Role of Surface Tension in Gas Nanobubble Stability Under Ultrasound

Ultrasound protein-copolymer microbubble library engineering through poly(vinylpyrrolidone-co-acrylic acid) structure

HYPOTHESIS

While albumin-coated microbubbles are routine contrast agents for ultrasound imaging, their short duration of contrast enhancement limits their use, yet can be improved by incorporating protein-copolymer hybrids into microbubble shells. The incorporation of N-vinyl-2-pyrrolidone and acrylic acid copolymer (P(VP-AA)) has been shown to enhance the performance of bovine serum albumin (BSA)-coated microbubbles. However, the impact of the copolymer structural properties on key microbubble characteristics (i.e., concentration, mean diameter and acoustic response) remains poorly understood. Therefore, we hypothesize that the copolymer structure affects its capacity to form micelle-like nanoaggregates, protein-copolymer hybrids, and microbubble shells, ultimately influencing the physicochemical and acoustic properties of the microbubbles.

EXPERIMENTS

Here we evaluate the production and performance of BSA@P(VP-AA) microbubbles synthesized using a series of P(VP-AA) copolymers with -C8H17 and -C18H37 end groups and molecular weight cutoffs between 3.5 and 15 kDa. Both simulation and experimental data demonstrate that interactions between BSA and the copolymers significantly influence the performance of the resulting microbubbles across the library of 60 formulations.

FINDINGS

The introduction of -C8H17 terminated copolymers into microbubble shells resulted in up to 200-fold higher concentration, 7-fold greater acoustic response, and 5-fold longer ultrasound contrast enhancement compared to plain BSA microbubbles. The enhanced acoustic performance was sustained during in vivo cardiac ultrasound imaging, without altering liver accumulation after copolymer introduction. These findings underscore how optimizing copolymer structure (specifically the terminal end group and molecular weight) can tailor the formation and performance of protein-copolymer-coated microbubbles, offering valuable insights for designing ultrasound contrast agents.

CsxWO3@NBs as a Multi-Image Guided Photothermal/Photodynamic Combination Therapy Platform for the Treatment of Hepatocellular Carcinoma

Purpose

Effective cancer treatment relies on the precise deployment of clinical imaging techniques to accurately treat tumors. One highly representative technology among these is multi-imaging guided phototherapy. This work introduces a new and innovative theranostic drug that combines near-infrared (NIR) irradiation-induced photodynamic therapy (PDT) and photothermal therapy (PTT) to treat malignancies. Moreover, it can be utilized as a contrasting substance for X-ray computed tomography (CT) imaging and contrast-enhanced ultrasound (CEUS) to aid in the administration of therapy.

Methods

Cesium tungsten bronze nanobubbles (CsxWO3@NBs) were constructed via a water-controlled solvothermal synthesis and thin film hydration of phospholipid. Various methods, including dynamic light scattering, transmission electron microscopy, and X-ray photoelectron spectroscopy, were used to analyze and describe the size, shape, and chemical characteristics of the nanoparticles. In this study, hepatoma cell lines HepG2 and HUH7 were employed in vitro, and xenotransplantation mouse models were used to assess their antitumor effects. A series of in vitro and in vivo trials were conducted to assess the effectiveness of combining photodynamic and photothermal therapies, as well as using CEUS and CT imaging.

Results

The CsxWO3@NBs exhibit photothermal effects and the generation of reactive oxygen species (ROS) under laser irradiation, thereby enabling effective photothermal and photodynamic combinatorial therapy. Following combined treatment, the activity and invasive capacity of hepatocellular carcinoma cells were markedly diminished, the development rate of the tumor was noticeably reduced, and the level of biological toxicity was low. Additionally, CsxWO3@NBs possess the capacity to serve as both a CT imaging agent and a contrast-enhanced ultrasound agent.

Conclusion

CsxWO3@NBs represent a promising theranostic agent for image-guided cancer therapy.

Heterogeneous Kinetics of Nanobubble Ultrasound Contrast Agent and Angiogenic Signaling in Head and Neck Cancer

Recently developed nanobubble ultrasound contrast agents are a promising tool for imaging and drug delivery in tumors. To better understand their unusual kinetics, we implemented a novel pixel clustering analysis, which provides unique information by accounting for spatial heterogeneity. By combining ultrasound results with proteomics of the imaged tumors, we show that this analysis is highly predictive of protein expression and that specific types of nanobubble time-intensity curve are associated with upregulation of different metabolic pathways. We applied this method to study the effects of two proteins, EphB4 and ephrinB2, which control tumor angiogenesis through bidirectional juxtacrine signaling, in mouse models of head and neck cancer. We show that ephrinB2 expression by endothelial cells and EphB4 expression by cancer cells have similar effects on tumor vasculature, despite sometimes opposite effects on tumor growth. This implicates a cancer-cell-intrinsic effect of EphB4 forward signaling and not angiogenesis in EphB4's action as a tumor suppressor.

Interfacial Thermal Fluctuations Stabilize Bulk Nanobubbles

Consensus on bulk nanobubble stability remains elusive, despite accepted indirect evidence for longevity. We develop a nanobubble evolution model by incorporating thermal capillary wave theory that reveals that dense nanobubbles generated by acoustic cavitation tend to shrink and intensify interfacial thermal fluctuations; this significantly reduces surface tension to neutralize enhanced Laplace pressure, and secures their stabilization at a finite size. A stability criterion emerges: thermal fluctuation intensity scales superlinearly with curvature: sqrt[⟨h^{2}⟩]∝(1/R)^{n}, n>1. The model prolongs the time frame for nanobubble contraction to 2 orders of magnitude beyond classical theory estimates, and captures the equilibrium radius (90-215 nm) within the experimental range.

A Comprehensive Review: Versatile Imaging Probe Based on Chemical Materials for Biomedical Applications

Imaging probe and contrast agents play significant role in combating cancer. Based on special chemical materials, imaging probe can convert cancer symptoms into information-rich images with high sensitivity and signal amplification, accompanying with detection, diagnosis, drug delivery and treatment. In the paper, some inorganic and organic chemical materials as imaging probe, including Ultrasound imaging (US), Optical imaging (OP), Photoacoustic imaging (PA), X-ray Computed Tomography (CT), Magnetic Resonance imaging (MRI), Radionuclide imaging (RNI) probe, as well as multi-modality imaging probe for diagnosis and therapy of tumour were introduced. The sophisticated and comprehensive chemical materials as imaging probe were highlighted in detail. Meanwhile, the advantages and disadvantages of the imaging probe were compared. In order to provide some reference and help researchers for construction imaging probe for tumour diagnosis and treatment, it attempts to exhaustively cover the whole field. Finally, the prospect and challenge for imaging probe were discussed.

Influence of the liquid ionic strength on the resonance frequency and shell parameters of lipid-coated microbubbles

The correct measurement of the resonance frequency and shell properties of coated microbubbles (MBs) is essential in understanding and optimizing their response to ultrasound (US) exposure parameters. In diagnostic and therapeutic ultrasound, MBs are typically surrounded by blood; however, the influence of the medium charges on the MB resonance frequency has not been systematically studied using controlled measurements. This study aims to measure the medium charge interactions on MB behavior by measuring the frequency-dependent attenuation of the same size MBs in mediums with different charge densities. In-house lipid-coated MBs with C3F8 gas core were formulated. The MBs were isolated to a mean size of 2.35 μm using differential centrifugation. MBs were diluted to ≈8×105 MBs/mL in distilled water (DW), Phosphate-Buffered Saline solution (PBS1x) and PBS10x. The frequency-dependent attenuation of the MBs solutions was measured using an aligned pair of PVDF transducers with a center frequency of 10MHz and 100% bandwidth in the linear oscillation regime (7 kPa pressure amplitude). The MB shell properties were estimated by fitting the linear equation to experiments. Using a pendant drop tension meter, the surface tension at the equilibrium of ≈6 mm diameter size drops of the same MB shell was measured inside DW, PBS1x and PBS10x. The surface tension at the C3F8/solution interface was estimated by fitting the Young-Laplace equation from the recorded images. The frequency of the peak attenuation at different salinity levels was 13, 7.5 and 6.25 MHz in DW, PBS1x and PBS-10x, respectively. The attenuation peak increased by ≈140% with increasing ion density. MBs' estimated shell elasticity decreased by 64% between DW and PBS-1x and 36% between PBS-1x and PBS-10x. The drop surface tension reduced by 10.5% between DW and PBS-1x and by 5% between PBS-1x and PBS-10x, respectively. Reduction in the shell stiffness is consistent with the drop surface tension measurements. The shell viscosity was reduced by ≈40% between DW and PBS-1x and 42% between PBS-1x and PBS-10x. The reduction in the fitted stiffness and viscosity is possibly due to the formation of a densely charged layer around the shell, further reducing the effective surface tension on the MBs. The changes in the resonance frequency and estimated shell parameters were significant and may potentially help to better understand and explain bubble behavior in applications.

Life at the interface: Engineering bio-nanomaterials through interfacial molecular self-assembly

Interfacial self-assembly describes the directed organization of molecules and colloids at phase boundaries. Believed to be fundamental to the inception of primordial life, interfacial assembly is exploited by a myriad of eukaryotic and prokaryotic organisms to execute physiologic activities and maintain homeostasis. Inspired by these natural systems, chemists, engineers, and materials scientists have sought to harness the thermodynamic equilibria at phase boundaries to create multi-dimensional, highly ordered, and functional nanomaterials. Recent advances in our understanding of the biophysical principles guiding molecular assembly at gas-solid, gas-liquid, solid-liquid, and liquid-liquid interphases have enhanced the rational design of functional bio-nanomaterials, particularly in the fields of biosensing, bioimaging and biotherapy. Continued development of non-canonical building blocks, paired with deeper mechanistic insights into interphase self-assembly, holds promise to yield next generation interfacial bio-nanomaterials with unique, and perhaps yet unrealized, properties. This article is categorized under: Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Microbubble Biointerfacing by Regulation of the Platelet Membrane Surfactant Activity at the Gas-Liquid Interface for Acute Thrombosis Targeting

Biointerfacing nanomaterials with cell membranes has been successful in the functionalization of nanoparticles or nanovesicles, but microbubble functionalization remains challenging due to the unique conformation of the lipid monolayer structure at the gas-liquid interface that provides insufficient surfactant activity. Here, we describe a strategy to rationally regulate the surfactant activity of platelet membrane vesicles by adjusting the ratio of proteins to lipids through fusion with synthetic phospholipids (i.e., liposomes). A "platesome" with the optimized protein-to-lipid ratio can be assembled at the gas-liquid interface in the same manner as pulmonary surfactants to stabilize a microsized gas bubble. Platesome microbubbles (PMBs) inherited 61.4 % of the platelet membrane vesicle proteins and maintained the active conformation of integrin αIIbβ3 without the talin 1 for fibrin binding. We demonstrated that the PMBs had good stability, long circulation, and superior functionality both in vitro and in vivo. Moreover, by molecular ultrasound imaging, the PMBs provide up to 11.8 dB of ultrasound signal-to-noise ratio enhancement for discriminating between acute and chronic thrombi. This surface tension regulating strategy may provide a paradigm for biointerfacing microbubbles with cell membranes, offering a potential new approach for the construction of molecular ultrasound contrast agents for the diagnosis of different diseases.

High throughput microfluidic nanobubble generation by microporous membrane integration and controlled bubble shrinkage

Microfluidics has recently been proposed as a viable method for producing bulk nanobubbles for use in various applications. The portability, compact size, and capacity to precisely control fluids on a small scale are a few of the benefits of microfluidics that may be exploited to create customized bulk nanobubbles. However, despite the potential of microfluidic nanobubble generation, low throughput and limited nanobubble concentration remain challenging for microfluidics. Here, we integrate a microporous silicon membrane into a polydimethylsiloxane microfluidic chip to generate bulk nanobubbles in the 100-140 nm diameter range with a concentration of up to 108 mL-1. We investigate the nanobubble size and morphology using several characterisation techniques, including transmission electron microscopy, resonance mass measurement, dynamic light scattering, and the Tyndall effect. This new nanobubble generation technique can increase nanobubble concentration by ∼ 23 times compared to earlier microfluidic nanobubble generation platforms, which should increase the feasibility of translation to medical applications.

Novel Nanoprobe with Combined Ultrasonography/Chemical Exchange Saturation Transfer Magnetic Resonance Imaging for Precise Diagnosis of Tumors

Given that cancer mortality is usually due to a late diagnosis, early detection is crucial to improve the patient's results and prevent cancer-related death. Imaging technology based on novel nanomaterials has attracted much attention for early-stage cancer diagnosis. In this study, a new block copolymer, poly(ethylene glycol)-poly(l-lactide) diblock copolymer (PEG-PLLA), was synthesized by the ring-opening polymerization method and thoroughly characterized using Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance spectroscopy (H-NMR), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The obtained PEG-PLLA was used to prepare nanoparticles encapsulated with perfluoropentane and salicylic acid by the emulsion-solvent evaporation method, resulting in a new dual-mode nano-image probe (PEG-PLLA@SA·PFP). The zeta potential and mean diameter of the obtained nanoparticles were measured using dynamic light scattering (DLS) with a Malvern Zetersizer Nano. The in vitro biocompatibility of the PEG-PLLA nanoparticles was evaluated with cell migration, hemolysis, and cytotoxicity assays. Ultrasonic imaging was performed using an ultrasonic imaging apparatus, and chemical exchange saturation transfer (CEST) MRI was conducted on a 7.0 T animal scanner. The results of IR and NMR confirmed that the PEG-PLLA was successfully synthesized. The particle size and negative charge of the nanoparticles were 223.8 ± 2.5 nm and -39.6 ± 1.9 mV, respectively. The polydispersity of the diameter was 0.153 ± 0.020. These nanoparticles possessed good stability at 4 °C for about one month. The results of cytotoxicity, cell migration, and hemolysis assays showed that the carrier material was biocompatible. Finally, PEG-PLLA nanoparticles were able to significantly enhance the imaging effect of tumors by the irradiation of ultrasound and saturation by a radiofrequency pulse, respectively. In conclusion, these nanoparticles exhibit promising dual-mode capabilities for US/CEST MR imaging.

Monitoring effects of hydrodynamic cavitation pretreatment of sodium oleate on the aggregation of fine diaspore particles through small-angle laser scattering

Hydrodynamic cavitation (HC) enhanced fine particle aggregation could be largely due to the generation of tiny bubbles and their role in bridging particles. However, the lack of adequate characterizations of aggregates severally limits our further understanding of the associated aggregation behaviors. In this study, the aggregation of fine diaspore particles was comparatively investigated in sodium oleate (NaOl) solutions with and without HC pretreatment through the small-angle laser scattering (SALS) technique in a shear-induced aggregation (SIA) system. Results showed that HC pretreatment caused the formation of bulk nanobubbles (BNBs), which significantly modified the particle interactions and thereby modified the size and mass fractal dimension (Df) of aggregates under different SIA conditions. Although HC pretreatment did not noticeably alter the gradual change trend of aggregate size and structure characteristics under specific variables, BNBs bridging facilitated the aggregation process towards the diffusion-limited cluster aggregation model, resulting in the formation of larger but looser aggregates. This effect was more pronounced under relatively high NaOl concentrations. Apart from BNBs, the aggregation was also affected by cavitation bubbles formed during shear cavitation, which was more significant under high stirring intensity conditions (i.e., 1800 rpm) than the low stirring intensity conditions (i.e., 600 rpm).

Visually controlled pulsatile release of insulin from chitosan poly-acrylic acid nanobubbles triggered by focused ultrasound

Insulin therapy is the most effective way to control the blood glucose value of diabetic patients. The most effective administration route for insulin is subcutaneous injection because bioavailability for non-injection administration is low and unstable. However, patients often need a multiple daily insulin injection regimen to control basal and postprandial blood glucose, which causes various complications. Controlled pulsatile drug release technology using ultrasound as an external stimulus source is a very promising method to avoid multiple injections of insulin. However, most of the drug-loaded microbubbles used for ultrasound-mediated treatment have a short half-life, which limits their use in controlled pulsatile drug release. More importantly, how to control insulin release is still a challenge. In this paper, chitosan poly-acrylic acid nanobubbles as drug carriers of insulin were prepared to achieve a visually controlled pulsatile release of insulin triggered by focused ultrasound. The experimental results in vivo demonstrated that nanobubbles were stable enough to achieve long-term visualization for 7 days after intramuscular injection in rats. Under the guidance of ultrasound imaging, it is visible to find the position and observe the gray values change of nanobubbles. Thus, when triggered by focused ultrasound, the amount of insulin could be accurately pulsatile released from nanobubbles. In vivo experiments in rats showed that the visually controlled pulsatile release of insulin could be achieved for a long time, up to 3 consecutive days. The blood glucose level could be repeatedly reduced by focused ultrasound irradiation with just one injection. Our research provided a promising way for visually controlled pulsatile release of insulin, which would significantly reduce the injection frequency of insulin.

Clean production and characterization of nanobubbles using laser energy deposition

We have demonstrated the production of laser bulk nanobubbles (BNB) with ambient radii typically below 500 nm. The gaseous nature of the nanometric objects was confirmed by a focused acoustic pulse that expands the gas cavities to a size that can be visualized with optical microscopy. The BNBs were produced on demand by a collimated high-energy laser pulse in a "clean" way, meaning that no solid particles or drops were introduced in the sample by the generation method. This is a clear advantage relative to the other standard BNB production techniques. Accordingly, the role of nanometric particles in laser bubble production is discussed. The characteristics of the nanobubbles were evaluated with two alternative methods. The first one measures the response of the BNBs to acoustic pulses of increasing amplitude to estimate their rest radius through the calculation of the dynamics Blake threshold. The second one is based on the bubble dissolution dynamics and the correlation of the bubble's lifetime with its initial size. The high reproducibility of the present system in combination with automated data acquisition and analysis constitutes a sound tool for studying the effects of the liquid and gas properties on the stability of the BNBs solution.

A Study of Mechanisms of Nanobubble-Enhanced Flotation of Graphite

This study was conducted to investigate the mechanisms of enhanced microfine flake graphite (MFG) flotation by nanobubbles generated based on the principle of hydrodynamic cavitation. The effects of nanobubbles on graphite surface properties were characterized in terms of the flotation kinetics, collector adsorption behavior, Zeta potential, IR spectra, contact angle, etc. The results show that the surface nanobubbles increased the hydrophobic attraction and reduced the electrostatic repulsion between the graphite particles and collector molecules, significantly improving the flotation selectivity and the kinetic flotation rate and promoting the agglomeration of MFG.

Ultrasound-assisted fabrication of biopolymer materials: A review

There is an urgent need to develop technologies that can physically manipulate the structure of biocompatible and green polymer materials in order to tune their performance in an efficient, repeatable, easy-to-operate, chemical-free, non-contact, and highly controllable manner. Ultrasound technology produces a cavitation effect that promotes the generation of free radicals, the fracture of chemical chain segments and a rapid change of morphology. The cavitation effects are accompanied by thermal, chemical, and biological effects that interact with the material being studied. With its high efficiency, cleanliness, and reusability applications, ultrasound has a vast range of opportunity within the field of natural polymer-based materials. This work expounds the basic principle of ultrasonic cavitation and analyzes the influence that ultrasonic strength, temperature, frequency and induced liquid surface tension on the physical and chemical properties of biopolymer materials. The mechanism and the influence that ultrasonic modification has on materials is discussed, with highlighted details on the agglomeration, degradation, morphology, structure, and the mechanical properties of these novel materials from naturally derived polymers.

Hemodynamic Mimic Shear Stress for Platelet Membrane Nanobubbles Preparation and Integrin αIIbβ3 Conformation Regulation

Platelet (PLT) membrane biomimetic nanomaterials have become promising theranostic platforms due to their good biocompatibility and effectiveness. However, in order to achieve precise regulation of cell membrane components, novel controllable construction approaches need to be developed. Inspired by the interaction mechanism among platelet production, activation, and dynamic biomechanical signals in blood circulation, here a platelet nanobubbles (PNBs) with reassembled platelet membrane with ideal echogenicity was fabricated using an adjustable pressure-induced shear stress method. The results demonstrate that the high shear stress during PNBs fabrication led to the enrichment of platelet membrane lipid rafts and proteins, as well as their reassembly on the gas-liquid interface. More importantly, the conformation of platelet integrin α_IIbβ₃ was transformed into a shear stress-induced intermediate affinity state, which gives PNBs enhanced adhesion ability to the vascular endothelial injury. Taken together, these PNBs have great application potential in the specifically targeted ultrasound diagnosis of vascular endothelial injury.

Effects of shell-integrated Sudan Black dye on the acoustic activity and ultrasound imaging properties of lipid-shelled nanoscale ultrasound contrast agents

SIGNIFICANCE

An effective contrast agent for concurrent multimodal photoacoustic (PA) and ultrasound (US) imaging must have both high optical absorption and high echogenicity. Integrating a highly absorbing dye into the lipid shell of gas core nanobubbles (NBs) adds PA contrast to existing US contrast agents but may impact agent ultrasonic response.

AIM

We report on the development and ultrasonic characterization of lipid-shell stabilized C3F8 NBs with integrated Sudan Black (SB) B dye in the shell as dual-modal PA-US contrast agents.

APPROACH

Perfluoropropane NBs stabilized with a lipid shell including increasing concentrations of SB B dye were formulated by amalgamation (SBNBs). Physical properties of SBNBs were characterized using resonant mass measurement, transmission electron microscopy and pendant drop tensiometry. Concentrated bubble solutions were imaged for 8 min to assess signal decay. Diluted bubble solutions were stimulated by a focused transducer to determine the response of individual NBs to long cycle (30 cycle) US. For assessment of simultaneous multimodal contrast, bulk populations of SBNBs were imaged using a PA and US imaging platform.

RESULTS

We produced high agent yield (∼1011) with a mean diameter of ∼200 to 300 nm depending on SB loading. A 40% decrease in bubble yield was measured for solutions with 0.3 and 0.4  mg  /  ml SB. The addition of SB to the shell did not substantially affect NB size despite an increase in surface tension by up to 8  mN  /  m. The bubble decay rate increased after prolonged exposure (8 min) by dyed bubbles in comparison to their undyed counterparts (2.5-fold). SB in bubble shells increased gas exchange across the shell for long cycle US. PA imaging of these agents showed an increase in power (up to 10 dB) with increasing dye.

CONCLUSIONS

We added PA contrast function to NBs. The addition of SB increased gas exchange across the NB shell. This has important implications in their use as multimodal agents.

SonoVue® vs. Sonazoid™ vs. Optison™: Which Bubble Is Best for Low-Intensity Sonoporation of Pancreatic Ductal Adenocarcinoma?

The use of ultrasound and microbubbles to enhance therapeutic efficacy (sonoporation) has shown great promise in cancer therapy from in vitro to ongoing clinical studies. The fastest bench-to-bedside translation involves the use of ultrasound contrast agents (microbubbles) and clinical diagnostic scanners. Despite substantial research in this field, it is currently not known which of these microbubbles result in the greatest enhancement of therapy within the applied conditions. Three microbubble formulations-SonoVue^®, Sonazoid™, and Optison™-were physiochemically and acoustically characterized. The microbubble response to the ultrasound pulses used in vivo was simulated via a Rayleigh-Plesset type equation. The three formulations were compared in vitro for permeabilization efficacy in three different pancreatic cancer cell lines, and in vivo, using an orthotopic pancreatic cancer (PDAC) murine model. The mice were treated using one of the three formulations exposed to ultrasound from a GE Logiq E9 and C1-5 ultrasound transducer. Characterisation of the microbubbles showed a rapid degradation in concentration, shape, and/or size for both SonoVue^® and Optison™ within 30 min of reconstitution/opening. Sonazoid™ showed no degradation after 1 h. Attenuation measurements indicated that SonoVue^® was the softest bubble followed by Sonazoid™ then Optison™. Sonazoid™ emitted nonlinear ultrasound at the lowest MIs followed by Optison™, then SonoVue^®. Simulations indicated that SonoVue^® would be the most effective bubble using the evaluated ultrasound conditions. This was verified in the pre-clinical PDAC model demonstrated by improved survival and largest tumor growth inhibition. In vitro results indicated that the best microbubble formulation depends on the ultrasound parameters and concentration used, with SonoVue^® being best at lower intensities and Sonazoid™ at higher intensities.

Nanosized Contrast Agents in Ultrasound Molecular Imaging

Applying nanosized ultrasound contrast agents (nUCAs) in molecular imaging has received considerable attention. nUCAs have been instrumental in ultrasound molecular imaging to enhance sensitivity, identification, and quantification. nUCAs can achieve high performance in molecular imaging, which was influenced by synthetic formulations and size. This review presents an overview of nUCAs from different synthetic formulations with a discussion on imaging and detection technology. Then we also review the progress of nUCAs in preclinical application and highlight the recent challenges of nUCAs.

Blinking Acoustic Nanodroplets Enable Fast Super-resolution Ultrasound Imaging

The advent of localization-based super-resolution ultrasound (SRUS) imaging creates a vista for precision vasculature and hemodynamic measurements in brain science, cardiovascular diseases, and cancer. As blinking fluorophores are crucial to super-resolution optical imaging, blinking acoustic contrast agents enabling ultrasound localization microscopy have been highly sought, but only with limited success. Here we report on the discovery and characterization of a type of blinking acoustic nanodroplets (BANDs) ideal for SRUS. BANDs of 200-500 nm diameters comprise a perfluorocarbon-filled core and a shell of DSPC, Pluronic F68, and DSPE-PEG2000. When driven by clinically safe acoustic pulses (MI < 1.9) provided by a diagnostic ultrasound transducer, BANDs underwent reversible vaporization and reliquefaction, manifesting as "blinks", at rates of up to 5 kHz. By sparse activation of perfluorohexane-filled BANDs-C₆ at high concentrations, only 100 frames of ultrasound imaging were sufficient to reconstruct super-resolution images of a no-flow tube through either cumulative localization or temporal radiality autocorrelation. Furthermore, the use of high-density BANDs-C_6-4 (1 × 10⁸/mL) with a 1:9 admixture of perfluorohexane and perfluorobutane supported the fast SRUS imaging of muscle vasculature in live animals, at 64 μm resolution requiring only 100 frames per layer. We anticipate that the BANDs developed here will greatly boost the application of SRUS in both basic science and clinical settings.

Biomedical nanobubbles and opportunities for microfluidics

The use of bulk nanobubbles in biomedicine is increasing in recent years, which is attributable to the array of therapeutic and diagnostic tools promised by developing bulk nanobubble technologies. From cancer drug delivery and ultrasound contrast enhancement to malaria detection and the diagnosis of acute donor tissue rejection, the potential applications of bulk nanobubbles are broad and diverse. Developing these technologies to the point of clinical use may significantly impact the quality of patient care. This review compiles and summarizes a representative collection of the current applications, fabrication techniques, and characterization methods of bulk nanobubbles in biomedicine. Current state-of-the-art generation methods are not designed to create nanobubbles of high concentration and low polydispersity, both characteristics of which are important for several bulk nanobubble applications. To date, microfluidics has not been widely considered as a tool for generating nanobubbles, even though the small-scale precision and real-time control offered by microfluidics may overcome the challenges mentioned above. We suggest possible uses of microfluidics for improving the quality of bulk nanobubble populations and propose ways of leveraging existing microfluidic technologies, such as organ-on-a-chip platforms, to expand the experimental toolbox of researchers working to develop biomedical nanobubbles.

The use of bulk nanobubbles in biomedicine is increasing in recent years. This translates into new opportunities for microfluidics, which may enable the generation of higher quality nanobubbles that lead to advances in diagnostics and therapeutics.

Oxygen transport to mammalian cell and bacteria using nano-sized liposomes encapsulating oxygen molecules

Hypoxic microenvironments emerge as tumor grow, leading to the over-expression and stabilization of hypoxia-inducible factor 1-alpha (HIF-1α). HIF-1α lowers the sensitization against chemotherapy, radiation therapy and photodynamic therapy in cancer. In this study, nano-sized oxygen carrier, namely oxygen dissolved nanoliposome (ODL) was synthesized, and oxygen was efficiently delivered to different types of mammalian cells to help relieve hypoxia. ODL confirmed that oxygen was released without inducing toxicity to cells. After artificially creating hypoxia in cancer cells, normal cells, and immune cells; various parameters such as cell morphology, HIF-1α expression, and degree of hypoxia were examined. The cellular environment was found to be altered by treatment with the ODL. Under hypoxia, the shape of the cells changed, and the cells began to die. After treatment with the ODL, the degree of hypoxia was reduced, indicating that HIF-1α expression and the rate of cell death decreased. Furthermore, bacteria proliferation was observed with the ODL. Therefore, ODL can be used for oxygen delivery platform in cancer therapy. ODL has a potential application in other microorganisms which needs future research.

Stable Thermally-Modulated Nanodroplet Ultrasound Contrast Agents

Liquid perfluorocarbon-based nanodroplets are stable enough to be used in extravascular imaging, but provide limited contrast enhancement due to their small size, incompressible core, and small acoustic impedance mismatch with biological fluids. Here we show a novel approach to overcoming this limitation by using a heating-cooling cycle, which we will refer to as thermal modulation (TM), to induce echogenicity of otherwise stable but poorly echogenic nanodroplets without triggering a transient phase shift. We apply thermal modulation to high-boiling point tetradecafluorohexane (TDFH) nanodroplets stabilized with a bovine serum albumin (BSA) shell. BSA-TDFH nanodroplets with an average diameter under 300 nanometers showed an 11.9 ± 5.4 mean fold increase in echogenicity on the B-mode and a 13.9 ± 6.9 increase on the nonlinear contrast (NLC) mode after thermal modulation. Once activated, the particles maintained their enhanced echogenicity (p < 0.001) for at least 13 h while retaining their nanoscale size. Our data indicate that thermally modulated nanodroplets can potentially serve as theranostic agents or sensors for various applications of contrast-enhanced ultrasound.

Bursting Microbubbles: How Nanobubble Contrast Agents Can Enable the Future of Medical Ultrasound Molecular Imaging and Image-Guided Therapy

The field of medical ultrasound has undergone a significant evolution since the development of microbubbles as contrast agents. However, due to their size, microbubbles remain in the vasculature, and therefore have limited clinical applications. Building a better - and smaller - bubble can expand the applications of contrast-enhanced ultrasound by allowing bubbles to extravasate from blood vessels - creating new opportunities. In this review, we summarize recent research on the formulation and use of NBs as imaging agents and as therapeutic vehicles. We discuss the ongoing debates in the field and reluctance to accepting NBs as an acoustically active construct and a potentially impactful clinical tool that can help shape the future of medical ultrasound. We hope that the overview of key experimental and theoretical findings in the NB field presented in this paper provides a fundamental framework that will help clarify NB-ultrasound interactions and inspire engagement in the field.

Stimuli-responsive nanobubbles for biomedical applications

Stimuli-responsive nanobubbles have received increased attention for their application in spatial and temporal resolution of diagnostic techniques and therapies, particularly in multiple imaging methods, and they thus have significant potential for applications in the field of biomedicine. This review presents an overview of the recent advances in the development of stimuli-responsive nanobubbles and their novel applications. Properties of both internal- and external-stimuli responsive nanobubbles are highlighted and discussed considering the potential features required for biomedical applications. Furthermore, the methods used for synthesis and characterization of nanobubbles are outlined. Finally, novel biomedical applications are proposed alongside the advantages and shortcomings inherent to stimuli-responsive nanobubbles.

Nonlinear dynamics and bifurcation structure of ultrasonically excited lipid coated microbubbles

In many applications, microbubbles (MBs) are encapsulated by a lipid coating to increase their stability. However, the complex behavior of the lipid coating including buckling and rupture sophisticates the dynamics of the MBs and as a result the dynamics of the lipid coated MBs (LCMBs) are not well understood. Here, we investigate the nonlinear behavior of the LCMBs by analyzing their bifurcation structure as a function of acoustic pressure. We show that, the LC can enhance the generation of period 2 (P2), P3, higher order subharmonics (SH), superharmonics and chaos at very low excitation pressures (e.g. 1 kPa). For LCMBs sonicated by their SH resonance frequency and in line with experimental observations with increasing pressure, P2 oscillations exhibit three stages: generation at low acoustic pressures, disappearance and re-generation. Within non-destructive oscillation regimes and by pressure amplitude increase, LCMBs can also exhibit two saddle node (SN) bifurcations resulting in possible abrupt enhancement of the scattered pressure. The first SN resembles the pressure dependent resonance phenomenon in uncoated MBs and the second SN resembles the pressure dependent SH resonance. Depending on the initial surface tension of the LCMBs, the nonlinear behavior may also be suppressed for a wide range of excitation pressures.

Toward Precisely Controllable Acoustic Response of Shell-Stabilized Nanobubbles: High Yield and Narrow Dispersity

Understanding the pressure dependence of the nonlinear behavior of ultrasonically excited phospholipid-stabilized nanobubbles (NBs) is important for optimizing ultrasound exposure parameters for implementations of contrast enhanced ultrasound, critical to molecular imaging. The viscoelastic properties of the shell can be controlled by the introduction of membrane additives, such as propylene glycol as a membrane softener or glycerol as a membrane stiffener. We report on the production of high-yield NBs with narrow dispersity and different shell properties. Through precise control over size and shell structure, we show how these shell components interact with the phospholipid membrane, change their structure, affect their viscoelastic properties, and consequently change their acoustic response. A two-photon microscopy technique through a polarity-sensitive fluorescent dye, C-laurdan, was utilized to gain insights on the effect of membrane additives to the membrane structure. We report how the shell stiffness of NBs affects the pressure threshold (P_t) for the sudden amplification in the scattered acoustic signal from NBs. For narrow size NBs with 200 nm mean size, we find P_t to be between 123 and 245 kPa for the NBs with the most flexible membrane as assessed using C-Laurdan, 465-588 kPa for the NBs with intermediate stiffness, and 588-710 kPa for the NBs with stiff membranes. Numerical simulations of the NB dynamics are in good agreement with the experimental observations, confirming the dependence of acoustic response to shell properties, thereby substantiating further the development in engineering the shell of ultrasound contrast agents. The viscoelastic-dependent threshold behavior can be utilized for significantly and selectively enhancing the diagnostic and therapeutic ultrasound applications of potent narrow size NBs.

Perfluoropentane-filled chitosan poly-acrylic acid nanobubbles with high stability for long-term ultrasound imaging in vivo

Reducing the size of ultrasound contrast agents (UCAs) will decrease the intensity of the ultrasound echogenic signals and reduce the stability of the bubbles. Therefore, it is a challenge to design nanobubbles that are less than 200 nm in size and that have both good imaging abilities and high stability for long-term imaging in vivo. In this work, we successfully prepared perfluoropentane-filled chitosan poly-acrylic acid (PFP-CS-PAA) nanobubbles with a size of about 100 nm via a direct simple core-template-free strategy. In vitro tests demonstrated that the nanobubbles showed satisfactory imaging capabilities in non-linear harmonic imaging mode and had significantly better stability than commercial Sonovue® lipid microbubbles. It was valuable to discover that the prepared PFP-CS-PAA nanobubbles could exhibit good imaging quality in rat livers for 10 min after intravenous injection. Also, the PFP-CS-PAA nanobubbles could maintain imaging capabilities in nude mouse tumors for 7 days after intratumoral injection, which indicated that these nanobubbles could keep their stability for a long time in vivo. To the best of our knowledge, the ultrasound imaging persistence time in vivo was the longest of currently reported polymer nanobubbles that are smaller than 200 nm. This new nanosized UCA with high stability has great potential for long-term ultrasound imaging in vivo. Tumor cellular uptake and histological analysis revealed that PFP-CS-PAA nanobubbles could be taken up into tumor cells, but no intracellular uptake was observed in the case of Sonovue®. Animal fluorescence imaging in vivo demonstrated that PFP-CS-PAA nanobubbles could be effectively cleared after intravenous injection within 168 h. MTT assays indicated that PFP-CS-PAA nanobubbles had appropriate biocompatibility. Abnormal levels of blood urea nitrogen were detected after the intravenous administration of PFP-CS-PAA nanobubbles to rats, and body-weight gain was inhibited for up to 6 d, but, after that, body weights recovered their tendency to increase.

Optoacoustic/Fluorescent/Acoustic Imaging Probe Based on Air-Filled Bubbles Functionalized with Gold Nanorods and Fluorescein Isothiocyanate

Liquid/surfactant/gas interfaces are promising objects for nanoengineered multimodal contrasts, which can be used for biomedical imaging in preclinical and clinical applications. Microbubbles with the gaseous core and shell made of lipids/proteins have already acted as ultrasound (US) contrast agents for angiography. In the present work, microbubbles with a shell composed of Span 60 and Tween 80 surfactants functionalized with fluorescein isothiocyanate and gold nanorods to achieve a multimodal combination of US, fluorescence, and optoacoustic imaging are described. Optimal conditions for microbubble generation by studying the surface tension of the initial solutions and analyzing the size, stability, and charge of the resulting bubbles were found. By controlling and modifying bubbles' surface properties, an increase in stability and storage time can be achieved. The functionalization of bubbles with gold nanoparticles and a dye by using an optimally selected sonication protocol was performed. The biomedical application's potential in imaging modalities of functionalized microbubbles using a medical US device with a frequency of 50 MHz, fluorescence tomography, and raster-scanning optoacoustic mesoscopy measurements was evaluated. The obtained results are important for optimum stabilization and functionalization of gas/liquid interfaces and the following applications in the multimodal biomedical imaging.

Effect of Nanobubble-Based Ultrasound Imaging Technology on the Treatment of Ureteral Stenosis

In order to explore the effect of nanobubble-based ultrasound imaging technology on the treatment of ureteral stenosis, a total of 120 patients, who were confirmed as ureteral stenosis by surgery, pathology or multiple imaging examinations at a designated hospital of the study from December 2015 to December 2018, were selected as research objects and were divided into three groups of targeted nanobubble (TN) group, blank nanobubble (BN) group and control (CT) group with 40 cases in each group. The TN group utilized the nanobubbles with a particle size of (499.52±72.87) nm as carriers to compare and analyze patients' ultrasound images for the predisposition and etiology of ureteral stenosis and the sonogram variations of hydronephrosis, renal pelvis; the BN group utilized the blank nanobubble with a particle size of (446.71±45.36) nm as carriers to perform ultrasound imaging and diagnostic analysis of ureteral stenosis; the CT group directly conducted ureteral stenosis treatment with ultrasound imaging technology. The results showed that the total coincidence rates of the targeted diagnosis for ureteral stenosis of the TN, BN and CT group were 94.38%, 87.52%, and 67.94%, respectively; the coincidence rates of different examination methods for different diagnostic parts were different and the diagnostic coincidence rates of TN group for pelvic ureteral transition area, end of ureter, and the area between pelvic ureteral transition area and end of ureter were 82.91%, 79.66%, and 75.17%, respectively; the diagnostic coincidence rates of BN group for those were 80.32%, 94.77%, respectively and 92.18% and the CT group were 58.66%, 72.14%, and 66.48%, respectively; the diagnosis coincidence rates for ureteral stenosis etiology of the TN, BN and CT group were 93.81%, 82.66% and 64.57%, respectively. Therefore, it was believed that the nanobubble-based ultrasound examination can accurately diagnose the site of ureteral stenosis through the exploration of hydronephrosis and ureteral dilatation with the advantages of simplicity, no pain, repeatable examination, and no impact on renal function, and having high clinical value for diagnosing ureteral stenosis.

Stability of Engineered Micro or Nanobubbles for Biomedical Applications

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

Hybrid nanocapsules for in situ TEM imaging of gas evolution reactions in confined liquids

Liquid cell transmission electron microscopy (TEM) enables the direct observation of dynamic physical and chemical processes in liquids at the nanoscale. Quantitative investigations into reactions with fast kinetics and/or multiple reagents will benefit from further advances in liquid cell design that facilitate rapid in situ mixing and precise control over reagent volumes and concentrations. This work reports the development of inorganic-organic nanocapsules for high-resolution TEM imaging of nanoscale reactions in liquids with well-defined zeptoliter volumes. These hybrid nanocapsules, with 48 nm average diameter, consist of a thin layer of gold coating a lipid vesicle. As a model reaction, the nucleation, growth, and diffusion of nanobubbles generated by the radiolysis of water is investigated inside the nanocapsules. When the nanobubbles are sufficiently small (10-25 nm diameter), they are mobile in the nanocapsules, but their movement deviates from Brownian motion, which may result from geometric confinement by the nanocapsules. Gases and fluids can be transported between two nanocapsules when they fuse, demonstrating in situ mixing without using complex microfluidic schemes. The ability to synthesize nanocapsules with controlled sizes and to monitor dynamics simultaneously inside multiple nanocapsules provides opportunities to investigate nanoscale processes such as single nanoparticle synthesis in confined volumes and biological processes such as biomineralization and membrane dynamics.

Nested Nanobubbles for Ultrasound-Triggered Drug Release

Because of their size (1-10 μm), microbubble-based drug delivery agents suffer from confinement to the vasculature, limiting tumor penetration and potentially reducing the drug efficacy. Nanobubbles (NBs) have emerged as promising candidates for ultrasound-triggered drug delivery because of their small size, allowing drug delivery complexes to take advantage of the enhanced permeability and retention effect. In this study, we describe a simple method for production of nested-nanobubbles (Nested-NBs) by encapsulation of NBs (∼100 nm) within drug-loaded liposomes. This method combines the efficient and well-established drug-loading capabilities of liposomes while utilizing NBs as an acoustic trigger for drug release. Encapsulation was characterized using transmission electron microscopy with an encapsulation efficiency of 22 ± 2%. Nested-NBs demonstrated echogenicity using diagnostic B-mode imaging, and acoustic emissions were monitored during high-intensity focused ultrasound (HIFU) in addition to monitoring of model drug release. Results showed that although the encapsulated NBs were destroyed by pulsed HIFU [peak negative pressure (PNP) 1.54-4.83 MPa], signified by loss of echogenicity and detection of inertial cavitation, no model drug release was observed. Changing modality to continuous wave (CW) HIFU produced release across a range of PNPs (2.01-3.90 MPa), likely because of a synergistic effect of mechanical and increased thermal stimuli. Because of this, we predict that our NBs contain a mixed population of both gaseous and liquid core particles, which upon CW HIFU undergo rapid phase conversion, triggering liposomal drug release. This hypothesis was investigated using previously described models to predict the existence of droplets and their phase change potential and the ability of this phase change to induce liposomal drug release.

Microbubble Agents: New Directions

Microbubble ultrasound contrast agents have now been in use for several decades and their safety and efficacy in a wide range of diagnostic applications have been well established. Recent progress in imaging technology is facilitating exciting developments in techniques such as molecular, 3-D and super resolution imaging and new agents are now being developed to meet their specific requirements. In parallel, there have been significant advances in the therapeutic applications of microbubbles, with recent clinical trials demonstrating drug delivery across the blood-brain barrier and into solid tumours. New agents are similarly being tailored toward these applications, including nanoscale microbubble precursors offering superior circulation times and tissue penetration. The development of novel agents does, however, present several challenges, particularly regarding the regulatory framework. This article reviews the developments in agents for diagnostic, therapeutic and "theranostic" applications; novel manufacturing techniques; and the opportunities and challenges for their commercial and clinical translation.

Increasing Doxorubicin Loading in Lipid-Shelled Perfluoropropane Nanobubbles via a Simple Deprotonation Strategy

Drug delivery to solid tumors using echogenic nanobubbles (NBs) and ultrasound (US) has recently gained significant interest. The approach combines attributes of nanomedicine and the enhanced permeation and retention (EPR) effect with the documented benefits of ultrasound to improve tumor drug distribution and treatment outcomes. However, optimized drug loading strategies, the drug-carrying capacity of NBs and their drug delivery efficiency have not been explored in depth and remain unclear. Here, we report for the first time on the development of a novel deprotonated hydrophobic doxorubicin-loaded C3F8 nanobubble (hDox-NB) for more effective US-mediated drug delivery. In this study, the size distribution and yield of hDox-NBs were measured via resonant mass measurement, while their drug-loading capacity was determined using a centrifugal filter technique. In vitro acoustic properties including contrast-imaging enhancement, initial echogenic signal, and decay were assessed and compared to doxorubicin hydrochloride loaded-NBs (Dox.HCl-NBs). In addition, in vitro therapeutic efficacy of hDox-NBs was evaluated by cytotoxicity assay in human ovarian cancer cells (OVCAR-3). The results showed that the hDox-NBs were small (300.7 ± 4.6 nm), and the drug loading content was significantly enhanced (2 fold higher) compared to Dox.HCl-NBs. Unexpectedly, the in vitro acoustic performance was also improved by inclusion of hDox into NBs. hDox-NB showed higher initial US signal and a reduced signal decay rate compared to Dox.HCl-NBs. Furthermore, hDox-NBs combined with higher intensity US exhibited an excellent therapeutic efficacy in human ovarian cancer cells as shown in a reduction in cell viability. These results suggest that hDox-NBs could be considered as a promising theranostic agent to achieve a more effective noninvasive US-mediated drug delivery for cancer treatment.

Theoretical and Experimental Gas Volume Quantification of Micro- and Nanobubble Ultrasound Contrast Agents

The amount of gas in ultrasound contrast agents is related to their acoustic activity. Because of this relationship, gas volume has been used as a key variable in normalizing the in vitro and in vivo acoustic behavior of lipid shell-stabilized bubbles with different sizes and shell components. Despite its importance, bubble gas volume has typically only been theoretically calculated based on bubble size and concentration that is typically measured using the Coulter counter for microbubbles and nanoparticle tracking analysis (NTA) for nanoscale bubbles. However, while these methods have been validated for the analysis of liquid or solid particles, their application in bubble analysis has not been rigorously studied. We have previously shown that resonant mass measurement (RMM) may be a better-suited technique for sub-micron bubble analysis, as it can measure both buoyant and non-buoyant particle size and concentration. Here, we provide validation of RMM bubble analysis by using headspace gas chromatography/mass spectrometry (GC/MS) to experimentally measure the gas volume of the bubble samples. This measurement was then used as ground truth to test the accuracy of theoretical gas volume predictions based on RMM, NTA (for nanobubbles), and Coulter counter (for microbubbles) measurements. The results show that the headspace GC/MS gas volume measurements agreed well with the theoretical predictions for the RMM of nanobubbles but not NTA. For nanobubbles, the theoretical gas volume using RMM was 10% lower than the experimental GC/MS measurements; meanwhile, using NTA resulted in an 82% lower predicted gas volume. For microbubbles, the experimental gas volume from the GC/MS measurements was 27% lower compared to RMM and 72% less compared to the Coulter counter results. This study demonstrates that the gas volume of nanobubbles and microbubbles can be reliably measured using headspace GC/MS to validate bubble size measurement techniques. We also conclude that the accuracy of theoretical predictions is highly dependent on proper size and concentration measurements.

Contrast enhanced ultrasound imaging by nature-inspired ultrastable echogenic nanobubbles

Advancement of ultrasound molecular imaging applications requires not only a reduction in size of the ultrasound contrast agents (UCAs) but also a significant improvement in the in vivo stability of the shell-stabilized gas bubble. The transition from first generation to second generation UCAs was marked by an advancement in stability as air was replaced by a hydrophobic gas, such as perfluoropropane and sulfur hexafluoride. Further improvement can be realized by focusing on how well the UCAs shell can retain the encapsulated gas under extreme mechanical deformations. Here we report the next generation of UCAs for which we engineered the shell structure to impart much better stability under repeated prolonged oscillation due to ultrasound, and large changes in shear and turbulence as it circulates within the body. By adapting an architecture with two layers of contrasting elastic properties similar to bacterial cell envelopes, our ultrastable nanobubbles (NBs) withstand continuous in vitro exposure to ultrasound with minimal signal decay and have a significant delay on the onset of in vivo signal decay in kidney, liver, and tumor. Development of ultrastable NBs can potentially expand the role of ultrasound in molecular imaging, theranostics, and drug delivery.

Time-intensity-curve Analysis and Tumor Extravasation of Nanobubble Ultrasound Contrast Agents

Our group recently presented a simple strategy using the non-ionic surfactant, Pluronic, as a size control excipient to produce nanobubbles in the 100-nm range, which exhibited stability and echogenicity on par with clinically available microbubbles. The objective of the present study was to evaluate biodistribution and extravasation of the Pluronic-stabilized lipid nanobubbles compared with microbubbles in 2 experimental tumor models in mice. Standard lipid-stabilized perfluoropropane bubbles (Pluronic L10) and lipid-stabilized perfluoropropane nanobubbles were intravenously injected into mice bearing either an orthotopic mouse breast cancer (BC4 T1) or subcutaneous mouse ovarian cancer (OVCAR-3) through the tail vein to perform perfusion dynamic studies. No significant differences between the nanobubble and microbubble groups were observed in the peak enhancement of the 3 tested regions (tumor, liver and kidney). However, the decay rates of nanobubble in the tumor and kidney of BC4 T1-bearing mice, as well as in mice with OVRCAR-3 tumors were significantly slower than those of the microbubble. To quantify extravasation, fluorescently labeled bubbles were intravenously injected into mice bearing the same tumors. Histologic analysis showed that nanobubbles were retained in tumor tissue to a greater extent compared with microbubbles in both tumor models at the 3-h time point. Our results demonstrate unique nanobubble behavior compared with microbubbles and support augmented application of these agents in ultrasound molecular imaging and drug delivery beyond the tumor vasculature.

Influence of Nanobubble Concentration on Blood-Brain Barrier Opening Using Focused Ultrasound Under Real-Time Acoustic Feedback Control

Real-time acoustic feedback control based on harmonic emissions of stimulated microbubbles may serve as a way to achieve reliable blood-brain barrier (BBB) opening with focused ultrasound in the brain. Previously, we demonstrated BBB opening was possible using sub-micron bubbles (aka nanobubbles) and produced comparable results to commercially available microbubbles (Optison, Definity, etc.). The harmonic emissions and acoustic control were observed to be more consistent using nanobubbles, which warrants further study of BBB opening using these agents. This study examined the stimulated acoustic emissions of nanobubbles at different concentrations both in vitro and in vivo and evaluated BBB opening under real-time acoustic feedback control across concentrations. Original nanobubbles (10¹¹ bubbles/mL) have long in vitro persistence (7.3 ± 3.3 min) and circulation time in rats (approximately 10 min) under exposures in this study, and both degraded with dilutions. With all three tested dilutions (1:1, 1:10 and 1:100), successful BBB opening was reliably achieved under real-time feedback control.