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

Progress and potential of nanobubbles for ultrasound-mediated drug delivery

INTRODUCTION

Despite much progress, nanomedicine-based drug therapies in oncology remain limited by systemic toxicity and insufficient particle accumulation in the tumor. To address these barriers, formulations responsive to external physical stimuli have emerged. One most promising system is the ultrasound stimulation of drug-loaded, gas-core particles (bubbles). Ultrasound induces bubble cavitation for cell and tissue permeabilization, triggers on-demand drug release, and provides opportunities for real-time imaging of delivery.

AREAS COVERED

Here, we focus on shell-stabilized, gas-core nanoparticles (also termed nanobubbles or ultrafine bubbles) and their role in ultrasound-mediated therapeutic delivery to tumors. This review frames the advantages of nanobubbles within the ongoing deficits in nanomedicine, describes mechanisms of ultrasound-mediated therapy, and details formulation techniques for nanobubble delivery systems. It then highlights the past decade of research in nanobubble-facilitated drug delivery for cancer therapy and anticipates new directions in the field.

EXPERT OPINION

Nanobubble ultrasound contrast agents offer a spatiotemporally triggerable therapeutic coupled with a safe, accessible imaging modality. Nanobubbles can be loaded with diverse therapeutic cargoes to treat disease and overcome numerous barriers limiting delivery to solid tumors. Close attention to formulation, characterization methods, acoustic testing parameters, and the biological mechanisms of nanobubble delivery will facilitate preclinical research toward clinical adoption.

Microfluidic nanobubbles produced using a micromixer for ultrasound imaging and gene delivery

Ultrasound (US)-mediated delivery is considered relatively safe and achieves tissue-specific targeting by simply adjusting the application site of the physical energy. Moreover, combining US with micro- or nanobubbles (MBs or NBs), which serve as US contrast agents, enhances the delivery of drugs, genes, and nucleic acids which also functioning as a tool for US. The performance of US-responsive MBs and NBs, including their therapeutic outcomes, is influenced by the bubble manufacturing methods. Furthermore, productivity and scalability must also be considered for clinical applications. Among various NBs fabrication techniques, microfluidic technology has emerged as a promising approach. However, the potential of NBs generated by microfluidics for drug delivery remains unexplored. In this study, US-responsive NBs were prepared using a microfluidic device, providing a single step gas-filling operation and rapid production method not only for US imaging but also for gene delivery. The effectiveness of these NBs was subsequently evaluated. The preparation conditions for the microfluidic NBs (MF-NBs) were optimized based on their physical properties, including particle size, number concentration, and their performance as US agents. Gene delivery capability was assessed in various tissues, including muscles, heart, kidney, and brain. The results demonstrated that MF-NBs exhibit high monodispersity, enhance US imaging, achieve widespread distribution following administration (including in brain tissue), and enable gene delivery to irradiated areas. These findings suggest that MF-NBs, with their high productivity and uniformity, are promising candidates for practical applications in US imaging, gene delivery, and nucleic acid delivery systems.

Real-time quantification of microfluidic hydrogel crosslinking via gas-phase electrophoresis

This study presents a novel approach for the controlled synthesis and real-time characterization of crosslinked hyaluronic acid (HA) hydrogels utilizing a microfluidic platform coupled with hyphenated electrospray-differential mobility analysis (ES-DMA). By precisely controlling key synthesis parameters within the microfluidic environment, including pH, temperature, reaction time, and the molar ratio of HA to crosslinker (1,4-butanediol diglycidyl ether, BDDE), we successfully synthesized HA hydrogels with tailored size and properties. The integrated ES-DMA system provides rapid, in-line analysis of hydrogel particle size and distribution, enabling real-time monitoring and optimization of the synthesis process. Furthermore, small-angle x-ray scattering (SAXS) was employed to complement ES-DMA analysis, providing valuable insights into the internal structure and extent of crosslinking within the synthesized hydrogels. The evolution of the number-based particle size distribution revealed a strong correlation with the synthesis conditions, demonstrating the high degree of controllability achieved by this integrated approach. This novel methodology offers a promising platform for the high-throughput synthesis of uniform and well-defined hydrogel nanoparticles with enhanced traceability, paving the way for advancements in various applications including drug delivery, tissue engineering, and biomaterials.

A Machine Vision Perspective on Droplet-Based Microfluidics

Microfluidic droplets, with their unique properties and broad applications, are essential in in chemical, biological, and materials synthesis research. Despite the flourishing studies on artificial intelligence-accelerated microfluidics, most research efforts have focused on the upstream design phase of microfluidic systems. Generating user-desired microfluidic droplets still remains laborious, inefficient, and time-consuming. To address the long-standing challenges associated with the accurate and efficient identification, sorting, and analysis of the morphology and generation rate of single and double emulsion droplets, a novel machine vision approach utilizing the deformable detection transformer (DETR) algorithm is proposed. This method enables rapid and precise detection (detection relative error < 4% and precision > 94%) across various scales and scenarios, including real-world and simulated environments. Microfluidic droplets identification and analysis (MDIA), a web-based tool powered by Deformable DETR, which supports transfer learning to enhance accuracy in specific user scenarios is developed. MDIA characterizes droplets by diameter, number, frequency, and other parameters. As more training data are added by other users, MDIA's capability and universality expand, contributing to a comprehensive database for droplet microfluidics. The work highlights the potential of artificial intelligence in advancing microfluidic droplet regulation, fabrication, label-free sorting, and analysis, accelerating biochemical sciences and materials synthesis engineering.

Clinical Applications of Micro/Nanobubble Technology in Neurological Diseases

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

A Promising Therapeutic Strategy of Combining Acoustically Stimulated Nanobubbles and Existing Cancer Treatments

In recent years, ultrasound-stimulated microbubbles (USMBs) have gained great attention because of their wide theranostic applications. However, due to their micro-size, reaching the targeted site remains a challenge. At present, ultrasound-stimulated nanobubbles (USNBs) have attracted particular interest, and their small size allows them to extravasate easily in the blood vessels penetrating deeper into the tumor vasculature. Incorporating USNBs with existing cancer therapies such as chemotherapy, immunotherapy, and/or radiation therapy in several preclinical models has been demonstrated to have a profound effect on solid tumors. In this review, we provide an understanding of the composition and formation of nanobubbles (NBs), followed by the recent progress of the therapeutic combinatory effect of USNBs and other cancer therapies in cancer treatment.

Air trap and removal on a pressure driven PDMS-based microfluidic device

With the development of microfluidic technology, microfluidic chips have played a positive role in applications such as cell culture, microfluidic PCR, and nanopore gene sequencing. However, the presence of bubbles interferes with fluid flow and has a significant impact on experimental results. There are many reasons for the generation of bubbles in microfluidic chips, such as pressure changes inside the chip, air vibration inside the chip, and the open chip guiding air into the chip when driving fluid. This study designed and prepared a microfluidic device based on polydimethylsiloxane. First, air was actively introduced into the microfluidic chip, and bubbles were captured through the microfluidic device to simulate the presence of bubbles inside the chip in biological experiments. To remove bubbles trapped in the microfluidic chip, distilled water, distilled water containing surfactants, and mineral oil were pumped into the microfluidic chip. We compared and discussed the bubble removal efficiency under different driving fluids, driving pressures, and open/closed channel configurations. This study helps to understand the mechanism of bubble formation and removal in microfluidic devices, optimize chip structure design and experimental reagent selection, prevent or eliminate bubbles, and reduce the impact of bubbles on experiments.

Nanostructured Substrate-Mediated Bubble Degassing in Microfluidic Systems

Microfluidic platforms have been widely used in a variety of fields owing to their numerous advantages. The prevention and prompt removal of air bubbles from microchannels are important to ensuring the optimal functioning of microfluidic devices. The entrapment of bubbles in the microchannels can result in flow instability and device performance disruption. Active and passive methods are the primary categories of degassing technologies. Active methods rely on external equipment, and passive methods operate autonomously without any external sources. This study proposed a passive degassing method that employs a nanoscale surface morphology integrated into the substrate of a microfluidic device. Nanostructures exhibit a microchannel geometry and are fabricated based on surface micromachining technology using silver ink and chemical etching. Consequently, the gas permeability is enhanced, resulting in effective degassing through the nanostructure. The performance of this degassing method was characterized under varying substrate permeabilities and input pressure conditions, and it was found that increased permeability facilitates the degassing performance. Furthermore, the applicability of our method was demonstrated by using a serpentine channel design prone to gas entrapment, particularly in the corner regions. The nanostructured substrate exhibited significantly improved degassing performance under the given pressure conditions in comparison to the glass substrate.