Increasing the nonlinear character of microbubble oscillations at low acoustic pressures

Thermocapillary migration of a drop with a thermally conducting stagnant cap

Hypothesis The thermocapillary migration of a spherical drop with a stagnant cap in the presence of a constant applied temperature gradient can be strongly affected by the finite thermal conductivity of the stagnant cap. Numerics The heat conduction of the stagnant cap is analytically modeled. The effects of the additional interfacial stresses generated by the disturbances to the local temperature field due to the presence of the cap at the fluid-fluid interface and the corresponding velocity of migration of the drop are evaluated by solving for the temperature and hydrodynamic field equations in and around the drop. An asymptotic model is derived to predict the terminal velocity in the presence of an infinitely conducting stagnant cap. Findings The effects of the surface conductivity and size of the stagnation region alongside the bulk thermal conductivities and viscosities of the drop and surrounding media are evaluated. The terminal velocity of the drop is shown to have a monotonic dependence on the conductivity of the stagnant cap. The bounds to the terminal velocity increment due to the stagnant cap are derived. These bounds can be of significance to multiphysics problems involving particle laden drops, Pickering emulsions and other multi-phase technologies where the conductivity of the surface adsorbents is non-negligible.

An Acoustic Device for Ultra High-Speed Quantification of Cell Strain During Cell-Microbubble Interaction

Microbubbles utilize high-frequency oscillations under ultrasound stimulation to induce a range of therapeutic effects in cells, often through mechanical stimulation and permeabilization of cells. One of the largest challenges remaining in the field is the characterization of interactions between cells and microbubbles at therapeutically relevant frequencies. Technical limitations, such as employing sufficient frame rates and obtaining sufficient image resolution, restrict the quantification of the cell's mechanical response to oscillating microbubbles. Here, a novel methodology was developed to address many of these limitations and improve the image resolution of cell-microbubble interactions at high frame rates. A compact acoustic device was designed to house cells and microbubbles as well as a therapeutically relevant acoustic field while being compatible with a Shimadzu HPV-X camera. Cell viability tests confirmed the successful culture and proliferation of cells, and the attachment of DSPC- and cationic DSEPC-microbubbles to osteosarcoma cells was quantified. Microbubble oscillation was observed within the device at a frame rate of 5 million FPS, confirming suitable acoustic field generation and ultra high-speed image capture. High spatial resolution in these images revealed observable deformation in cells following microbubble oscillation and supported the first use of digital image correlation for strain quantification in a single cell. The novel acoustic device provided a simple, effective method for improving the spatial resolution of cell-microbubble interaction images, presenting the opportunity to develop an understanding of the mechanisms driving the therapeutic effects of oscillating microbubbles upon ultrasound exposure.

Novel Magnetic Elastic Phase-Change Nanodroplets as Dual Mode Contrast Agent for Ultrasound and Magnetic Resonance Imaging

Recently, dual-mode imaging systems merging magnetic resonance imaging (MRI) and ultrasound (US) have been developed. Designing a dual-mode contrast agent is complex due to different mechanisms of enhancement. Herein, we describe novel phase change nanodroplets (PCNDs) with perfluoropentane encapsulated in a pre-polyglycerol sebacate (pre-PGS) shell loaded with polyethylene glycol (PEG)-coated iron oxide nanoparticles as having a dual-mode contrast agent effect. Iron oxide nanoparticles were prepared via the chemical co-precipitation method and PCNDs were prepared via the solvent displacement technique. PCNDs showed excellent enhancement in the in vitro US much more than Sonovue^® microbubbles. Furthermore, they caused a susceptibility effect resulting in a reduction of signal intensity on MRI. An increase in the concentration of nanoparticles caused an increase in the MR contrast effect but a reduction in US intensity. The concentration of nanoparticles in a shell of PCNDs was optimized to obtain a dual-mode contrast effect. Biocompatibility, hemocompatibility, and immunogenicity assays showed that PCNDs were safe and non-immunogenic. Another finding was the dual-mode potential of unloaded PCNDs as T1 MR and US contrast agents. Results suggest the excellent potential of these PCNDs for use as dual-mode contrast agents for both MRI and US.

Modelling of magnetic microbubbles to evaluate contrast enhanced magnetomotive ultrasound in lymph nodes - a pre-clinical study

OBJECTIVES

Despite advances in MRI the detection and characterisation of lymph nodes in rectal cancer remains complex, especially when assessing the response to neoadjuvant treatment. An alternative approach is functional imaging, previously shown to aid characterisation of cancer tissues. We report proof of concept of the novel technique Contrast-Enhanced Magneto-Motive Ultrasound (CE-MMUS) to recover information relating to local perfusion and lymphatic drainage, and interrogate tissue mechanical properties through magnetically induced deformations.

METHODS

The feasibility of the proposed application was explored using a combination of experimental animal and phantom ultrasound imaging, along with finite element analysis. First, contrast-enhanced ultrasound imaging on one wild type mouse recorded lymphatic drainage of magnetic microbubbles after bolus injection. Second, tissue phantoms were imaged using MMUS to illustrate the force- and elasticity dependence of the magnetomotion. Third, the magnetomechanical interactions of a magnetic microbubble with an elastic solid were simulated using finite element software.

RESULTS

Accumulation of magnetic microbubbles in the inguinal lymph node was verified using contrast enhanced ultrasound, with peak enhancement occurring 3.7 s post-injection. The magnetic microbubble gave rise to displacements depending on force, elasticity, and bubble radius, indicating an inverse relation between displacement and the latter two.

CONCLUSION

Combining magnetic microbubbles with MMUS could harness the advantages of both techniques, to provide perfusion information, robust lymph node delineation and characterisation based on mechanical properties.

ADVANCES IN KNOWLEDGE

(a) Lymphatic drainage of magnetic microbubbles visualised using contrast-enhanced ultrasound imaging and (b) magnetomechanical interactions between such bubbles and surrounding tissue could both contribute to (c) robust detection and characterisation of lymph nodes.

Metformin-Loaded Polymer-Based Microbubbles/Nanoparticles Generated for the Treatment of Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a chronic metabolic disease that is increasingly common all over the world with a high risk of progressive hyperglycemia and high microvascular and macrovascular complications. The currently used drugs in the treatment of T2DM have insufficient glucose control and can carry detrimental side effects. Several drug delivery systems have been investigated to decrease the side effects and frequency of dosage, and also to increase the effect of oral antidiabetic drugs. In recent years, the use of microbubbles in biomedical applications has greatly increased, and research into microactive carrier bubbles continues to generate more and more clinical interest. In this study, various monodisperse polymer nanoparticles at different concentrations were produced by bursting microbubbles generated using a T-junction microfluidic device. Morphological analysis by scanning electron microscopy, molecular interactions between the components by FTIR, drug release by UV spectroscopy, and physical analysis such as surface tension and viscosity measurement were carried out for the particles generated and solutions used. The microbubbles and nanoparticles had a smooth outer surface. When the microbubbles/nanoparticles were compared, it was observed that they were optimized with 0.3 wt % poly(vinyl alcohol) (PVA) solution, 40 kPa pressure, and a 110 μL/min flow rate, thus the diameters of the bubbles and particles were 100 ± 10 μm and 70 ± 5 nm, respectively. Metformin was successfully loaded into the nanoparticles in these optimized concentrations and characteristics, and no drug crystals and clusters were seen on the surface. Metformin was released in a controlled manner at pH 1.2 for 60 min and at pH 7.4 for 240 min. The process and structures generated offer great potential for the treatment of T2DM.

An Ultrasound Enhancing Agent with Nonlinear Acoustic Activity that Depends on the Presence of an Electric Field

The nonlinear acoustic properties of microbubble ultrasound enhancing agents have allowed for the development of subharmonic, second harmonic, and contrast-pulse sequence ultrasound imaging modes, which enhance the quality, reduce the noise, and improve the diagnostic capabilities of clinical ultrasound. This study details acoustic scattering responses of perfluorobutane (PFB) microbubbles, an un-nested perfluoropentane (PFP) nanoemulsion, and two nested PFP nanoemulsions-one comprising a negatively charged phospholipid bilayer and another comprising a zwitterionic phospholipid bilayer-when excited at 1 or 2.25 MHz over a peak negative pressure range of 200 kPa to 4 MPa in the absence and presence of a 1-Hz, 1-V/cm electric field. The only sample that exhibited an increase in nonlinear activity in the presence of an electric field at both excitation frequencies was the negatively charged nested PFP nanoemulsion; the most pronounced effect was observed at an excitation of 2.25 MHz. Interestingly, the application of an electric field not only increased the nonlinear acoustic activity of the negatively charged nested PFP nanoemulsion but increased it beyond that seen when the nanoemulsion is un-nested and on the same scale as PFB microbubbles.

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.

A versatile method for the preparation of particle-loaded microbubbles for multimodality imaging and targeted drug delivery

Microbubbles are currently in clinical use as ultrasound contrast agents and under active investigation as mediators of ultrasound therapy. To improve the theranostic potential of microbubbles, nanoparticles can be attached to the bubble shell for imaging, targeting and/or enhancement of acoustic response. Existing methods for fabricating particle-loaded bubbles, however, require the use of polymers, oil layers or chemical reactions for particle incorporation; embed/attach the particles that can reduce echogenicity; impair biocompatibility; and/or involve multiple processing steps. Here, we describe a simple method to embed nanoparticles in a phospholipid-coated microbubble formulation that overcomes these limitations. Magnetic nanoparticles are used to demonstrate the method with a range of different microbubble formulations. The size distribution and yield of microbubbles are shown to be unaffected by the addition of the particles. We further show that the microbubbles can be retained against flow using a permanent magnet, can be visualised by both ultrasound and magnetic resonance imaging (MRI) and can be used to transfect SH-SY5Y cells with fluorescent small interfering RNA under the application of a magnetic field and ultrasound field.

Characterisation of Liposome-Loaded Microbubble Populations for Subharmonic Imaging

Therapeutic microbubbles could make an important contribution to the diagnosis and treatment of cancer. Acoustic characterisation was performed on microfluidic generated microbubble populations that either were bare or had liposomes attached. Through the use of broadband attenuation techniques (3-8 MHz), the shell stiffness was measured to be 0.72 ± 0.01 and 0.78 ± 0.05 N/m and shell friction was 0.37 ± 0.05 and 0.74 ± 0.05 × 10^-6 kg/s for bare and liposome-loaded microbubbles, respectively. Acoustic scatter revealed that liposome-loaded microbubbles had a lower subharmonic threshold, occurring from a peak negative pressure of 50 kPa, compared with 200 kPa for equivalent bare microbubbles. It was found that liposome loading had a negligible effect on the destruction threshold for this microbubble type, because at a mechanical index >0.4 (570 kPa), 80% of both populations were destroyed.

Marriage Strategy of Structure and Composition Designs for Intensifying Ultrasound & MR & CT Trimodal Contrast Imaging

Despite great efforts having been devoted to the design of multimodal imaging probe, almost all design principles of nanotheranostic agents subordinate to simple assemblies of building blocks, resulting in complex preparation process and discounted ability, that is, 1 + 1 < 2. In this report, a novel design strategy, marriage of structure design and composition design that can maximize imaging ability of each building block, ultimately achieving 1 + 1 ≥ 2, has been established. Moreover, a high-efficient ultrasound (US) & MR & CT trimodal contrast agent acts as model to instantiate this design strategy, wherein nanoparticles-induced nonlinear scattering and rattle-type structure-induced double scattering enhancing US imaging, and uniform distribution of Mn(2+) paramagentic centers and "core-satellite" structure of Au atoms favoring enhanced MR imaging and CT imaging, respectively have been validated, achieving optimization of structure design. Importantly, the selected components, silica, Au and MnO are endowed with excellent biocompatibility, displaying the marriage strategy of composition design with aforementioned structure optimization. In in vivo evaluations, such a biocompatible trimodal probe is demonstrated of excellent performance in intensifying CT, MR and US imaging in vivo, especially after positively charged modification by PEI promoting more probes retained in tumor. More importantly, as a universal design strategy, the involved principles in constructing such a US&MR&CT trimodal imaging probe promise great potentials in guiding designs of other materials-based multimodal imaging probe.

Controlled assembly of magnetic nanoparticles on microbubbles for multimodal imaging

Magnetic microbubbles (MMBs) consisting of microbubbles (MBs) and magnetic nanoparticles (MNPs) were synthesized for use as novel markers for improving multifunctional biomedical imaging. The MMBs were fabricated by assembling MNPs in different concentrations on the surfaces of MBs. The relationships between the structure, magnetic properties, stability of the MMBs, and their use in magnetic resonance/ultrasound (MR/US) dual imaging applications were determined. The MNPs used were NPs of 3-aminopropyltriethoxysilane (APTS)-functionalized superparamagnetic iron oxide γ-Fe2O3 (SPIO). SPIO was assembled on the surfaces of polymer MBs using a "surface-coating" approach. An analysis of the underlying mechanism showed that the synergistic effects of covalent coupling, electrostatic adsorption, and aggregation of the MNPs allowed them to be unevenly assembled in large amounts on the surfaces of the MBs. With an increase in the MNP loading amount, the magnetic properties of the MMBs improved significantly; in this way, the shell structure and mechanical properties of the MMBs could be modified. For surface densities ranging from 2.45 × 10(-7) μg per MMB to 8.45 × 10(-7) μg per MMB, in vitro MR/US imaging experiments showed that, with an increase in the number of MNPs on the surfaces of the MBs, the MMBs exhibited better T2 MR imaging contrast, as well as an increase in the US contrast for longer durations. In vivo experiments also showed that, by optimizing the structure of the MMBs, enhanced MR/US dual-modality image signals could be obtained for mouse tumors. Therefore, by adjusting the shell composition of MBs through the assembly of MNPs in different concentrations, MMBs with good magnetic and acoustic properties for MR/US dual-modality imaging contrast agents could be obtained.

Assembly of jammed colloidal shells onto micron-sized bubbles by ultrasound

Stabilization of gas bubbles in water by applying solid particles is a promising technique to ensure long-term stability of the dispersion against coarsening. However, the production of large quantities of particle stabilized bubbles is challenging. The delivery of particles to the interface must occur rapidly compared to the typical time scale of coarsening during production. Furthermore, the production route must be able to overcome the energy barriers for interfacial adsorption of particles. Here we demonstrate that ultrasound can be applied to agitate a colloidal dispersion and supply sufficient energy to ensure particle adsorption onto the air-water interface. With this technique we are able to produce micron-sized bubbles, solely stabilized by particles. The interface of these bubbles is characterized by a colloidal shell, a monolayer of particles which adopt a hexagonal packing. The particles are anchored to the interface owing to partial wetting and experience lateral compression due to bubble shrinkage. The combination of both effects stops coarsening once the interface is jammed with particles. As a result, stable bubbles are formed. Individual particles can desorb from the interface upon surfactant addition, though. The latter fact confirms that the particle shell is not covalently linked due to thermal sintering, but is solely held together by capillary interaction. In summary, we show that our ultrasound approach allows for the straightforward creation of micron-sized particle stabilized bubbles with high stability towards coarsening.

Formation of protein and protein-gold nanoparticle stabilized microbubbles by pressurized gyration

A one-pot single-step novel process has been developed to form microbubbles up to 250 μm in diameter using a pressurized rotating device. The microbubble diameter is shown to be a function of rotational speed and working pressure of the processing system, and a modified Rayleigh-Plesset equation has been derived to explain the bubble-forming mechanism. A parametric plot is constructed to identify a rotating speed and working pressure regime, which allows for continuous bubbling. Bare protein (lysozyme) microbubbles generated in this way exhibit a morphological change, resulting in microcapsules over a period of time. Microbubbles prepared with gold nanoparticles at the bubble surface showed greater stability over a time period and retained the same morphology. The functionalization of microbubbles with gold nanoparticles also rendered optical tunability and has promising applications in imaging, biosensing, and diagnostics.

Microfluidic preparation of polymer nanospheres

In this work, solid polymer nanospheres with their surface tailored for drug adhesion were prepared using a V-shaped microfluidic junction. The biocompatible polymer solutions were infused using two channels of the microfluidic junction which was also simultaneously fed with a volatile liquid, perfluorohexane using the other channel. The mechanism by which the nanospheres are generated is explained using high speed camera imaging. The polymer concentration (5-50 wt%) and flow rates of the feeds (50-300 µl min^-1) were important parameters in controlling the nanosphere diameter. The diameter of the polymer nanospheres was found to be in the range of 80-920 nm with a polydispersity index of 11-19 %. The interior structure and surfaces of the nanospheres prepared were studied using advanced microscopy and showed the presence of fine pores and cracks on surface which can be used as drug entrapment locations.

Measuring absolute blood pressure using microbubbles

Gas microbubbles are highly compressible, which makes them very efficient sound scatterers. As another consequence of their high compressibility, the radii of the microbubbles are affected by the pressure of the fluid around them, which changes their resonance frequency. Although the pressures present within the human body cause only minor variations in the radii of uncoated microbubbles (∼0.2% per 10 mmHg) and, therefore, very small variations in the resonance frequency (∼1 kHz per 10 mmHg), it was found in the work described here, through both simulations and in vitro measurements, that large changes in resonance frequency can occur in phospholipid-coated microbubbles for small blood pressure variations because of the exotic buckling dynamics of phospholipid monolayers (up to 240 kHz per 10 mmHg). This method should allow non-invasive measurement of the gauge blood pressure in deep blood vessels as long as the microbubble physical properties are well controlled.

Self-assembly of actin scaffolds on lipid microbubbles

Microbubbles offer unique properties as combined carriers of therapeutic payloads and diagnostic agents. Here we report on the development of novel microbubble architectures that in addition to the usual lipid shell have an actin cytoskeletal cortex assembled on their exterior. We show, using atomic force microscopy that this biomimetic coating creates a thin mesh that allows tuning of the mechanical properties of microbubbles and that the nature of actin assembly is determined by the fluidity of the lipid layer. Further, we show that it is possible to attach payloads and targeting-ligands to the actin scaffold. Resistance to gas permeation showed that the additional actin layer reduces gas diffusion across the shell and thus increases bubble lifetime. This study demonstrates a one step method to creating more complex microbubble architectures, which would be capable of further modification and tuning through the inclusion of actin binding proteins.

Silver nanoparticle-embedded microbubble as a dual-mode ultrasound and optical imaging probe

Microbubbles (MBs) coupled with nanoparticles represent a new class of multifunctional probe for multiscale biomedical imaging and drug delivery. In this study, we describe the development of multifunctional, microscale microbubble probes that are composed of a nitrogen gas core and a biocompatible polymer shell harboring silver nanoparticles (AgNPs). Ultrasound imaging studies show that the presence of AgNPs in the MB significantly improves the contrast of ultrasound images. The AgNPs within individual MB can be also imaged by using dark-field microscopy (DFM), which suggests that AgNPs in the polymer shell adopt multiple structural forms. AgNPs are released from the polymer shell following a brief exposure to an ultrasonic field and are subsequently taken up by living cells. AgNPs within labeled cells are imaged by DFM, while surface-enhanced Raman scattering is used to identify specific cytoplasmic biomolecules that bind to the surface of the AgNP. Collectively, these studies demonstrate the application of multifunctional MBs for micrometer scale contrast-enhanced ultrasound imaging, as vehicles for the ultrasound-based delivery of optical probes and drugs to cells, and for imaging of chemical sensing of individual nanopartiles within cells and tissue.

Au-nanoparticle coated mesoporous silica nanocapsule-based multifunctional platform for ultrasound mediated imaging, cytoclasis and tumor ablation

Au nanoparticles-coated, perfluorohexane-encapsulated and PEGylated mesoporous silica nanocapsule-based enhancement agents (MSNC@Au-PFH-PEG, abb. as MAPP) have been synthesized, for the ultrasound-induced cytoclasis, contrast-intensified ultrasound (US) imaging and US-guided high intensity focused ultrasound (HIFU) surgical therapy. Both the US-induced thermal effect and US triggered release of loaded model drug with MAPP under US exposure indicated the excellent US sensitivity of MAPP and its applicability for the combined chemo-/thermal therapy and future potential for HIFU ablation; US imaging under different modes verify the attractive US contrast intensification by using MAPP; US-guided HIFU therapy ex vivo and in vivo with MAPP is found to be highly efficient on rabbit VX2 xenograft tumor ablation due to the high thermal energy accumulation and increased mechanical/thermal effects from US-induced PFH bubble cavitations. MAPP can be promisingly used as an inorganic theranostic platform for contrast-intensified US imaging, combined chemotherapy and efficient HIFU tumor ablation under the guidance by the intensified US.

Acoustical properties of individual liposome-loaded microbubbles

A comparison between phospholipid-coated microbubbles with and without liposomes attached to the microbubble surface was performed using the ultra-high-speed imaging camera (Brandaris 128). We investigated 73 liposome-loaded microbubbles (loaded microbubbles) and 41 microbubbles without liposome loading (unloaded microbubbles) with a diameter ranging from 3-10 μm at frequencies ranging from 0.6-3.8 MHz and acoustic pressures ranging from 5-100 kPa. The experimental data showed nearly the same shell elasticity for the loaded and unloaded bubbles, but the shell viscosity was higher for loaded bubbles compared with unloaded bubbles. For loaded bubbles, a higher pressure threshold for the bubble vibrations was noticed. In addition, an "expansion-only" behavior was observed for up to 69% of the investigated loaded bubbles, which mostly occurred at low acoustic pressures (≤30 kPa). Finally, fluorescence imaging showed heterogeneity of liposome distributions of the loaded bubbles.

Effects of gold nanoparticles on the stability of microbubbles

Surfactant-coated microbubbles are utilized in a wide variety of applications, from wastewater purification to contrast agents in medical ultrasound imaging. In many of these applications, the stability of the microbubbles is crucial to their effectiveness. Controlling this, however, represents a considerable challenge. In this study, the potential for stabilizing microbubbles using solid nanoparticles adsorbed onto their surfaces was explored. A new theoretical model has been developed to describe the influence of interfacially adsorbed solid particles upon the dissolution of a gas bubble in a liquid. The aim of this work was to test experimentally the prediction of the model that the presence of the nanoparticles would inhibit gas diffusion and coalescence/disproportionation, thus increasing the life span of the bubbles. Near-monodisperse microbubbles (~100 μm diameter) were prepared using a microfluidic device and coated with a surfactant, with and without the addition of a suspension of spherical gold nanoparticles (~15 nm diameter). The experimental results confirmed the theoretical predictions that as the surface concentration of gold nanoparticles increased the bubbles underwent negligible changes in their size and size distribution over a period of 30 days at the ambient temperature and pressure. Under the same conditions, bubbles coated with the same surfactant but no nanoparticles survived only a matter of hours.

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.

Forced vibrations of a bubble in a liquid-filled elastic vessel

There is increasing demand for accurate characterization of the in vivo behavior of microbubble agents used for ultrasound imaging and therapy. This study examines bubble-vessel interaction, in particular the propagation of disturbances along the vessel wall. Finite element simulations of a 3 μm radius microbubble suspended in a viscous liquid and enclosed in a 4 μm radius elastic vessel were performed, and the results compared with existing analytical results for wave propagation in elastic liquid-filled tubes. The vessel wall was shown to have a significant effect upon the amplitude of bubble oscillation and hence acoustic radiation from it, as well as distension of the vessel wall. It was found that the most important factor was the ratio of the excitation frequency to the natural "ring" frequency of the vessel which in turn depends upon its dimensions and mechanical properties. As this ratio increases, the motion of the vessel wall becomes increasingly localized to the site of the bubble. It was also shown that the validity of the results obtained using the applied model of vessel elasticity is limited to frequencies below the ring frequency, and this should be taken into account in the development of protocols for ultrasound safety and/or therapeutic procedures.

"Compression-only" behavior: a second-order nonlinear response of ultrasound contrast agent microbubbles

Oscillating phospholipid-coated ultrasound contrast agent microbubbles display a so-called "compression-only" behavior, where it is observed that the bubbles compress efficiently while their expansion is suppressed. Here, a theoretical understanding of the source of this nonlinear behavior is provided through a weakly nonlinear analysis of the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499-3505 (2005)]. It is shown that the radial dynamics of the bubble can be considered as a superposition of a linear response at the fundamental driving frequency and a second-order nonlinear low-frequency response that describes the negative offset of the mean bubble radius. The analytical solution deduced from the weakly nonlinear analysis shows that the compression-only behavior results from a rapid change of the shell elasticity with bubble radius. In addition, the radial dynamics of single phospholipid-coated microbubbles was recorded as a function of both the amplitude and the frequency of the driving pressure pulse. The comparison between the experimental data and the theory shows that the magnitude of compression-only behavior is mainly determined by the initial phospholipids concentration on the bubble surface, which slightly varies from bubble to bubble.

Nonlinear shell behavior of phospholipid-coated microbubbles

The influence of the stabilizing phospholipid-coating on the nonlinear dynamics of ultrasound contrast agent microbubbles is investigated. We record the radial dynamics of individual microbubbles with an ultra high-speed camera as a function of both driving pressure and frequency. The viscoelastic shell was found to enhance the nonlinear bubble response at acoustic pressures as low as 10 kPa. For increasing acoustic pressures a decrease of the frequency of maximum response was observed for a distinct class of bubbles, leading to a pronounced skewness of the resonance curve, which we show to be the origin of the "thresholding" behavior (Emmer et al. 2007). For the other bubbles, the frequency of maximum response was found to lie just above the resonance frequency of an uncoated microbubble and to be independent of the applied acoustic pressure. The shell-buckling bubble model (Marmottant et al. 2005), which accounts for buckling and rupture of the shell, captures both cases for a unique set of the shell parameters, the relevant parameter being the phospholipid concentration at the bubble interface.

Microbubbles loaded with nanoparticles: a route to multiple imaging modalities

We report a single-step approach to producing small and stable bubbles functionalized with nanoparticles. The strategy includes the following events occurring in sequence: (i) a microfluidic generation of bubbles from a mixture of CO(2) and a minute amount of gases with low solubility in water, in an aqueous solution of a protein, a polysaccharide, and anionic nanoparticles; (ii) rapid dissolution of CO(2) leading to the shrinkage of bubbles and an increase in acidity of the medium in the vicinity of the bubbles; and (iii) co-deposition of the biopolymers and nanoparticles at the bubble-liquid interface. The proposed approach yielded microbubbles with a narrow size distribution, long-term stability, and multiple functions originating from the attachment of metal oxide, metal, or semiconductor nanoparticles onto the bubble surface. We show the potential applications of these bubbles in ultrasound and magnetic resonance imaging.

Microfluidic assembly of monodisperse, nanoparticle-incorporated perfluorocarbon microbubbles for medical imaging and therapy

New medical imaging contrast agents that permit multiple imaging and therapy applications using a single agent can result in more accurate diagnosis and local treatment of diseased tissue. Solid nanoparticles (NPs) (5-150 nm in size) have emerged as promising imaging and therapy agents, as have micrometer-scale, perfluorocarbon gas-filled microbubbles (MBs) used in patients as intravascular ultrasound contrast agents. We propose that the modular combination of small, solid NPs and larger, highly compressible MBs into a single agent is an effective way to attain the desired complementary and hybrid properties of two very different agents. Presented here is a new strategy for the simple and robust incorporation of various medical NPs with monodisperse MBs based upon the controlled pH-based regulation of the electrostatic attraction between NPs and the MB shell. Using this simple approach, microfluidic-generated, protein-lipid-coated, perfluorobutane MBs (with size control down to 3 microm) were incorporated with silica-coated NPs, including CdSe/ZnS quantum dots, gold nanorods, iron oxide NPs, and Gd-loaded mesoporous silica NPs. The silica interface permits NP inclusion within MBs to be independent of NP composition, morphology, and size. Significantly, the NP-incorporated MBs (NP-MBs) diluted in saline were detectable using low-pressure ultrasound, and the monodisperse MB platform can be produced at high-throughput, sufficient for in vivo usage (10(6) MB/sec). The modular synthesis of a variety of NP-MBs can facilitate flexible, user-defined, multifunctional imaging and therapy agents tailored for specific applications and disease types.

Creation of cavitation activity in a microfluidic device through acoustically driven capillary waves

We present a study on achieving intense acoustic cavitation generated by ultrasonic vibrations in polydimethylsiloxane (PDMS) based microfluidic devices. The substrate to which the PDMS is bonded was forced into oscillation with a simple piezoelectric transducer attached at 5 mm from the device to a microscopic glass slide. The transducer was operated at 100 kHz with driving voltages ranging between 20 V and 230 V. Close to the glass surface, pressure and vibration amplitudes of up to 20 bar and 400 nm were measured respectively. It is found that this strong forcing leads to the excitation of nonlinear surface waves when gas-liquid interfaces are present in the microfluidic channels. Also, it is observed that nuclei leading to intense inertial cavitation are generated by the entrapment of gas pockets at those interfaces. Subsequently, cavitation bubble clusters with void fractions of more than 50% are recorded with high-speed photography at up to 250,000 frames/s. The cavitation clusters can be sustained through the continuous injection of gas using a T-junction in the microfluidic device.

Effects of nonlinear propagation in ultrasound contrast agent imaging

This paper investigates two types of nonlinear propagation and their effects on image intensity and contrast-to-tissue ratio (CTR) in contrast ultrasound images. Previous studies have shown that nonlinear propagation can occur when ultrasound travels through tissue and microbubble clouds, making tissue farther down the acoustic path appear brighter in pulse inversion (PI) images, thus reducing CTR. In this study, the effect of nonlinear propagation through tissue or microbubbles on PI image intensity and CTR are compared at low mechanical index. A combination of simulation and experiment with SonoVue microbubbles were performed using a microbubble dynamics model, a laboratory ultrasound system and a clinical prototype scanner. The results show that, close to the bubble resonance frequency, nonlinear propagation through a bubble cloud of a few centimeter thickness with a modest concentration (1:10000 dilution of SonoVue microbubbles) is much more significant than through tissue-mimicking material. Consequently, CTR in regions distal to the imaging probe is greatly reduced for nonlinear propagation through the bubble cloud, with as much as a 12-dB reduction compared with nonlinear propagation through tissue-mimicking material. Both types of nonlinear propagation cause only a small change in bubble PI signals at the bubble resonance frequency. When the driving frequency increases beyond bubble resonance, nonlinear propagation through bubbles is greatly reduced in absolute values. However because of a greater reduction in nonlinear scattering from bubbles at higher frequencies, the corresponding CTR is much lower than that at bubble resonance frequency.

Cavitation and contrast: The use of bubbles in ultrasound imaging and therapy

Microbubbles and cavitation are playing an increasingly significant role in both diagnostic and therapeutic applications of ultrasound. Microbubble ultrasound contrast agents have been in clinical use now for more than two decades, stimulating the development of a range of new contrast-specific imaging techniques which offer substantial benefits in echocardiography, microcirculatory imaging, and more recently, quantitative and molecular imaging. In drug delivery and gene therapy, microbubbles are being investigated/developed as vehicles which can be loaded with the required therapeutic agent, traced to the target site using diagnostic ultrasound, and then destroyed with ultrasound of higher intensity energy burst to release the material locally, thus avoiding side effects associated with systemic administration, e.g. of toxic chemotherapy. It has moreover been shown that the motion of the microbubbles increases the permeability of both individual cell membranes and the endothelium, thus enhancing therapeutic uptake, and can locally increase the activity of drugs by enhancing their transport across biologically inaccessible interfaces such as blood clots or solid tumours. In high-intensity focused ultrasound (HIFU) surgery and lithotripsy, controlled cavitation is being investigated as a means of increasing the speed and efficacy of the treatment. The aim of this paper is both to describe the key features of the physical behaviour of acoustically driven bubbles which underlie their effectiveness in biomedical applications and to review the current state of the art.

Preparation of suspensions of phospholipid-coated microbubbles by coaxial electrohydrodynamic atomization

The use of phospholipid-coated microbubbles for medical applications is gaining considerable attention. However, the preparation of lipid-coated microbubble suspensions containing the ideal size and size distribution of bubbles still represents a considerable challenge. The most commonly used preparation methods of sonication and mechanical agitation result in the generation of polydisperse microbubbles with diameters ranging from less than 1 microm to greater than 50 microm. Efforts have been made via distinctly different techniques such as microfluidic and electrohydrodynamic bubbling to prepare lipid-coated microbubbles with diameters less than 10 microm and with a narrow size distribution, and recent results have been highly promising. In this paper, we describe a detailed investigation of the latter method that essentially combines liquid and air flow, and an applied electric field to generate microbubbles. A parametric plot was constructed between the air flow rate (Qg) and the lipid suspension flow rate (Ql) to identify suitable flow rate regimes for the preparation of phospholipid-coated microbubbles with a mean diameter of 6.6 microm and a standard deviation of 2.5 microm. The parametric plot has also helped in developing a scaling equation between the bubble diameter and the ratio Qg/Ql. At ambient temperature (22 degrees C), these bubbles were very stable with their size remaining almost unchanged for 160 min. The influence of higher temperatures such as the human body temperature (37 degrees C) on the size and stability of the microbubbles was also explored. It was found that the mean bubble diameter fell rapidly to begin with but then stabilized at 1-2 microm after 20 min.

Stability of microbubbles prepared by co-axial electrohydrodynamic atomisation

Previous studies have indicated that microbubbles prepared by co-axial electrohydrodynamic atomisation (CEHDA) are less stable than those prepared by other methods such as sonication and microfluidic techniques. The aim of this investigation was to determine the reasons for this observation and how this might be addressed in future work. Microbubbles were prepared by CEHDA using (i) a glycerol-air system, (ii) a glycerol-Tween 80-air system and (iii) a glycerol-zirconia-air system and also by simple agitation of (i) and (ii), in order to compare the effect upon the dissolution rate of microbubbles of different materials and processing methods. Both theoretical examination and the experimental results indicated that all three quantities were important in controlling the rate of microbubble dissolution, namely: surface tension at the gas/liquid interface, the effective diffusivity of gas through this interface and the initial concentration of gas dissolved in the surrounding liquid. However, it was the difference in gas concentration in the surrounding liquid that was indicated as the primary reason for the differences in stability observed with different processing methods. It was concluded, therefore, that improved stability could be achieved for microbubbles prepared using CEHDA by saturating the collecting fluid with gas and/or maintaining a high concentration of microbubbles during collection.

Generation of multilayered structures for biomedical applications using a novel tri-needle coaxial device and electrohydrodynamic flow

In this short communication, we describe the scope and flexibility of using a novel device containing three coaxially arranged needles to form a variety of novel morphologies. Different combinations of materials are subjected to controlled flow through the device under the influence of an applied electric field. The resulting electrohydrodynamic flow allows us to prepare double-layered bubbles, porous encapsulated threads and nanocapsules containing three layers. The ability to process such multilayered structures is very significant for biomedical engineering applications, for example, generating capsules for drug delivery, which can provide multistage controlled release.

Generation of multilayered structures for biomedical applications using a novel tri-needle coaxial device and electrohydrodynamic flow

In this short communication, we describe the scope and flexibility of using a novel device containing three coaxially arranged needles to form a variety of novel morphologies. Different combinations of materials are subjected to controlled flow through the device under the influence of an applied electric field. The resulting electrohydrodynamic flow allows us to prepare double-layered bubbles, porous encapsulated threads and nanocapsules containing three layers. The ability to process such multilayered structures is very significant for biomedical engineering applications, for example, generating capsules for drug delivery, which can provide multistage controlled release.