Microbubble Oscillation Research Articles

The influence of surface adsorption on microbubble dynamics

In a pure liquid, the behaviour of a gas or vapour microbubble is determined primarily by its size, the ambient pressure and the properties of the surrounding liquid. In practice, however, adsorption of a dissolved substance from the surrounding liquid onto the microbubble surface will often take place, producing a thin coating which can significantly affect both the microbubble's stability and its dynamic response. This can have important implications in a wide range of applications, including underwater acoustics, cavitation detection, medical imaging and drug delivery. The aim of this paper is to review the existing theoretical treatments of coated microbubbles and to present and discuss some recent developments. It will be shown that the presence of the coating can substantially modify the amplitude of microbubble volumetric oscillation, resonance characteristics and relative amplitude in tension and compression. Finally, the need for improved understanding of the dynamic behaviour of surface coatings at high frequencies will be discussed.

Increasing the nonlinear character of microbubble oscillations at low acoustic pressures

The nonlinear response of gas bubbles to acoustic excitation is an important phenomenon in both the biomedical and engineering sciences. In medical ultrasound imaging, for example, microbubbles are used as contrast agents on account of their ability to scatter ultrasound nonlinearly. Increasing the degree of nonlinearity, however, normally requires an increase in the amplitude of excitation, which may also result in violent behaviour such as inertial cavitation and bubble fragmentation. These effects may be highly undesirable, particularly in biomedical applications, and the aim of this work was to investigate alternative means of enhancing nonlinear behaviour. In this preliminary report, it is shown through theoretical simulation and experimental verification that depositing nanoparticles on the surface of a bubble increases the nonlinear character of its response significantly at low excitation amplitudes. This is due to the fact that close packing of the nanoparticles restricts bubble compression.

Characterization of cell membrane response to ultrasound activated microbubbles

Contrast agents for ultrasound imaging, composed of tiny gas microbubbles, have become a reality in clinical routine. They are extensively used in radiology for detection and characterization of various tumors and in cardiology for left ventricular opacification. Recent experimental studies showed that ultrasound waves in combination with contrast agent microbubbles increase transiently cell membrane permeability in a process known as sonoporation. This effect is thought to allow foreign molecules to enter the cell. In that context, we explored the cell membrane's responses to microbubbles' oscillations as the mechanism is not completely understood. Breast cancer cell line in combination with contrast microbubbles were used. Ultrasound was applied using a transducer of 1 MHz center frequency transmitting a 10-cycle burst of different acoustic pressures repeated every 100 mus. Patch-clamp technique in whole cell configuration was used to explore transmembrane ion exchange through the variations in membrane potential. To characterize the activated ion channels, the variations of the intracellular calcium (Ca(2+)) concentration were explored using a fluorescent marker. The results revealed that ultrasound stimulation induces a rapid hyperpolarization of cell membrane potential when the microbubble is in direct contact with the cell, but the potential returned to its initial value when ultrasound stimulation stopped. The change in cell membrane potential indicates the activation of specific ion channels and depends on the quality of microbubble adhesion to the cell membrane. Microbubbles were shown to induce a mechanical stretch activating BKca channels. Simultaneous Ca(2+) measurements indicate a slow and progressive Ca(2+) increase that is likely a consequence of BKca channels opening not a cause. These results demonstrate that microbubbles' oscillations under ultrasound activation entail modulation of cellular function and signaling by t- riggering the modulation of ionic transports through the cell membrane. Cells response to the mechanical stretch caused by gentle microbubble oscillations is characterized by the opening of BKca stretch channels and a Ca(2+) flux, which might potentially trigger other cellular responses responsible for membrane sonopermeabilization.

Ultrasound-Directed Drug Delivery

It has been proven, that the cellular uptake of drugs and genes is increased, when the region of interest is under ultrasound insonification, and even more when a contrast agent is present. This increased uptake has been attributed to the formation of transient porosities in the cell membrane, which are big enough for the transport of drugs into the cell (sonoporation). Owing to this technique, new ultrasound contrast agents that incorporate a therapeutic compound have become of interest. Combining ultrasound contrast agents with therapeutic substances, such a chemotherapeutics and virus vectors, may lead to a simple and economic method to instantly cure upon diagnosis, using conventional ultrasound scanners. There are two hypotheses for explaining the sonoporation phenomenon, the first being microbubble oscillations near a cell membrane, the second being microbubble jetting through the cell membrane. Based on modeling, high-speed photography, and recent cellular uptake measurements, it is concluded that microbubble jetting behavior is less likely to be the dominant sonoporation mechanism. Ultrasound-directed drug delivery using microbubbles is a promising method that has great potential in the treatment of malignant disorders.

Ultrasound-Driven Microbubble Oscillation and Translation Within Small Phantom Vessels

The use of ultrasound radiation force to manipulate microbubbles in blood vessels has attracted recent interest as a method to increase the efficiency of ultrasonic molecular imaging and drug delivery. However, recent studies indicate that microbubble oscillation is diminished within small blood vessels, and therefore we investigate microbubble oscillation and translation within 12 μm vessels using high-speed photography. With each 0.1- to 1-MPa ultrasound pulse, microbubbles (radius of 1, 1.5 and 2 μm) within 12 μm tubes translate 5 to 10 times less than those within 200 μm tubes. Application of a pulse train with a high pulse repetition frequency displaces bubbles to the wall of 12- and 200- μm tubes within an interval (∼1 s) that is reasonable for clinical translation. Modeling of coupled oscillation and translation for unconstrained microbubbles, based on a modified Rayleigh-Plesset (RP) and the trajectory equations, is compared with experimental observations and demonstrates agreement for the larger displacements observed within the 200 μm tubes. This study has implications for contrast-assisted ultrasound applications, aiding the manipulation of targeted microbubbles and for further theoretical understanding of the complex bubble dynamics within constrained vessel. (E-mail: kwferrara@ucdavis.edu)

Direct observations of ultrasound microbubble contrast agent interaction with the microvessel wall

Many thousands of contrast ultrasound studies have been conducted in clinics around the world. In addition, the microbubbles employed in these examinations are being widely investigated to deliver drugs and genes. Here, for the first time, the oscillation of these microbubbles in small vessels is directly observed and shown to be substantially different than that predicted by previous models and imaged within large fluid volumes. Using pulsed ultrasound with a center frequency of 1 MHz and peak rarefactional pressure of 0.8 or 2.0 MPa, microbubble expansion was significantly reduced when microbubbles were constrained within small vessels in the rat cecum (p<0.05). A model for microbubble oscillation within compliant vessels is presented that accurately predicts oscillation and vessel displacement within small vessels. As a result of the decreased oscillation in small vessels, a large resting microbubble diameter resulting from agent fusion or a high mechanical index was required to bring the agent shell into contact with the endothelium. Also, contact with the endothelium was observed during asymmetrical collapse, not during expansion. These results will be used to improve the design of drug delivery techniques using microbubbles.

Multifunctional Nanoparticles for Combining Ultrasonic Tumor Imaging and Targeted Chemotherapy

Drug delivery in polymeric micelles combined with tumor irradiation by ultrasound results in effective drug targeting, but this technique requires prior tumor imaging. A technology that combined ultrasound imaging with ultrasound-mediated nanoparticle-based targeted chemotherapy could therefore have important applications in cancer treatment. Mixtures of drug-loaded polymeric micelles and perfluoropentane (PFP) nano/microbubbles stabilized by the same biodegradable block copolymer were prepared. Size distribution of nanoparticles was measured by dynamic light scattering. Cavitation activity (oscillation, growth, and collapse of microbubbles) under ultrasound was assessed based on the changes in micelle/microbubble volume ratios. The effect of the nano/microbubbles on the ultrasound-mediated cellular uptake of doxorubicin (Dox) in MDA MB231 breast tumors in vitro and in vivo (in mice bearing xenograft tumors) was determined by flow cytometry. Statistical tests were two-sided. Phase state and nanoparticle sizes were sensitive to the copolymer/perfluorocarbon volume ratio. At physiologic temperatures, nanodroplets converted into nano/microbubbles. Doxorubicin was localized in the microbubble walls formed by the block copolymer. Upon intravenous injection into mice, Dox-loaded micelles and nanobubbles extravasated selectively into the tumor interstitium, where the nanobubbles coalesced to produce microbubbles with a strong, durable ultrasound contrast. Doxorubicin was strongly retained in the microbubbles but released in response to therapeutic ultrasound. Microbubbles cavitated under the action of tumor-directed ultrasound, which enhanced intracellular Dox uptake by tumor cells in vitro to a statistically significant extent relative to that observed with unsonicated microbubbles (drug uptake ratio = 4.60; 95% confidence interval [CI] = 1.70 to 12.47; P = .017) and unsonicated micelles (drug uptake ratio = 7.97; 95% CI = 3.72 to 17.08; P = .0032) and resulted in tumor regression in the mouse model. Multifunctional nanoparticles that are tumor-targeted drug carriers, long-lasting ultrasound contrast agents, and enhancers of ultrasound-mediated drug delivery have been developed and deserve further exploration as cancer therapeutics.

“Compression-Only” Behavior of Phospholipid-Coated Contrast Bubbles

Ultrasound contrast agents oscillate approximately linearly up to a certain pressure range where nonlinearity sets in. Nonlinear microbubble oscillations are exploited in ultrasound pulse-echo imaging as this improves the contrast-to-tissue ratio. Here we report the observation of a highly nonlinear response of phospholipid-coated contrast agents at pressures as low as 50 kPa, termed “compression-only” behavior, where the microbubbles only compress, yet hardly expand. Time-resolved bubble dynamics recorded through ultra high-speed imaging revealed that nearly 40% of the coated bubbles show “compression-only” behavior. (E-mail: n.dejong@erasmusmc.nl)

Microbubble shape oscillations excited through an ultrasound‐driven parametric instability

An ultrasonically driven air bubble can become shape‐unstable through a parametric instability. Here, we report time‐resolved optical observations of shape oscillations (mode n=2 to 6) of micron‐sized single air bubbles for a range of acoustic pressures. The observed mode number n was found to be linearly related to the resting radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which as expected is minimum at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small, nonspherical linear perturbation into a Rayleigh‐Plesset‐type equation model which includes a physical thermal damping mechanism describing heat and mass transport in the thin boundary layer at the bubble‐to‐water interface. Indeed, a parametric instability is responsible for the shape oscillations, and the Rayleigh‐Plesset‐type equation captures the experimental observations in great detail.

Acoustic response of compliable microvessels containing ultrasound contrast agents

The existing models of the dynamics of ultrasound contrast agents (UCAs) have largely been focused on an UCA surrounded by an infinite liquid. Preliminary investigations of a microbubble's oscillation in a rigid tube have been performed using linear perturbation, under the assumption that the tube diameter is significantly larger than the UCA diameter. In the potential application of drug and gene delivery, it may be desirable to fragment the agent shell within small blood vessels and in some cases to rupture the vessel wall, releasing drugs and genes at the site. The effect of a compliant small blood vessel on the UCA's oscillation and the microvessel's acoustic response are unknown. The aim of this work is to propose a lumped-parameter model to study the interaction of a microbubble oscillation and compliable microvessels. Numerical results demonstrate that in the presence of UCAs, the transmural pressure through the blood vessel substantially increases and thus the vascular permeability is predicted to be enhanced. For a microbubble within an 8 to 40 µm vessel with a peak negative pressure of 0.1 MPa and a centre frequency of 1 MHz, small changes in the microbubble oscillation frequency and maximum diameter are observed. When the ultrasound pressure increases, strong nonlinear oscillation occurs, with an increased circumferential stress on the vessel. For a compliable vessel with a diameter equal to or greater than 8 µm, 0.2 MPa PNP at 1 MHz is predicted to be sufficient for microbubble fragmentation regardless of the vessel diameter; however, for a rigid vessel 0.5 MPa PNP at 1 MHz may not be sufficient to fragment the bubbles. For a centre frequency of 1 MHz, a peak negative pressure of 0.5 MPa is predicted to be sufficient to exceed the stress threshold for vascular rupture in a small (diameter less than 15 µm) compliant vessel. As the vessel or surrounding tissue becomes more rigid, the UCA oscillation and vessel dilation decrease; however the circumferential stress is predicted to increase. Decreasing the vessel size or the centre frequency increases the circumferential stress. For the two frequencies considered in this work, the circumferential stress does not scale as the inverse of the square root of the acoustic frequency va as in the mechanical index, but rather has a stronger frequency dependence, 1/va.For more information on this article, see medicalphysicsweb.org

Nonlinear oscillation of microbubble in microtubes for ultrasound imaging and drug delivery

Ultrasound-assisted microbubble drug delivery is under intensive investigation due to the potential for enhanced local delivery without systemic adverse effects. The efforts in modeling bubble oscillation have been largely focused on using various modified forms of the Rayleigh-Plesset bubble dynamics equation, of which the cornerstone assumption is that a bubble oscillates in an unbounded liquid field and remains spherical until it collapses. Here, nonlinear oscillation of a bubble confined inside a microtube with diameters comparable with capillaries is studied numerically and experimentally. In numerical simulation, the liquid is treated as incompressible viscid fluid and contrast agent is treated as gas bubble obeying the polytropic law. Microbubble oscillation is shown to decrease in a small vessel as compared with oscillation in an infinite fluid or a larger vessel diameter. For the same bubble size, the natural oscillation frequency of a bubble deceases with the decrease of tube size. The natural frequency of a bubble with an equilibrium diameter of 2.5 μm decreases from 3.56 MHz, in unbounded liquid field, to 1.16 MHz in an 8-μm tube. Decreasing the ultrasound center frequency can increase the amplitude of bubble oscillation and thereby produce a higher pressure on the tube.

Education in medical acoustics

The field of medical acoustics has grown tremendously in the last 30 years. Sophisticated ultrasound scanners are utilized to image a developing fetus in the womb or blood flow in the vessels. High-pressure acoustic pulses travel through muscle, fat, and capillary beds to pulverize kidney stones with minimal damage to the intervening tissue. Gas-based microbubbles stabilized with polymeric or liposomal shells are injected into the vasculature to enhance the contrast in ultrasound images or to deliver life-saving drugs. Due to the interdisciplinary nature of medical acoustics, educating the next generation of clinicians, engineers, and researchers within the field has become more challenging. Students must gain some understanding of a variety of concepts, including the derivation of the wave equation, the effect of biological media on the propagation of ultrasound (attenuation, scatter, etc.), signal-processing techniques for converting scattered sound into images, and the effect of viscoelastic shells on the forced oscillation of microbubbles. Armed with this knowledge, medical acousticians will continue to develop innovative diagnostic and therapeutic applications for ultrasound.

Microbubble oscillation in tubes with diameters of 12, 25, and 195 microns

Ultrasound contrast agents are often used to measure flow rate in the microvasculature; however, the oscillation of these agents in capillary-sized tubes has not been directly observed. Here, oscillations of microbubbles are examined in microvessel phantoms with diameters similar to those of capillaries. High-speed camera images demonstrate the effects of ultrasonic pressure and tube diameter and length on microbubble expansion and fragmentation occurrence. Microbubble displacement due to radiation force is also demonstrated in a phantom microvessel.

Relaxation Behavior of Microbubbles in Ultrasonic Field

The nonlinear oscillation of microbubbles under the influence of ultrasound was investigated experimentally and theoretically. The radius-time curves of bubbles including afterbounces calculated using the Rayleigh–Plesset equation with polytropic relations and with the Keller–Miksis equation using analytical solutions for the Navier–Stokes equations of the gases were compared with the results observed from light scattering. This study revealed that the dynamic behavior of microbubbles in an ultrasonic field, such as the expansion ratio of the maximum to the equilibrium radius and the bouncing motion after the first collapse (minimum), depends crucially on the relaxation time of the bubble motion with respect to the characteristic time of the applied ultrasound, and that the relaxation time decreases as the equilibrium radius of the microbubbles increases.

High-frequency dynamics of ultrasound contrast agents.

Ultrasound contrast agents enhance echoes from the microvasculature and enable the visualization of flow in smaller vessels. Here, we optically and acoustically investigate microbubble oscillation and echoes following insonation with a 10 MHz center frequency pulse. A high-speed camera system with a temporal resolution of 10 ns, which provides two-dimensional (2-D) frame images and streak images, is used in optical experiments. Two confocally aligned transducers, transmitting at 10 MHz and receiving at 5 MHz, are used in acoustical experiments in order to detect subharmonic components. Results of a numerical evaluation of the modified Rayleigh-Plesset equation are used to predict the dynamics of a microbubble and are compared to results of in vitro experiments. From the optical observations of a single microbubble, nonlinear oscillation, destruction, and radiation force are observed. The maximum bubble expansion, resulting from insonation with a 20-cycle, 10-MHz linear chirp with a peak negative pressure of 3.5 MPa, has been evaluated. For an initial diameter ranging from 1.5 to 5 microm, a maximum diameter less than 8 microm is produced during insonation. Optical and acoustical experiments provide insight into the mechanisms of destruction, including fragmentation and active diffusion. High-frequency pulse transmission may provide the opportunity to detect contrast echoes resulting from a single pulse, may be robust in the presence of tissue motion, and may provide the opportunity to incorporate high-frequency ultrasound into destruction-replenishment techniques.

Asymmetric oscillation of adherent targeted ultrasound contrast agents

With a lipid shell containing biotin, micron-sized bubbles bound to avidin on a porous and flexible cellulose boundary were insonified by ultrasound. The oscillation of these targeted microbubbles was observed by high-speed photography and compared to the oscillation of free-floating microbubbles. Adherent microbubbles were observed to oscillate asymmetrically in the plane normal to the boundary, and nearly symmetrically in the plane parallel to the boundary, with a significantly smaller maximum expansion in each dimension for bound than free bubbles. With sufficient transmitted pressure, a jet was produced traveling toward the boundary.

Medical bubbles

Ultrasound contrast agents consist of gas bubbles with equilibrium radii to . These medical bubbles are small enough to be transported intravascularly and to pass through capillary vessels. Because their resonance frequencies coincide with those applied in ultrasonic imaging, they are suitable markers for the detection of perfused areas. Most of the ultrasound contrast agents that have been developed for clinical diagnostics contain slowly diffusing gas microbubbles encapsulated by highly elastic shells, to prevent them from dissolving too quickly during application. After intravascular injection of an agent into the circulation, microbubbles will pass the site of interest which is insonified by the clinician. Upon insonification, the microbubbles will generate a characteristic response, which is detected by the ultrasound scanner used. The resulting echographic image can then be interpreted by the clinician. Most studies on microbubble behavior have been based on the acoustic response from an ensemble of bubbles. With high‐speed photography fast enough to surpass the bubble resonance periods, the observation of individual microbubble behavior has become possible. In this thesis, we made use of fast‐framing camera systems operating at three million frames per second and higher, to observe dynamic behavior of individual microbubbles subjected to ultrasound. Multiple frames were captured during a single ultrasonic cycle. To account for the presence of a shell encapsulating the gas microbubble, the physical properties shell stiffness and shell friction have been accounted for in models describing microbubble oscillation. These properties have been measured for ensembles of ultrasound contrast agent microbubbles. From our results, however, it is suggested that the shell properties may differ between individual bubbles, because optically identical bubbles reveal different oscillating behavior. The presence of a shell appears to be less of interest for ultrasound contrast agent microbubble phenomena observed at high acoustic pressures: The physical mechanisms of microbubble coalescence, fragmentation, translation, and jetting (microsyringing) are comparable to those of free gas bubbles in the millimeter range. Because of the fast‐framing, we are the first to notice repeated coalescence and fragmentation. Irregular shapes of insonified bubbles were previously interpreted and published in literature as modes of shape instability of a single bubble. However, these shapes may also be accounted for by coalescence of bubbles or bubble fragments. We investigated the influence of the lipid shell on the coalescence by computing the film drainage for immobile bubble surfaces resulting in a laminar flow, and for mobile bubbles surfaces resulting in a plug flow. The coalescence of lipid‐encapsulated microbubbles appeared to be unimpeded by shells. Previously, release of gas from encapsulated microbubbles had been assumed from acoustical measurements. We demonstrated such release from encapsulated microbubbles with a rigid albumin shell during insonification. After this so‐called “sonic cracking,” the free gas dissolves in the surrounding fluid. The release of gas from encapsulations may find an application in noninvasive pressure measurements. Medical bubbles will continue to play an important role in ultrasonic imaging, but they have potential therapeutic applications, too. The experiments have been performed at the Department of Experimental Echocardiography, Erasmus MC, Rotterdam, The Netherlands. The Ph.D. defense was held at the University of Twente, Enschede, The Netherlands, on 17 September, 2004. The project has been supported by the Technology Foundation STW (RKG.5104) and the Interuniversity Cardiology Institute of the Netherlands.

Microbubble oscillations and stability for drug delivery

Ultrasound contrast agents have been developed from micron size bubbles whose gas core is enclosed by a polymer, lipid or protein shell. Furthermore, specific designs have been developed for drug delivery in which the ultrasound contrast agent acts as drug delivery vehicle. A drug may be suspended in the shell of these agents which is released at a particular site as the microbubble undergoes destruction. Localized delivery depends on the destruction of a sufficient number of bubbles within a confined geometry typically a capillary or small vessel. Experimental evidence suggests the composition and thickness of the shell play important role in the break-up. Break-up scenarios of buckling have been suggested for elastic shell agents and shape stability for fluid shell agents. However, these have not been rigorously examined theoretically. The formulations of contrast agent designed for drug delivery are highlighted including a description of a double polymeric layer design. Shape stability equations for fluid shell agents are derived and analyzed in limiting cases. Furthermore, biological issues related to potential endothelial cell transport and interactions are briefly discussed.

Coupled nonlinear oscillations of microbubbles

The coupling effects on the acoustic signature from nonlinear oscillations of a group of microbubbles is investigated. In general, exploring this phenomenon would require solving a set of (linearly) coupled nonlinear ordinary differential equations (\lowercase {ODE}s). However, assuming that the initial conditions of all bubbles are identical and that all bubbles are equi-distant from each other simplifies the governing equations to just a single \lowercase {ODE}. Numerical data obtained by solving this \lowercase {ODE} is used to investigate the effects of bubble population size on the subharmonics and ultraharmonics of the system. As the number of bubbles is increased, the natural frequency and the damping of the system decreases. There is a slight shift in the frequencies at which the maximum bubble oscillation occur. The amplitude of oscillations near the main resonance are significantly reduced as the number of bubble increases.

Behaviour of a bubble cluster in an ultrasound field

AbstractUltrasound medical applications, such as an ultrasound imaging with micro‐bubble contrast agents, and high intensity focused ultrasound (HIFU) therapy have attracted much attention in recent years. These applications have a close relation to the motion of micro‐bubbles, so that it is essential to understand their dynamics. The bubble motion is influenced by the internal phenomena and its interaction with the surrounding medium, such as the thermal diffusion, the mist formation, the mass diffusion, the heat and mass transfer through the bubble wall. It is important that the oscillation of micro bubbles generates a strong acoustic pressure for these medical applications. The strong pressure also appears in the case of a bubble cluster, like a cloud cavitation. As these dynamics of bubbles are strongly influenced by the thermal phenomena inside them, it is necessary to construct the model taking these phenomena into account to analyse the behaviour of the bubble cluster precisely. Thus, the following effects are considered: the evaporation and condensation of the liquid at the bubble wall, the heat transfer through the bubble wall, and the compressibility of the liquid. Then the spherical bubble cluster is numerically simulated. When the frequency of the ultrasound is sufficiently high, the bubble cluster hardly oscillates. On the contrary, when the frequency of the ultrasound is at the resonance of the bubble cluster, the pressure wave generates the shock wave and it focuses to the cluster centre. As a result, the pressure inside the bubble at the cluster centre becomes much higher than that of a single bubble. Though this extreme high pressure causes the severe cavitation erosion, it is thought that this high energy concentration has the potential to be utilized for medical applications. Copyright © 2004 John Wiley &amp; Sons, Ltd.