Stabilization and fabrication of microbubbles: applications for medical purposes and functional materials

Clinical Applications of Micro/Nanobubble Technology in Neurological Diseases

Nanomedicine, leveraging the unique properties of nanoparticles, has revolutionized the diagnosis and treatment of neurological diseases. Among various nanotechnological advancements, ultrasound-mediated drug delivery using micro- and nanobubbles offers promising solutions to overcome the blood-brain barrier (BBB), enhancing the precision and efficacy of therapeutic interventions. This review explores the principles, current clinical applications, challenges, and future directions of ultrasound-mediated drug delivery systems in treating stroke, brain tumors, neurodegenerative diseases, and neuroinflammatory disorders. Additionally, ongoing clinical trials and potential advancements in this field are discussed, providing a comprehensive overview of the impact of nanomedicine on neurological diseases.

Progress in application of nanomedicines for enhancing cancer sono-immunotherapy

Cancer immunotherapy has significant potential as a cancer treatment since it boosts the immune system and prevents immune escape to get rid of or fight cancers. However, its clinical applicability is still limited because of the low response rate and immune-related side effects. Recently ultrasound has been shown to alter the tumor immune microenvironment, enhance the effectiveness of other antitumor therapies, and cause tumors to become more sensitive to immunotherapy, thus providing new insights into cancer treatment. Nanomedicines are also anticipated to have a positive impact on improving the immunological effects and enhancing ultrasound effect for cancer therapy. Therefore, designing effective nanomedicines enhanced ultrasound effect for augmenting sono-immunotherapy has been a pivot on anticancer therapy. In this review, the immunological impacts of various ultrasound therapeutic modalities, ultrasound parameters, and their underlying mechanisms are discussed. Moreover, we highlight the recent progress of nanomedicines synergistically enhancing sono-immunotherapy. Finally, we put forward opportunities and challenges on sono-immunotherapy.

A Systematic Study on Long-acting Nanobubbles: Current Advancement and Prospects on Theranostic Properties

Delivery of diagnostic drugs via nanobubbles (NBs) has shown to be an emerging field of study. Due to their small size, NBs may more easily travel through constricted blood vessels and precisely target certain bodily parts. NB is considered the major treatment for cancer treatment and other diseases which are difficult to diagnose. The field of NBs is dynamic and continues to grow as researchers discover new properties and seek practical applications in various fields. The predominant usage of NBs in novel drug delivery is to enhance the bioavailability, and controlled drug release along with imaging properties NBs are important because they may change interfacial characteristics including surface force, lubrication, and absorption. The quick diffusion of gas into the water was caused by a hypothetical film that was stimulated and punctured by a strong acting force at the gas/water contact of the bubble. In this article, various prominent aspects of NBs have been discussed, along with the long-acting nature, and the theranostical aspect which elucidates the potential marketed drugs along with clinical trial products. The article also covers quality by design aspects, different production techniques that enable method-specific therapeutic applications, increasing the floating time of the bubble, and refining its properties to enhance the prepared NB's quality. NB containing both analysis and curing properties makes it special from other nano-carriers. This work includes all the possible methods of preparing NB, its application, all marketed drugs, and products in clinical trials.

Two-in-one sensor of refractive index and Raman scattering using hollow-core microstructured optical waveguides for colloid characterization

Hollow-core microstructured optical waveguides (HC-MOW) have recently emerged in sensing technologies, including the gas and liquid detection for industrial as well as clinical applications. Antiresonant HC-MOW provide capabilities for applications in refractive index (RI) sensing, while the long optical path for analyte-light interaction in HC-MOW leads to increased sensitivity of sensor based on Raman scattering signal measurements. In this study, we developed a two-in-one sensor device using HC-MOW for RI and Raman scattering detection. The performance of the sensor was evaluated by characterizing protein-copolymer multicomponent colloids, specifically, bovine serum albumin (BSA) and poly(N - vinyl-2 -pyrrolidone-co-acrylic acid) P(VP-AA) nano-sized complexes and microbubbles of the corresponding shell. Monocomponent solutions showed linear dependencies of RI and characteristic Raman peak intensities on mass concentration. Multicomponent Raman sensing of BSA@P(VP-AA) complexes and microbubbles revealed that changes in P(VP-AA) characteristic peak intensities can describe interactions between components needed to produce colloid systems. RI sensing of multicomponent colloids demonstrated linear dependence on total mass concentrations for BSA@P(VP-AA) complexes, while corresponding BSA@P(VP-AA) microbubbles can be detected with concentrations as high as 4.0 × 108 MB/mL. Therefore, the developed two-in-one sensor of RI and Raman scattering can be used the robust characterization of albumin-based colloids designed for therapeutic and diagnostic needs.

Assembly of Metal-Phenolic Networks onto Microbubbles for One-Step Generation of Functional Microcapsules

The one-step assembly of metal-phenolic networks (MPNs) onto particle templates can enable the facile, rapid, and robust construction of hollow microcapsules. However, the required template removal step may affect the refilling of functional species in the hollow interior space or the in situ encapsulation of guest molecules during the formation of the shells. Herein, a simple strategy for the one-step generation of functional MPNs microcapsules is proposed. This method uses bovine serum albumin microbubbles (BSA MBs) as soft templates and carriers, enabling the efficient pre-encapsulation of guest species by leveraging the coordination assembly of tannic acid (TA) and FeIII ions. The addition of TA and FeIII induces a change in the protein conformation of BSA MBs and produces semipermeable capsule shells, which allow gas to escape from the MBs without template removal. The MBs-templated strategy can produce highly biocompatible capsules with controllable structure and size, and it is applicable to produce other MPNs systems like BSA-TA-CuII and BSA-TA-NiII . Finally, those MBs-templated MPNs capsules can be further functionalized or modified for the loading of magnetic nanoparticles and the pre-encapsulation of model molecules through covalence or physical adsorption, exhibiting great promise in biomedical applications.

Sonosensitive Cavitation Nuclei-A Customisable Platform Technology for Enhanced Therapeutic Delivery

Ultrasound-mediated cavitation shows great promise for improving targeted drug delivery across a range of clinical applications. Cavitation nuclei-sound-sensitive constructs that enhance cavitation activity at lower pressures-have become a powerful adjuvant to ultrasound-based treatments, and more recently emerged as a drug delivery vehicle in their own right. The unique combination of physical, biological, and chemical effects that occur around these structures, as well as their varied compositions and morphologies, make cavitation nuclei an attractive platform for creating delivery systems tuned to particular therapeutics. In this review, we describe the structure and function of cavitation nuclei, approaches to their functionalization and customization, various clinical applications, progress toward real-world translation, and future directions for the field.

Sono-processes: Emerging systems and their applicability within the (bio-)medical field

Sonochemistry, although established in various fields, is still an emerging field finding new effects of ultrasound on chemical systems and are of particular interest for the biomedical field. This interdisciplinary area of research explores the use of acoustic waves with frequencies ranging from 20 kHz to 1 MHz to induce physical and chemical changes. By subjecting liquids to ultrasonic waves, sonochemistry has demonstrated the ability to accelerate reaction rates, alter chemical reaction pathways, and change physical properties of the system while operating under mild reaction conditions. It has found its way into diverse industries including food processing, pharmaceuticals, material science, and environmental remediation. This review provides an overview of the principles, advancements, and applications of sonochemistry with a particular focus on the domain of (bio-)medicine. Despite the numerous benefits sonochemistry has to offer, most of the research in the (bio-)medical field remains in the laboratory stage. Translation of these systems into clinical practice is complex as parameters used for medical ultrasound are limited and toxic side effects must be minimized in order to meet regulatory approval. However, directing attention towards the applicability of the system in clinical practice from the early stages of research holds significant potential to further amplify the role of sonochemistry in clinical applications.

Current perspectives of artificial oxygen carriers as red blood cell substitutes: a review of old to cutting-edge technologies using in vitro and in vivo assessments

Background

Several circumstances such as accidents, surgery, traumatic hemorrhagic shock, and other causalities cause major blood loss. Allogenic blood transfusion can be resuscitative for such conditions; however, it has numerous ambivalent effects, including supply shortage, needs for more time, cost for blood grouping, the possibility of spreading an infection, and short shelf-life. Hypoxia or ischemia causes heart failure, neurological problems, and organ damage in many patients. To address this emergent medical need for resuscitation and to treat hypoxic conditions as well as to enhance oxygen transportation, researchers aspire to achieve a robust technology aimed to develop safe and feasible red blood cell substitutes for effective oxygen transport.

Area covered

This review article provides an overview of the formulation, storage, shelf-life, clinical application, side effects, and current perspectives of artificial oxygen carriers (AOCs) as red blood cell substitutes. Moreover, the pre-clinical (in vitro and in vivo) assessments for the evaluation of the efficacy and safety of oxygen transport through AOCs are key considerations in this study. With the most significant technologies, hemoglobin- and perfluorocarbon-based oxygen carriers as well as other modern technologies, such as synthetically produced porphyrin-based AOCs and oxygen-carrying micro/nanobubbles, have also been elucidated.

Expert opinion

Both hemoglobin- and perfluorocarbon-based oxygen carriers are significant, despite having the latter acting as safeguards; they are cost-effective, facile formulations which penetrate small blood vessels and remove arterial blockages due to their nano-size. They also show better biocompatibility and longer half-life circulation than other similar technologies.

Resonance-Induced Reduction of Interfacial Tension of Water-Methane and Improvement of Methane Solubility in Water

Molecular dynamics simulations were performed to study the effect of the periodic oscillating electric field on the interface between water and methane. We propose a new strategy that utilizes oscillating electric fields to reduce the interfacial tension (IFT) between water and methane and increase the solubility of methane in water simultaneously. These are attributed to the hydrogen bond resonance induced by an electric field with a frequency close to the natural frequency of the hydrogen bond. The resonance breaks the hydrogen bond network among water molecules to the maximum, which destroys the hydration shell and reduces the cohesive action of water, thus resulting in the decrease of IFT and the increase of methane solubility. As the frequency of the electric field is close to the optimum resonant frequency of hydrogen bonds, IFT decreases from 56.43 to 5.66 mN/m; water and methane are miscible because the solubility parameter of water reduces from 47.63 to 2.85 MPa^1/2, which is close to that of methane (3.43 MPa^1/2). Our results provide a new idea for reducing the water-gas IFT and improving the solubility of insoluble gas in water and theoretical guidance in the fields of natural gas exploitation, hydrate generation, and nanobubble nucleation.

Microbubble–Nanoparticle Complexes for Ultrasound-Enhanced Cargo Delivery

Targeted delivery of therapeutics to specific tissues is critically important for reducing systemic toxicity and optimizing therapeutic efficacy, especially in the case of cytotoxic drugs. Many strategies currently exist for targeting systemically administered drugs, and ultrasound-controlled targeting is a rapidly advancing strategy for externally-stimulated drug delivery. In this non-invasive method, ultrasound waves penetrate through tissue and stimulate gas-filled microbubbles, resulting in bubble rupture and biophysical effects that power delivery of attached cargo to surrounding cells. Drug delivery capabilities from ultrasound-sensitive microbubbles are greatly expanded when nanocarrier particles are attached to the bubble surface, and cargo loading is determined by the physicochemical properties of the nanoparticles. This review serves to highlight and discuss current microbubble–nanoparticle complex component materials and designs for ultrasound-mediated drug delivery. Nanocarriers that have been complexed with microbubbles for drug delivery include lipid-based, polymeric, lipid–polymer hybrid, protein, and inorganic nanoparticles. Several schemes exist for linking nanoparticles to microbubbles for efficient nanoparticle delivery, including biotin–avidin bridging, electrostatic bonding, and covalent linkages. When compared to unstimulated delivery, ultrasound-mediated cargo delivery enables enhanced cell uptake and accumulation of cargo in target organs and can result in improved therapeutic outcomes. These ultrasound-responsive delivery complexes can also be designed to facilitate other methods of targeting, including bioactive targeting ligands and responsivity to light or magnetic fields, and multi-level targeting can enhance therapeutic efficacy. Microbubble–nanoparticle complexes present a versatile platform for controlled drug delivery via ultrasound, allowing for enhanced tissue penetration and minimally invasive therapy. Future perspectives for application of this platform are also discussed in this review.

Effects of Nano-Aerators on Microbial Communities and Functions in the Water, Sediment, and Shrimp Intestine in Litopenaeus vannamei Aquaculture Ponds

Nanobubble technology has promising development and application prospects in the fields of sewage treatment, soil and groundwater remediation, animal and plant growth, and biomedicine. However, few studies have investigated its effect on shrimp aquaculture. In this study, we investigated the effect of nano-aerators on microbial communities of the water, sediment, and shrimp gut in a Litopenaeus vannamei aquaculture pond using 16S rRNA high-throughput sequencing. The results indicated that the nano-aerator significantly increased the microbial community diversity and species abundance in the pond, and the microbial community diversity of the pond sediment increased under short-term aeration conditions. Compared to that with ordinary aerators, nano-aerators increased the proportion of beneficial bacteria, such as Exiguobacterium and Acinetobacter, in the water and sediment microbial communities. Moreover, the proportions of beneficial bacteria in the gut, including Rhodobacter, Oscillospira, and Faecalibacterium, were all increased by using the nano-aerator. Therefore, our findings suggest that nano-aerators could promote the activity of beneficial bacteria in aquaculture ecosystems, thereby regulating water quality, reducing disease incidence, and improving aquaculture efficiency and benefits. Our findings provide new insights into the effects of nano-aerators on microbes in crustacean culture ponds.

Preparation of Nanoparticle-Loaded Microbubbles via an Electrohydrodynamic Atomization Process

Microbubbles have been widely used in many research fields due to their outstanding physicochemical properties and unique structural characteristics, especially as ultrasonic contrast agents and drug delivery carriers. However, the stability of conventional microbubbles is generally poor, which limits the development of their applications. Loading nanoparticle to microbubbles has great potential in enhancing the stability of microbubbles. This paper reports for the first time the feasibility of one-step preparation of nanoparticle-loaded microbubbles by coaxial electrohydrodynamic atomization. Bovine serum albumin (BSA) was used as the model material of the bubble shell layer to study the effect of the loading of nanoparticles on the stability of microbubbles. The results show that the concentration of nanoparticles has a significant impact on the stability of microbubbles, and loading an appropriate amount of nanoparticles is helpful in improving the stability of microbubbles. The results also show that nanoparticle-loaded microbubbles with a size distribution in the range of 120–200 μm can be prepared under optimal conditions.

Advances in the Sensing and Treatment of Wound Biofilms

Wound biofilms represent a particularly challenging problem in modern medicine. They are increasingly antibiotic resistant and can prevent the healing of chronic wounds. However, current treatment and diagnostic options are hampered by the complexity of the biofilm environment. In this review, we present new chemical avenues in biofilm sensors and new materials to treat wound biofilms, offering promise for better detection, chemical specificity, and biocompatibility. We briefly discuss existing methods for biofilm detection and focus on novel, sensor-based approaches that show promise for early, accurate detection of biofilm formation on wound sites and that can be translated to point-of-care settings. We then discuss technologies inspired by new materials for efficient biofilm eradication. We focus on ultrasound-induced microbubbles and nanomaterials that can both penetrate the biofilm and simultaneously carry active antimicrobials and discuss the benefits of those approaches in comparison to conventional methods.

Advances in the Sensing and Treatment of Wound Biofilms

Wound biofilms represent a particularly challenging problem in modern medicine. They are increasingly antibiotic resistant and can prevent the healing of chronic wounds. However, current treatment and diagnostic options are hampered by the complexity of the biofilm environment. In this review, we present new chemical avenues in biofilm sensors and new materials to treat wound biofilms, offering promise for better detection, chemical specificity, and biocompatibility. We briefly discuss existing methods for biofilm detection and focus on novel, sensor-based approaches that show promise for early, accurate detection of biofilm formation on wound sites and that can be translated to point-of-care settings. We then discuss technologies inspired by new materials for efficient biofilm eradication. We focus on ultrasound-induced microbubbles and nanomaterials that can both penetrate the biofilm and simultaneously carry active antimicrobials and discuss the benefits of those approaches in comparison to conventional methods.

A comprehensive update of micro- and nanobubbles as theranostics in oncology

Advances in diagnostic and imaging capabilities have allowed cancers to be detected earlier and characterized more robustly. These strategies have recently branched into theranostics whereby contrast agents traditionally used for imaging have been co-loaded with therapeutics to simultaneously diagnose and treat cancers in a patient-specific manner. Microbubbles (MB) and nanobubbles (NB) are contrast agents which can be modulated to meet the theranostic needs particularly in the realm of oncology. The current review focuses on the ultrasound-responsive MB/NB platforms used as a theranostic tool in oncology. We discuss in detail the key parameters that influence the utility of MB/NB formulations and implications of such treatment modalities. Recent advances in composition strategies, latest works in the pre-clinical stages and multiple paradigm-shifting innovations in the field of MB/NB are discussed in-depth in this review. The clinical application of MB/NB is currently limited to diagnostic imaging. Surface chemistry modification strategies will help tune the formulations toward therapeutic applications. It is also anticipated that MB/NB will see increased use to deliver gas therapeutics. Scalability and stability considerations will be at the forefront as these particles get introduced into the clinical theranostic toolbox.

Theranostic nanobubbles towards smart nanomedicines

Targeted therapy and early accurate detection of malignant lesions are essential for the effectiveness of treatment and prognosis in cancer patients. The development of gaseous system as a versatile platform for the fabricated nanobubbles, has attracted much interest in improving the efficacy of ultrasound therapeutic, diagnostic, and theranostic platforms. Nano-sized bubble, as an ultrasound contrast agent, with spherical gas-filled structures exhibited contrast enhancement capability due to their inherent EPR effect. Additionally, nanobubbles exhibited good stability with extended retention time in the blood stream. The current review summarized various nanobubbles and discussed about the crucial parameters affecting the stability of ultrafine bubbles. Furthermore, therapeutic and theranostic gaseous systems for fighting against cancer were described.

Microbubble technology for natural rubber latex foam production: The use of various gas-filled microbubbles

In recent years, microbubble technology has attracted great attention in many application fields including water treatment, food processing, oil recovery, surface cleaning, and therapeutic applications. In this paper, microbubbles (MBs) of air, nitrogen, and argon were applied to produce natural rubber latex foams (NRLFs). The bubbles were generated by flowing the gas through a porous diffuser and latex. The effect of gas source on cellular structure, density, elasticity, indentation hardness, and flammability of the bubbled foams was discussed. Argon MBs offered the latex foams with fine cell diameters and uniform cell size distribution resulting in enhanced elasticity and physical properties of the foams. Indentation hardness index and limiting oxygen index value depended significantly on the gas used. By using the microbubble technique, the future prospects in NRLF production can be expected due to its ability in controllable cellular structure.

The promising shadow of microbubble over medical sciences: from fighting wide scope of prevalence disease to cancer eradication

Microbubbles are typically 0.5-10 μm in size. Their size tends to make it easier for medication delivery mechanisms to navigate the body by allowing them to be swallowed more easily. The gas included in the microbubble is surrounded by a membrane that may consist of biocompatible biopolymers, polymers, surfactants, proteins, lipids, or a combination thereof. One of the most effective implementation techniques for tiny bubbles is to apply them as a drug carrier that has the potential to activate ultrasound (US); this allows the drug to be released by US. Microbubbles are often designed to preserve and secure medicines or substances before they have reached a certain area of concern and, finally, US is used to disintegrate microbubbles, triggering site-specific leakage/release of biologically active drugs. They have excellent therapeutic potential in a wide range of common diseases. In this article, we discussed microbubbles and their advantageous medicinal uses in the treatment of certain prevalent disorders, including Parkinson's disease, Alzheimer's disease, cardiovascular disease, diabetic condition, renal defects, and finally, their use in the treatment of various forms of cancer as well as their incorporation with nanoparticles. Using microbubble technology as a novel carrier, the ability to prevent and eradicate prevalent diseases has strengthened the promise of effective care to improve patient well-being and life expectancy.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

Aqueous and Oil Foams Stabilized by Surfactant Crystals: New Concepts and Perspectives

Surfactant crystals can stabilize liquid foams. The crystals are adsorbed at bubble surfaces, slowing down coarsening and coalescence. Excess crystals in the liquid channels between bubbles arrest drainage, leading to ultrastable foams. The melting of crystals upon raising the temperature allows thermoresponsive foams to be designed. In the case of oil foams, the stabilization by crystals received substantial renewed interest in the last 5 years due to their potential applications, particularly in the food industry. For aqueous foams, several reports exist on foams stabilized by crystals. However, these two kinds of liquid foams possess similarities in terms of stabilization mechanisms and the design of surfactant crystal systems. This field will certainly grow in the coming years, and it will contribute to the engineering of new soft materials not only for food but also for cosmetics, pharmaceuticals, and biomedical applications.

Bubbles in microfluidics: an all-purpose tool for micromanipulation

In recent decades, the integration of microfluidic devices and multiple actuation technologies at the microscale has greatly contributed to the progress of related fields. In particular, microbubbles are playing an increasingly important role in microfluidics because of their unique characteristics that lead to specific responses to different energy sources and gas-liquid interactions. Many effective and functional bubble-based micromanipulation strategies have been developed and improved, enabling various non-invasive, selective, and precise operations at the microscale. This review begins with a brief introduction of the morphological characteristics and formation of microbubbles. The theoretical foundations and working mechanisms of typical micromanipulations based on acoustic, thermodynamic, and chemical microbubbles in fluids are described. We critically review the extensive applications and the frontline advances of bubbles in microfluidics, including microflow patterns, position and orientation control, biomedical applications, and development of bubble-based microrobots. We lastly present an outlook to provide directions for the design and application of microbubble-based micromanipulation tools and attract the attention of relevant researchers to the enormous potential of microbubbles in microfluidics.

Trapping and control of bubbles in various microfluidic applications

As a simple, clean and effective tool, micro bubbles have enabled advances in various lab on a chip (LOC) applications recently. In bubble-based microfluidic applications, techniques for capturing and controlling the bubbles play an important role. Here we review active and passive techniques for bubble trapping and control in microfluidic applications. The active techniques are categorized based on various types of external forces from optical, electric, acoustic, mechanical and thermal fields. The passive approaches depend on surface tension, focusing on optimization of microgeometry and modification of surface properties. We discuss control techniques of size, location and stability of microbubbles and show how these bubbles are employed in various applications. To finalize, by highlighting the advantages of these approaches along with the current challenges, we discuss the future prospects of bubble trapping and control in microfluidic applications.

Alleviating tumor hypoxia with perfluorocarbon-based oxygen carriers

Hypoxia is a major impediment to many foremost cancer treatments that require O₂ for generation of tumoricidal reactive oxygen species. Liquid perfluorocarbons (PFCs) are inert gas solvents that help alleviate this oxygen deficit situation. PFC nanoemulsions have demonstrated oxygen delivery to tissues. The lifetime of ¹O₂ in PFCs is considerably expanded. PFC nanodroplets extravasate and accumulate in tumors. Alternatively, PFCs stabilize injectable O₂ microbubbles. On-demand local O₂ delivery is facilitated by ultrasound. Liquid PFC nanodroplets that convert into microbubbles upon activation provide another shuttle for O₂-delivery. PFC nanocarriers can also be enriched with fluorescent dyes, radiopaque materials, photo(sono)sensitizers, loaded with chemotherapeutics, and fitted with targeting devices, or stimuli-responsive functions for image-guided theranostics. We review recent literature on PFC-based O₂ carriers to enhance the efficacy of radiotherapy, photo(sono)dynamic therapy and chemotherapy. Of particular relevance to this series of reviews, PFC-based carriers may provide novel strategies to promote T-cell trafficking into tumors to improve immune responses.

Mass transfer between microbubbles

HYPOTHESIS

The role of interfacial coatings in gas transport dynamics in foam coarsening is often difficult to quantify. The complexity of foam coarsening measurements or gas transport measurements between bubbles requires assumptions about the liquid thin film thickness profile in order to explore the effects of interfacial coatings on gas transport. It should be possible to independently quantify the effects from changes in film thickness and interfacial permeability by using both atomic force microscopy and optical microscopy to obtain time snapshots of this dynamic process. Further, it is expected that the surfactant and polymer interfacial coatings will affect the mass transfer differently.

EXPERIMENTS

We measure the mass transfer between the same nitrogen microbubbles pairs in an aqueous solution using two methods simultaneously. First, we quantify the bubble volume changes with time via microscopy and second, we use Atomic Force Microscopy to measure the film thickness and mass transfer resistances using a model for the gas transport.

FINDINGS

Modelling of the interface deformation, surface forces and mass transfer across the thin film agrees with independent measurements of changes in bubble size. We demonstrate that an anionic surfactant does not provide a barrier to mass transfer, but does enhance mass transfer above the critical micelle concentration. In contrast, a polymer monolayer at the interface does restrict mass transfer.

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.

Ultrasound-responsive lipid microbubbles for drug delivery: A review of preparation techniques to optimise formulation size, stability and drug loading

Lipid-shelled microbubbles have received extensive interest to enhance ultrasound-responsive drug delivery outcomes due to their high biocompatibility. While therapeutic effectiveness of microbubbles is well established, there remain limitations in sample homogeneity, stability profile and drug loading properties which restrict these formulations from seeing widespread use in the clinical setting. In this review, we evaluate and discuss the most encouraging leads in lipid microbubble design and optimisation. We examine current applications in drug delivery for the systems and subsequently detail shell compositions and preparation strategies that improve monodispersity while retaining ultrasound responsiveness. We review how excipients and storage techniques help maximise stability and introduce different characterisation and drug loading techniques and evaluate their impact on formulation performance. The review concludes with current quality control measures in place to ensure lipid microbubbles can be reproducibly used in drug delivery.

Ultrasound-mediated therapies for the treatment of biofilms in chronic wounds: a review of present knowledge

Bacterial biofilms are an ever-growing concern for public health, featuring both inherited genetic resistance and a conferred innate tolerance to traditional antibiotic therapies. Consequently, there is a growing interest in novel methods of drug delivery, in order to increase the efficacy of antimicrobial agents. One such method is the use of acoustically activated microbubbles, which undergo volumetric oscillations and collapse upon exposure to an ultrasound field. This facilitates physical perturbation of the biofilm and provides the means to control drug delivery both temporally and spatially. In line with current literature in this area, this review offers a rounded argument for why ultrasound-responsive agents could be an integral part of advancing wound care. To achieve this, we will outline the development and clinical significance of biofilms in the context of chronic infections. We will then discuss current practices used in combating biofilms in chronic wounds and then critically evaluate the use of acoustically activated gas microbubbles as an emerging treatment modality. Moreover, we will introduce the novel concept of microbubbles carrying biologically active gases that may facilitate biofilm dispersal.

Effective delivery of mycophenolic acid by oxygen nanobubbles for modulating immunosuppression

Immunosuppressive drugs are crucial for preventing acute graft rejection or autoimmune diseases. They are generally small molecules that require suitable drug carriers for ensuring stability, bioavailability, and longer half-life. Mycophenolic acid (MPA) is an extensively studied immunosuppressive drug. However, it requires suitable carriers for overcoming clinical limitations. Currently, lipid-shelled micro- and nanobubbles are being thoroughly investigated for diagnostic and therapeutic applications, as they possess essential properties, such as injectability, smaller size, gaseous core, high surface area, higher drug payload, and enhanced cellular penetration. Phospholipids are biocompatible and biodegradable molecules, and can be functionalized according to specific requirements.

Methods: In this study, we synthesized oxygen nanobubbles (ONBs) and loaded the hydrophobic MPA within the ONBs to generate ONB/MPA. Peripheral blood mononuclear cells (PBMCs) were treated with ONB/MPA to determine the suppression of immune response by measuring cytokine release. In vivo murine experiments were performed to evaluate the effectiveness of ONB/MPA in the presence of inflammatory stimulants.

Results: Our results suggest that ONBs successfully delivered MPA and reduced the release of cytokines, thereby controlling inflammation and significantly increasing the survival rate of animals.

Conclusion: This method can be potentially used for implantation and for treating autoimmune diseases, wherein immunosuppression is desired.

Review on Microbubbles and Microdroplets Flowing through Microfluidic Geometrical Elements

Two-phase flows are found in several industrial systems/applications, including boilers and condensers, which are used in power generation or refrigeration, steam generators, oil/gas extraction wells and refineries, flame stabilizers, safety valves, among many others. The structure of these flows is complex, and it is largely governed by the extent of interphase interactions. In the last two decades, due to a large development of microfabrication technologies, many microstructured devices involving several elements (constrictions, contractions, expansions, obstacles, or T-junctions) have been designed and manufactured. The pursuit for innovation in two-phase flows in these elements require an understanding and control of the behaviour of bubble/droplet flow. The need to systematize the most relevant studies that involve these issues constitutes the motivation for this review. In the present work, literature addressing gas-liquid and liquid-liquid flows, with Newtonian and non-Newtonian fluids, and covering theoretical, experimental, and numerical approaches, is reviewed. Particular focus is given to the deformation, coalescence, and breakup mechanisms when bubbles and droplets pass through the aforementioned microfluidic elements.

Combining Nanoparticles with Colloidal Bubbles: A Short Review

Ultrasound and magneto-responsive nanosized drug delivery systems have been designed as novel carriers for controlled release. Colloidal bubbles (CBs) could be designed to incorporate different materials, such as protein, lipid, polymer, surfactants, and even nanoparticles in their shell, which makes them suitable for a wide range of drug delivery applications. The interior of CBs may be filled with different gases, which is essential for conferring the characteristics of an ultrasounds contrasting agent. Manipulating the core of CBs enhances features such as stability and duration of the echogenic effect. Thus CBs derivatized with nanoparticles combine functional properties of CBs and NPs to yield a versatile theranostics platform technology.

Recent advances in micro- and nano-bubbles for atherosclerosis applications

Micro- and nano-bubbles have been developed as powerful multimodal theranostic agents for atherosclerosis treatment.

Anti-Tumor Drug-Loaded Oxygen Nanobubbles for the Degradation of HIF-1α and the Upregulation of Reactive Oxygen Species in Tumor Cells

Hypoxia is a key concern during the treatment of tumors, and hypoxia-inducible factor 1 alpha (HIF-1α) has been associated with increased tumor resistance to therapeutic modalities. In this study, doxorubicin-loaded oxygen nanobubbles (Dox/ONBs) were synthesized, and the effectiveness of drug delivery to MDA-MB-231 breast cancer and HeLa cells was evaluated. Dox/ONBs were characterized using optical and fluorescence microscopy, and size measurements were performed through nanoparticle tracking analysis (NTA). The working mechanism of Dox was evaluated using reactive oxygen species (ROS) assays, and cellular penetration was assessed with confocal microscopy. Hypoxic conditions were established to assess the effect of Dox/ONBs under hypoxic conditions compared with normoxic conditions. Our results indicate that Dox/ONBs are effective for drug delivery, enhancing oxygen levels, and ROS generation in tumor-derived cell lines.

Vibration Characteristics and Persistence of Poloxamer- or Phospholipid-Coated Single Microbubbles under Ultrasound Irradiation

Microbubbles shelled with soft materials are expected to find applications as ultrasound-sensitive drug delivery systems, including through sonoporation. Microbubbles with specific vibrational characteristics and long intravascular persistence are required for clinical uses. To achieve this aim, the kinetics of the microbubble shell components at the gas/liquid interface while being subjected to ultrasound need to be better understood. This paper investigates the vibration characteristics and lifetime of single microbubbles coated with a poloxamer surfactant, Pluronic F-68, and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) under ultrasound irradiation. Air- and perfluorohexane (PFH)-filled microbubbles coated with Pluronic F-68 and DMPC at several concentrations (0 to 10^-2 mol L^-1) were produced. An optical measurement system using a laser Doppler vibrometer and microscope was used to observe the radial vibration mode of single microbubbles. The vibrational displacement amplitude and resonance radius of Pluronic- or DMPC-coated microbubbles were found to depend very little on the concentrations. The resonance radius was around 65 μm at 38.8 kHz under all the experimental conditions investigated. The lifetime of the microbubbles was investigated simultaneously by measuring their temporal change in volume, and it was increased with Pluronic concentration. Remarkably, the oscillation amplitude of the bubble has an effect on the bubble lifetime. In other words, larger oscillation under the resonance condition accelerates the diffusion of the inner gas.

Formulation and Characterization of Chemically Cross-linked Microbubble Clusters

The purpose of this study is to introduce a new concept of chemically cross-linked microbubble clusters (CCMCs), which are individual microbubble ultrasound contrast agents (UCAs) physically tethered together. We demonstrate a facile means of their production, characterize their size and stability, and describe how they can potentially be used in biomedical applications. By tethering UCAs together into CCMCs, we propose that novel methods of ultrasound mediated imaging and therapy can be developed through unique interbubble interactions in an ultrasound field. One of the major challenges in generating CCMCs is controlling aggregate sizes and maintaining stability against Ostwald ripening and coalescence. In this study, we demonstrate that chemically cross-linked microbubble clusters can produce small (<10 μm) quasi-stable complexes that slowly fuse into bubbles with individual gas cores. Furthermore, we demonstrate that this process can be driven with low-intensity ultrasound pulses, enabling a rapid fusion of clusters which could potentially be used to develop novel ultrasound contrast imaging and drug delivery strategies in future studies. The development of novel microbubble clusters presents a simple yet robust process for generating novel UCAs with a design that could allow for more versatility in contrast-enhanced ultrasound (CEUS), molecular imaging, and drug delivery applications. Additionally, microbubble clustering is a unique way to control size, shell, and gas compositions that can be used to study bubble ripening and coalescence in a highly controlled environment or study the behavior of mixed-microbubble populations.

Nanoparticle-Coated Microbubbles for Combined Ultrasound Imaging and Drug Delivery

Biomedical microbubbles stabilized by a coating of magnetic or drug-containing nanoparticles show great potential for theranostics applications. Nanoparticle-coated microbubbles can be made to be stable, to be echogenic, and to release the cargo of drug-containing nanoparticles with an ultrasound trigger. This Article reviews the design principles of nanoparticle-coated microbubbles for ultrasound imaging and drug delivery, with a particular focus on the physical chemistry of nanoparticle-coated interfaces; the formation, stability, and dynamics of nanoparticle-coated bubbles; and the conditions for controlled nanoparticle release in ultrasound. The emerging understanding of the modes of nanoparticle expulsion and of the transport of expelled material by microbubble-induced flow is paving the way toward more efficient nanoparticle-mediated drug delivery. This Article highlights the knowledge gap that still remains to be addressed before we can control these phenomena.

Tunable microfluidic standing air bubbles and its application in acoustic microstreaming

Microbubbles are often used in chemistry, biophysics, and medicine. Properly controlled microbubbles have been proved beneficial for various applications by previous scientific endeavors. However, there is still a plenty of room for further development of efficient microbubble handling methods. Here, this paper introduces a tunable, stable, and robust microbubble interface handling mechanism, named as microfluidic standing air bubbles (μSABs), by studying the multiphysical phenomena behind the gas-liquid interface formation and variation. A basic μSAB system consists specially structured fluidic channels, pneumatic channels, and selectively permeable porous barriers between them. The μSABs originate inside the crevice structures on the fluidic channel walls in a repeatable and robust manner. The volumetric variation of the μSAB is a multiphysical phenomenon that dominated by the air diffusion between the pneumatic channel and the bubble. Theoretical analysis and experimental data illustrate the coupling processes of the repeatable and linear μSAB volumetric variation when operated under common handling conditions (control pneumatic pressure: -90 kPa to 200 kPa). Furthermore, an adjustable acoustic microstreaming is demonstrated as an application using the alterable μSAB gas-liquid interface. Derived equations and microscopic observations elucidate the mechanism of the continuous and linear regulation of the acoustic microstreaming using varying μSAB gas-liquid interfaces. The μSAB system provides a new tool to handle the flexible and controllable gas-liquid interfaces in a repeatable and robust manner, which makes it a promising candidate for innovative biochemical, biophysical, and medical applications.

Microbubble Formulations: Synthesis, Stability, Modeling and Biomedical Applications

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

Surface Composition and Preparation Method for Oxygen Nanobubbles for Drug Delivery and Ultrasound Imaging Applications

Phospholipids have been widely investigated for the preparation of liposomes, and micro and nanobubbles. They comprise biocompatible and biodegradable molecules and offer simple preparation with a variety of functions in diagnostic and therapeutic applications. Phospholipids require emulsifiers and surfactants to assemble in the form of bubbles. These surfactants determine the size, zeta potential, and other characteristics of particles. Polyethylene glycol (PEG) and its various derivatives have been employed by researchers to synthesize micro and nanobubbles. The stability of phospholipid-shelled nanobubbles has been reported by various researchers owing to the reduction of surface tension by surfactants in the shell. Nanobubbles have been employed to deliver oxygen to tissues and hypoxic cells. In this study, we investigated the effects of different ratios of phospholipids to PEG on the size, distribution, and characterization of oxygen nanobubbles (ONBs). ONBs were synthesized using a sonication technique. We analyzed and compared the sizes, numbers of generated particles, and zeta potentials of different compositions of ONBs using dynamic light scattering and nanoparticle tracking analysis. Then, we employed these oxygen nanobubbles to enhance the cellular microenvironment and cell viability. ONBs were also investigated for ultrasound imaging.

Engineering Theranostic Microbubbles Using Microfluidics for Ultrasound Imaging and Therapy: A Review

Microbubbles interact with ultrasound in various ways to enable their applications in ultrasound imaging and diagnosis. To generate high contrast and maximize therapeutic efficacy, microbubbles of high uniformity are required. Microfluidic technology, which enables precise control of small volumes of fluid at the sub-millimeter scale, has provided a versatile platform on which to produce highly uniform microbubbles for potential applications in ultrasound imaging and diagnosis. Here, we describe fundamental microfluidic principles and the most common types of microfluidic devices used to produce sub-10 μm microbubbles, appropriate for biomedical ultrasound. Bubbles can be engineered for specific applications by tailoring the bubble size, inner gas and shell composition and by functionalizing for additional imaging modalities, therapeutics or targeting ligands. To translate the laboratory-scale discoveries to widespread clinical use of these microfluidic-based microbubbles, increased bubble production is needed. We present various strategies recently developed to improve scale-up. We conclude this review by describing some outstanding problems in the field and presenting areas for future use of microfluidics in ultrasound.

Oxygen-Carrying Micro/Nanobubbles: Composition, Synthesis Techniques and Potential Prospects in Photo-Triggered Theranostics

Microbubbles and nanobubbles (MNBs) can be prepared using various shells, such as phospholipids, polymers, proteins, and surfactants. MNBs contain gas cores due to which they are echogenic and can be used as contrast agents for ultrasonic and photoacoustic imaging. These bubbles can be engineered in various sizes as vehicles for gas and drug delivery applications with novel properties and flexible structures. Hypoxic areas in tumors develop owing to an imbalance of oxygen supply and demand. In tumors, hypoxic regions have shown more resistance to chemotherapy, radiotherapy, and photodynamic therapies. The efficacy of photodynamic therapy depends on the effective accumulation of photosensitizer drug in tumors and the availability of oxygen in the tumor to generate reactive oxygen species. MNBs have been shown to reverse hypoxic conditions, degradation of hypoxia inducible factor 1α protein, and increase tissue oxygen levels. This review summarizes the synthesis methods and shell compositions of micro/nanobubbles and methods deployed for oxygen delivery. Methods of functionalization of MNBs, their ability to deliver oxygen and drugs, incorporation of photosensitizers and potential application of photo-triggered theranostics, have also been discussed.

Engineering oxygen nanobubbles for the effective reversal of hypoxia

Hypoxia, which results from an inadequate supply of oxygen, is a major cause of concern in cancer therapy as it is associated with a reduction in the effectiveness of chemotherapy and radiotherapy in cancer treatment. Overexpression and stabilization of hypoxia-inducible factor 1α (HIF-1α) protein in tumours, due to hypoxia, results in poor prognosis and increased patient mortality. To increase oxygen tension in hypoxic areas, micro- and nanobubbles have been investigated by various researchers. In the present research, lipid-shelled oxygen nanobubbles (ONBs) were synthesized through a sonication method to reverse hypoxic conditions created in a custom-made hypoxic chamber. Release of oxygen gas from ONBs in deoxygenated water was evaluated by measuring dissolved oxygen. Hypoxic conditions were evaluated by performing in vitro experiments on MDA-MB231 breast cancer cells through the expression of HIF-1α and the fluorescence of image-iT™ hypoxia reagent. The results indicated the degradation of HIF-1α after the introduction of ONBs. We propose that ONBs are successful in reversing hypoxia, downregulating HIF-1α, and improving cellular conditions, leading to further medical applications.