The Optimized Fabrication of Nanobubbles as Ultrasound Contrast Agents for Tumor Imaging

Effective strategies to enhance the diagnosis and treatment of RCC: The application of biocompatible materials

Renal cell carcinoma (RCC) is recognized as one of the three primary malignant tumors affecting the urinary system, posing a significant risk to human health and life. Despite advancements in understanding RCC, challenges persist in its diagnosis and treatment, particularly in early detection and diagnosis due to issues of low specificity and sensitivity. Consequently, there is an urgent need for the development of effective strategies to enhance diagnostic accuracy and treatment outcomes for RCC. In recent years, with the extensive research on materials for applications in the biomedical field, some materials have been identified as promising for clinical applications, e.g., in the diagnosis and treatment of many tumors, including RCC. Herein, we summarize the latest materials that are being studied and have been applied in the early diagnosis and treatment of RCC. While focusing on their adjuvant effects, we also discuss their technical principles and safety, thus highlighting the value and potential of their application. In addition, we also discuss the limitations of the application of these materials and possible future directions, providing new insights for improving RCC diagnosis and treatment.

Smart nanocarriers for enzyme-activated prodrug therapy

Exogenous enzyme-activated prodrug therapy (EPT) is a potential cancer treatment strategy that delivers non-human enzymes into or on the surface of the cell and subsequently converts a non-toxic prodrug into an active cytotoxic substance at a specific location and time. The development of several pharmacological pairs based on EPT has been the focus of anticancer research for more than three decades. Numerous of these pharmacological pairs have progressed to clinical trials, and a few have achieved application in specific cancer therapies. The current review highlights the potential of enzyme-activated prodrug therapy as a promising anticancer treatment. Different microbial, plant, or viral enzymes and their corresponding prodrugs that advanced to clinical trials have been listed. Additionally, we discuss new trends in the field of enzyme-activated prodrug nanocarriers, including nanobubbles combined with ultrasound (NB/US), mesoscopic-sized polyion complex vesicles (PICsomes), nanoparticles, and extracellular vesicles (EVs), with special emphasis on smart stimuli-triggered drug release, hybrid nanocarriers, and the main application of nanotechnology in improving prodrugs.

Radical generation and bactericidal activity of nanobubbles produced by ultrasonic irradiation of carbonated water

Our previous study showed that nanobubbles (NBs) encapsulating CO2 gas have bactericidal activity due to reactive oxygen species (ROS) (Yamaguchi et al., 2020). Here, we report that bulk NBs encapsulating CO2 can be efficiently generated by ultrasonically irradiating carbonated water using a piezoelectric transducer with a frequency of 1.7 MHz. The generated NBs were less than 100 nm in size and had a lifetime of 500 h. Furthermore, generation of ROS in the NB suspension was investigated using electron spin resonance spectroscopy and fluorescence spectrometry. The main ROS was found to be the hydroxyl radical, which is consistent with our previous observations. The bactericidal activity lasted for at least one week. Furthermore, a mist generated by atomizing the NB suspension with ultrasonic waves was confirmed to have the same bactericidal activity as the suspension itself. We believe that the strong, persistent bactericidal activity and radical generation phenomenon are unique to NBs produced by ultrasonic irradiation of carbonated water. We propose that entrapped CO2 molecules strongly interact with water at the NB interface to weaken the interface, and high-pressure CO2 gas erupts from this weakened interface to generate ROS with bactericidal activity.

Exploring the Theranostic Applications and Prospects of Nanobubbles

Anticancer medications as well as additional therapeutic compounds, have poor clinical effectiveness due to their diverse distribution, non-selectivity for malignant cells, and undesirable off-target side effects. As a result, ultrasound-based targeted delivery of therapeutic compounds carried in sophisticated nanocarriers has grown in favor of cancer therapy and control. Nanobubbles are nanoscale bubbles that exhibit unique physiochemical properties in both their inner core and outer shell. Manufacturing nanobubbles primarily aims to enhance therapeutic agents' bioavailability, stability, and targeted delivery. The small size of nanobubbles allows for their extravasation from blood vessels into surrounding tissues and site-specific release through ultrasound targeting. Ultrasound technology is widely utilized for therapy due to its speed, safety, and cost-effectiveness, and micro/nanobubbles, as ultrasound contrast agents, have numerous potential applications in disease treatment. Thus, combining ultrasound applications with NBs has recently demonstrated increased localization of anticancer molecules in tumor tissues with triggered release behavior. Consequently, an effective therapeutic concentration of drugs/genes is achieved in target tumor tissues with ultimately increased therapeutic efficacy and minimal side effects on other non-cancerous tissues. This paper provides a brief overview of the production processes for nanobubbles, along with their key characteristics and potential therapeutic uses.

Two-dimensional water-molecule-cluster layers at nanobubble interfaces

HYPOTHESIS

Bulk nanobubbles (NBs) have high surface charge densities and long lifetimes. Despite several attempts to understand the lifetime of NBs, their interfacial layer structure remains unknown. It is hypothesized that a specific interfacial layer exists with a hydrogen bond network that stabilizes NBs.

EXPERIMENTS

In situ infrared reflectance-absorption spectroscopy and density functional theory were used to determine the interfacial layer structure of NBs. Furthermore, nuclear magnetic resonance spectroscopy was used to examine the interfacial layer hardness of bubbles filled with N2, O2, and CO2, which was expected to depend on the encapsulated gas species.

FINDINGS

The interfacial layer was composed of three-, four-, and five-membered ring clusters of water molecules. An interface model was proposed in which a two-dimensional layer of clusters with large electric dipole moments is oriented toward the endohedral gas, and the hydrophobic surface is adjacent to the free water. The interfacial layer hardness was dependent on the interaction with the gas (N2 > O2 > CO2), which supports the proposed interface model. These findings can be generalized to the structure of water at gas-water interfaces.

Ultrasound-Responsive Nanobubbles for Combined siRNA-Cerium Oxide Nanoparticle Delivery to Bone Cells

This study aims to present an ultrasound-mediated nanobubble (NB)-based gene delivery system that could potentially be applied in the future to treat bone disorders such as osteoporosis. NBs are sensitive to ultrasound (US) and serve as a controlled-released carrier to deliver a mixture of Cathepsin K (CTSK) siRNA and cerium oxide nanoparticles (CeNPs). This platform aimed to reduce bone resorption via downregulating CTSK expression in osteoclasts and enhance bone formation via the antioxidant and osteogenic properties of CeNPs. CeNPs were synthesized and characterized using transmission electron microscopy and X-ray photoelectron spectroscopy. The mixture of CTSK siRNA and CeNPs was adsorbed to the surface of NBs using a sonication method. The release profiles of CTSK siRNA and CeNPs labeled with a fluorescent tag molecule were measured after low-intensity pulsed ultrasound (LIPUS) stimulation using fluorescent spectroscopy. The maximum release of CTSK siRNA and the CeNPs for 1 mg/mL of NB-(CTSK siRNA + CeNPs) was obtained at 2.5 nM and 1 µg/mL, respectively, 3 days after LIPUS stimulation. Then, Alizarin Red Staining (ARS) was applied to human bone marrow-derived mesenchymal stem cells (hMSC) and tartrate-resistant acid phosphatase (TRAP) staining was applied to human osteoclast precursors (OCP) to evaluate osteogenic promotion and osteoclastogenic inhibition effects. A higher mineralization and a lower number of osteoclasts were quantified for NB-(CTSK siRNA + CeNPs) versus control +RANKL with ARS (p < 0.001) and TRAP-positive staining (p < 0.01). This study provides a method for the delivery of gene silencing siRNA and CeNPs using a US-sensitive NB system that could potentially be used in vivo and in the treatment of bone fractures and disorders such as osteoporosis.

Green Cleaning of 3D-Printed Polymeric Products by Micro-/Nano-Bubbles

3D printing technology has been used to directly produce various actual products, ranging from engines and medicines to toys, especially due to its advantage in producing items of complicated, porous structures, which are inherently difficult to clean. Here, we apply micro-/nano-bubble technology to the removal of oil contaminants from 3D-printed polymeric products. Micro-/nano-bubbles show promise in the enhancement of cleaning performance with or without ultrasound, which is attributed to their large specific surface area enhancing the adhesion sites of contaminants, and their high Zeta potential which attracts contaminant particles. Additionally, bubbles produce tiny jets and shock waves at their rupture, driven by coupled ultrasound, which can remove sticky contaminants from 3D-printed products. As an effective, efficient, and environmentally friendly cleaning method, micro-/nano-bubbles can be used in a range of applications.

On Nanobubble Dynamics under an Oscillating Pressure Field during Salting-out Effects and Its DLVO Potential

We have investigated the origin, stability, and nanobubble dynamics under an oscillating pressure field followed by the salting-out effects. The higher solubility ratio (salting-out parameter) of the dissolved gases and pure solvent nucleates nanobubbles during the salting-out effect, and the oscillating pressure field enhances the nanobubble density further as solubility varies linearly with gas pressure by Henry's law. A novel method for refractive index estimation is developed to differentiate nanobubbles and nanoparticles based on the scattering intensity of light. The electromagnetic wave equations have been numerically solved and compared with the Mie scattering theory. The scattering cross-section of the nanobubbles was estimated to be smaller than the nanoparticles. The DLVO potentials of the nanobubbles predict the stable colloidal system. The zeta potential of nanobubbles varied by generating nanobubbles in different salt solutions, and it is characterized by particle tracking, dynamic light scattering, and cryo-TEM. The size of nanobubbles in salt solutions was reported to be higher than that in pure water. The novel mechanical stability model is proposed by considering both ionic cloud and electrostatic pressure at the charged interface. The ionic cloud pressure is derived by electric flux balance, and it is found to be twice the electrostatic pressure. The mechanical stability model for a single nanobubble predicts the existence of stable nanobubbles in the stability map.

Novel technologies to improve the treatment of endodontic microbial infections: Inputs from a drug delivery perspective

Endodontic microbial infections are still a challenge for an effective treatment for being biofilm-mediated and very refractory to conventional therapies. Biomechanical preparation and chemical irrigants cannot fully eradicate biofilms due to the anatomic structure of the root canal system. Instruments employed in biomechanical preparation and irrigants solution cannot reach the narrow and deepest portion of root canals, especially the apical thirds. In addition, aside from the dentin surface, biofilms can also infiltrate dentine tubules and periapical tissues, compromising treatment success. Therefore, different technologies have been investigated to achieve a more effective outcome in the control of endodontic infections. However, these technologies continue to face great difficulties in reaching the apical region and eradicating biofilms to avoid the recurrence of infection. Here, we present an overview of the fundamentals of endodontics infections and review technologies currently available for root canal treatment. We discuss them from a drug delivery perspective, highlighting each technology's strength to envision the best use of these technologies.

Microbubbles for human diagnosis and therapy

Microbubbles (MBs) were observed for the first time in vivo as a curious consequence of quick saline injection during ultrasound (US) imaging of the aortic root, more than 50 years ago. From this serendipitous event, MBs are now widely used as contrast enhancers for US imaging. Their intrinsic properties described in this review, allow a multitude of designs, from shell to gas composition but also from grafting targeting agents to drug payload encapsulation. Indeed, the versatile MBs are deeply studied for their dual potential in imaging and therapy. As presented in this paper, new generations of MBs now opens perspectives for targeted molecular imaging along with the development of new US imaging systems. This review also presents an overview of the different therapeutic strategies with US and MBs for cancer, cardiovascular diseases, and inflammation. The overall aim is to overlap those fields in order to find similarities in the MBs application for treatment enhancement associated with US. To conclude, this review explores the new scales of MBs technologies with nanobubbles development, and along concurrent advances in the US imaging field. This review ends by discussing perspectives for the booming future uses of MBs.

The Combined Effect of Nanobubble-IR783-HPPH-Affibody Complex and Laser on HER2-Positive Breast Cancer

Introduction

Nanobubble is an innovative ultrasound contrast agent that triggers the development of targeted imaging of HER2-positive breast cancer by combining with HER2 affibody and IR783. HPPH is a second-generation photosensitiser that is effective in treating tumours. Hence, the nanobubble-IR783-HPPH-affibody (NIHA) complex demonstrates considerable potential in the treatment of HER2-positive breast cancer.

Methods

We fabricated the NIHA complex via an advanced thin-film hydration method and detected its characteristics such as particle size, morphology, stability, and cytotoxicity. Moreover, the effect of NIHA complex with laser on HER2-positive breast cancer was confirmed via in vitro and in vivo experiments.

Results

The NIHA complex was spheroid, stable and exhibited no cytotoxicity; moreover, its particle size was 524.8 ± 53.3 nm (n = 5). In combination with laser treatment, NIHA complex reduced the cell viability and tumour volume, induced apoptosis of HER2-positive breast cancer cells, and prolonged survival of nude mice.

Conclusion

The newly prepared NIHA complex with laser treatment has the potential on treating HER2-positive breast cancer.

Nanomedicine for renal cell carcinoma: imaging, treatment and beyond

The kidney is a vital organ responsible for maintaining homeostasis in the human body. However, renal cell carcinoma (RCC) is a common malignancy of the urinary system and represents a serious threat to human health. Although the overall survival of RCC has improved substantially with the development of cancer diagnosis and management, there are various reasons for treatment failure. Firstly, without any readily available biomarkers, timely diagnosis has been greatly hampered. Secondly, the imaging appearance also varies greatly, and its early detection often remains difficult. Thirdly, chemotherapy has been validated as unavailable for treating renal cancer in the clinic due to its intrinsic drug resistance. Concomitant with the progress of nanotechnological methods in pharmaceuticals, the management of kidney cancer has undergone a transformation in the recent decade. Nanotechnology has shown many advantages over widely used traditional methods, leading to broad biomedical applications ranging from drug delivery, prevention, diagnosis to treatment. This review focuses on nanotechnologies in RCC management and further discusses their biomedical translation with the aim of identifying the most promising nanomedicines for clinical needs. As our understanding of nanotechnologies continues to grow, more opportunities to improve the management of renal cancer are expected to emerge.

Medical Imaging Technology and Imaging Agents

Medical imaging is a technology that studies the interaction between human body and irradiations of X-ray, ultrasound, magnetic field, etc. and represents anatomical structures of human organs/tissues with the implication of irradiation attenuation in the form of grayscales. With these medical images, detailed information on health status and disease diagnosis may be judged by clinical physicians to determine an appropriate therapy approach. This chapter will give a systematic introduction on the modalities, classifications, basic principles, and biomedical applications of traditional medical imaging along with the types, construction, and major features of the corresponding contrast agents or imaging probes.

Radiopaque Iodosilane-Coated Lipid Hybrid Nanoparticle Contrast Agent for Dual-Modality Ultrasound and X-ray Bioimaging

Here, we report the synthesis of robust hybrid iodinated silica-lipid nanoemulsions (HSLNEs) for use as a contrast agent for ultrasound and X-ray applications. We engineered iodinated silica nanoparticles (SNPs), lipid nanoemulsions, and a series of HSLNEs by a low-energy spontaneous nanoemulsification process. The formation of a silica shell requires sonication to hydrolyze and polymerize/condensate the iodomethyltrimethoxysilane at the oil/water interface of the nanoemulsion droplets. The resulting nanoemulsions (NEs) exhibited a homogeneous spherical morphology under transmission electron microscopy. The particles had diameters ranging from 20 to 120 nm with both negative and positive surface charges in the absence and presence of cetyltrimethylammonium bromide (CTAB), respectively. Unlike CTAB-coated nanoformulations, the CTAB-free NEs showed excellent biocompatibility in murine RAW macrophages and human U87-MG cell lines in vitro. The maximum tolerated dose assessment was evaluated to verify their safety profiles in vivo. In vitro X-ray and ultrasound imaging and in vivo computed tomography were used to monitor both iodinated SNPs and HSLNEs, validating their significant contrast-enhancing properties and suggesting their potential as dual-modality clinical agents in the future.

Albumin microbubbles conjugated with zinc and aluminum phthalocyanine dyes for enhanced photodynamic activity

Gas-liquid interfaces are reaching a particular interest in biomedicine. Microbubbles, ultrasound contrast agents of clinical routine, gained increasing attention as theranostic platforms due to the preserved acoustic response, drug conjugation capabilities, and applicability in biological barrier opening. A combination of microbubbles and photodynamic therapy agents can enhance the photodynamic effect, yet the evaluation of agent conjugation on microbubble stabilization and photodynamic effect is needed. Hence, two commercially available phthalocyanine photosensitizers - Holosens® (ZnPc) and Photosens® (AlPc) - were coupled with bovine serum albumin before microbubble synthesis. We demonstrated an albumin: phthalocyanine ratio of 1:1 and covalent attachment for ZnPc, a ratio of 1:3 with electrostatic binding for AlPc. Submicron-sized microbubbles (air- and SF6- filled) had a diameter of 0.8 µm. Albumin-phthalocyanine conjugates increased the microbubble concentration and shelf-life stability compared to plain ones. We hypothesized that phthalocyanine fluorescence lifetime values decreased after conjugation with microbubbles due to narrow distance between conjugates in the shell. Agents based on AlPc demonstrated higher photodynamic activity than agents based on ZnPc, and microbubbles preserved acoustic stability in human blood plasma. The biodistribution of AlPc-conjugated microbubbles was evaluated. We conclude that our microbubble platforms demonstrate greater photodynamic activity and prolonged stability for further applications in photodynamic therapy.

Effect of fine bubbles for washing of monolith type porous ceramic membranes treating oil-in-water emulsions

Produced water generated in the recovery of crude oil contains oil and high concentrations of salts, organic matter, and suspended solids and must therefore be treated appropriately prior to disposal. Monolithic ceramic membranes have high oil removal rates and have the advantage of being compact, having a long life, and withstanding chemicals, heat, and high cleaning pressures. Membrane fouling, however, is a significant drawback to membrane filtration. Scrubbing using air bubbles generated by a diffuser is generally used to physically clean membranes. However, monolithic ceramic membranes cannot be scrubbed using air bubbles because their fluid channels are only a few millimeters wide. Membrane washing efficiency was therefore evaluated using fine bubbles smaller than the diameter of the channels. In dead-end filtration, flushing the membrane surface with air-microbubble water or air-ultra-fine bubble (UFB) water after backwashing and air-blowing (conventional cleaning) of the channels was more efficient than conventional cleaning. Flushing with UFB water was not influenced by changes in pH that changed the zeta potential of the UFB. Membrane fouling was suppressed in crossflow filtration by mixing UFB water with feed water. There was no significant change in the diameter of the oil droplets in the feed water before and after UFB mixing. The ZP of the oil droplets peaked at around -20 mV before UFB mixing. However, the peak shifted to around -25 to -29 mV after UFB mixing.

Biomimetic nanobubbles for triple-negative breast cancer targeted ultrasound molecular imaging

Triple-negative breast cancer (TNBC) is a highly heterogeneous breast cancer subtype with poor prognosis. Although anatomical imaging figures prominently for breast lesion screening, TNBC is often misdiagnosed, thus hindering early medical care. Ultrasound (US) molecular imaging using nanobubbles (NBs) capable of targeting tumor cells holds great promise for improved diagnosis and therapy. However, the lack of conventional biomarkers in TNBC impairs the development of current targeted agents. Here, we exploited the homotypic recognition of cancer cells to synthesize the first NBs based on TNBC cancer cell membrane (i.e., NB_CCM) as a targeted diagnostic agent. We developed a microfluidic technology to synthesize NB_CCM based on the self-assembly property of cell membranes in aqueous solutions. In vitro, optimal NB_CCM had a hydrodynamic diameter of 683 ± 162 nm, showed long-lasting US contrast enhancements and homotypic affinity. In vivo, we demonstrated that NB_CCM showed increased extravasation and retention in a TNBC mouse model compared to non-targeted NBs by US molecular imaging. Peak intensities and areas under the curves from time-intensity plots showed a significantly enhanced signal from NB_CCM compared to non-targeted NBs (2.1-fold, P = 0.004, and, 3.6-fold, P = 0.0009, respectively). Immunofluorescence analysis further validated the presence of NB_CCM in the tumor microenvironment. Circumventing the challenge for universal cancer biomarker identification, our approach could enable TNBC targeting regardless of tumor tissue heterogeneity, thus improving diagnosis and potentially gene/drug targeted delivery. Ultimately, our approach could be used to image many cancer types using biomimetic NBs prepared from their respective cancer cell membranes.

Nanobubbles are Non-Echogenic for Fundamental-Mode Contrast-Enhanced Ultrasound Imaging

Microbubbles (1-10 μm diameter) have been used as conventional ultrasound contrast agents (UCAs) for applications in contrast-enhanced ultrasound (CEUS) imaging. Nanobubbles (<1 μm diameter) have recently been proposed as potential extravascular UCAs that can extravasate from the leaky vasculature of tumors or sites of inflammation. However, the echogenicity of nanobubbles for CEUS remains controversial owing to prior studies that have shown very low ultrasound backscatter. We hypothesize that microbubble contamination in nanobubble formulations may explain the discrepancy. To test our hypothesis, we examined the size distributions of lipid-coated nanobubble and microbubble suspensions using multiple sizing techniques, examined their echogenicity in an agar phantom with fundamental-mode CEUS at 7 MHz and 330 kPa peak negative pressure, and interpreted our results with simulations of the modified Rayleigh-Plesset model. We found that nanobubble formulations contained a small contamination of microbubbles. Once the contribution from these microbubbles is removed from the acoustic backscatter, the acoustic contrast of the nanobubbles was shown to be near noise levels. This result indicates that nanobubbles have limited utility as UCAs for CEUS.

Ultrasound and magnetic resonance imaging of cyclic arginine glycine aspartic acid-gadopentetic acid-polylactic acid in human breast cancer by targeting αvβ3 in xenograft-bearing nude mice

Effective early detection shows the potential to reduce breast cancer mortality. This study aimed to establish a targeted contrast agent for Magnetic Resonance Imaging (MRI)/ultrasound dual-modality molecular radiography for breast cancer. The cyclic arginine-glycine-aspartate-gadopentetic acid-polylactic acid (cRGD and Gd-DTPA) coated by multi-functional blank poly (lactic-co-glycolic acid) (PLGA) nanoparticles) was successfully constructed by chemical synthesis method with high stability. The safety of cRGD-Gd-DTPA-PLGA was demonstrated in vitro and in vivo, and their affinity to breast cancer cells was revealed. Moreover, MRI/ultrasound dual-modality molecular radiography in vitro showed that as the concentration of contrast agent increased, the echo enhancement and signal intensity of MRI imaging were also elevated. The mouse models of human breast cancer also indicated significant target enhancements of cRGD-Gd-DTPA-PLGA magnetic nanoparticles in the mouse tumor. Thus, cRGD-Gd-DTPA-PLGA magnetic nanoparticles were suggested as qualified MRI/ultrasound dual-modality molecular radiography contrast agent. We further explored the targeting mechanism of cRGD-Gd-DTPA-PLGA in breast cancer. The results showed that αvβ3 was highly expressed in breast cancer tissues, and cRGD-Gd-DTPA-PLGA used for MRI/ultrasound dual-modality molecular radiography by targeting αvβ3. Additionally, we found that the signal-to-noise ratio of MRI was positively correlated with microvessel density (MVD). The cRGD-Gd-DTPA-PLGA dynamicly and quantitatively monitored breast cancer by monitoring the state of neovascularization. In conclusion, in the present study, we successfully constructed the cRGD-Gd-DTPA-PLGA magnetic nanoparticles for MRI/ultrasound dual-modality molecular radiography. The cRGD-Gd-DTPA-PLGA showed potential in early detection and diagnosis of metastasis, and dynamic evaluation of the efficacy of molecular targeted therapy of integrin αvβ3.

Ultrasound-sensitive siRNA-loaded nanobubbles fabrication and antagonism in drug resistance for NSCLC

Due to the lack of safe, effective, and gene-targeted delivery technology. In this study, we have prepared nanobubbles loaded PDLIM5 siRNA (PDLIM5siRNA-NBs) to investigate the transfection efficiency and their antagonism in drug resistance in combination with ultrasound irradiation for non-small-cell lung cancer (NSCLC). Research results show that the PDLIM5 siRNA are effectively bound to the shell of NBs with a mean diameter of 191.6 ± 0.50 nm and a Zeta potential of 11.8 ± 0.68 mV. And the ultrasonic imaging indicated that the PDLIM5 siRNA NBs maintain the same signals as the microbubbles (SonoVue). Under the optimized conditions of 0.5 W/m² ultrasound intensity and 1 min irradiation duration, the highest transfection efficiency of PC9GR cells was 90.23 ± 1.45%, which resulted in the inhibition of PDLIM5 mRNA and protein expression. More importantly, the anti-tumor effect of fabricated PDLIM5siRNA-NBs with the help of ultrasound irradiation has been demonstrated to significantly inhibit tumor cell growth and promote apoptosis. Therefore, NBs carrying PDLIM5siRNA may have the potential to act as gene vectors combined with ultrasound irradiation to antagonize drug resistance for NSCLC.

Review of Oxygenation with Nanobubbles: Possible Treatment for Hypoxic COVID-19 Patients

The coronavirus disease (COVID-19) pandemic, which has spread around the world, caused the death of many affected patients, partly because of the lack of oxygen arising from impaired respiration or blood circulation. Thus, maintaining an appropriate level of oxygen in the patients' blood by devising alternatives to ventilator systems is a top priority goal for clinicians. The present review highlights the ever-increasing application of nanobubbles (NBs), miniature gaseous vesicles, for the oxygenation of hypoxic patients. Oxygen-containing NBs can exert a range of beneficial physiologic and pharmacologic effects that include tissue oxygenation, as well as tissue repair mechanisms, antiinflammatory properties, and antibacterial activity. In this review, we provide a comprehensive survey of the application of oxygen-containing NBs, with a primary focus on the development of intravenous platforms. The multimodal functions of oxygen-carrying NBs, including antimicrobial, antiinflammatory, drug carrying, and the promotion of wound healing are discussed, including the benefits and challenges of using NBs as a treatment for patients with acute hypoxemic respiratory failure, particularly due to COVID-19.

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.

Coarsening behavior of bulk nanobubbles in water

In recent years, minuscule gas bubbles called bulk nanobubbles (BNBs) have drawn increasing attention due to their unique properties and broad applicability in various technological fields, such as biomedical engineering, water treatment, and nanomaterials. However, questions remain regarding the stability and behavior of BNBs. In the present work, BNBs were generated in water using a gas-liquid mixing method. NB analysis was performed using a nanoparticle tracking analysis (NTA) method to investigate the coarsening behavior of BNBs in water over time. The diameters of the BNBs increased, and their cubic radii increased linearly (r³ ~ t) over time. While the concentration of BNBs decreased, the total volume of BNBs remained the same. The size distribution of the BNBs broadened, and the concentration of larger BNBs increased over time. These results indicate that relatively small BNBs disappeared due to dissolution and larger BNBs grew through mass transfer between BNBs instead of coalescence. In other words, BNBs underwent Ostwald ripening: gas molecules from smaller BNBs diffused through the continuous phase to be absorbed into larger BNBs.

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.

Liposomes-based nanoplatform enlarges ultrasound-related diagnostic and therapeutic precision

Ultrasound (US) is notable in the medical field as a safe and effective imaging modality due to its lack of ionizing radiation, non-invasive approach, and real-time monitoring capability. Accompanying recent progress in nanomedicine, US has been providing hope of theranostic capability not only for imaging-based diagnosis but also for US-based therapy by taking advantage of the bioeffects induced by US. Cavitation, sonoporation, thermal effects, and other cascade effects stimulated by acoustic energy conversion have contributed to medical problem-solving in the past decades although to varying degrees of efficacy in comparisons to other methods. Recently, the usage of liposomes-based nanoplatform fuels the development of nanomedicine and provides novel clinical strategies for antitumor, thrombolysis, and controlled drug release. Merging of novel liposome-based nanoplatforms and US-induced reactions has promise for a new blueprint for future medicine. In the present review article, the value of liposome-based nanoplatforms in US-related diagnosis and therapy will be discussed and summarized along with potential future directions for further investigations.

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.

High-Frequency Array-Based Nanobubble Nonlinear Imaging in a Phantom and In Vivo

There has been growing interest in nanobubbles (NBs) for vascular and extravascular ultrasound contrast imaging and therapeutic applications. Studies to date have generally utilized low frequencies (<12 MHz), high concentrations (>10⁹ mL^-1), and uncalibrated B-mode or contrast-mode on commercial systems without reporting investigations on NB signatures upon which the imaging protocols should be based. We recently demonstrated that low concentrations (10⁶ mL^-1) of porphyrin-lipid-encapsulated NBs scatter nonlinearly at low (2.5, 8 MHz) and high (12.5, 25, 30 MHz) frequencies in a pressure threshold-dependent manner that is advantageous for amplitude modulation (AM) imaging. Here, we implement pressure-calibrated AM at high frequency on a commercial preclinical array system to enhance sensitivity to nonlinear scattering of three phospholipid-based NB formulations. With this approach, improvements in contrast to tissue ratio relative to B-mode between 12.4 and 22.8 dB are demonstrated in a tissue-mimicking phantom, and between 6.7 and 14.8 dB in vivo.

Nanobubble technology in anaerobic digestion: A review

Nanobubble technology has significant potential to improve the anaerobic digestion (AD) process by ameliorating the rate-limiting steps of hydrolysis and methanogenesis, as well as providing process stability by reducing sulfide and volatile fatty acid (VFA) levels. Nanobubbles (NB) can enhance substrate accessibility, digestibility, and enzymatic activity due to their minuscule size, high electrostatic interaction, and ability to generate reactive oxygen species. Air- and O₂-NB can create a microaerobic environment for higher efficiency of the electron transport system, thereby reducing VFAs through enhanced facultative bacterial activity. Additionally, H₂- and CO₂-NB can improve hydrogenotrophic methanogenesis. Recently, several studies have employed NB technology in the AD process. There is, however, a lack of concise, synthesized information on NB applications to the AD process. This review provides an in-depth discussion on the NB-integrated AD process and the putative mechanisms involved. General discussions on other potential applications and future research directions are also provided.

Microfluidic Generation of Monodisperse Nanobubbles by Selective Gas Dissolution

Nanotechnology currently enables the fabrication of uniform solid nanoparticles and liquid nano-emulsions, but not uniform gaseous nanobubbles (NBs). In this article, for the first time, a method based on microfluidics that directly produces monodisperse NBs is reported. Specifically, a two-component gas mixture of water-soluble nitrogen and water-insoluble octafluoropropane as the gas phase are used in a microfluidic bubble generator. First, monodisperse microbubbles (MBs) with a classical microfluidic flow-focusing junction is generated, then the MBs shrink down to ≈100 nm diameter, due to the dissolution of the water-soluble components in the gas mixture. The degree of shrinkage is controlled by tuning the ratio of water-soluble to water-insoluble gas components. This technique maintains the monodispersity of the NBs, and enables precise control of the final NB size. It is found that the monodisperse NBs show better homogeneity than polydisperse NBs in in vitro ultrasound imaging experiments. Proof-of-concept in vivo kidney imaging is performed in live mice, demonstrating enhanced contrast using the monodisperse NBs. The NB monodispersity and imaging results make microfluidically generated NBs promising candidates as ultrasound contrast and molecular imaging agents.

Preparation of multifunctional nanobubbles and their application in bimodal imaging and targeted combination therapy of early pancreatic cancer

Pancreatic cancer will gradually become the second leading cause of cancer death due to its poor suitability for surgical treatment, frequent recurrence and metastasis, and insensitivity to radiotherapy and chemotherapy. Strategies for precise early detection and effective targeted treatment of pancreatic cancer are urgently needed. Because of its unique advantages, molecular targeted contrast-enhanced ultrasound imaging (CEUI) has generated new opportunities to overcome this challenge. The aim of this study was to explore multifunctional nanobubbles named IR780-NBs-DTX as novel ultrasound contrast agents (UCAs) for dual-mode targeted imaging and photothermal ablation combined with chemotherapy for pancreatic cancer. An optimized “film hydration method” was used to prepare IR780-NBs-DTX in this research. The characteristics and ability of the new UCAs were detected via in vitro, in vivo and ex vivo experiments. The initial dose of 0.15 mg IR-780 iodide/1.0 mg DTX was considered to be the best formula for IR780-NBs-DTX, and the concentration of 6 ×10⁶ bubbles/mL was best for CEUI. The excellent characteristics of IR780-NBs-DTX, including a uniform nanoscale particle size (349.8± 159.1 nm, n= 3), good performance in dual-mode imaging, high stability and reliable biocompatibility, were also proven. In the in vitro cell experiments, IR780-NBs-DTX targeted more pancreatic cancer cells than the control treatments, and the targeting rate was approximately 95.6± 1.7%. Under irradiation with an 808 nm laser, most cells died. Furthermore, the in vivo study demonstrated that IR780-NBs-DTX could precisely detect pancreatic cancer through near infrared fluorescence (NIRF) imaging and CEUI, and the tumor almost disappeared at 18 days after combined treatment. In ex vivo experiments, immunohistochemistry (IHC) and immunofluorescence (IF) showed that the expression of HSP70 increased and that of PCNA decreased, and many apoptotic tumor cells were observed by TUNEL staining in the IR780-NBs-DTX group. The newly prepared IR780-NBs-DTX are novel nanosized UCAs with high efficiency for dual-mode molecular targeted imaging and combined therapy, and they may have future potential applications in the precise detection and effective targeted therapy of small and metastatic lesions in the early stage of pancreatic cancer.

Destabilization of ultrafine bubbles in water using indirect ultrasonic irradiation

Ultrafine bubble (UFB) is a bubble with a diameter of less than 1 μm. Little attention has been paid to the defoaming and removal of UFBs. This study proposes a method to destabilize UFBs by using indirect ultrasonic irradiation. Besides, the destabilization mechanism of UFB was investigated. The ultrasonic frequency was 1.6 MHz and the dissipated power was 30 W. UFB dispersions were prepared using two different types of bubble generators: pressurized dissolution method and swirling liquid flow method. The effects of ultrasonic irradiation on the stability of UFBs were evaluated by particle tracking analysis (PTA) and electrophoretic zeta potential measurement. Results showed that the indirect ultrasonic irradiation for 30 min reduced the number concentration of UFBs by 90% regardless of the generation method. This destabilization was attributed to a decrease in the magnitude of zeta potential of UFBs due to the changes in pH and electrical conductivity. These changes in the electrochemical properties were caused by the formation of nitric acid. To study the destabilization mechanism, the pH of the UFB dispersions were modified by titration; the chemical and mechanical effects of ultrasound were separately examined. It was found that not only the chemical effect caused by the formation of nitric acid but also the mechanical effect contributed to the destabilization of UFB. Feasibility studies were also performed for UFBs in an aqueous surfactant solution and UFBs in a solid particle dispersion. The proposed method selectively destabilized UFBs in the solutions.

Molecular imaging of orthotopic prostate cancer with nanobubble ultrasound contrast agents targeted to PSMA

Ultrasound imaging is routinely used to guide prostate biopsies, yet delineation of tumors within the prostate gland is extremely challenging, even with microbubble (MB) contrast. A more effective ultrasound protocol is needed that can effectively localize malignancies for targeted biopsy or aid in patient selection and treatment planning for organ-sparing focal therapy. This study focused on evaluating the application of a novel nanobubble ultrasound contrast agent targeted to the prostate specific membrane antigen (PSMA-targeted NBs) in ultrasound imaging of prostate cancer (PCa) in vivo using a clinically relevant orthotopic tumor model in nude mice. Our results demonstrated that PSMA-targeted NBs had increased extravasation and retention in PSMA-expressing orthotopic mouse tumors. These processes are reflected in significantly different time intensity curve (TIC) and several kinetic parameters for targeted versus non-targeted NBs or LUMASON MBs. These, may in turn, lead to improved image-based detection and diagnosis of PCa in the future.

High frequency ultrasound nonlinear scattering from porphyrin nanobubbles

Emerging contrast imaging studies have highlighted the potential of nanobubbles for both intravascular and extravascular applications. Reports to date on nanobubbles have generally utilized low frequencies (<12 MHz), high concentrations (>10⁹ mL^-1), and B-mode or contrast-mode on preclinical and clinical systems. However, none of these studies directly examined nanobubble acoustic signatures systematically to implement nonlinear imaging schemes in a methodical manner based on nanobubble behaviour. Here, nanobubble nonlinear behaviour is investigated at high frequencies (12.5, 25, 30 MHz) and low concentration (10⁶ mL^-1) in a channel phantom, with different pulse types in single- and multi-pulse sequences to examine behaviour under conditions relevant to high frequency imaging. Porphyrin nanobubbles are demonstrated to initiate nonlinear scattering at high frequencies in a pressure-threshold dependent manner, as previously observed at low frequencies. This threshold behaviour was then utilized to demonstrate enhanced nanobubble imaging with pulse inversion, amplitude modulation, and a combination of the two, progressing towards the improved sensitivity and expanded utility of these ultrasound contrast agents.

Current advances in ultrasound-combined nanobubbles for cancer-targeted therapy: a review of the current status and future perspectives

The non-specific distribution, non-selectivity towards cancerous cells, and adverse off-target side effects of anticancer drugs and other therapeutic molecules lead to their inferior clinical efficacy. Accordingly, ultrasound-based targeted delivery of therapeutic molecules loaded in smart nanocarriers is currently gaining wider acceptance for the treatment and management of cancer. Nanobubbles (NBs) are nanosize carriers, which are currently used as effective drug/gene delivery systems because they can deliver drugs/genes selectively to target sites. Thus, combining the applications of ultrasound with NBs has recently demonstrated increased localization of anticancer molecules in tumor tissues with triggered release behavior. Consequently, an effective therapeutic concentration of drugs/genes is achieved in target tumor tissues with ultimately increased therapeutic efficacy and minimal side-effects on other non-cancerous tissues. This review illustrates present developments in the field of ultrasound-nanobubble combined strategies for targeted cancer treatment. The first part of this review discusses the composition and the formulation parameters of NBs. Next, we illustrate the interactions and biological effects of combining NBs and ultrasound. Subsequently, we explain the potential of NBs combined with US for targeted cancer therapeutics. Finally, the present and future directions for the improvement of current methods are proposed.

NBs combined with ultrasound demonstrated the ability to enhance the targeting of anticancer agents and improve the efficacy.

The dance of the nanobubbles: detecting acoustic backscatter from sub-micron bubbles using ultra-high frequency acoustic microscopy

Nanobubbles have gained attention for their use as highly stable ultrasound (US) contrast agents, but assessment of individual nanobubble size remains a challenge. Current sizing techniques require either extensive sample preparation or depend on assumed values of nanobubble density that are not well characterized. An US based approach would be desirable; however, probing individual nanobubbles using US transducers at clinical frequencies is not feasible due to the comparatively long acoustic wavelengths employed. Here we present a technique which can be used to estimate nano- or microbubble size by virtue of the amount of motion detected in an M-Mode image acquired using an acoustic microscope equipped with a 200 MHz transducer. A sample of a bubble-containing solution is incorporated into a phantom composed of molten agarose. The solidified agarose gel contains pores with well-defined sizes dictated by the agarose concentration. Bubbles in the gel matrix that are smaller in diameter than the gel pore size are capable of undergoing stochastic motion which manifests as intensity fluctuations in M-Mode images. Conversely, bubbles which are larger than the agarose pores become trapped and produce static M-Mode intensity patterns. In this study, agarose gels with concentrations ranging from 0.25% to 1.25% (mean pore sizes ranging from 2.68 μm to 0.34 μm) were loaded with either nanobubbles (mean diameter 0.326 μm) or microbubbles (mean diameter 2.71 μm) and imaged at 200 MHz. In the nanobubble loaded gels, M-Mode fluctuations were clearly visible up to a gel concentration of 1% (pore size of 0.39 μm). In contrast, the microbubble loaded gels exhibited minimal M-Mode fluctuation even at agarose concentrations of 0.25% (2.68 μm pore size). Autocorrelation curves generated from the M-Mode data demonstrated a clear trend of curve flattening (loss of motion) when the pore size was comparable to mean bubble diameter, indicating that individual bubbles trapped in the agarose pores are the main source of acoustic backscatter. In the future, decay parameters extracted from the autocorrelation curves could potentially be used as indicators of mean bubble diameter for bubble populations of unknown size.

Molecular Ultrasound Imaging

In the last decade, molecular ultrasound imaging has been rapidly progressing. It has proven promising to diagnose angiogenesis, inflammation, and thrombosis, and many intravascular targets, such as VEGFR2, integrins, and selectins, have been successfully visualized in vivo. Furthermore, pre-clinical studies demonstrated that molecular ultrasound increased sensitivity and specificity in disease detection, classification, and therapy response monitoring compared to current clinically applied ultrasound technologies. Several techniques were developed to detect target-bound microbubbles comprising sensitive particle acoustic quantification (SPAQ), destruction-replenishment analysis, and dwelling time assessment. Moreover, some groups tried to assess microbubble binding by a change in their echogenicity after target binding. These techniques can be complemented by radiation force ultrasound improving target binding by pushing microbubbles to vessel walls. Two targeted microbubble formulations are already in clinical trials for tumor detection and liver lesion characterization, and further clinical scale targeted microbubbles are prepared for clinical translation. The recent enormous progress in the field of molecular ultrasound imaging is summarized in this review article by introducing the most relevant detection technologies, concepts for targeted nano- and micro-bubbles, as well as their applications to characterize various diseases. Finally, progress in clinical translation is highlighted, and roadblocks are discussed that currently slow the clinical translation.

Concurrent visual and acoustic tracking of passive and active delivery of nanobubbles to tumors

Background: There has been growing interest in nanobubbles for their potential to extend bubble-mediated ultrasound approaches beyond that of their larger microbubble counterparts. In particular, the smaller scale of nanobubbles may enable them to access the tumor extravascular compartment for imaging and therapy in closer proximity to cancer cells. Compelling preliminary demonstrations of the imaging and therapeutic abilities of nanobubbles have thus emerged, with emphasis on their ability to extravasate. However, studies to date rely on indirect histologic evidence that cannot confirm whether the structures remain intact beyond the vasculature - leaving their extravascular potential largely untapped.

Methods: Nanobubble acoustic scattering was assessed using a recently reported ultra-stable formulation at low concentration (10⁶ mL^-1) and frequency (1 MHz), over a range of pressures (100-1500 kPa) in a channel phantom. The pressure-dependent response was utilized as a basis for in vivo experiments where ultrasound transmitters and receivers were integrated into a window chamber for simultaneous intravital multiphoton microscopy and acoustic monitoring in tumor-affected microcirculation. Microscopy and acoustic data were utilized to assess passive and active delivery of nanobubbles and determine whether they remained intact beyond the vasculature.

Results: Nanobubbles exhibit pressure-dependent nonlinear acoustic scattering. Nanobubbles are also found to have prolonged acoustic vascular pharmacokinetics, and passively extravasate intact into tumors. Ultrasound stimulation of nanobubbles is shown to actively enhance the delivery of both intact nanobubbles and shell material, increasing their spatial bioavailability deeper into the extravascular space. A range of acute vascular effects were also observed.

Conclusion: This study presents the first direct evidence that nanobubbles passively and actively extravasate intact in tumor tissue, and is the first to directly capture acute vascular events from ultrasound-stimulation of nanobubbles. The insights gained here demonstrate an important step towards unlocking the potential of nanobubbles and extending ultrasound-based applications.

Ultrasound-Responsive Carriers for Therapeutic Applications

Ultrasound (US)-responsive carriers have emerged as promising theranostic candidates because of their ability to enhance US-contrast, promote image-guided drug delivery, cause on-demand pulsatile release of drugs in response to ultrasound stimuli, as well as to enhance the permeability of physiological barriers such as the stratum corneum, the vascular endothelium, and the blood-brain barrier (BBB). US-responsive carriers include microbubbles MBs, liposomes, droplets, hydrogels, and nanobubble-nanoparticle complexes and have been explored for cavitation-mediated US-responsive drug delivery. Recently, a transient increase in the permeability of the BBB by microbubble (MB)-assisted low-frequency US has shown promise in enhancing the delivery of therapeutic agents in the case of neurological disorders. Further, the periodic mechanical stimulus generated by US-responsive MBs have also been explored in tissue engineering and has directly influenced the differentiation of mesenchymal stem cells into cartilage. This Review discusses the various types of US-responsive carriers and explores their emerging roles in therapeutics ranging from drug delivery to tissue engineering.

Oxygenation and synchronous control of nitrogen and phosphorus release at the sediment-water interface using oxygen nano-bubble modified material

Synchronously controlling the nitrogen (N) and phosphorus (P) release from sediments is an important basis for eutrophication management in lakes, but it is still a technical challenge at present. Loading nano-bubbles on the surface of natural minerals to increase dissolved oxygen(DO) level at the sediment-water interface (SWI)provides a possible solution to this problem. In this study, oxygen nano-bubble modified mineral (ONBMM) technology was developed, and its efficiency of oxygenation at the SWI and effect on the removal of internal nutrient input were evaluated under simulated conditions. The results showed that ONBMM effectively improved DO levels near the SWI; the highest concentration reached 6.55 mgL^-1. Meanwhile, adding ONBMM remarkably reduced the concentrations of total P(TP), total N(TN) and ammonia N(NH₃-N) in the overlying water. Compared with the control group, the fluxes of TP, NH₃-N, and TN loading from sediments in simulation cores treated with ONBMM reduced by 96.4%, 51.1%, and 24.9%, respectively. The high-resolution data obtained by DGT showed that ONBMM effectively inhibited the reduction and release of FeP through increasing the oxygen level at the SWI. The results of 16S rRNA high-throughput sequencing showed that adding ONBMM strengthened the role of nitrobacteria, denitrifying bacteria, and ammonia oxidation bacteria at the SWI. The ONBMM technology provides a new tool to achieve oxygenation at the SWI and in situ control of internal pollution in eutrophic lakes.

Simultaneous Intravital Optical and Acoustic Monitoring of Ultrasound-Triggered Nanobubble Generation and Extravasation

Ultrasound-activated nanobubbles are being widely investigated as contrast agents and therapeutic vehicles. Nanobubbles hold potential for accessing the tumor extravascular compartment, though this relies on clinically debated passive accumulation for which evidence to date is indirect. We recently reported ultrasound-triggered conversion of high payload porphyrin-encapsulated microbubbles to nanobubbles, with actively enhanced permeability for local delivery. This platform holds implications for optical/ultrasound-based imaging and therapeutics. While promising, it remains to be established how nanobubbles are generated and whether they extravasate intact. Here, insights into the conversion process are reported, complemented by novel simultaneous intravital and acoustic monitoring in tumor-affected functional circulation. The first direct acoustic evidence of extravascular intact nanobubbles are presented. These insights collectively advance this delivery platform and multimodal micro- and nanobubbles, extending their utility for imaging and therapeutics within and beyond the vasculature.

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.

Anti-G250 nanobody-functionalized nanobubbles targeting renal cell carcinoma cells for ultrasound molecular imaging

Traditional imaging examinations have difficulty in identifying benign and malignant changes in renal masses. This difficulty may be solved by ultrasound molecular imaging based on targeted nanobubbles, which could specifically enhance the ultrasound imaging of renal cell carcinomas (RCC) so as to discriminate benign and malignant renal masses. In this study, we aimed to prepare anti-G250 nanobody-functionalized targeted nanobubbles (anti-G250 NTNs) by coupling anti-G250 nanobodies to lipid nanobubbles and to verify their target specificity and binding ability to RCC cells that express G250 antigen and their capacity to enhance ultrasound imaging of RCC xenografts. Anti-G250 nanobodies were coupled to the lipid nanobubbles using the biotin-streptavidin bridge method. The average particle diameter of the prepared anti-G250 NTNs was 446 nm. Immunofluorescence confirmed that anti-G250 nanobodies were uniformly distributed on the surfaces of nanobubbles. In vitro experiments showed that the anti-G250 NTNs specifically bound to G250-positive 786-O cells and HeLa cells with affinities of 88.13% ± 4.37% and 71.8% ± 5.7%, respectively, and that they did not bind to G250-negative ACHN cells. The anti-G250 NTNs could significantly enhance the ultrasound imaging of xenograft tumors arising from 786-O cells and HeLa cells compared with blank nanobubbles, while the enhancement was not significant for xenograft tumors arising from ACHN cells. Immunofluorescence of tumor tissue slices confirmed that the anti-G250 NTNs could enter the tissue space through tumor blood vessels and bind to tumor cells specifically. In conclusion, anti-G250 nanobody-functionalized targeted nanobubbles could specifically bind to G250-positive RCC cells and enhance the ultrasound imaging of G250-positive RCC xenografts. This study has high-potential clinical application value for the diagnosis and differential diagnosis of renal tumors.

Proving and interpreting the spontaneous formation of bulk nanobubbles in aqueous organic solvent solutions: effects of solvent type and content

We show that the mixing of organic solvents with pure water leads to the spontaneous formation of suspended nano-entities which exhibit long-term stability on the scale of months. A wide range of solvents representing different functional groups are studied: methanol, ethanol, propanol, acetone, DMSO and formamide. We use various physical and chemical analytical techniques to provide compounded evidence that the nano-entities observed in all these aqueous solvent solutions must be gas-filled nanobubbles as they cannot be attributed to solvent nanodroplets, impurities or contamination. The nanobubble suspensions are characterized in terms of their bubble size distribution, bubble number density and zeta potential. The bubble number density achieved is a function of the type of solvent. It increases sharply with solvent content, reaching a maximum at an intermediate solvent concentration, before falling off to zero. We show that, whilst bulk nanobubbles can exist in pure water, they cannot exist in pure organic solvents and they disappear at some organic solvent-water ratio depending on the type of solvent. The gas solubility of the solvent relative to water as well as the molecular structure of the solvent are determining factors in the formation and stability of bulk nanobubbles. These phenomena are discussed and interpreted in the light of the experimental results obtained.

Optimized Anti-Prostate-Specific Membrane Antigen Single-Chain Variable Fragment-Loaded Nanobubbles as a Novel Targeted Ultrasound Contrast Agent for the Diagnosis of Prostate Cancer

OBJECTIVES

To prepare optimized prostate-specific membrane antigen (PSMA) single-chain variable fragment (scFv)-loaded nanobubbles (NBs) as a novel targeted ultrasound (US) contrast agent for diagnosis and treatment of prostate cancer (PCa).

METHODS

Prostate-specific membrane antigen scFv-loaded NBs were prepared by membrane hydration and biotin-streptavidin conjugation. Flow cytometry was used to observe the binding rate of the targeted NBs to PSMA-expressing cells. Contrast-enhanced US was used to monitor targeted and nontargeted NBs administered to nude mice with 22RV1, LNCaP, and PC-3 xenograft tumors. The specific binding ability of the targeted NBs was further examined by fluorescence imaging of tumor cryosections.

RESULTS

Uniformly sized targeted NBs were successfully prepared (mean ± SD, 485.3 ± 28.4 nm). The NBs showed good stability and bound specifically to LNCaP and 22RV1 cells with high PSMA expression in vitro but did not bind to PC-3 cells without PSMA expression. The targeted NBs presented good US enhancement, and the results of the in vivo xenograft tumor nude mouse model showed that the peak contrast intensity in LNCaP and 22RV1 cells was significantly higher for the targeted NBs than the nontargeted NBs (P < .05), whereas there was no significant difference in PC-3 cells. Immunofluorescence results obtained from tumor sections confirmed that the targeted NBs were capable of targeting PSMA-expressing tumor cells.

CONCLUSIONS

These novel PSMA scFv-loaded NBs have proven to be an excellent US contrast agent for imaging PSMA-expressing PCa and have the potential to not only enable efficient and safe molecular imaging but also to serve as a delivery system for targeted PCa therapies.

Formation and Stability of Bulk Nanobubbles by Vibration

Vibration is a very common process in nature, industry, biology, etc. Thus, whether vibration could induce the formation of nanoscale bubbles in water or not is very important for some chemical or biological reactions. In this paper, we designed a control experiment to simulate the vibration process to explore the production and stability of bulk nanobubbles. Experimental results showed that the vibration could indeed induce the formation of a certain number of bulk nanobubbles in water. In addition, the formation of bulk nanobubbles depended on the frequency and time of vibration. The existence of gas-liquid interface played an important role for the bulk nanobubbles formation because that external air is a possible important gas source. Our findings would be helpful to explore the mystical behavior of nanobubbles in natural processes.

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 Quantification: From Kinetic Modeling to Machine Learning

Ultrasound contrast agents (UCAs) have opened up immense diagnostic possibilities by combined use of indicator dilution principles and dynamic contrast-enhanced ultrasound (DCE-US) imaging. UCAs are microbubbles encapsulated in a biocompatible shell. With a rheology comparable to that of red blood cells, UCAs provide an intravascular indicator for functional imaging of the (micro)vasculature by quantitative DCE-US. Several models of the UCA intravascular kinetics have been proposed to provide functional quantitative maps, aiding diagnosis of different pathological conditions. This article is a comprehensive review of the available methods for quantitative DCE-US imaging based on temporal, spatial and spatiotemporal analysis of the UCA kinetics. The recent introduction of novel UCAs that are targeted to specific vascular receptors has advanced DCE-US to a molecular imaging modality. In parallel, new kinetic models of increased complexity have been developed. The extraction of multiple quantitative maps, reflecting complementary variables of the underlying physiological processes, requires an integrative approach to their interpretation. A probabilistic framework based on emerging machine-learning methods represents nowadays the ultimate approach, improving the diagnostic accuracy of DCE-US imaging by optimal combination of the extracted complementary information. The current value and future perspective of all these advances are critically discussed.