による固体壁付着マイクロバブル挙動の可視化

放電誘起水中衝撃波の干渉による固体壁付着マイクロバブル挙動の可視化

Nonlinear acoustic characteristics of multilayer magnetic microbubbles

The combination of superparamagnetic iron oxide nanoparticles (SPIOs) with ultrasonic contrast agent (UCA) microbubble is called magnetic microbubble (MMB) and has been used to produce multimodal contrast agents to enhance medical ultrasound and magnetic resonance imaging. The nanoparticles are either covalently linked to the shell or physically entrapped into the shell. Considering the effect of the volume fraction of SPIOs on the shell density and viscosity, a nonlinear dynamic equation of magnetic microbubbles (MMBs) with multilayer membrane structure is constructed based on the basic theory of bubble dynamics. The influences of the driving sound pressure and frequency, particle volume fraction, shell thickness and surface tension on the acoustic-dynamics behavior of microbubbles are numerically analyzed. The results show that when the volume fraction of magnetic particles is small and &lt;i&gt;α&lt;/i&gt; ≤ 0.1, the acoustic properties of magnetic microbubbles are similar to those of ordinary UCA microbubbles. The acoustic response of the microbubble depends on its initial size and driving pressure. The critical sound pressure of microbubble vibration instability is lowest when the driving sound field frequency is twice the magnetic microbubble resonance frequency &lt;i&gt;f&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt; (&lt;i&gt;f&lt;/i&gt; = 2&lt;i&gt;f&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;). The presence of magnetic particles inhibits the bubbles from expanding and contracting, but the inhibition effect is very limited. The surface tension parameter &lt;i&gt;K&lt;/i&gt; of the outer film material and thickness of the shell also affect the vibration of the microbubble. When &lt;i&gt;K&lt;/i&gt; and film thickness are 0.2–0.4 N/m and 50–150 nm respectively, it is observed that the bubble has an unstable vibration response region.

Dissolution and Shrinking of a Single Microbubble in Stationary Liquid with Surfactants

AbstractFor a variety of applications of microbubbles, it is necessary to clarify the dissolution dynamics and the lifetime of a microbubble. Although various models have been proposed, predicting the dissolution rate of a microbubble in a static fluid with a surfactant is still difficult. In this study, dissolution behavior was predicted with taking into account the mass transfer inhibition effect due to a surfactant at the bubble surface. The shrinking behavior of an oxygen microbubble was observed with several surfactants and the experimental results were in good agreement with model predictions.

Effects of shell chemistry on acoustic response and dissolution behavior of lipid-coated microbubbles

Microbubbles have been widely used as contrast agents for ultrasound imaging. They are also being investigated for therapeutic applications including as a vehicle for ultrasound activated targeted drug delivery. Here the effects of the shell chemistry on the acoustic response and dissolution behavior of microbubbles have been investigated. Microbubbles encapsulated by a lipid shell (DPPC and DPPE-PEG2000) were prepared with different PEG molar ratios (0% to 20%) investigating their concentration, size distribution, stability and material properties. Note that different PEG concentrations were previously shown to result in different PEG surface configuration: mushroom for lower PEG concentrations and brush for higher ones. The microbubbles in the brush regime seem to generate higher fundamental and subharmonic scattering components at 2.25 MHz excitation, while the higher harmonics seems unaffected. Microbubbles in the mushroom regime showed different dissolution behavior with longer growth period before dissolution. This is believed to be resulted from a less permeable shell in comparison to shells in brush regime. The relation between acoustic response, dissolution behavior and material properties of the shell will be discussed.

Behavior of microbubbles in homogeneous stratified turbulence

Dynamics behavior of rising microbubbles in homogeneous stratified turbulence is investigated by direct numerical simulation. Stratification creates a predominantly horizontal oscillatory motion in a fluid, inducing bubbles' zigzag pattern. Despite this oscillating motion, the horizontal dispersion of a single bubble is rather suppressed. The horizontal separation of paired bubbles displays a power-law growth in the Batchelor and Richardson regimes.

High-Resolution Imaging of Intracellular Calcium Fluctuations Caused by Oscillating Microbubbles

Ultrasound insonification of microbubbles can locally enhance drug delivery, but the microbubble–cell interaction remains poorly understood. Because intracellular calcium (Cai2+) is a key cellular regulator, unraveling the Cai2+ fluctuations caused by an oscillating microbubble provides crucial insight into the underlying bio-effects. Therefore, we developed an optical imaging system at nanometer and nanosecond resolution that can resolve Cai2+ fluctuations and microbubble oscillations. Using this system, we clearly distinguished three Cai2+ uptake profiles upon sonoporation of endothelial cells, which strongly correlated with the microbubble oscillation amplitude, severity of sonoporation and opening of cell–cell contacts. We found a narrow operating range for viable drug delivery without lethal cell damage. Moreover, adjacent cells were affected by a calcium wave propagating at 15 μm/s. With the unique optical system, we unraveled the microbubble oscillation behavior required for drug delivery and Cai2+ fluctuations, providing new insight into the microbubble–cell interaction to aid clinical translation.