Concentration-Dependent Viscoelasticity of Poloxamer-Shelled Microbubbles.

The oscillation of shelled microbubbles during exposure to ultrasound is influenced by the mechanical properties of the shell components. The oscillation behavior of bubbles coated with various phospholipids and other amphiphiles has been studied. However, there have been few investigations of how the adsorption conditions of the shell molecules relate to the viscoelastic properties of the shell and influence the oscillation behavior of the bubbles. In the present study, we investigated the oscillation characteristics of microbubbles coated with a poloxamer surfactant, that is, Pluronic F-68, at several concentrations after the adsorption kinetics of the surfactant at the gas-water interface had reached equilibrium. The dilatational viscoelasticity of the shell during exposure to ultrasound was analyzed in the frequency domain from the attenuation characteristics of the acoustic pulses propagated in the bubble suspension. At Pluronic F-68 concentrations lower than 2.0 × 10-2 mol L-1, the attenuation characteristics typically exhibited a sharp peak. At concentrations higher than 2.0 × 10-2 mol L-1, the peak flattened. The dilatational elasticity and viscosity of the shell were estimated by fitting the theoretical model to the experimental values, which revealed that both the elasticity and viscosity increased markedly at approximately 2.0 × 10-2 mol L-1. This suggests that the adsorption properties of Pluronic F-68 strongly affect the oscillation characteristics of microbubbles of a size suitable for medical ultrasound diagnostics.

Anti-cancer activity of microbubble conjugated with Sorafenib containing liposome and IL4R-targeting peptide in kidney cancer cells.

Microbubble-based cancer treatment is a promising new approach that utilizes tiny gas-filled bubbles to deliver cancer drugs directly to tumor sites. This study aims to investigate the anti-cancer effect of the novel microbubble (MB) complex conjugated with sorafenib containing liposome and interleukin 4 receptor (IL4R) targeting peptide in kidney cancer cells. MBs were synthesized by using a solvent with an emulsion evaporation technique. To target kidney tumor cells, the produced MBs were conjugated with sorafenib (SOR) loaded liposomes and peptide ligands for (IL4RTP). The anti-cancer effect of the MB complex was accessed by WST-1 assay, confocal microscopy analysis, and western blotting analysis. The finally prepared IL4RTP (MB-Lipo(SOR)-IL4RTP) showed an average size of 1,600 nm. A498, a kidney cancer cell line that expresses IL4Rα strongly, had an uptake of the MB-Lipo(SOR)-IL4RTP when exposed to frequency ultrasonic energy. Additionally, MB-Lipo(SOR)-IL4RTP suppressed the growth of A498 cells in an IL4R-dependent manner. This cell proliferation assay results were validated by western blotting analysis of the signal transduction proteins such as FOXO3, phosphorylated Erk, total Erk, and p27. Taken together, these findings show that MB-Lipo(SOR)-IL4RTP exerts the effective targeting capacity for A498 kidney cancer cells via regulation of Erk phosphorylation as a promising ultrasound contrast and therapeutic agent for treating kidney cancers.

A comparative study on detection of polymer-shelled microbubbles by different excitation pulses.

Contrast agents are widely used in ultrasound imaging. Many imaging techniques have been developed to improve the contrast between tissue and the agents, based on the nonlinear response of microbubbles. In this study, heterodyne excitation was introduced and was compared with traditional sinusoidal signal and chirp excitation for visualizing polymer-shelled microbubbles and degassed water in a tissue-mimicking phantom. Pulse inversion technique was implemented under plane wave (PW) and focused imaging mode. Image enhancement was evaluated by contrast-to-tissue ratio (CTR) at different transmitting peak negative pressures (PNPs). Experimental results showed that heterodyne excitation had a better suppression effect on tissue signals in PW imaging. The CTR reached an approximation of 17 dB at a low peak negative pressure, which was much higher compared to other excitations. In focused wave imaging, a saturation threshold of CTR was observed for the sinusoidal wave burst and chirp excitation at high PNPs. Heterodyne excitation showed considerable contrast-to-noise ratio under both imaging modes. The response of a polymer-shelled microbubble under heterodyne excitation was simulated. Simulations suggest that in future work, specific filters are required to extract the nonlinear components, such as at the two-peak frequencies around fundamental frequency, to achieve a better image enhancement effect.

A PVDF Receiver for Acoustic Monitoring of Microbubble-Mediated Ultrasound Brain Therapy.

The real-time monitoring of spectral characteristics of microbubble (MB) acoustic emissions permits the prediction of increases in blood-brain barrier (BBB) permeability and of tissue damage in MB-mediated focused ultrasound (FUS) brain therapy. Single-element passive cavitation detectors provide limited spatial information regarding MB activity, greatly affecting the performance of acoustic control. However, an array of receivers can be used to spatially map cavitation events and thus improve treatment control. The spectral content of the acoustic emissions provides additional information that can be correlated with the bio-effects, and wideband receivers can thus provide the most complete spectral information. Here, we develop a miniature polyvinylidene fluoride (PVDF thickness = 110 µm, active area = 1.2 mm2) broadband receiver for the acoustic monitoring of MBs. The receiver has superior sensitivity (2.36-3.87 V/MPa) to those of a commercial fibre-optic hydrophone in the low megahertz frequency range (0.51-5.4 MHz). The receiver also has a wide -6 dB acceptance angle (54 degrees at 1.1 MHz and 13 degrees at 5.4 MHz) and the ability to detect subharmonic and higher harmonic MB emissions in phantoms. The overall acoustic performance of this low-cost receiver indicates its suitability for the eventual use within an array for MB monitoring and mapping in preclinical studies.

Treatment of high-salinity brine containing dissolved organic matters by vacuum membrane distillation: A fouling mitigation approach via microbubble aeration.

In this study, a laboratory-scale vacuum membrane distillation (VMD) system coupled with microbubble aeration (MBA) was developed for the treatment of high-salinity brine containing organic matters. Herein, at the beginning, feedwater only containing model organics such as humic acid (HA), bovine serum albumin (BSA) and sodium alginate (SA) was utilized to investigate the organic-fouling behavior, results indicated that the permeate flux was not affected by a thin and loose contaminated layer deposited on the membrane surface. Furthermore, dissolved organics in the feed brine inhibited the occurrence of membrane wetting due to the existence of a compact and protective crystals/organic-fouling layer, which can prevent the intrusion of scaling ions into membrane substrates. Besides, organics in the feedwater have a high tendency to adsorb on the membrane surface based on molecular dynamics simulations, thus, forming an organic-fouling layer prior to inorganic scaling. Finally, the effect of MBA on fouling alleviation was evaluated in VMD system, nearly 50% of salt precipitation from fouled membrane was effectively removed with the introduction of MBA, which can be ascribed to a combination of mechanisms, including surface shear forces and electrostatic attractions induced by microbubbles, meanwhile, about 2.2% of the total energy was only consumed, when using MBA. Together, these results demonstrated that MBA was a promising approach to alleviate membrane fouling in VMD.

Modification of the distribution of humic acid complexations by introducing microbubbles to membrane distillation process for effective membrane fouling alleviation.

Membrane fouling caused by inorganic ions and natural organic matters (NOMs) has been a severe issue in membrane distillation. Microbubble aeration (MB) is a promising technology to control membrane fouling. In this study, MB aeration was introduced to alleviate humic acid (HA) composited fouling during the treatment of simulative reverse osmosis concentrate (ROC) by vacuum membrane distillation (VMD). The objective of this work was to explore the HA fouling inhibiting effect by MB aeration and discuss its mechanism from the interfacial point of view. The results showed that VMD was effective for treating ROC, followed by a severe membrane fouling aggravated with the addition of 100 mg/L HA in feed solution, resulting in 45.7% decline of membrane flux. Analysis using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and zeta potential distribution of charged particles proved the coexistence of HA and inorganic cations (especially Ca2+), resulting in more serious membrane fouling. The introduction of MB aeration exhibited excellent alleviating effect on HA-inorganic salt fouling, with the normalized flux increased from 19.7% to 37.0%. The interfacial properties of MBs played an important role, which altered the zeta potential distributions of charged particles in HA solution, indicating that MBs adhere the HA complexations. Furthermore, this mitigating effect was limited at high inorganic cations concentration. Overall, MBs could change the potential characteristics of HA complexes, which also be used for other similar membrane fouling alleviation.

Antimicrobial and cleaning effects of ultrasonic-mediated plasma-loaded microbubbles on Enterococcus faecalis biofilm: an in vitro study.

BACKGROUND: Enterococcus faecalis (E. faecalis) is the most frequently isolated bacteria from teeth with root canal treatment failure. This study aims to evaluate the disinfection effect of ultrasonic-mediated cold plasma-loaded microbubbles (PMBs) on 7d E. faecalis biofilm, the mechanical safety and the mechanisms. METHODS: The PMBs were fabricated by a modified emulsification process and the key reactive species, nitric oxide (NO) and hydrogen peroxide (H2O2) were evaluated. The 7d E. faecalis biofilm on human tooth disk was constructed and divided into the following groups: PBS, 2.5%NaOCl, 2%CHX, and different concentrations of PMBs (108 mL-1, 107 mL-1). The disinfection effects and elimination effects were verified with confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM). Microhardness and roughness change of dentin after PMBs treatment were verified respectively. RESULTS: The concentration of NO and H2O2 in PMBs increased by 39.99% and 50.97% after ultrasound treatment (p < 0.05) respectively. The CLSM and SEM results indicate that PMBs with ultrasound treatment could remove the bacteria and biofilm components effectively, especially those living in dentin tubules. The 2.5% NaOCl presented an excellent effect against biofilm on dishes, but the elimination effect on dentin tubules is limited. The 2% CHX group exhibits significant disinfection effect. The biosafety tests indicated that there is no significant changes on microhardness and roughness after PMBs with ultrasound treatment (p > 0.05). CONCLUSION: PMBs combined with ultrasound treatment exhibited significant disinfection effect and biofilm removal effect, the mechanical safety is acceptable.

Extrusion: A New Method for Rapid Formulation of High-Yield, Monodisperse Nanobubbles.

Shell-stabilized gas microbubbles (MB) and nanobubbles (NB) are frequently used for biomedical ultrasound imaging and therapeutic applications. While it is widely recognized that monodisperse bubbles can be more effective in these applications, the efficient formulation of uniform bubbles at high concentrations is difficult to achieve. Here, it is demonstrated that a standard mini-extruder setup, commonly used to make vesicles or liposomes, can be used to quickly and efficiently generate monodisperse NBs with high yield. In this highly reproducible technique, the NBs obtained have an average diameter of 0.16 ± 0.05 µm and concentration of 6.2 ± 1.8 × 1010  NBs mL-1 compared to 0.32 ± 0.1 µm and 3.2 ± 0.7 × 1011  mL-1 for NBs made using mechanical agitation. Parameters affecting the extrusion and NB generation process including the temperature, concentration of the lipid solution, and the number of passages through the extruder are also examined. Moreover, it is demonstrated that extruded NBs show a strong acoustic response in vitro and a strong and persistent US signal enhancement under nonlinear contrast enhanced ultrasound imaging in mice. The extrusion process is a new, efficient, and scalable technique that can be used to easily produce high yield smaller monodispersed nanobubbles.

Engineering the Acoustic Response and Drug Loading Capacity of PBCA-Based Polymeric Microbubbles with Surfactants.

Gas-filled microbubbles (MB) are routinely used in the clinic as ultrasound contrast agents. MB are also increasingly explored as drug delivery vehicles based on their ultrasound stimuli-responsiveness and well-established shell functionalization routes. Broadening the range of MB properties can enhance their performance in both imaging and drug delivery applications. This can be promoted by systematically varying the reagents used in the synthesis of MB, which in the case of polymeric MB include surfactants. We therefore set out to study the effect of key surfactant characteristics, such as the chemical structure, molecular weight, and hydrophilic-lipophilic balance on the formation of poly(butyl cyanoacrylate) (PBCA) MB, as well as on their properties, including shell thickness, drug loading capacity, ultrasound contrast, and acoustic stability. Two different surfactant families (i.e., Triton X and Tween) were employed, which show opposite molecular weight vs hydrophilic-lipophilic balance trends. For both surfactant types, we found that the shell thickness of PBCA MB increased with higher-molecular-weight surfactants and that the resulting MB with thicker shells showed higher drug loading capacities and acoustic stability. Furthermore, the higher proportion of smaller polymer chains of the Triton X-based MB (as compared to those of the Tween-based ones) resulted in lower polymer entanglement, improving drug loading capacity and ultrasound contrast response. These findings open up new avenues to fine-tune the shell properties of polymer-based MB for enhanced ultrasound imaging and drug delivery applications.

The Influence of Nanobubble Size and Stability on Ultrasound Enhanced Drug Delivery.

Lipid-shelled nanobubbles (NBs) are emerging as potential dual diagnostic and therapeutic agents. Similar to their micron-scale counterparts, microbubbles (1-10 µm), they can act as ultrasound contrast agents as well as locally enhance therapeutic uptake. Recently, it has been shown that the reduced size of NBs (<1 µm) promotes increased uptake and accumulation in tumor interstitial space, which can enhance their diagnostic and therapeutic performance. However, accurate characterization of NB size and concentration is challenging and may limit their translation into clinical use. Their submicron nature limits accuracy of conventional microscopy techniques, while common light scattering techniques fail to distinguish between subpopulations present in NB samples (i.e., bubbles and liposomes). Due to the difficulty in the characterization of NBs, relatively little is known about the influence of size on their therapeutic performance. In this study, we describe a novel method of using a commercially available nanoparticle tracking analysis system, to distinguish between NBs and liposomes based on their differing optical properties. We used this technique to characterize three NB populations of varying size, isolated via centrifugation, and subsequently used this to assess their potential for enhancing localized delivery. Confocal fluorescence microscopy and image analysis were used to quantify the ultrasound enhanced uptake of fluorescent dextran into live colorectal cancer cells. Our results showed that the amount of localized uptake did not follow the expected trends, in which larger NB populations out-perform smaller NBs, at matched concentration. To understand this observed behavior, the stability of each NB population was assessed. It was found that dilution of the NB samples from their stock concentration influences their stability, and it is hypothesized that both the total free lipid and interbubble distance play a role in NB lifetime, in agreement with previously proposed theories and models.

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.

Use of Nitric Oxide Donor-Loaded Microbubble Destruction by Ultrasound in Thrombus Treatment.

In the presence of a vascular thrombus, the recovery of blood flow and vascular recanalization are very important to prevent tissue damage. An alternative procedure to thrombolysis is required for patients who are unable to receive surgery or thrombolytic drugs due to other physical conditions. Recently, the performance of thrombolysis combined with microbubbles has become an attractive and effective therapeutic procedure. Indeed, in a recent study, we demonstrated that, upon exposure to ultrasound, liposomes loaded with nitric oxide release agonists conjugated to microbubbles; therefore, there is potential to release the agonist in a controlled manner into specific tissues. This means that the effect of the agonist is potentiated, decreasing interactions with other tissues, and reducing the dose required to induce nitric-oxide-dependent vasodilation. In the present study, we hypothesized that a liposome microbubble delivery system can be used as a hydrophilic agonist carrier for the nitric oxide donor spermine NONOate, to elicit femoral vasodilation and clot degradation. Therefore, we used spermine-NONOate-loaded microbubbles to evaluate the effect of ultrasound-mediated microbubble disruption (UMMD) on thromboembolic femoral artery recanalization. We prepared spermine NONOate-loaded microbubbles and tested their effect on ex vivo preparations, hypothesizing that ultrasound-induced microbubble disruption is associated with the vasorelaxation of aortic rings. Thrombolysis was demonstrated in aorta blood-flow recovery after disruption by spermine NONOate-loaded microbubbles via ultrasound application in the region where the thrombus is located. Our study provides an option for the clinical translation of NO donors to therapeutic applications.

Fluorinated Copolypeptide-Stabilized Microbubbles with Maleimide-Decorated Surfaces as Long-Term Ultrasound Contrast Agents.

Gas-filled microbubbles (MBs) have been clinically used as ultrasound (US) contrast agents for disease diagnosis and treatment. However, it remains a great challenge to resolve the dilemma of stability and contrast enhancement of MBs. Herein, amphiphilic copolypeptides bearing fluorinated blocks are synthesized to stabilize perfluorocarbon (PFC)-filled MBs, exhibiting unique stability under both long-term storage and US imaging conditions. The fluorinated inner layer reduces the internal Laplace pressure and greatly improves the stability of MBs, which can be further reinforced by crosslinking of the dipropargyl-containing middle blocks. To overcome the suppressed nonlinear oscillation of polymer shells, maleimide groups are introduced onto the surface of MBs, enabling in situ reaction with plasma proteins to enhance second harmonic signals without compromising the stability of MBs, conferring better US imaging performance than that of SonoVueTM MBs.

Biomimetic PLGA Microbubbles Coated with Platelet Membranes for Early Detection of Myocardial Ischaemia-Reperfusion Injury.

Early diagnosis of myocardial ischaemia-reperfusion (MI/R) injury is important for protecting the myocardium and improving patient prognoses. Fortunately, the platelet membrane possesses the ability to target the region of MI/R injury. Therefore, we hypothesized that platelet membrane-coated particles (PMPs) could be used to detect early MI/R injury by ultrasound imaging. We designed PMPs with a porous polylactic-co-glycolic acid (PLGA) core coated with a platelet membrane shell. Red blood cell membrane-coated particles (RMPs) were fabricated as controls. Transmission electron microscopy (TEM) and fluorescence microscopy were applied to confirm the membrane coatings of the PMPs and RMPs. In vitro imaging of the PMPs and RMPs was verified. Moreover, binding experiments were designed to examine the targeting ability of the PMPs. Finally, we assessed the signal intensity of the adherent PMPs in the risk area and remote area by ultrasound imaging based on an MI/R rat model. The platelet membrane equipped the PMPs with an accurate targeting ability. Compared with RMPs, PMPs showed significantly more adhesion to human umbilical vein endothelial cells and collagen IV in vitro. Both PMPs and RMPs exhibited good enhancement ability in vitro and in vivo. Furthermore, the signal intensity of PMPs in the risk area was significantly higher than that in remote areas. These results were further validated by an immunofluorescence assay and ex vivo fluorescence imaging. In summary, ultrasound imaging with PMPs can detect early MI/R injury in a noninvasive manner.

Deriving acoustic properties for perfluoropentane droplets with viscoelastic cellulose nanofiber shell via numerical simulations.

Perfluoropentane droplets with cellulose nanofibers (CNF) shells have demonstrated better stability and easier surface modification as ultrasound contrast agents and drug delivery vehicles. This paper presents a theoretical model assuming a four-phase state "inverse antibubble," with the core filled with gas perfluoropentane surrounded by liquid perfluoropentane. A continuous, incompressible, and viscoelastic stabilizing layer separates the core from the surrounding water. A parametric study is performed to predict the frequency-dependent attenuation coefficient, the speed of sound, and the resonance frequency of the droplets which have a mean diameter of 2.47 ± 0.95 µm. Results reveal that the CNF-stabilized perfluoropentane droplets can be modeled in a Rayleigh-Plesset like equation. We conclude that the shell strongly influences the acoustic behavior of the droplets and the resonance frequency largely depends on the initial gas cavity radius. More specifically, the peak attenuation coefficient and peak-to-peak speed of sound decrease with increasing shear modulus, shear viscosity, and shell thickness, while they increase with increasing gas cavity radius and concentration. The resonance frequency increases as shear modulus and shell thickness increase, while it decreases as shear viscosity and gas cavity radius increase. It is worth mentioning that droplet concentration has no effect on the resonance frequency.

Size and Refractive Index Determination of Subwavelength Particles and Air Bubbles by Holographic Nanoparticle Tracking Analysis.

Determination of size and refractive index (RI) of dispersed unlabeled subwavelength particles is of growing interest in several fields, including biotechnology, wastewater monitoring, and nanobubble preparations. Conventionally, the size distribution of such samples is determined via the Brownian motion of the particles, but simultaneous determination of their RI remains challenging. This work demonstrates nanoparticle tracking analysis (NTA) in an off-axis digital holographic microscope (DHM) enabling determination of both particle size and RI of individual subwavelength particles from the combined information about size and optical phase shift. The potential of the method to separate particle populations is demonstrated by analyzing a mixture of three types of dielectric particles within a narrow size range, where conventional NTA methods based on Brownian motion alone would fail. Using this approach, the phase shift allowed individual populations of dielectric beads overlapping in either size or RI to be clearly distinguished and quantified with respect to these properties. The method was furthermore applied for analysis of surfactant-stabilized micro- and nanobubbles, with RI lower than that of water. Since bubbles induce a phase shift of opposite sign to that of solid particles, they were easily distinguished from similarly sized solid particles made up of undissolved surfactant. Surprisingly, the dependence of the phase shift on bubble size indicates that only those with 0.15-0.20 µm radius were individual bubbles, whereas larger bubbles were actually clusters of bubbles. This label-free means to quantify multiple parameters of suspended individual submicrometer particles offers a crucial complement to current characterization strategies, suggesting broad applicability for a wide range of nanoparticle systems.

Drug Loading in Poly(butyl cyanoacrylate)-Based Polymeric Microbubbles.

Microbubbles (MB) are routinely used ultrasound (US) contrast agents that have recently attracted increasing attention as stimuli-responsive drug delivery systems. To better understand MB-based drug delivery, we studied the role of drug hydrophobicity and molecular weight on MB loading, shelf-life stability, US properties, and drug release. Eight model drugs, varying in hydrophobicity and molecular weight, were loaded into the shell of poly(butyl cyanoacrylate) (PBCA) MB. In the case of drugs with progesterone as a common structural backbone (i.e., for corticosteroids), loading capacity and drug release correlated well with hydrophobicity and molecular weight. Conversely, when employing drugs with no structural similarity (i.e., four different fluorescent dyes), loading capacity and release did not correlate with hydrophobicity and molecular weight. All model drug-loaded MB formulations could be equally efficiently destroyed upon exposure to US. Together, these findings provide valuable insights on how the physicochemical properties of (model) drug molecules affect their loading and retention in and US-induced release from polymeric MB, thereby facilitating the development of drug-loaded MB formulations for US-triggered drug delivery.

Multi-scale microporous silica microcapsules from gas-in water-in oil emulsions.

Controlling the surface area, pore size and pore volume of microcapsules is crucial for modulating their activity in applications including catalytic reactions, delivery strategies or even cell culture assays, yet remains challenging to achieve using conventional bulk techniques. Here we describe a microfluidics-based approach for the formation of monodisperse silica-coated micron-scale porous capsules of controllable sizes. Our method involves the generation of gas-in water-in oil emulsions, and the subsequent rapid precipitation of silica which forms around the encapsulated gas bubbles resulting in hollow silica capsules with tunable pore sizes. We demonstrate that by varying the gas phase pressure, we can control both the diameter of the bubbles formed and the number of internal bubbles enclosed within the silica microcapsule. Moreover, we further demonstrate, using optical and electron microscopy, that these silica capsules remain stable under particle drying. Such a systematic manner of producing silica-coated microbubbles and porous microparticles thus represents an attractive class of biocompatible material for biomedical and pharmaceutical related applications.

In-vitro sonothrombolysis using thick-shelled polymer microbubbles - a comparison with thin-shelled microbubbles.

BACKGROUND: Vascular thrombosis can be treated pharmacologically, however, serious shortcomings such as bleeding may occur. Several studies suggest that sonothrombolysis can induce lysis of the clots using ultrasound. Moreover, intravenously injected thin-shelled microbubbles (MBs) combined with ultrasound can further improve clot lysis. Thick-shelled MBs have been used for drug delivery, targeting and multimodal imaging. However, their capability to enhance sonothrombolysis is unknown. In this study, using an in-vitro set-up, the enhancement of clot lysis using ultrasound and thick-shelled MBs was investigated. Thin-shelled MBs was used for comparison. METHOD: The main components in the in-vitro set-up was a vessel mimicking phantom, a pressure mearing system and programmable ultrasound machine. Blood clots were injected and entrapped on a pore mesh in the vessel phantom. Four different protocols for ultrasound transmission and MB exposure (7 blood clots/protocol) were considered together with a control test were no MBs and ultrasound were used. For each protocol, ultrasound exposure of 20 min was used. The upstream pressure of the partially occluded mesh was continuously measured to assess clot burden. At the end of each protocol blood clots were removed from the phantom and the clot mass loss was computed. RESULTS: For the thick-shelled MBs no difference in clot mass loss compared with the control tests was found. A 10% increase in the clot mass loss compared with the control tests was found when using thin-shelled MBs and low pressure/long pulses ultrasound exposure. Similarly, in terms of upstream pressure over exposure time, no differences were found when using the thick-shelled MBs, whereas thin-shelled MBs showed a 15% decrease achieved within the first 4 min of ultrasound exposure. CONCLUSION: No increase in clot lysis was achieved using thick-shelled MBs as demonstrated by no significant change in clot mass or upstream pressure. Although thick-shelled MBs are promising for targeting and drug delivery, they do not enhance clot lysis when considering the ultrasound sequences used in this study. On the other hand, ultrasound in combination with thin-shelled MBs can facilitate thrombolysis when applying long ultrasound pulses with low pressure.

Closed-loop control of targeted ultrasound drug delivery across the blood-brain/tumor barriers in a rat glioma model.

Cavitation-facilitated microbubble-mediated focused ultrasound therapy is a promising method of drug delivery across the blood-brain barrier (BBB) for treating many neurological disorders. Unlike ultrasound thermal therapies, during which magnetic resonance thermometry can serve as a reliable treatment control modality, real-time control of modulated BBB disruption with undetectable vascular damage remains a challenge. Here a closed-loop cavitation controlling paradigm that sustains stable cavitation while suppressing inertial cavitation behavior was designed and validated using a dual-transducer system operating at the clinically relevant ultrasound frequency of 274.3 kHz. Tests in the normal brain and in the F98 glioma model in vivo demonstrated that this controller enables reliable and damage-free delivery of a predetermined amount of the chemotherapeutic drug (liposomal doxorubicin) into the brain. The maximum concentration level of delivered doxorubicin exceeded levels previously shown (using uncontrolled sonication) to induce tumor regression and improve survival in rat glioma. These results confirmed the ability of the controller to modulate the drug delivery dosage within a therapeutically effective range, while improving safety control. It can be readily implemented clinically and potentially applied to other cavitation-enhanced ultrasound therapies.