Ultrasound-induced dissolution of lipid-coated and uncoated gas bubbles

Elastic Deformation of Soft Tissue-Mimicking Materials Using a Single Microbubble and Acoustic Radiation Force

Mechanical effects of microbubbles on tissues are central to many emerging ultrasound applications. Here, we investigated the acoustic radiation force a microbubble exerts on tissue at clinically relevant therapeutic ultrasound parameters. Individual microbubbles administered into a wall-less hydrogel channel (diameter: 25-100 µm, Young's modulus: 2-8.7 kPa) were exposed to an acoustic pulse (centre frequency: 1 MHz, pulse length: 10 ms, peak-rarefactional pressures: 0.6-1.0 MPa). Using high-speed microscopy, each microbubble was tracked as it pushed against the hydrogel wall. We found that a single microbubble can transiently deform a soft tissue-mimicking material by several micrometres, producing tissue loading-unloading curves that were similar to those produced using other indentation-based methods. Indentation depths were linked to gel stiffness. Using a mathematical model fitted to the deformation curves, we estimated the radiation force on each bubble (typically tens of nanonewtons) and the viscosity of the gels. These results provide insight into the forces exerted on tissues during ultrasound therapy and indicate a potential source of bio-effects.

Stability of Engineered Micro or Nanobubbles for Biomedical Applications

A micro/nanobubble (MNB) refers to a bubble structure sized in a micrometer or nanometer scale, in which the core is separated from the external environment and is normally made of gas. Recently, it has been confirmed that MNBs can be widely used in angiography, drug delivery, and treatment. Thus, MNBs are attracting attention as they are capable of constructing a new contrast agent or drug delivery system. Additionally, in order to effectively use an MNB, the method of securing its stability is also being studied. This review highlights the factors affecting the stability of an MNB and the stability of the MNB within the ultrasonic field. It also discusses the relationship between the stability of the bubble and its applicability in vivo.

Biomolecular Ultrasound Imaging of Phagolysosomal Function

Phagocytic clearance and lysosomal processing of pathogens and debris are essential functions of the innate immune system. However, the assessment of these functions in vivo is challenging because most nanoscale contrast agents compatible with noninvasive imaging techniques are made from nonbiodegradable synthetic materials that do not undergo regular lysosomal degradation. To overcome this challenge, we describe the use of an all-protein contrast agent to directly visualize and quantify phagocytic and lysosomal activities in vivo by ultrasound imaging. This contrast agent is based on gas vesicles (GVs), a class of air-filled protein nanostructures naturally expressed by buoyant microbes. Using a combination of ultrasound imaging, pharmacology, immunohistology, and live-cell optical microscopy, we show that after intravenous injection, GVs are cleared from circulation by liver-resident macrophages. Once internalized, the GVs undergo lysosomal degradation, resulting in the elimination of their ultrasound contrast. By noninvasively monitoring the temporal dynamics of GV-generated ultrasound signal in circulation and in the liver and fitting them with a pharmacokinetic model, we can quantify the rates of phagocytosis and lysosomal degradation in living animals. We demonstrate the utility of this method by showing how these rates are perturbed in two models of liver dysfunction: phagocyte deficiency and nonalcoholic fatty liver disease. The combination of proteolytically degradable nanoscale contrast agents and quantitative ultrasound imaging thus enables noninvasive functional imaging of cellular degradative processes.

Simultaneous Intravital Optical and Acoustic Monitoring of Ultrasound-Triggered Nanobubble Generation and Extravasation

Ultrasound-activated nanobubbles are being widely investigated as contrast agents and therapeutic vehicles. Nanobubbles hold potential for accessing the tumor extravascular compartment, though this relies on clinically debated passive accumulation for which evidence to date is indirect. We recently reported ultrasound-triggered conversion of high payload porphyrin-encapsulated microbubbles to nanobubbles, with actively enhanced permeability for local delivery. This platform holds implications for optical/ultrasound-based imaging and therapeutics. While promising, it remains to be established how nanobubbles are generated and whether they extravasate intact. Here, insights into the conversion process are reported, complemented by novel simultaneous intravital and acoustic monitoring in tumor-affected functional circulation. The first direct acoustic evidence of extravascular intact nanobubbles are presented. These insights collectively advance this delivery platform and multimodal micro- and nanobubbles, extending their utility for imaging and therapeutics within and beyond the vasculature.

Rapid leakage from PEGylated liposomes triggered by bubbles

Liposomes are applicable to fabrication of colloidal carriers of drugs and proteins. Physicochemical stimuli-triggered leakage from liposomes offers a wide variety of applications in biochemical and biomedical fields. In this work, effects of bubbles on the characteristics of PEGylated liposomes encapsulating 5(6)-carboxyfluorescein were examined. The liposomes were composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1-10 mol% 1,2-distearoyl-sn-glycero-3-phosphoethanolamine conjugated with poly(ethylene glycol) (DSPE-PEG). The mean molecular mass M_r,PEG of the PEG moiety was 550 or 5000. A bubble column was used for generating air bubbles at a superficial gas velocity of 0.58-0.88 cm s^-1. Leakage from the PEGylated liposomes was remarkably accelerated at 25 or 40 °C by introducing air to a liposome suspension at pH 7.4, whereas the dye molecules practically remained encapsulated in the liposomes being suspended in static liquid. The apparent rate constant for the dye release from the liposomes composed of DOPC and 1 mol% DSPE-PEG (M_r,PEG = 5000) being suspended in the gas-liquid flow was 168 times larger than that obtained with respect to the same liposomes in static liquid. Leakage from non-PEGylated liposomes was not pronounced even in the gas-liquid flow. Furthermore, the release rate of the dye from the PEGylated liposomes in liquid shear flow (no bubble) was clearly smaller than that in the gas-liquid flow, meaning that the interaction between bubbles and the liposomes was responsible for the observed rapid leakage. Adsorption of the PEGylated lipids to bubbles was indicated to induce leaky lipid bilayers, which was discussed on the basis of the conformational state of the PEG moiety.

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.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

Microbubbles are increasingly being used in biomedical applications such as ultrasonic imaging and targeted drug delivery. Microbubbles typically range from 0.1 to 10 µm in size and consist of a protective shell made of lipids or proteins. The shell encapsulates a gaseous core containing gases such as oxygen, sulfur hexafluoride or perfluorocarbons. This review is a consolidated account of information available in the literature on research related to microbubbles. Efforts have been made to present an overview of microbubble synthesis techniques; microbubble stability; microbubbles as contrast agents in ultrasonic imaging and drug delivery vehicles; and side effects related to microbubble administration in humans. Developments related to the modeling of microbubble dissolution and stability are also discussed.

Highly Uniform Perfluoropropane-Loaded Cerasomal Microbubbles As a Novel Ultrasound Contrast Agent

Microbubbles are widely used as ultrasound contrast agents owing to their excellent echoing characteristics under ultrasound radiation. However, their short sonographic duration and wide size distribution still hinder their application. Herein, we present a hard-template approach to produce perfluoropropane-loaded cerasomal microbubbles (PLCMs) with uniform size and long sonographic duration. The preparation of PLCMs includes deposition of Si-lipids onto functionalized CaCO3 microspheres, removal of their CaCO3 cores and mild infusion of perfluoropropane. In vitro and in vivo experiments showed that PLCMs had excellent echoing characteristics under different ultrasound conditions. More importantly, PLCMs could be imaged for much longer than SonoVue (commercially used microbubbles) under the same ultrasound parameters and concentrations. Our results demonstrated that PLCMs have great potential for use as a novel contrast agent in ultrasound imaging.

On the relationship between microbubble fragmentation, deflation and broadband superharmonic signal production

Acoustic angiography imaging of microbubble contrast agents uses the superharmonic energy produced from excited microbubbles and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. To elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, whereas weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when populations of stationary microbubbles were imaged with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1-μm bubbles than for 4-μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in decreases in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. Although the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate to maintain adequate contrast during in vivo imaging.

Inertial cavitation threshold of nested microbubbles

Cavitation of ultrasound contrast agents (UCAs) promotes both beneficial and detrimental bioeffects in vivo (Radhakrishnan et al., 2013) [1]. The ability to determine the inertial cavitation threshold of UCA microbubbles has potential application in contrast imaging, development of therapeutic agents, and evaluation of localized effects on the body (Ammi et al., 2006) [2]. This study evaluates a novel UCA and its inertial cavitation behavior as determined by a home built cavitation detection system. Two 2.25 MHz transducers are placed at a 90° angle to one another where one transducer is driven by a high voltage pulser and the other transducer receives the signal from the oscillating microbubble. The sample chamber is placed in the overlap of the focal region of the two transducers where the microbubbles are exposed to a pulser signal consisting of 600 pulse trains per experiment at a pulse repetition frequency of 5 Hz where each train has four pulses of four cycles. The formulation being analyzed is comprised of an SF6 microbubble coated by a DSPC PEG-3000 monolayer nested within a poly-lactic acid (PLA) spherical shell. The effect of varying shell diameters and microbubble concentration on cavitation threshold profile for peak negative pressures ranging from 50 kPa to 2 MPa are presented and discussed in this paper. The nesting shell decreases inertial cavitation events from 97.96% for an un-nested microbubble to 19.09% for the same microbubbles nested within a 2.53 μm shell. As shell diameter decreases, the percentage of inertially cavitating microbubbles also decreases. For nesting formulations with average outer capsule diameters of 20.52, 14.95, 9.95, 5.55, 2.53, and 1.95 μm, the percentage of sample destroyed at 1 MPa was 51.02, 38.94, 33.25, 25.27, 19.09, and 5.37% respectively.

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.

Temperature-dependent biphasic shrinkage of lipid-coated bubbles in ultrasound

Lipid-coated microbubbles and emulsions are of interest as possible ultrasound-mediated drug delivery vehicles and for their interesting behaviors and fundamental properties. We and others have noted that bubbles coated with the long chain saturated phospholipid distearoylphosphatidylcholine (DSPC) rapidly shrink to a quasistable size when repeatedly insonated with short ultrasound pulses; such stability may adversely affect the bubble's subsequent ability to deliver its pharmacological cargo. Bubbles coated with the unsaturated lipid dioleoylphosphatidylcholine (DOPC) did not show stability but did undergo an abrupt change from rapid initial shrinkage to a slow persistent shrinkage, leading ultimately to dissolution or dispersion. As DOPC and DSPC differ not only in chain saturation but also phase behavior, we performed additional studies using dimyristoyl PC (DMPC) as a coat lipid and controlled the solution temperature to study bubble behavior on exposure to repeated ultrasound pulses for the same coat, in both fluid and gel phases. We find, first, that essentially all bubbles show an initially rapid shrinkage, in which gas loss exceeds the limit imposed by gas diffusion into the surrounding medium; this rapid shrinkage may be evidence of nanoscopic bubble fragmentation. Second, upon reaching a fraction of their initial size, bubbles begin a slower shrinkage with a shrinkage rate that depends on the resting phase state of the coat lipid: fluid DMPC monolayers give a more rapid shrinkage than gel phase. DOPC-coated bubbles showed no temperature-dependent responses in the same temperature range. The results are especially interesting in that bubble compression during the pulse is likely to adiabatically heat the bubble and fluidize the coat, regardless of its initial phase state; thus, some structural feature of the resting coat, such as defect lines in the gel phase, may be important in the subsequent response to the ~3 μs ultrasound pulse.

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.

Correspondence - Nonlinear oscillations of deflating bubbles

Phospholipid-coated ultrasound contrast agents may deflate or even collapse because of stress resulting from ultrasound-induced oscillations. In this work, we investigate the behavior of isolated contrast agent microbubbles during prolonged ultrasound excitation. Isolated microbubbles placed in a thin capillary tube were excited with hundreds of ultrasound pulses at a low mechanical index, and their oscillations were recorded using the Brandaris-128 ultra-high-speed camera. Results show that microbubbles undergo an irreversible, non-destructive deflation process. Such deflation seems to occur in discrete steps rather than as a continuous process; furthermore, the dynamics of the bubble change during deflation: radial oscillations, both symmetric and asymmetric around the resting radius of the bubble, occur at various stages of the deflation process. Strongly asymmetric oscillations, such as compression-only and expansion-only behavior, were also observed: notably, expansion-only behavior is associated with a rapid size reduction, whereas compression-only behavior mostly occurs without a noticeable change of the bubble radius. We hypothesize that bubble deflation results from at least two distinct phenomena, namely diffusive gas loss and lipid material shedding from the encapsulating shell.

Accounting for the stability of microbubbles to multi-pulse excitation using a lipid-shedding model

Interest in microbubble ultrasound contrast agents as therapeutic and quantitative imaging tools has increased the need for accurate modeling of their behavior. Experiments have shown that some bubbles shrink significantly over the course of a single pulse but that the bubbles may eventually reach a stable size after many insonations. Here, it is shown from dimensional arguments that diffusion phenomena are negligible on the time scales that characterize a typical ultrasound pulse. Subsequently, a new model describing both a lipid-shedding mechanism and a nonlinear surface viscosity is developed and shown to provide a more accurate description of the observed experimental behavior.