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

Application of high intensity focused ultrasound combined with nanomaterials in anti-tumor therapy

High intensity focused ultrasound (HIFU) has demonstrated its safety, efficacy and noninvasiveness in the ablation of solid tumor. However, its further application is limited by its inherent deficiencies, such as postoperative recurrence caused by incomplete ablation and excessive intensity affecting surrounding healthy tissues. Recent research has indicated that the integration of nanomaterials with HIFU exhibits a promising synergistic effect in tumor ablation. The concurrent utilization of nanomaterials with HIFU can help overcome the limitations of HIFU by improving targeting and ablation efficiency, expanding operation area, increasing operation accuracy, enhancing stability and bio-safety during the process. It also provides a platform for multi-therapy and multi-mode imaging guidance. The present review comprehensively expounds upon the synergistic mechanism between nanomaterials and HIFU, summarizes the research progress of nanomaterials as cavitation nuclei and drug carriers in combination with HIFU for tumor ablation. Furthermore, this review highlights the potential for further exploration in the development of novel nanomaterials that enhance the synergistic effect with HIFU on tumor ablation.

Decorrelation Time Mapping as an Analysis Tool for Nanobubble-Based Contrast Enhanced Ultrasound Imaging

Nanobubbles (NBs; ~100-500 nm diameter) are preclinical ultrasound (US) contrast agents that expand applications of contrast enhanced US (CEUS). Due to their sub-micron size, high particle density, and deformable shell, NBs in pathological states of heightened vascular permeability (e.g. in tumors) extravasate, enabling applications not possible with microbubbles (~1000-10,000 nm diameter). A method that can separate intravascular versus extravascular NB signal is needed as an imaging biomarker for improved tumor detection. We present a demonstration of decorrelation time (DT) mapping for enhanced tumor NB-CEUS imaging. In vitro models validated the sensitivity of DT to agent motion. Prostate cancer mouse models validated in vivo imaging potential and sensitivity to cancerous tissue. Our findings show that DT is inversely related to NB motion, offering enhanced detail of NB dynamics in tumors, and highlighting the heterogeneity of the tumor environment. Average DT was high in tumor regions (~9 s) compared to surrounding normal tissue (~1 s) with higher sensitivity to tumor tissue compared to other mapping techniques. Molecular NB targeting to tumors further extended DT (11 s) over non-targeted NBs (6 s), demonstrating sensitivity to NB adherence. From DT mapping of in vivo NB dynamics we demonstrate the heterogeneity of tumor tissue while quantifying extravascular NB kinetics and delineating intra-tumoral vasculature. This new NB-CEUS-based biomarker can be powerful in molecular US imaging, with improved sensitivity and specificity to diseased tissue and potential for use as an estimator of vascular permeability and the enhanced permeability and retention (EPR) effect in tumors.

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

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

Using imaging modalities to predict nanoparticle distribution and treatment efficacy in solid tumors: The growing role of ultrasound

Nanomedicine in oncology has not had the success in clinical impact that was anticipated in the early stages of the field's development. Ideally, nanomedicines selectively accumulate in tumor tissue and reduce systemic side effects compared to traditional chemotherapeutics. However, this has been more successful in preclinical animal models than in humans. The causes of this failure to translate may be related to the intra- and inter-patient heterogeneity of the tumor microenvironment. Predicting whether a patient will respond positively to treatment prior to its initiation, through evaluation of characteristics like nanoparticle extravasation and retention potential in the tumor, may be a way to improve nanomedicine success rate. While there are many potential strategies to accomplish this, prediction and patient stratification via noninvasive medical imaging may be the most efficient and specific strategy. There have been some preclinical and clinical advances in this area using MRI, CT, PET, and other modalities. An alternative approach that has not been studied as extensively is biomedical ultrasound, including techniques such as multiparametric contrast-enhanced ultrasound (mpCEUS), doppler, elastography, and super-resolution processing. Ultrasound is safe, inexpensive, noninvasive, and capable of imaging the entire tumor with high temporal and spatial resolution. In this work, we summarize the in vivo imaging tools that have been used to predict nanoparticle distribution and treatment efficacy in oncology. We emphasize ultrasound imaging and the recent developments in the field concerning CEUS. The successful implementation of an imaging strategy for prediction of nanoparticle accumulation in tumors could lead to increased clinical translation of nanomedicines, and subsequently, improved patient outcomes. This article is categorized under: Diagnostic Tools In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery Nanomedicine for Oncologic Disease Therapeutic Approaches and Drug Discovery Emerging Technologies.

Ultrasound-responsive microbubbles and nanodroplets: A pathway to targeted drug delivery

Ultrasound-responsive agents have shown great potential as targeted drug delivery agents, effectively augmenting cell permeability and facilitating drug absorption. This review focuses on two specific agents, microbubbles and nanodroplets, and provides a sequential overview of their drug delivery process. Particular emphasis is given to the mechanical response of the agents under ultrasound, and the subsequent physical and biological effects on the cells. Finally, the state-of-the-art in their pre-clinical and clinical implementation are discussed. Throughout the review, major challenges that need to be overcome in order to accelerate their clinical translation are highlighted.

Efficient ultrasound-mediated drug delivery to orthotopic liver tumors - Direct comparison of doxorubicin-loaded nanobubbles and microbubbles

Liver metastasis is a major obstacle in treating aggressive cancers, and current therapeutic options often prove insufficient. To overcome these challenges, there has been growing interest in ultrasound-mediated drug delivery using lipid-shelled microbubbles (MBs) and nanobubbles (NBs) as promising strategies for enhancing drug delivery to tumors. Our previous work demonstrated the potential of Doxorubicin-loaded C3F8 NBs (hDox-NB, 280 ± 123 nm) in improving cancer treatment in vitro using low-frequency unfocused therapeutic ultrasound (TUS). In this study, we investigated the pharmacokinetics and biodistribution of sonicated hDox-NBs in orthotopic rat liver tumors. We compared their delivery and therapeutic efficiency with size-isolated MBs (hDox-MB, 1104 ± 373 nm) made from identical shell material and core gas. Results showed a similar accumulation of hDox in tumors treated with hDox-MBs and unfocused therapeutic ultrasound (hDox-MB + TUS) and hDox-NB + TUS. However, significantly increased apoptotic cell death in the tumor and fewer off-target apoptotic cells in the normal liver were found upon the treatment with hDox-NB + TUS. The tumor-to-liver apoptotic ratio was elevated 9.4-fold following treatment with hDox-NB + TUS compared to hDox-MB + TUS, suggesting that the therapeutic efficacy and specificity are significantly increased when using hDox-NB + TUS. These findings highlight the potential of this approach as a viable treatment modality for liver tumors. By elucidating the behavior of drug-loaded bubbles in vivo, we aim to contribute to developing more effective liver cancer treatments that could ultimately improve patient outcomes and decrease off-target side effects.

Numerical investigation of the effect of bubble properties on the linear resonance frequency shift due to inter-bubble interactions in ultrasonically excited lipid coated microbubbles

Ultrasonically excited microbubbles (MBs) have numerous applications in various fields, such as drug delivery, and imaging. Ultrasonically excited MBs are known to be nonlinear oscillators that generate secondary acoustic emissions in the media when excited by a primary ultrasound wave. The propagation of acoustic waves in the liquid is limited to the speed of sound, resulting in each MB receiving the primary and secondary waves at different times depending on their distance from the ultrasound source and the distance between MBs. These delays are referred to as primary and secondary delays, respectively. A previous study demonstrated that the inclusion of secondary delays in a model describing the interactions between MBs exposed to ultrasound results in an increase in the linear resonance frequency of MBs as they approach each other. This work investigates the impact of various MB properties on the change in linear resonance frequency resulting from changes in inter-bubble distances. The effects of shell properties, including the initial surface tension, surface dilatational viscosity of the shell monolayer, elastic compression modulus of the shell, and the initial radius of the MBs, are examined. MB size is a significant factor influencing the rate of linear resonance frequency increase with increasing concentration. Moreover, it is found that the shell properties of MBs play a negligible role in the rate of change in linear resonance frequency of MBs as the inter-bubble distances change.The findings of this study have implications for various applications of MBs in the biomedical field. By understanding the impact of inter-bubble distances and shell properties on the linear resonance frequency of MBs, the utilization of MBs in applications reliant on their resonant behavior can be optimized.

Application of contrast-enhanced ultrasound in diagnosis and grading of bladder urothelial carcinoma

PURPOSE

To explore the application of contrast-enhanced ultrasound (CEUS) for the diagnosis and grading of bladder urothelial carcinoma (BUC).

METHODS

The results of a two-dimensional ultrasound, color Doppler ultrasound and CEUS, were analyzed in 173 bladder lesion cases. The ultrasound and surgical pathology results were compared, and their diagnostic efficacy was analyzed.

RESULTS

There were statistically significant differences between BUC and benign lesions in terms of color blood flow distribution intensity and CEUS enhancement intensity (both P < 0.05). The area under the time-intensity curve (AUC), rising slope, and peak intensity of BUC were significantly higher than those of benign lesions (all P < 0.05). The H/T (height H / basal width T)value of 0.63 was the critical value for distinguishing high- and low-grade BUC, had a diagnostic sensitivity of 80.0% and a specificity of 60.0%.

CONCLUSION

The combination of CEUS and TIC can help improve the diagnostic accuracy of BUC. There is a statistically significant difference between high- and low-grade BUC in contrast enhancement intensity (P < 0.05); The decrease of H/T value indicates the possible increase of the BUC grade.

Do Nanobubbles Exist in Pure Alcohol?

The existence of nanobubbles in pure water has been extensively debated in recent years, and it is speculated that nanobubbles may be ion-stabilized. However, nanobubbles in the alcohol-water mixture and pure alcohols are still controversial due to the lack of ions present in the alcohol system. This work tested the hypothesis that stable nanobubbles exist in pure alcohol. The ultrasound and oscillatory pressure fields are used to generate nanobubbles in pure alcohol. The size distribution, concentration, diameter, and scattering intensity of the nanobubbles were measured by nanoparticle tracking analysis. The light scattering method measures the zeta potential. The Mie scattering theory and electromagnetic wave simulation are utilized to estimate the refractive index (RI) of nanobubbles from the experimentally measured scattering light intensity. The average RI of the nanobubbles in pure alcohols produced by ultrasound and oscillating pressure fields was estimated to be 1.17 ± 0.03. Degassing the nanobubble sample reduces its concentration and increases its size. The average zeta potential of the nanobubbles in pure alcohol was measured to be -5 ± 0.9 mV. The mechanical stability model, which depends on force balance around a single nanobubble, also predicts the presence of nanobubbles in pure alcohol. The nanobubbles in higher-order alcohols were found to be marginally colloidally stable. In summary, both experimental and theoretical results suggest the existence of nanobubbles in pure alcohol.

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

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

Thermal Oscillations of Nanobubbles

Nanobubble cavitation is advancing technologies in enhanced wastewater treatment, cancer therapy and diagnosis, and microfluidic cleaning. Current macroscale models predict that nanobubble oscillations should be isothermal, yet recent studies suggest that they are adiabatic with an associated increase in natural frequency, which becomes challenging when characterizing nanobubble sizes using ultrasound in experiments. We derive a new theoretical model that considers the nonideal nature of the nanobubble's internal gas phase and nonequilibrium effects, by employing the van der Waals (vdW) equation of state and implementing a temperature jump term at the liquid-gas interface, respectively, finding excellent agreement with molecular dynamics (MD) simulations. Our results reveal how adiabatic behavior could be erroneously interpreted when analyzing the thermal response of the gas using the commonly employed polytropic process and explain instead how nanobubble oscillations are physically closer to their isothermal limit.

From Nanovesicles to Nanobubbles Based on Repeated Compression Method

Nanobubbles have been increasingly applied in biomedicine, which is attributed to their ability to work as ultrasound imaging contrast agents and powerful gene/drug carriers. Different production techniques or approaches have been developed to generate uniform and stable shelled nanobubbles. However, these shelled nanobubbles are usually prepared based on disordered shell materials, such as free phospholipids and polymers. In recent years, the continuous repeated compression method for a gas-liquid mixture has been developed to produce free and lipid-shelled nanobubbles. In this study, to explore the response of well-organized nanostructures to this method, the repeated compression method was used to treat preprepared liposomes and polymeric nanovesicles. Size distribution, morphologies, and ultrasound image contrast enhancement of these nanovesicles were determined before and after repeated compression. Results demonstrate that the presence of a phospholipid bilayer is vital to form liposome-based nanobubbles. And the low elastic modulus of the polymeric membrane is key to encapsulate gases into polymeric nanovesicles. Overall, it demonstrated the advantages of well-organized nanostructures to produce nanobubble structures, giving new insights into the preparation and understanding of nanobubbles.

Assessing Tumor Microenvironment Characteristics and Stratifying EPR with a Nanobubble Companion Nanoparticle via Contrast-Enhanced Ultrasound Imaging

The tumor microenvironment is characterized by dysfunctional endothelial cells, resulting in heightened vascular permeability. Many nanoparticle-based drug delivery systems attempt to use this enhanced permeability combined with impaired lymphatic drainage (a concept known as the 'enhanced permeability and retention effect' or EPR effect) as the primary strategy for drug delivery, but this has not proven to be as clinically effective as anticipated. The specific mechanisms behind the inconsistent clinical outcomes of nanotherapeutics have not been clearly articulated, and the field has been hampered by a lack of accessible tools to study EPR-associated phenomena in clinically relevant scenarios. While medical imaging has tremendous potential to contribute to this area, it has not been broadly explored. This work examines, for the first time, the use of multiparametric dynamic contrast-enhanced ultrasound (CEUS) with a novel nanoscale contrast agent to examine tumor microenvironment characteristics noninvasively and in real-time. We demonstrate that CEUS imaging can: (1) evaluate tumor microenvironment features and (2) be used to help predict the distribution of doxorubicin-loaded liposomes in the tumor parenchyma. CEUS using nanobubbles (NBs) was carried out in two tumor types of high (LS174T) and low (U87) vascular permeability, and time-intensity curve (TIC) parameters were evaluated in both models prior to injection of doxorubicin liposomes. Consistently, LS174T tumors showed significantly different TIC parameters, including area under the rising curve (2.7x), time to peak intensity (1.9x) and decorrelation time (DT, 1.9x) compared to U87 tumors. Importantly, the DT parameter successfully predicted tumoral nanoparticle distribution (r = 0.86 ± 0.13). Ultimately, substantial differences in NB-CEUS generated parameters between LS174T and U87 tumors suggest that this method may be useful in determining tumor vascular permeability and could be used as a biomarker for identifying tumor characteristics and predicting sensitivity to nanoparticle-based therapies. These findings could ultimately be applied to predicting treatment efficacy and to evaluating EPR in other diseases with pathologically permeable vasculature.

Imaging in inflammatory arthritis: progress towards precision medicine

Imaging techniques such as ultrasonography and MRI have gained ground in the diagnosis and management of inflammatory arthritis, as these imaging modalities allow a sensitive assessment of musculoskeletal inflammation and damage. However, these techniques cannot discriminate between disease subsets and are currently unable to deliver an accurate prediction of disease progression and therapeutic response in individual patients. This major shortcoming of today's technology hinders a targeted and personalized patient management approach. Technological advances in the areas of high-resolution imaging (for example, high-resolution peripheral quantitative computed tomography and ultra-high field MRI), functional and molecular-based imaging (such as chemical exchange saturation transfer MRI, positron emission tomography, fluorescence optical imaging, optoacoustic imaging and contrast-enhanced ultrasonography) and artificial intelligence-based data analysis could help to tackle these challenges. These new imaging approaches offer detailed anatomical delineation and an in vivo and non-invasive evaluation of the immunometabolic status of inflammatory reactions, thereby facilitating an in-depth characterization of inflammation. By means of these developments, the aim of earlier diagnosis, enhanced monitoring and, ultimately, a personalized treatment strategy looms closer.

Efficient ultrasound-mediated drug delivery to orthotopic liver tumors - Direct comparison of doxorubicin-loaded nanobubbles and microbubbles

Liver metastasis is a major obstacle in treating aggressive cancers, and current therapeutic options often prove insufficient. To overcome these challenges, there has been growing interest in ultrasound-mediated drug delivery using lipid-shelled microbubbles (MBs) and nanobubbles (NBs) as promising strategies for enhancing drug delivery to tumors. Our previous work demonstrated the potential of Doxorubicin-loaded C3F8 NBs (hDox-NB, 280 ± 123 nm) in improving cancer treatment in vitro using low-frequency ultrasound. In this study, we investigated the pharmacokinetics and biodistribution of sonicated hDox-NBs in orthotopic rat liver tumors. We compared their delivery and therapeutic efficiency with size-isolated MBs (hDox-MB, 1104 ± 373 nm). Results showed a similar accumulation of hDox in tumors treated with hDox-MBs and unfocused therapeutic ultrasound (hDox-MB+TUS) and hDox-NB+TUS. However, significantly increased apoptotic cell death in the tumor and fewer off-target apoptotic cells in the normal liver were found upon the treatment with hDox-NB+TUS. The tumor-to-liver apoptotic ratio was elevated 9.4-fold following treatment with hDox-NB+TUS compared to hDox-MB+TUS, suggesting that the therapeutic efficacy and specificity are significantly increased when using hDox-NB+TUS. These findings highlight the potential of this approach as a viable treatment modality for liver tumors. By elucidating the behavior of drug-loaded bubbles in vivo, we aim to contribute to developing more effective liver cancer treatments that could ultimately improve patient outcomes and decrease off-target side effects.

Real-time imaging of nanobubble ultrasound contrast agent flow, extravasation, and diffusion through an extracellular matrix using a microfluidic model

Lipid shell-stabilized nanoparticles with a perfluorocarbon gas-core, or nanobubbles, have recently attracted attention as a new contrast agent for molecular ultrasound imaging and image-guided therapy. Due to their small size (∼275 nm diameter) and flexible shell, nanobubbles have been shown to extravasate through hyperpermeable vasculature (e.g., in tumors). However, little is known about the dynamics and depth of extravasation of intact, acoustically active nanobubbles. Accordingly, in this work, we developed a microfluidic chip with a lumen and extracellular matrix (ECM) and imaging method that allows real-time imaging and characterization of the extravasation process with high-frequency ultrasound. The microfluidic device has a lumen and is surrounded by an extracellular matrix with tunable porosity. The combination of ultrasound imaging and the microfluidic chip advantageously produces real-time images of the entire length and depth of the matrix. This captures the matrix heterogeneity, offering advantages over other imaging techniques with smaller fields of view. Results from this study show that nanobubbles diffuse through a 1.3 μm pore size (2 mg mL-1) collagen I matrix 25× faster with a penetration depth that was 0.19 mm deeper than a 3.7 μm (4 mg mL-1) matrix. In the 3.7 μm pore size matrix, nanobubbles diffused 92× faster than large nanobubbles (∼875 nm diameter). Decorrelation time analysis was successfully used to differentiate flowing and extra-luminally diffusing nanobubbles. In this work, we show for the first time that combination of an ultrasound-capable microfluidic chip and real-time imaging provided valuable insight into spatiotemporal nanoparticle movement through a heterogeneous extracellular matrix. This work could help accurately predict parameters (e.g., injection dosage) that improve translation of nanoparticles from in vitro to in vivo environments.

Bubble-Based Drug Delivery Systems: Next-Generation Diagnosis to Therapy

Current radiologic and medication administration is systematic and has widespread side effects; however, the administration of microbubbles and nanobubbles (MNBs) has the possibility to provide therapeutic and diagnostic information without the same ramifications. Microbubbles (MBs), for instance, have been used for ultrasound (US) imaging due to their ability to remain in vessels when exposed to ultrasonic waves. On the other hand, nanobubbles (NBs) can be used for further therapeutic benefits, including chronic treatments for osteoporosis and cancer, gene delivery, and treatment for acute conditions, such as brain infections and urinary tract infections (UTIs). Clinical trials are also being conducted for different administrations and utilizations of MNBs. Overall, there are large horizons for the benefits of MNBs in radiology, general medicine, surgery, and many more medical applications. As such, this review aims to evaluate the most recent publications from 2016 to 2022 to report the current uses and innovations for MNBs.

Microfluidic nanobubbles: observations of a sudden contraction of microbubbles into nanobubbles

Microfluidic devices are often utilized to generate uniform-size microbubbles. In most microfluidic bubble generation experiments, once the bubbles are formed the gas inside the bubbles begin to dissolve into the surrounding aqueous environment. The bubbles shrink until they attain an equilibrium size dictated by the concentration and type of amphiphilic molecules stabilizing the gas-liquid interface. Here, we exploit this shrinkage mechanism, and control the solution lipid concentration and microfluidic geometry, to make monodisperse bulk nanobubbles. Interestingly, we make the surprising observation of a critical microbubble diameter above and below which the scale of bubble shrinkage dramatically changes. Namely, microbubbles generated with an initial diameter larger than the critical diameter shrinks to a stable diameter that is consistent with previous literature. However, microbubbles that are initially smaller than the critical diameter experience a sudden contraction into nanobubbles whose size is at least an order-of-magnitude below expectations. We apply electron microscopy and resonance mass measurement methods to quantify the size and uniformity of the nanobubbles, and probe the dependence of the critical bubble diameter on the lipid concentration. We anticipate that further analysis of this unexpected microbubble sudden contraction regime can lead to more robust technologies for making monodisperse nanobubbles.

Nanomaterial-based contrast agents

Medical imaging, which empowers the detection of physiological and pathological processes within living subjects, has a vital role in both preclinical and clinical diagnostics. Contrast agents are often needed to accompany anatomical data with functional information or to provide phenotyping of the disease in question. Many newly emerging contrast agents are based on nanomaterials as their high payloads, unique physicochemical properties, improved sensitivity and multimodality capacity are highly desired for many advanced forms of bioimaging techniques and applications. Here, we review the developments in the field of nanomaterial-based contrast agents. We outline important nanomaterial design considerations and discuss the effect on their physicochemical attributes, contrast properties and biological behaviour. We also describe commonly used approaches for formulating, functionalizing and characterizing these nanomaterials. Key applications are highlighted by categorizing nanomaterials on the basis of their X-ray, magnetic, nuclear, optical and/or photoacoustic contrast properties. Finally, we offer our perspectives on current challenges and emerging research topics as well as expectations for future advancements in the field.

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.

Ultrasound-mediated nano drug delivery for treating cancer: Fundamental physics to future directions

The application of biocompatible nanocarriers in medicine has provided several benefits over conventional treatment methods. However, achieving high treatment efficacy and deep penetration of nanocarriers in tumor tissue is still challenging. To address this, stimuli-responsive nano-sized drug delivery systems (DDSs) are an active area of investigation in delivering anticancer drugs. While ultrasound is mainly used for diagnostic purposes, it can also be applied to affect cellular function and the delivery/release of anticancer drugs. Therapeutic ultrasound (TUS) has shown potential as both a stand-alone anticancer treatment and a method to induce targeted drug release from nanocarrier systems. TUS approaches have been used to overcome various physiological obstacles, including endothelial barriers, the tumor microenvironment (TME), and immunological hurdles. Combining nanomedicine and ultrasound as a smart DDS can increase in situ drug delivery and improve access to impermeable tissues. Furthermore, smart DDSs can perform targeted drug release in response to distinctive TMEs, external triggers, or dual/multi-stimulus. This results in enhanced treatment efficacy and reduced damage to surrounding healthy tissue or organs at risk. Integrating DDSs and ultrasound is still in its early stages. More research and clinical trials are required to fully understand ultrasound's underlying physical mechanisms and interactions with various types of nanocarriers and different types of cells and tissues. In the present review, ultrasound-mediated nano-sized DDS, specifically focused on cancer treatment, is presented and discussed. Ultrasound interaction with nanoparticles (NPs), drug release mechanisms, and various types of ultrasound-sensitive NPs are examined. Additionally, in vitro, in vivo, and clinical applications of TUS are reviewed in light of the critical challenges that need to be considered to advance TUS toward an efficient, secure, straightforward, and accessible cancer treatment. This study also presents effective TUS parameters and safety considerations for this treatment modality and gives recommendations about system design and operation. Finally, future perspectives are considered, and different TUS approaches are examined and discussed in detail. This review investigates drug release and delivery through ultrasound-mediated nano-sized cancer treatment, both pre-clinically and clinically.

Acoustically-Activated Liposomal Nanocarriers to Mitigate the Side Effects of Conventional Chemotherapy with a Focus on Emulsion-Liposomes

To improve currently available cancer treatments, nanomaterials are employed as smart drug delivery vehicles that can be engineered to locally target cancer cells and respond to stimuli. Nanocarriers can entrap chemotherapeutic drugs and deliver them to the diseased site, reducing the side effects associated with the systemic administration of conventional anticancer drugs. Upon accumulation in the tumor cells, the nanocarriers need to be potentiated to release their therapeutic cargo. Stimulation can be through endogenous or exogenous modalities, such as temperature, electromagnetic irradiation, ultrasound (US), pH, or enzymes. This review discusses the acoustic stimulation of different sonosensitive liposomal formulations. Emulsion liposomes, or eLiposomes, are liposomes encapsulating phase-changing nanoemulsion droplets, which promote acoustic droplet vaporization (ADV) upon sonication. This gives eLiposomes the advantage of delivering the encapsulated drug at low intensities and short exposure times relative to liposomes. Other formulations integrating microbubbles and nanobubbles are also discussed.

Characterization of the interaction of nanobubble ultrasound contrast agents with human blood components

Nanoscale ultrasound contrast agents, or nanobubbles, are being explored in preclinical applications ranging from vascular and cardiac imaging to targeted drug delivery in cancer. These sub-micron particles are approximately 10x smaller than clinically available microbubbles. This allows them to effectively traverse compromised physiological barriers and circulate for extended periods of time. While various aspects of nanobubble behavior have been previously examined, their behavior in human whole blood has not yet been explored. Accordingly, herein we examined, for the first time, the short and long-term effects of blood components on nanobubble acoustic response. We observed differences in the kinetics of backscatter from nanobubble suspensions in whole blood compared to bubbles in phosphate buffered saline (PBS), plasma, or red blood cell solutions (RBCs). Specifically, after introducing nanobubbles to fresh human whole blood, signal enhancement, or the magnitude of nonlinear ultrasound signal, gradually increased by 22.8 ± 13.1% throughout our experiment, with peak intensity reached within 145 s. In contrast, nanobubbles in PBS had a stable signal with negligible change in intensity (−1.7 ± 3.2%) over 8 min. Under the same conditions, microbubbles made with the same lipid formulation showed a −56.8 ± 6.1% decrease in enhancement in whole blood. Subsequent confocal, fluorescent, and scanning electron microscopy analysis revealed attachment of the nanobubbles to the surface of RBCs, suggesting that direct interactions, or hitchhiking, of nanobubbles on RBCs in the presence of plasma may be a possible mechanism for the observed effects. This phenomenon could be key to extending nanobubble circulation time and has broad implications in drug delivery, where RBC interaction with nanoparticles could be exploited to improve delivery efficiency.

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Show distinct signal enhancement curve of nanobubbles (NB) in human whole blood

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Compare signal enhancement over time in blood with NBs and microbubbles (MBs)

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Examine the effect of blood components on NB and MB movement over time

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Demonstrate, with microscopy, that NBs localize to the RBC surface in whole blood

AMPK is required for recovery from metabolic stress induced by ultrasound microbubble treatment

Ultrasound-stimulated microbubble (USMB) treatment is a promising strategy for cancer therapy. USMB promotes drug delivery by sonoporation and enhanced endocytosis, and also impairs cell viability. However, USMB elicits heterogeneous effects on cell viability, with apparently minimal effects on a subset of cells. This suggests that mechanisms of adaptation following USMB allow some cells to survive and/or proliferate. Herein, we used several triple negative breast cancer cells to identify the molecular mechanisms of adaptation to USMB-induced stress. We found that USMB alters steady-state levels of amino acids, glycolytic intermediates, and citric acid cycle intermediates, suggesting that USMB imposes metabolic stress on cells. USMB treatment acutely reduces ATP levels and stimulates the phosphorylation and activation of AMP-activated protein kinase (AMPK). AMPK is required to restore ATP levels and support cell proliferation post-USMB treatment. These results suggest that AMPK and metabolic perturbations are likely determinants of the antineoplastic efficacy of USMB treatment.

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

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

Controlled Shrinkage of Microfluidically Generated Microbubbles by Tuning Lipid Concentration

Monodisperse microbubbles with diameters less than 10 μm are desirable in several ultrasound imaging and therapeutic delivery applications. However, conventional approaches to synthesize microbubbles, which are usually agitation-based, produce polydisperse bubbles that are less desirable because of their heterogeneous response when exposed to an ultrasound field. Microfluidics technology has the unique advantage of generating size-controlled monodisperse microbubbles, and it is now well established that the diameter of microfluidically made microbubbles can be tuned by varying the liquid flow rate, gas pressure, and dimensions of the microfluidic channel. It is also observed that once the microbubbles form, the bubbles shrink and eventually stabilize to a quasi-equilibrium diameter, and that the rate of stabilization is related to the lipid solution. However, how the lipid solution concentration affects the degree of bubble shrinkage, and the stable size of microbubbles, has not been thoroughly examined. Here, we investigate whether and how the lipid concentration affects the degree of microbubble shrinkage. Namely, we utilize a flow-focusing microfluidic geometry to generate monodisperse bubbles, and observe the effect of gas composition (2.5, 1.42, and 0.17 wt % octafluoropropane in nitrogen) and lipid concentration (1-16 mg/mL) on the degree of microbubble shrinkage. For the lipid system and gas utilized in these experiments, we observe a monotonic increase in the degree of microbubble shrinkage with decreasing lipid concentration, and no dependency on the gas composition. We hypothesize that the degree of shrinkage is related to lipid concentration by the self-assembly of lipids on the gas-liquid interface during bubble generation and subsequent lipid packing on the interface during shrinkage, which is arrested when a maximum packing density is achieved. We anticipate that this approach for creating and tuning the size of monodisperse microbubbles will find utility in biomedical applications, such as contrast-enhanced ultrasound imaging and ultrasound-triggered gene delivery.

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.

The influence of inter-bubble spacing on the resonance response of ultrasound contrast agent microbubbles

Ultrasound-driven microbubbles, typically between 1 and 8 µm in diameter, are resonant scatterers that are employed as diagnostic contrast agents and emerging as potentiators of targeted therapies. Microbubbles are administered in populations whereby their radial dynamics - key to their effectiveness - are greatly affected by intrinsic (e.g. bubble size) and extrinsic (e.g. boundaries) factors. In this work, we aim to understand how two neighbouring microbubbles influence each other. We developed a finite element model of a system of two individual phospholipid-encapsulated microbubbles vibrating in proximity to each other to study the effect of inter-bubble distance on microbubble radial resonance response. For the case of two equal-sized and identical bubbles, each bubble exhibits a decrease between 7 and 10% in the frequency of maximum response (f_MR) and an increase in amplitude of maximum response (A_MR) by 9-11% as compared to its isolated response in free-space, depending on the bubble size examined. For a system of two unequal-sized microbubbles, the large bubble shows no significant change, however the smaller microbubble shows an increase in f_MR by 7-11% and a significant decrease in A_MR by 38-52%. Furthermore, in very close proximity the small bubble shows a secondary off-resonance peak at the corresponding f_MR of its larger companion microbubble. Our work suggests that frequency-dependent microbubble response is greatly affected by the presence of another bubble, which has implications in both imaging and therapy applications. Furthermore, our work suggests a mechanism by which nanobubbles show significant off-resonance vibrations in the clinical frequency range, a behaviour that has been observed experimentally but heretofore unexplained.

Nanoscale Contrast Agents for Ultrasound Imaging of Musculoskeletal System

Musculoskeletal ultrasound (MSKUS) has been recognized as an important method for the evaluation of diseases of the musculoskeletal system, and contrast-enhanced ultrasound (CEUS) technology is becoming an important branch of it. The development of novel materials and tiny nano-formulations has further expanded ultrasound contrast agents (UCAs) into the field of nanotechnology. Over the years, nanoscale contrast agents have been found to play an unexpected role in the integration of precise imaging for diagnosis and treatment of numerous diseases. It has been demonstrated that nanoscale UCAs (nUCAs) have advantages in imaging over conventional contrast agents, including superior biocompatibility, serum stability, and longer lifetime. The potential value of nUCAs in the musculoskeletal system is that they provide more reliable and clinically valuable guidance for the diagnosis, treatment, and follow-up of related diseases. The frontier of advances in nUCAs, their applications, and insights in MSKUS are reviewed in this paper.

Low frequency nanobubble-enhanced ultrasound mechanotherapy for noninvasive cancer surgery

Scaling down the size of microbubble contrast agents to the nanometer level holds the promise for noninvasive cancer therapy. However, the small size of nanobubbles limits the obtained bioeffects as a result of ultrasound cavitation, when operating near the nanobubble resonance frequency. Here we show that coupled with low energy insonation at a frequency of 80 kHz, well below the resonance frequency of these agents, nanobubbles serve as noninvasive therapeutic warheads that trigger potent mechanical effects in tumors following a systemic injection. We demonstrate these capabilities in tissue mimicking phantoms, where a comparison of the acoustic response of micro- and nano-bubbles after insonation at a frequency of 250 or 80 kHz revealed that higher pressures were needed to implode the nanobubbles compared to microbubbles. Complete nanobubble destruction was achieved at a mechanical index of 2.6 for the 250 kHz insonation vs. 1.2 for the 80 kHz frequency. Thus, the 80 kHz insonation complies with safety regulations that recommend operation below a mechanical index of 1.9. In vitro in breast cancer tumor cells, the cell viability was reduced to 17.3 ± 1.7% of live cells. In vivo, in a breast cancer tumor mouse model, nanobubble tumor distribution and accumulation were evaluated by high frequency ultrasound imaging. Finally, nanobubble-mediated low frequency insonation of breast cancer tumors resulted in effective mechanical tumor ablation and tumor tissue fractionation. This approach provides a unique theranostic platform for safe, noninvasive and low energy tumor mechanotherapy.

The unique second wave phenomenon in contrast enhanced ultrasound imaging with nanobubbles

Investigation of nanobubble (NB) pharmacokinetics in contrast-enhanced ultrasound (CEUS) at the pixel level shows a unique phenomenon where the first pass of the contrast agent bolus is accompanied by a second wave. This effect has not been previously observed in CEUS with microbubbles. The objective of this study was to investigate this second-wave phenomenon and its potential clinical applications. Seven mice with a total of fourteen subcutaneously-implanted tumors were included in the experiments. After injecting a bolus of NBs, the NB-CEUS images were acquired to record the time-intensity curves (TICs) at each pixel. These TICs are fitted to a pharmacokinetic model which we designed to describe the observed second-wave phenomenon. The estimated model parameters are presented as parametric maps to visualize the characteristics of tumor lesions. Histological analysis was also conducted in one mouse to compare the molecular features of tumor tissue with the obtained parametric maps. The second-wave phenomenon is evidently shown in a series of pixel-based TICs extracted from either tumor or tissues. The value of two model parameters, the ratio of the peak intensities of the second over the first wave, and the decay rate of the wash-out process present large differences between malignant tumor and normal tissue (0.04 < Jessen-Shannon divergence < 0.08). The occurrence of a second wave is a unique phenomenon that we have observed in NB-CEUS imaging of both mouse tumor and tissue. As the characteristics of the second wave are different between tumor and tissue, this phenomenon has the potential to support the diagnosis of cancerous lesions.

Effects of diagnostic ultrasound with cRGD-microbubbles on simultaneous detection and treatment of atherosclerotic plaque in ApoE−/− mice

Background

Atherosclerotic vulnerable plaque is the leading cause of acute fatal cardiovascular events. Thus, early rapid identification and appropriate treatment of atherosclerotic plaque maybe can prevent fatal cardiovascular events. However, few non–invasive molecular imaging techniques are currently available for the simultaneous detection and targeted treatment of atherosclerotic plaques. We hypothesized that diagnostic ultrasound (DU) combined with cyclic Arg-Gly-Asp-modified microbubbles (MB_R) could provide targeted imaging and dissolution of activated platelets to identify advanced atherosclerotic plaques and improve plaque instability.

Methods

Three mouse models, apolipoprotein E-deficient mice on a hypercholesterolemic diet (HCD) or normal chow diet and wild-type mice on an HCD were used. The most appropriate ultrasonic mechanical index (MI) was determined based on the expression of GP IIb/IIIa in sham, DU alone and DUMB_R-treated groups at MI values of 0.5, 1.5, and 1.9. The video intensity (VI) values, activated platelets and plaque instability were analyzed by ultrasound molecular imaging, scanning electron microscopy and histopathological methods.

Results

We found that the VI values of ultrasound molecular imaging of MB_R were positively correlated with plaque GP IIb/IIIa expression, vulnerability index and necrotic center / fiber cap ratio. 24 h after treatment at different MIs, compared with those of the other groups, both the VI values and GP IIb/IIIa expression were significantly reduced in MI 1.5 and MI 1.9 DUMB_R-treated groups. The plaque vulnerability index and necrotic center / fiber cap ratio were significantly decreased in MI 1.5-treated group, which may be due to targeted dissolution of activated platelets, with a reduction in von Willebrand factor expression.

Conclusion

DUMB_R targeting GP IIb/IIIa receptors could rapidly detect advanced atherosclerotic plaques and simultaneously give targeted therapy by dissolving activated and aggregated platelets. This technology may represent a novel approach for the simultaneous identification and treatment of atherosclerotic plaques.

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

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

Dynamics of double bubbles under the driving of burst ultrasound

This paper investigates the pulsations and translation of bubbles in a double-bubble system driven by burst ultrasound. Results illustrate that for two identical bubbles, decreasing the frequency of burst or increasing its amplitude can enhance the pulsations and improve the translation velocities of bubbles. In a certain scope, large bubble brings about fast translation velocity, but the velocity will fall down for too large bubble, such as the bubble with ambient radius over about its resonance radius. When the ambient radii of two bubbles are different, translation of the large bubble is smaller than that of the small bubble. In addition, the effect of initial distance between bubbles is described as well. If burst serials are used, shortening the time interval between each burst and improving the acoustic amplitude of bursts are beneficial for the translations of bubbles.

Towards controlled drug delivery in brain tumors with microbubble-enhanced focused ultrasound

Brain tumors are particularly challenging malignancies, due to their location in a structurally and functionally distinct part of the human body - the central nervous system (CNS). The CNS is separated and protected by a unique system of brain and blood vessel cells which together prevent most bloodborne therapeutics from entering the brain tumor microenvironment (TME). Recently, great strides have been made through microbubble (MB) ultrasound contrast agents in conjunction with ultrasound energy to locally increase the permeability of brain vessels and modulate the brain TME. As we elaborate in this review, this physical method can effectively deliver a wide range of anticancer agents, including chemotherapeutics, antibodies, and nanoparticle drug conjugates across a range of preclinical brain tumors, including high grade glioma (glioblastoma), diffuse intrinsic pontine gliomas, and brain metastasis. Moreover, recent evidence suggests that this technology can promote the effective delivery of novel immunotherapeutic agents, including immune check-point inhibitors and chimeric antigen receptor T cells, among others. With early clinical studies demonstrating safety, and several Phase I/II trials testing the preclinical findings underway, this technology is making firm steps towards shaping the future treatments of primary and metastatic brain cancer. By elaborating on its key components, including ultrasound systems and MB technology, along with methods for closed-loop spatial and temporal control of MB activity, we highlight how this technology can be tuned to enable new, personalized treatment strategies for primary brain malignancies and brain metastases.

Biomedical nanobubbles and opportunities for microfluidics

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

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