Nonspherical shape oscillations of coated microbubbles in contact with a wall

Numerical simulation study on opening blood-brain barrier by ultrasonic cavitation

Experimental studies have shown that ultrasonic cavitation can reversibly open the blood-brain barrier (BBB) to assist drug delivery. Nevertheless, the majority of the present study focused on experimental aspects of BBB opening. In this study, we developed a three-bubble-liquid-solid model to investigate the dynamic behavior of multiple bubbles within the blood vessels, and elucidate the physical mechanism of drug molecules through endothelial cells under ultrasonic cavitation excitation. The results showed that the large bubbles have a significant inhibitory effect on the movement of small bubbles, and the vibration morphology of intravascular microbubbles was affected by the acoustic parameters, microbubble size, and the distance between the microbubbles. The ultrasonic cavitation can significantly enhance the unidirectional flux of drug molecules, and the unidirectional flux growth rate of the wall can reach more than 5 %. Microjets and shock waves emitted from microbubbles generate different stress distribution patterns on the vascular wall, which in turn affects the pore size of the vessel wall and the permeability of drug molecules. The vibration morphology of microbubbles is related to the concentration, arrangement and scale of microbubbles, and the drug permeation impact can be enhanced by optimizing bubble size and acoustic parameters. The results offer an extensive depiction of the factors influencing the blood-brain barrier opening through ultrasonic cavitation, and the model may provide a potential technique to actively regulate the penetration capacity of drugs through endothelial layer of the neurovascular system by regulating BBB opening.

Nonspherical oscillations of an encapsulated microbubble with interface energy under the acoustic field

Spherical instability in acoustically driven encapsulated microbubbles (EBs) suspended in a fluid can trigger nonspherical oscillations within them. We apply the interface energy model [N. Dash and G. Tamadapu, J. Fluid Mech. 932, A26 (2022b)] to investigate nonspherical oscillations of smaller radius microbubbles encapsulated with a viscoelastic shell membrane under acoustic field. Using the Lagrangian energy formulation, coupled governing equations for spherical and nonspherical modes are derived, incorporating interface energy effects, shell elasticity, and viscosity. Numerical simulations of governing equations revealed that the parametrically forced even mode excites even modes, while the odd modes excite both even and odd modes. The model demonstrates that finite amplitude nonspherical oscillations are identifiable in smaller radius EBs only when the interface parameters are introduced into the model; otherwise, they are not. Realizing that nonlinear mode coupling is responsible for saturation of instability resulting in stable nonspherical oscillations, we perform a steady-state and stability analysis using the slow-time equations obtained from Krylov-Bogoliubov perturbation method. Analytical expressions for modal amplitudes and stability thresholds are derived in terms of interface and material parameters. The stability curves are invaluable in determining the precise range of excitation pressure and frequency values required for the EB to exhibit finite amplitude nonspherical oscillations.

Focused ultrasound-assisted delivery of immunomodulating agents in brain cancer

Focused ultrasound (FUS) combined with intravascularly circulating microbubbles can transiently increase the permeability of the blood-brain barrier (BBB) to enable targeted therapeutic delivery to the brain, the clinical testing of which is currently underway in both adult and pediatric patients. Aside from traditional cancer drugs, this technique is being extended to promote the delivery of immunomodulating therapeutics to the brain, including antibodies, immune cells, and cytokines. In this manner, FUS approaches are being explored as a tool to improve and amplify the effectiveness of immunotherapy for both primary and metastatic brain cancer, a particularly challenging solid tumor to treat. Here, we present an overview of the latest groundbreaking research in FUS-assisted delivery of immunomodulating agents to the brain in pre-clinical models of brain cancer, and place it within the context of the current immunotherapy approaches. We follow this up with a discussion on new developments and emerging strategies for this rapidly evolving approach.

Coupling Two Ultra-high-Speed Cameras to Elucidate Ultrasound Contrast-Mediated Imaging and Therapy

Ultrasound contrast-mediated medical imaging and therapy both rely on the dynamics of micron- and nanometer-sized ultrasound cavitation nuclei, such as phospholipid-coated microbubbles and phase-change droplets. Ultrasound cavitation nuclei respond non-linearly to ultrasound on a nanosecond time scale that necessitates the use of ultra-high-speed imaging to fully visualize these dynamics in detail. In this study, we developed an ultra-high-speed optical imaging system that can record up to 20 million frames per second (Mfps) by coupling two small-sized, commercially available, 10-Mfps cameras. The timing and reliability of the interleaved cameras needed to achieve 20 Mfps was validated using two synchronized light-emitting diode strobe lights. Once verified, ultrasound-activated microbubble responses were recorded and analyzed. A unique characteristic of this coupled system is its ability to be reconfigured to provide orthogonal observations at 10 Mfps. Acoustic droplet vaporization was imaged from two orthogonal views, by which the 3-D dynamics of the phase transition could be visualized. This optical imaging system provides the temporal resolution and experimental flexibility needed to further elucidate the dynamics of ultrasound cavitation nuclei to potentiate the clinical translation of ultrasound-mediated imaging and therapy developments.

Pulsating Microbubble in a Micro-vessel and Mechanical Effect on Vessel Wall: A Simulation Study

Background:

Microbubbles are widely used in diagnostic ultrasound applications as contrast agents. Recently, many studies have shown that microbubbles have good potential for the use in therapeutic applications such as drug and gene delivery and opening of blood- brain barrier locally and transiently. When microbubbles are located inside an elastic microvessel and activated by ultrasound, they oscillate and induce mechanical stresses on the vessel wall. However, the mechanical stresses have beneficial therapeutic effects, they may induce vessel damage if they are too high. Microstreaming-induced shear stress is one of the most important wall stresses.

Objective:

The overall aim of this study is to simulate the interaction between confined bubble inside an elastic microvessel and ultrasound field and investigate the effective parameters on microstreaming-induced shear stress.

Material and Methods:

In this Simulation study, we conducted a 2D finite element simulation to study confined microbubble dynamics, also we investigated both acoustical and bubble material parameters on microbubble oscillation and wall stress.

Results:

Based on our results, for acoustic parameters in the range of therapeutic applications, the maximum shear stress was lower than 4 kPa. Shear stress was approximately independent from shell viscosity whereas it decreased by increasing the shell stiffness. Moreover, shear stress showed an increasing trend with acoustic pressure.

Conclusion:

Beside the acoustical parameters, bubble properties have important effects on bubble behavior so that the softer and larger bubbles are more appropriate for therapeutic application as they can decrease the required frequency and acoustic pressure while inducing the same biological effects.

Ultrasound-microbubble cavitation facilitates adeno-associated virus mediated cochlear gene transfection across the round-window membrane

The round window of the cochlea provides an ideal route for delivering medicines and gene therapy reagents that can cross the round window membrane (RWM) into the inner ear. Recombinant adeno-associated viruses (rAAVs) have several advantages and are recommended as viral vectors for gene transfection. However, rAAVs cannot cross an intact RWM. Consequently, ultrasound-mediated microbubble (USMB) cavitation is potentially useful, because it can sonoporate the cell membranes, and increase their permeability to large molecules. The use of USMB cavitation for drug delivery across the RWM has been tested in a few animal studies but has not been used in the context of AAV-mediated gene transfection. The currently available large size of the ultrasound probe appears to be a limiting factor in the application of this method to the RWM. In this study, we used home-made ultrasound probe with a decreased diameter to 1.5 mm, which enabled the easy positioning of the probe close to the RWM. In guinea pigs, we used this probe to determine that (1) USMB cavitation caused limited damage to the outer surface layer or the RWM, (2) an eGFP-gene carrying rAAV could effectively pass the USMB-treated RWM and reliably transfect cochlear cells, and (3) the hearing function of the cochlea remained unchanged. Our results suggest that USMB cavitation of the RWM is a good method for rAAV-mediated cochlear gene transfection with clear potential for clinical translation. We additionally discuss several advantages of the small probe size.

Effects of extracellular matrix rigidity on sonoporation facilitated by targeted microbubbles: Bubble attachment, bubble dynamics, and cell membrane permeabilization

In this study, we investigated the effects of extracellular matrix rigidity, an important physical property of microenvironments regulating cell morphology and functions, on sonoporation facilitated by targeted microbubbles, highlighting the role of microbubbles. We conducted mechanistic studies at the cellular level on physiologically relevant soft and rigid substrates. By developing a unique imaging strategy, we first resolved details of the 3D attachment configurations between targeted microbubbles and cell membrane. High-speed video microscopy then unveiled bubble dynamics driven by a single ultrasound pulse. Finally, we evaluated the cell membrane permeabilization using a small molecule model drug. Our results demonstrate that: (1) stronger targeted microbubble attachment was formed for cells cultured on the rigid substrate, while six different attachment configurations were revealed in total; (2) more violent bubble oscillation was observed for cells cultured on the rigid substrate, while one third of bubbles attached to cells on the soft substrate exhibited deformation shortly after ultrasound was turned off; (3) higher acoustic pressure was needed to permeabilize the cell membrane for cells on the soft substrate, while under the same ultrasound condition, acoustically-activated microbubbles generated larger pores as compared to cells cultured on the soft substrate. The current findings provide new insights to understand the underlying mechanisms of sonoporation in a physiologically relevant context and may be useful for the clinical translation of sonoporation.

Ultrasound Contrast Agent Modeling: A Review

Ultrasound is extensively used in medical imaging, being safe and inexpensive and operating in real time. Its scope of applications has been widely broadened by the use of ultrasound contrast agents (UCAs) in the form of microscopic bubbles coated by a biocompatible shell. Their increased use has motivated a large amount of research to understand and characterize their physical properties as well as their interaction with the ultrasound field and their surrounding environment. Here we review the theoretical models that have been proposed to study and predict the behavior of UCAs. We begin with a brief introduction on the development of UCAs. We then present the basics of free-gas-bubble dynamics upon which UCA modeling is based. We review extensively the linear and non-linear models for shell elasticity and viscosity and present models for non-spherical and asymmetric bubble oscillations, especially in the presence of surrounding walls or tissue. Then, higher-order effects such as microstreaming, shedding and acoustic radiation forces are considered. We conclude this review with promising directions for the modeling and development of novel agents.

Nonspherical modes nondegeneracy of a tethered bubble

When excited at sufficiently high acoustic pressures, a wall-attached bubble may exhibit asymmetric nonspherical modes. These vibration modes can be decomposed over the set of spherical harmonics Y_{nm}(θ,ϕ) for a degree n and order m. We experimentally capture the time-resolved dynamics of asymmetric bubble oscillations in a top-view configuration. A spatiotemporal modal analysis is performed and allowed recovering the set of zonal (m=0), tesseral (0<m<n), and sectoral (m=n) spherical harmonics that develop at the bubble interface. The analysis of the surface instability thresholds reveals that the frequencies of asymmetric modes differ from the standard Lamb spectrum. In addition, the nondegeneracy of asymmetric modes for a given degree n is evidenced by noncompletely overlapping resonance bands. Finally, the coexistence between zonal and sectoral modes is analyzed through their modal interaction, amplitude interplay and relation of phase, as well as their geometric compatibility.

The Role of Ultrasound-Driven Microbubble Dynamics in Drug Delivery: From Microbubble Fundamentals to Clinical Translation

In the last couple of decades, ultrasound-driven microbubbles have proven excellent candidates for local drug delivery applications. Besides being useful drug carriers, microbubbles have demonstrated the ability to enhance cell and tissue permeability and, as a consequence, drug uptake herein. Notwithstanding the large amount of evidence for their therapeutic efficacy, open issues remain. Because of the vast number of ultrasound- and microbubble-related parameters that can be altered and the variability in different models, the translation from basic research to (pre)clinical studies has been hindered. This review aims at connecting the knowledge gained from fundamental microbubble studies to the therapeutic efficacy seen in in vitro and in vivo studies, with an emphasis on a better understanding of the response of a microbubble upon exposure to ultrasound and its interaction with cells and tissues. More specifically, we address the acoustic settings and microbubble-related parameters (i.e., bubble size and physicochemistry of the bubble shell) that play a key role in microbubble-cell interactions and in the associated therapeutic outcome. Additionally, new techniques that may provide additional control over the treatment, such as monodisperse microbubble formulations, tunable ultrasound scanners, and cavitation detection techniques, are discussed. An in-depth understanding of the aspects presented in this work could eventually lead the way to more efficient and tailored microbubble-assisted ultrasound therapy in the future.

Growth control of leaf lettuce with exposure to underwater ultrasound and dissolved oxygen supersaturation

The growth rate of vegetables in plant factories can be regulated by environmental factors including light, temperature, and chemicals, which might give rise to mutation in leaf health. Here, we aim to devise a new way that allows for controlling the growth rate of plants in hydroponics as well as maintaining the product quality; we apply underwater ultrasound and dissolved oxygen supersaturation as external stimuli to plants. As an example, we examine the growth of leaf lettuce in hydroponics with exposure to 28-kHz ultrasound and dissolved oxygen supersaturation up to 36 mg/L at 20 °C. Our results show that exposure to the ultrasound of peak-to-peak pressure at 20 kPa or larger works as the growth inhibitor of the leaves and the roots, while the oxygen supersaturation as the growth promoter, without any degradation of chlorophyll in the leaves. This suggests that these external stimuli can be used in the growth control system of plant factories.

A Review of Phospholipid Encapsulated Ultrasound Contrast Agent Microbubble Physics

Ultrasound contrast agent microbubbles have expanded the utility of biomedical ultrasound from anatomic imaging to the assessment of microvascular blood flow characteristics and ultrasound-assisted therapeutic applications. Central to their effectiveness in these applications is their resonant and non-linear oscillation behaviour. This article reviews the salient physics of an oscillating microbubble in an ultrasound field, with particular emphasis on phospholipid-coated agents. Both the theoretical underpinnings of bubble vibration and the experimental evidence of non-linear encapsulated bubble dynamics and scattering are discussed and placed within the context of current and emerging applications.

SPIO labeling of endothelial cells using ultrasound and targeted microbubbles at diagnostic pressures

In vivo cell tracking of therapeutic, tumor, and endothelial cells is an emerging field and a promising technique for imaging cardiovascular disease and cancer development. Site-specific labeling of endothelial cells with the MRI contrast agent superparamagnetic iron oxide (SPIO) in the absence of toxic agents is challenging. Therefore, the aim of this in vitro study was to find optimal parameters for efficient and safe SPIO-labeling of endothelial cells using ultrasound-activated CD31-targeted microbubbles for future MRI tracking. Ultrasound at a frequency of 1 MHz (10,000 cycles, repetition rate of 20 Hz) was used for varying applied peak negative pressures (10–160 kPa, i.e. low mechanical index (MI) of 0.01–0.16), treatment durations (0–30 s), time of SPIO addition (-5 min– 15 min with respect to the start of the ultrasound), and incubation time after SPIO addition (5 min– 3 h). Iron specific Prussian Blue staining in combination with calcein-AM based cell viability assays were applied to define the most efficient and safe conditions for SPIO-labeling. Optimal SPIO labeling was observed when the ultrasound parameters were 40 kPa peak negative pressure (MI 0.04), applied for 30 s just before SPIO addition (0 min). Compared to the control, this resulted in an approximate 12 times increase of SPIO uptake in endothelial cells in vitro with 85% cell viability. Therefore, ultrasound-activated targeted ultrasound contrast agents show great potential for effective and safe labeling of endothelial cells with SPIO.

Acoustic Characterization of a Vessel-on-a-Chip Microfluidic System for Ultrasound-Mediated Drug Delivery

Ultrasound in the presence of gas-filled microbubbles can be used to enhance local uptake of drugs and genes. To study the drug delivery potential and its underlying physical and biological mechanisms, an in vitro vessel model should ideally include 3-D cell culture, perfusion flow, and membrane-free soft boundaries. Here, we propose an organ-on-a-chip microfluidic platform to study ultrasound-mediated drug delivery: the OrganoPlate. The acoustic propagation into the OrganoPlate was determined to assess the feasibility of controlled microbubble actuation, which is required to study the microbubble-cell interaction for drug delivery. The pressure field in the OrganoPlate was characterized non-invasively by studying experimentally the well-known response of microbubbles and by simulating the acoustic wave propagation in the system. Microbubble dynamics in the OrganoPlate were recorded with the Brandaris 128 ultrahigh-speed camera (17 million frames/s) and a control experiment was performed in an OptiCell, an in vitro monolayer cell culture chamber that is conventionally used to study ultrasound-mediated drug delivery. When insonified at frequencies between 1 and 2 MHz, microbubbles in the OrganoPlate experienced larger oscillation amplitudes resulting from higher local pressures. Microbubbles responded similarly in both systems when insonified at frequencies between 2 and 4 MHz. Numerical simulations performed with a 3-D finite-element model of ultrasound propagation into the OrganoPlate and the OptiCell showed the same frequency-dependent behavior. The predictable and homogeneous pressure field in the OrganoPlate demonstrates its potential to develop an in vitro 3-D cell culture model, well suited to study ultrasound-mediated drug delivery.

Dynamics of nonspherical microbubble oscillations above instability threshold

Time-resolved dynamics of nonspherical oscillations of micrometer-sized bubbles are captured and analyzed using high-speed imaging. The axisymmetry of the bubble shape is ensured with certainty for the first time from the recordings of two synchronous high-speed cameras located at 90^{∘}. The temporal dynamics of finite-amplitude nonspherical oscillations are then analyzed for various acoustic pressures above the instability threshold. The experimental results are compared with recent theories accounting for nonlinearities and mode coupling, highlighting particular effects inherent to these mechanisms (saturation of the instability, triggering of nonparametric shape modes). Finally, the amplitude of the nonspherical oscillations is given as function of the driving pressure both for quadrupolar and octupolar bubbles.

Mechanistic Insight into Sonoporation with Ultrasound-Stimulated Polymer Microbubbles

Sonoporation is emerging as a feasible, non-viral gene delivery platform for the treatment of cardiovascular disease and cancer. Despite promising results, this approach remains less efficient than viral methods. The objective of this work is to help substantiate the merit of polymeric microbubble sonoporation as a non-viral, localized cell permeation and payload delivery strategy by taking a ground-up approach to elucidating the fundamental mechanisms at play. In this study, we apply simultaneous microscopy of polymeric microbubble sonoporation over its intrinsic biophysical timescales-with sub-microsecond resolution to examine microbubble cavitation and millisecond resolution over several minutes to examine local macromolecule uptake through enhanced endothelial cell membrane permeability-bridging over six orders of magnitude in time. We quantified microbubble behavior and resulting sonoporation thresholds at transmit frequencies of 0.5, 1 and 2 MHz, and determined that sonic cracking is a necessary but insufficient condition to induce sonoporation. Further, sonoporation propensity increases with the extent of sonic cracking, namely, from partial to complete gas escape from the polymeric encapsulation. For the subset that exhibited complete gas escape from sonic cracking, a proportional relationship between the maximum projected gas area and resulting macromolecule uptake was observed. These results have revealed one aspect of polymeric bubble activity on the microsecond time scale that is associated with eliciting sonoporation in adjacent endothelial cells, and contributes toward an understanding of the physical rationale for sonoporation with polymer-encapsulated microbubble contrast agents.

High-Speed In Situ Observation System for Sonoporation of Cells With Size- and Position-Controlled Microbubbles

A high-speed in situ microscopic observation system developed for basic studies on mechanisms of sonoporation is introduced in this paper. The main part of the system is an inverted-type fluorescence microscope, and a high-speed camera of 20 MHz in a maximum framing rate was used to visualize the dynamics of cavitation bubbles that causes a sonoporation effect. Differential interference contrast and fluorescence techniques were used for sensitive visualization of cell changes during sonoporation. The system is also equipped with optical tweezers that can move a microbubble of several microns in size by using a donut-shaped light beam. In situ microscopic observation of sonoporation was carried out using a cell with a size- and position-controlled microbubble. The experimental results showed that the ability of cells to repair sonoporation-induced damage depends on their membrane tension, indicating the usefulness of the observation system as a basic tool for the investigation of sonoporation phenomena.

Characterization of Contrast Agent Microbubbles for Ultrasound Imaging and Therapy Research

The high efficiency with which gas microbubbles can scatter ultrasound compared with the surrounding blood pool or tissues has led to their widespread employment as contrast agents in ultrasound imaging. In recent years, their applications have been extended to include super-resolution imaging and the stimulation of localized bio-effects for therapy. The growing exploitation of contrast agents in ultrasound and in particular these recent developments have amplified the need to characterize and fully understand microbubble behavior. The aim in doing so is to more fully exploit their utility for both diagnostic imaging and potential future therapeutic applications. This paper presents the key characteristics of microbubbles that determine their efficacy in diagnostic and therapeutic applications and the corresponding techniques for their measurement. In each case, we have presented information regarding the methods available and their respective strengths and limitations, with the aim of presenting information relevant to the selection of appropriate characterization methods. First, we examine methods for determining the physical properties of microbubble suspensions and then techniques for acoustic characterization of both suspensions and single microbubbles. The next section covers characterization of microbubbles as therapeutic agents, including as drug carriers for which detailed understanding of their surface characteristics and drug loading capacity is required. Finally, we discuss the attempts that have been made to allow comparison across the methods employed by various groups to characterize and describe their microbubble suspensions and promote wider discussion and comparison of microbubble behavior.

Biophysical insight into mechanisms of sonoporation

This study presents a unique approach to understanding the biophysical mechanisms of ultrasound-triggered cell membrane disruption (i.e., sonoporation). We report direct correlations between ultrasound-stimulated encapsulated microbubble oscillation physics and the resulting cellular membrane permeability by simultaneous microscopy of these two processes over their intrinsic physical timescales (microseconds for microbubble dynamics and seconds to minutes for local macromolecule uptake and cell membrane reorganization). We show that there exists a microbubble oscillation-induced shear-stress threshold, on the order of kilopascals, beyond which endothelial cellular membrane permeability increases. The shear-stress threshold exhibits an inverse square-root relation to the number of oscillation cycles and an approximately linear dependence on ultrasound frequency from 0.5 to 2 MHz. Further, via real-time 3D confocal microscopy measurements, our data provide evidence that a sonoporation event directly results in the immediate generation of membrane pores through both apical and basal cell membrane layers that reseal along their lateral area (resealing time of ∼<2 min). Finally, we demonstrate the potential for sonoporation to indirectly initiate prolonged, intercellular gaps between adjacent, confluent cells (∼>30-60 min). This real-time microscopic approach has provided insight into both the physical, cavitation-based mechanisms of sonoporation and the biophysical, cell-membrane-based mechanisms by which microbubble acoustic behaviors cause acute and sustained enhancement of cellular and vascular permeability.

Surface waves on a soft viscoelastic layer produced by an oscillating microbubble

Ultrasound-driven bubbles can cause significant deformation of soft viscoelastic layers, for instance in surface cleaning and biomedical applications. The effect of the viscoelastic properties of a boundary on the bubble-boundary interaction has been explored only qualitatively, and remains poorly understood. We investigate the dynamic deformation of a viscoelastic layer induced by the volumetric oscillations of an ultrasound-driven microbubble. High-speed video microscopy is used to observe the deformation produced by a bubble oscillating at 17-20 kHz in contact with the surface of a hydrogel. The localised oscillating pressure applied by the bubble generates surface elastic (Rayleigh) waves on the gel, characterised by elliptical particle trajectories. The tilt angle of the elliptical trajectories varies with increasing distance from the bubble. Unexpectedly, the direction of rotation of the surface elements on the elliptical trajectories shifts from prograde to retrograde at a distance from the bubble that depends on the viscoelastic properties of the gel. To explain these behaviours, we develop a simple three-dimensional model for the deformation of a viscoelastic solid by a localised oscillating force. By using as input for the model the values of the shear modulus obtained from the propagation velocity of the Rayleigh waves, we find good qualitative agreement with the experimental observations.

Fluid Viscosity Affects the Fragmentation and Inertial Cavitation Threshold of Lipid-Encapsulated Microbubbles

Ultrasound and microbubble optimization studies for therapeutic applications are often conducted in water/saline, with a fluid viscosity of 1 cP. In an in vivo context, microbubbles are situated in blood, a more viscous fluid (∼4 cP). In this study, ultrahigh-speed microscopy and passive cavitation approaches were employed to investigate the effect of fluid viscosity on microbubble behavior at 1 MHz subject to high pressures (0.25-2 MPa). The propensity for individual microbubble (n = 220) fragmentation was found to significantly decrease in 4-cP fluid compared with 1-cP fluid, despite achieving similar maximum radial excursions. Microbubble populations diluted in 4-cP fluid exhibited decreased wideband emissions (up to 10.2 times), and increasingly distinct harmonic emission peaks (e.g., ultraharmonic) with increasing pressure, compared with those in 1-cP fluid. These results suggest that in vitro studies should consider an evaluation using physiologic viscosity perfusate before transitioning to in vivo evaluations.

Influence of Surface Wettability on Microbubble Formation

The production and utilization of microbubbles are rapidly becoming of major importance in a number of global applications, from biofuel production to medical imaging contrast agents. Many aspects of bubble formation have been studied, with diffuser characteristics (such as pore size, pore orientation) and gas flow rate all being shown to influence the bubble formation process. However, very little attention has been paid to the influence of surface wettability of the diffuser and the detailed role it plays at the triple interface of gas-liquid-diffuser. Here, we investigate how the wettability of the diffuser surface impacts upon the dynamics of the bubble formation process and examine the effect both at the orifice and upon the bubble cloud produced as a result of the engineered wetting variations. Experimental data shown here indicate the presence of a switching point at a contact angle of θ = 90°, where bubble size vastly changes. When a surface exhibits a contact angle below 90°, bubbles emitted from it are considerably smaller than those emitted from a surface with an angle in excess of 90°. This effect is observable over flow rates ranging from 2.5 to 60 mL min(-1) from a single pore, an array of controlled pores, and the industrially relevant and commercially available sintered metals and sintered ceramic diffusers. It is also observed for both thiol and silane modified surfaces, encompassing a range of contact angles from 10° to 110°. In addition, the importance of the diffuser plate's surface topography is discussed, with elevated roughness acting to reduce the effect of surface chemistry in some instances.

In vitro methods to study bubble-cell interactions: Fundamentals and therapeutic applications

Besides their use as contrast agents for ultrasound imaging, microbubbles are increasingly studied for a wide range of therapeutic applications. In particular, their ability to enhance the uptake of drugs through the permeabilization of tissues and cell membranes shows great promise. In order to fully understand the numerous paths by which bubbles can interact with cells and the even larger number of possible biological responses from the cells, thorough and extensive work is necessary. In this review, we consider the range of experimental techniques implemented in in vitro studies with the aim of elucidating these microbubble-cell interactions. First of all, the variety of cell types and cell models available are discussed, emphasizing the need for more and more complex models replicating in vivo conditions together with experimental challenges associated with this increased complexity. Second, the different types of stabilized microbubbles and more recently developed droplets and particles are presented, followed by their acoustic or optical excitation methods. Finally, the techniques exploited to study the microbubble-cell interactions are reviewed. These techniques operate over a wide range of timescales, or even off-line, revealing particular aspects or subsequent effects of these interactions. Therefore, knowledge obtained from several techniques must be combined to elucidate the underlying processes.

Non-linear response and viscoelastic properties of lipid-coated microbubbles: DSPC versus DPPC

For successful in vivo contrast-enhanced ultrasound imaging (CEUS) and ultrasound molecular imaging, detailed knowledge of stability and acoustical properties of the microbubbles is essential. Here, we compare these aspects of lipid-coated microbubbles that have either 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) as their main lipid; the other components were identical. The microbubbles were investigated in vitro over the frequency range 1-4 MHz at pressures between 10 and 100 kPa, and their response to the applied ultrasound was recorded using ultrahigh-speed imaging (15 Mfps). Relative to DPPC-coated microbubbles, DSPC-coated microbubbles had (i) higher acoustical stability; (ii) higher shell elasticity as derived using the Marmottant model (DSPC: 0.26 ± 0.13 N/m, DPPC: 0.06 ± 0.06 N/m); (iii) pressure amplitudes twice as high at the second harmonic frequency; and (iv) a smaller amount of microbubbles that responded at the subharmonic frequency. Because of their higher acoustical stability and higher non-linear response, DSPC-coated microbubbles may be more suitable for contrast-enhanced ultrasound.

Sounds of marine seeps: a study of bubble activity near a rigid boundary

A passive acoustic method for detecting environmentally dangerous gas leaks from pipelines and methane naturally leaking from the seabed has been investigated. Gas escape involves the formation and release of bubbles of different sizes. Each bubble emits a sound at a specific frequency. Determination of the bubble radius from the frequency of its signature passive acoustic emission by use of so-called Minnaert formula has a restricted area of applicability near the seabed. The point is that the inertial mass and the damping constant of the birthing bubble are markedly different from those of a free bubble. The theoretical model for the bubble volume oscillations near the seabed has been proposed and an analytical solution has been derived. It was shown that the bispherical coordinates provide separation of variables and are more suitable for analysis of the volume oscillations of these constrained bubbles. Explicit formulas have been derived, which describe the dependence of the bubble emission near a rigid wall on its size and the separation distance between the bubble and the boundary.

Lipid shedding from single oscillating microbubbles

Lipid-coated microbubbles are used clinically as contrast agents for ultrasound imaging and are being developed for a variety of therapeutic applications. The lipid encapsulation and shedding of the lipids by acoustic driving of the microbubble has a crucial role in microbubble stability and in ultrasound-triggered drug delivery; however, little is known about the dynamics of lipid shedding under ultrasound excitation. Here we describe a study that optically characterized the lipid shedding behavior of individual microbubbles on a time scale of nanoseconds to microseconds. A single ultrasound burst of 20 to 1000 cycles, with a frequency of 1 MHz and an acoustic pressure varying from 50 to 425 kPa, was applied. In the first step, high-speed fluorescence imaging was performed at 150,000 frames per second to capture the instantaneous dynamics of lipid shedding. Lipid detachment was observed within the first few cycles of ultrasound. Subsequently, the detached lipids were transported by the surrounding flow field, either parallel to the focal plane (in-plane shedding) or in a trajectory perpendicular to the focal plane (out-of-plane shedding). In the second step, the onset of lipid shedding was studied as a function of the acoustic driving parameters, for example, pressure, number of cycles, bubble size and oscillation amplitude. The latter was recorded with an ultrafast framing camera running at 10 million frames per second. A threshold for lipid shedding under ultrasound excitation was found for a relative bubble oscillation amplitude >30%. Lipid shedding was found to be reproducible, indicating that the shedding event can be controlled.

Low-amplitude non-linear volume vibrations of single microbubbles measured with an "acoustical camera"

Contrast-enhanced ultrasound imaging is based on the detection of non-linear vibrational responses of a contrast agent after its intravenous administration. Improving contrast-enhanced images requires an accurate understanding of the vibrational response to ultrasound of the lipid-coated gas microbubbles that constitute most ultrasound contrast agents. Variations in the volume of microbubbles provide the most efficient radiation of ultrasound and, therefore, are the most important bubble vibrations for medical diagnostic ultrasound imaging. We developed an "acoustical camera" that measures the dynamic volume change of individual microbubbles when excited by a pressure wave. In the work described here, the technique was applied to the characterization of low-amplitude non-linear behaviors of BR14 microbubbles (Bracco Research, Geneva, Switzerland). The amplitude dependence of the resonance frequency and the damping, the prevalence of efficient subharmonic and ultraharmonic vibrations and the amplitude dependence of the response at the fundamental frequency and at the second harmonic frequency were investigated. Because of the large number of measurements, we provide a statistical characterization of the low-amplitude non-linear properties of the contrast agent.

Acoustic behavior of microbubbles and implications for drug delivery

Ultrasound contrast agents are valuable in diagnostic ultrasound imaging, and they increasingly show potential for drug delivery. This review focuses on the acoustic behavior of flexible-coated microbubbles and rigid-coated microcapsules and their contribution to enhanced drug delivery. Phenomena relevant to drug delivery, such as non-spherical oscillations, shear stress, microstreaming, and jetting will be reviewed from both a theoretical and experimental perspective. Further, the two systems for drug delivery, co-administration and the microbubble as drug carrier system, are reviewed in relation to the microbubble behavior. Finally, future prospects are discussed that need to be addressed for ultrasound contrast agents to move from a pre-clinical tool into a clinical setting.

Study of non-spherical bubble oscillations near a surface in a weak acoustic standing wave field

The interaction of acoustically driven bubbles with a wall is important in many applications of ultrasound and cavitation, as the close boundary can severely alter the bubble dynamics. In this paper, the non-spherical surface oscillations of bubbles near a surface in a weak acoustic standing wave field are investigated experimentally and numerically. The translation, the volume, and surface mode oscillations of bubbles near a flat glass surface were observed by a high speed camera in a standing wave cell at 46.8 kHz. The model approach is based on a modified Keller-Miksis equation coupled to surface mode amplitude equations in the first order, and to the translation equations. Modifications are introduced due to the adjacent wall. It was found that a bubble's oscillation mode can change in the presence of the wall, as compared to the bubble in the bulk liquid. In particular, the wall shifts the instability pressure thresholds to smaller driving frequencies for fixed bubble equilibrium radii, or to smaller equilibrium radii for fixed excitation frequency. This can destabilize otherwise spherical bubbles, or stabilize bubbles undergoing surface oscillations in the bulk. The bubble dynamics observed in experiment demonstrated the same trend as the theoretical results.

An optical system for detecting 3D high-speed oscillation of a single ultrasound microbubble

As contrast agents, microbubbles have been playing significant roles in ultrasound imaging. Investigation of microbubble oscillation is crucial for microbubble characterization and detection. Unfortunately, 3-dimensional (3D) observation of microbubble oscillation is challenging and costly because of the bubble size-a few microns in diameter-and the high-speed dynamics under MHz ultrasound pressure waves. In this study, a cost-efficient optical confocal microscopic system combined with a gated and intensified charge-coupled device (ICCD) camera were developed to detect 3D microbubble oscillation. The capability of imaging microbubble high-speed oscillation with much lower costs than with an ultra-fast framing or streak camera system was demonstrated. In addition, microbubble oscillations along both lateral (x and y) and axial (z) directions were demonstrated. Accordingly, this system is an excellent alternative for 3D investigation of microbubble high-speed oscillation, especially when budgets are limited.

Ultrafast dynamics of the acoustic vaporization of phase-change microdroplets

Acoustically sensitive emulsions are a promising tool for medical applications such as localized drug delivery. The physical mechanisms underlying the ultrasound-triggered nucleation and subsequent vaporization of these phase-change emulsions are largely unexplored. Here, the acoustic vaporization of individual micron-sized perfluoropentane (PFP) droplets is studied at a nanoseconds timescale. Highly diluted emulsions of PFP-in-water and oil-in-PFP-in-water droplets, ranging from 3.5 to 11 μm in radius, were prepared and the nucleation and growth of the vapor bubbles was imaged at frame rates of up to 20 Mfps. The droplet vaporization dynamics was observed to have three distinct regimes: (1) prior to nucleation, a regime of droplet deformation and oscillatory translations within the surrounding fluid along the propagation direction of the applied ultrasound; (2) a regime characterized by the rapid growth of a vapor bubble enhanced by ultrasound-driven rectified heat transfer; and (3) a final phase characterized by a relatively slow expansion, after ultrasound stops, that is fully dominated by heat transfer. A method to measure the moment of inception of the nucleation event with respect to the phase of the ultrasound wave is proposed. A simple physical model captures quantitatively all of the features of the subsequent vapor bubble growth.

Rapid shrinkage of lipid-coated bubbles in pulsed ultrasound

Many researchers have observed rapid shrinkage of lipid-coated microbubbles subjected to brief, MHz ultrasound pulses. The shrinkage is sometimes, but not always, accompanied by the shedding of visible fragments of the coat. It has been suggested that the shedding of the lipid coat alone is sufficient to explain the rapid shrinkage, as that loss increases bubble surface tension and, thus, internal pressure, increasing gas loss even between pulses. We have determined, however, that the shedding of the coat lipid must also entrain some of the gas content of the bubble, to account for the observed shrinkage rates. The evidence for this is that insonated bubbles typically shrink much faster than the Epstein-Plesset (diffusion) limit for gas dissolution and diffusion, whereas uncoated quiescent bubbles shrink more slowly. We have also modeled the diffusion of gas in the moving liquid surrounding the bubble and find no advective enhancement of diffusive loss of gas from the bubble. Thus, bubble gas loss through diffusion alone is insufficient to account for rapid shrinkage.

Secondary bjerknes forces deform targeted microbubbles

In this study, we investigated the effect of secondary Bjerknes forces on targeted microbubbles using high-speed optical imaging. We observed that targeted microbubbles attached to an underlying surface and subject to secondary Bjerknes forces deform in the direction of their neighboring bubble, thereby tending toward a prolate shape. The deformation induces an elastic restoring force, causing the bubbles to recoil back to their equilibrium position; typically within 100 μs after low-intensity ultrasound application. The temporal dynamics of the recoil was modeled as a simple mass-spring system, from which a value for the effective spring constant k of the order 10(-3) Nm(-1) was obtained. Moreover, the translational dynamics of interacting targeted microbubbles was predicted by a hydrodynamic point particle model, including a value of the spring stiffness k of the very same order as derived experimentally from the recoiling curves. For higher acoustic pressures, secondary Bjerknes forces rupture the molecular adhesion of the bubbles to the surface. We used this mutual attraction to quantify the binding force between a single biotinylated microbubble and an avidin-coated surface, which was found to be between 0.9 and 2 nanonewtons (nN). The observation of patches of lipids left at the initial binding site suggests that lipid anchors are pulled out of the microbubble shell, rather than biotin molecules unbinding from avidin. Understanding the effect of ultrasound application on targeted microbubbles is crucial for further advances in the realm of molecular imaging.

The effect of binding on the subharmonic emissions from individual lipid-encapsulated microbubbles at transmit frequencies of 11 and 25 MHz

Targeted microbubble imaging at ultrasound frequencies above 5 MHz has applications in both a preclinical context for a range of disease processes and clinically for the assessment of atherosclerosis and superficial tumors. Although the feasibility of ultrasound molecular imaging has been well demonstrated for a range of target molecules, little work has examined the effects of binding on microbubble oscillations, which is of potential relevance to improving the sensitivity, specificity, and quantification of bound-bubble detection. In this study we investigated the influence of binding on the subharmonic response of bubbles at transmit frequencies of 11 and 25 MHz. Individual bubbles were situated adjacent to a boundary in either a bound or an unbound state, optically sized and acoustically interrogated with pressures ranging from 0.02 to 1.2 MPa. At 11 MHz, unbound bubbles (n = 53) were found to have strong subharmonic activity for sizes between 2.4 and 2.6 μm, whereas bound bubbles (n = 50) were most active from 2.6 to 3.0 μm. Destruction thresholds were found to be lower for bound bubbles. At 25 MHz, bound-bubble (n = 57) activity was found to peak at 1.9 μm as compared to 2.1 μm in the unbound cases (n = 53), with a 20% increase in amplitude. Comparison with simulations indicates that both unbound and bound bubbles undergo compression-only behavior at 11 MHz, and expansion-dominated behavior at 25 MHz. Subharmonic emissions elicited from 0 radian transmit pulses were found to be π/2 radians out of phase with those elicited from a π radian transmit pulse, suggesting inefficient subharmonic preservation from pulse inversion schemes. With the appropriate postprocessed phase correction, an increase in the subharmonic amplitude of up to 60% was shown, depending on the bubble size and transmit frequency.

20 years of ultrasound contrast agent modeling

The merits of ultrasound contrast agents (UCAs) were already known in the 1960s. It was, however, not until the 1990s that UCAs were clinically approved and marketed. In these years, it was realized that the UCAs are not just efficient ultrasound scatterers, but that their main constituent, the coated gas microbubble, acts as a nonlinear resonator and, as such, is capable of generating harmonic energy. Subharmonic, ultraharmonic, and higher harmonic frequencies of the transmitted ultrasound frequency have been reported. This opened up new prospects for their use and several detection strategies have been developed to exploit this harmonic energy to discriminate the contrast bubbles from surrounding tissue. This insight created a need for tools to study coated bubble behavior in an ultrasound field and the first models were developed. Since then, 20 years have elapsed, in which a broad range of UCAs and UCA models have been developed. Although the models have helped in understanding the responses of coated bubbles, the influence of the coating has not been fully elucidated to date and UCA models are still being improved. The aim of this review paper is to offer an overview in these developments and indicate future directions for research.

Bursting bubbles and bilayers

This paper discusses various interactions between ultrasound, phospholipid monolayer-coated gas bubbles, phospholipid bilayer vesicles, and cells. The paper begins with a review of microbubble physics models, developed to describe microbubble dynamic behavior in the presence of ultrasound, and follows this with a discussion of how such models can be used to predict inertial cavitation profiles. Predicted sensitivities of inertial cavitation to changes in the values of membrane properties, including surface tension, surface dilatational viscosity, and area expansion modulus, indicate that area expansion modulus exerts the greatest relative influence on inertial cavitation. Accordingly, the theoretical dependence of area expansion modulus on chemical composition - in particular, poly (ethylene glyclol) (PEG) - is reviewed, and predictions of inertial cavitation for different PEG molecular weights and compositions are compared with experiment. Noteworthy is the predicted dependence, or lack thereof, of inertial cavitation on PEG molecular weight and mole fraction. Specifically, inertial cavitation is predicted to be independent of PEG molecular weight and mole fraction in the so-called mushroom regime. In the “brush” regime, however, inertial cavitation is predicted to increase with PEG mole fraction but to decrease (to the inverse 3/5 power) with PEG molecular weight. While excellent agreement between experiment and theory can be achieved, it is shown that the calculated inertial cavitation profiles depend strongly on the criterion used to predict inertial cavitation. This is followed by a discussion of nesting microbubbles inside the aqueous core of microcapsules and how this significantly increases the inertial cavitation threshold. Nesting thus offers a means for avoiding unwanted inertial cavitation and cell death during imaging and other applications such as sonoporation. A review of putative sonoporation mechanisms is then presented, including those involving microbubbles to deliver cargo into a cell, and those - not necessarily involving microubbles - to release cargo from a phospholipid vesicle (or reverse sonoporation). It is shown that the rate of (reverse) sonoporation from liposomes correlates with phospholipid bilayer phase behavior, liquid-disordered phases giving appreciably faster release than liquid-ordered phases. Moreover, liquid-disordered phases exhibit evidence of two release mechanisms, which are described well mathematically by enhanced diffusion (possibly via dilation of membrane phospholipids) and irreversible membrane disruption, whereas liquid-ordered phases are described by a single mechanism, which has yet to be positively identified. The ability to tune release kinetics with bilayer composition makes reverse sonoporation of phospholipid vesicles a promising methodology for controlled drug delivery. Moreover, nesting of microbubbles inside vesicles constitutes a truly “theranostic” vehicle, one that can be used for both long-lasting, safe imaging and for controlled drug delivery.

Collective bubble dynamics near a surface in a weak acoustic standing wave field

The transport of bubbles to a neighboring surface is very important in surface chemistry, bioengineering, and ultrasonic cleaning, etc. This paper proposes a multi-bubble transport method by using an acoustic standing wave field and establishes a model that explains the multi-bubble translation by expressing the balance between Bjerknes forces and hydrodynamic forces on a bubble in a liquid medium. Results indicated that the influence of primary Bjerknes force, secondary Bjerknes force, and buoyancy force on the bubble translation depends on the position of the target bubble in the acoustic field. Moreover, it was found that increasing the size of a bubble or pressure amplitude can accelerate the bubble motion and enhance the bubble-bubble interaction. The secondary Bjerknes force between two bubbles can switch from an attractive one when they oscillate in phase to a repulsive one when the bubble oscillations are out of phase. These findings provide an insight into the multi-bubble translation near a surface and can be applied to future bubble motion control studies, especially in drug delivery, sonoporation, and ultrasonic cleaning.

Modeling non-spherical oscillations and stability of acoustically driven shelled microbubbles

The oscillation and destruction of microbubbles under ultrasound excitation form the basis of contrast enhanced ultrasound imaging and microbubble assisted drug and gene delivery. A typical microbubble has a size of a few micrometers and consists of a gas core encapsulated by a shell. These bubbles can be driven into surface mode oscillations, which not only contribute to the measured acoustic signal but can lead to bubble destruction. Existing models of surface model oscillations have not considered the effects of a bubble shell. In this study a model was developed to study the surface mode oscillations in shelled bubbles. The effects of shell viscosity and elasticity on the surface mode oscillations were modeled using a Boussinesq-Scriven approach. Simulation was conducted using the model with various bubble sizes and driving acoustic pressures. The occurrence of surface modes and the number of ultrasound cycles needed for the occurrence were calculated. The simulation results show a significant difference between shelled bubbles and shell free bubbles. The shelled bubbles have reduced surface mode amplitudes and a narrower bubble size range within which these modes develop compared to shell free bubbles. The clinical implications were also discussed.

Theoretical and experimental characterisation of magnetic microbubbles

In addition to improving image contrast, microbubbles have shown great potential in molecular imaging and drug/gene delivery. Previous work by the authors showed that considerable improvements in gene transfection efficiency were obtained using microbubbles loaded with magnetic nanoparticles under simultaneous exposure to ultrasound and magnetic fields. The aim of this study was to characterise the effect of nanoparticles on the dynamic and acoustic response of the microbubbles. High-speed video microscopy indicated that the amplitude of oscillation was very similar for magnetic and nonmagnetic microbubbles of the same size for the same ultrasound exposure (0.5 MHz, 100 kPa, 12-cycle pulse) and that this was minimally affected by an imposed magnetic field. The linear scattering to attenuation ratio (STAR) was also similar for suspensions of both bubble types although the nonlinear STAR was ~50% lower for the magnetic microbubbles. Both the video and acoustic data were supported by the results from theoretical modelling.

Development and application of endothelium-targeted microparticles for molecular magnetic resonance imaging

Molecular imaging of disease states can enhance diagnosis allowing for accurate and more effective treatment. By specifically targeting molecules differentially expressed in disease states, researchers and clinicians have a means of disease characterization at a cellular or tissue level. Targeted micron-sized particles of iron oxide (MPIO) have been used as molecule-specific contrast agents for use with magnetic resonance imaging (MRI), and early evidence suggests they may be suitable for use with other imaging modalities. Targeting of MPIO to markers of disease is commonly achieved through the covalent attachment of antibodies to the surface of the particles, providing an imaging agent that is both highly specific and which binds with high affinity. When comparing micron-sized particles with nanometre-sized particles, the former provide substantial signal dropout in MRI and confer the sensitivity to detect low levels of target. Furthermore, larger particles appear to bind to targets more potently than smaller particles. Animal models have also demonstrated favorable blood clearance characteristics of MPIO, which are important in achieving favorable signal over background and to attain clearance and disposal. Although the current generation of commercially available MPIO are not suitable for administration into humans, future work may focus on the development of biodegradable and nonimmunogenic MPIO that may allow the use of these imaging agents in a clinical setting.