Droplet-based microfluidics in biomedical applications

Recent advances in microfluidic technology of arterial thrombosis investigations

Microfluidic technology has emerged as a powerful tool in studying arterial thrombosis, allowing researchers to construct artificial blood vessels and replicate the hemodynamics of blood flow. This technology has led to significant advancements in understanding thrombosis and platelet adhesion and aggregation. Microfluidic models have various types and functions, and by studying the fabrication methods and working principles of microfluidic chips, applicable methods can be selected according to specific needs. The rapid development of microfluidic integrated system and modular microfluidic system makes arterial thrombosis research more diversified and automated, but its standardization still needs to be solved urgently. One key advantage of microfluidic technology is the ability to precisely control fluid flow in microchannels and to analyze platelet behavior under different shear forces and flow rates. This allows researchers to study the physiological and pathological processes of blood flow, shedding light on the underlying mechanisms of arterial thrombosis. In conclusion, microfluidic technology has revolutionized the study of arterial thrombosis by enabling the construction of artificial blood vessels and accurately reproducing hemodynamics. In the future, microfluidics will place greater emphasis on versatility and automation, holding great promise for advancing antithrombotic therapeutic and prophylactic measures.

Dielectrophoretic separation and purification: From colloid and biological particles to droplets

It is indispensable to realize the high level of purification and separation, so that objective particles, such as malignant cells, harmful bacteria, and special proteins or biological molecules, could satisfy the high precise measurement in the pharmaceutical analysis, clinical diagnosis, targeted therapy, and food defense. In addition, this could reveal the intrinsic nature and evolution mechanisms of individual biological variations. Consequently, many techniques related to optical tweezers, microfluidics, acoustophoresis, and electrokinetics can be broadly used to achieve micro- and nano-scale particle separations. Dielectrophoresis (DEP) has been used for various manipulation, concentration, transport, and separation processes of biological particles owing to its early development, mature theory, low cost, and high throughput. Although numerous reviews have discussed the biological applications of DEP techniques, comprehensive descriptions of micro- and nano-scale particle separations feature less frequently in the literature. Therefore, this review summarizes the current state of particle separation attention to relevant technological developments and innovation, including theoretical simulation, microchannel structure, electrode material, pattern and its layout. Moreover, a brief overview of separation applications using DEP in combination with other technologies is also provided. Finally, conclusions, future guidelines, and suggestions for potential promotion are highlighted.

Antibacterial sequential growth factor delivery from alginate/gelatin methacryloyl microspheres for bone regeneration

Autologous or allogeneic bone tissue grafts remain the mainstay of treatment for clinical bone defects. However, the risk of infection and donor scarcity in bone grafting pose challenges to the process. Therefore, the development of excellent biomaterial grafts is of great clinical importance for the repair of bone defects. In this study, we used gas-assisted microfluidics to construct double-cross-linked hydrogel microspheres with good biological function based on the ionic cross-linking of Cu2+ with alginate and photo-cross-linking of gelatin methacryloylamide (GelMA) by loading vascular endothelial growth factor (VEGF) and His-tagged bone morphogenetic protein-2 (BMP2) (AGMP@VEGF&BMP2). The Cu2+ component in the microspheres showed good antibacterial and drug-release behavior, whereas VEGF and BMP2 effectively promoted angiogenesis and bone tissue repair. In in vitro and in vivo experiments, the dual cross-linked hydrogel microspheres showed good biological function and biocompatibility. These results demonstrate that AGMP@VEGF&BMP2 microspheres could be used as a bone defect graft substitute to promote effective healing of bone defects and may be applied to other tissue engineering studies.

The effects of droplet stabilization by surfactants and nanoparticles on leakage, cross-talk, droplet stability, and cell adhesion

Partially fluorinated nanoparticles (FNPs) have been proposed as a promising alternative for stabilising aqueous droplets in fluorinated oils. The exceptional energetic stability of FNPs at the droplet interface holds the potential for minimising leakage, enhancing stability, and promoting improved cell adhesion. However, their lower diffusion coefficient compared to surfactants presents challenges in achieving rapid droplet stabilisation, which is important in microfluidics applications. While several studies have focused on some of these aspects, a comprehensive study and direct comparison with conventional fluorosurfactants is still missing. In this manuscript, we undertake an examination and comparison of four crucial facets of both FNP- and surfactant-stabilised droplets: leakage of compounds, emulsion stability, droplet formation dynamics and cell adhesion. Contrary to what has previously been claimed, our findings demonstrate that FNPs only reduce leakage and cross-talk in very specific cases (e.g., resorufin), failing to provide enhanced compartmentalisation for highly hydrophobic dyes (e.g., rhodamine dyes). On the other hand, FNP-stabilised droplets indeed exhibit greater long-term stability compared to their surfactant-stabilised counterparts. Regarding the size of droplets generated via a diversity of microfluidic methods, no significant differences were observed between FNP-stabilised and surfactant-stabilised droplets. Finally, the previously reported improvements in cell adhesion and spreading on FNP-stabilised interfaces is limited to flat oil/water (o/w) interfaces and could not be observed within droplets. These comprehensive analyses shed light on the nuanced performance of FNPs and commercial fluorosurfactants as stabilising agents for aqueous droplets in fluorinated oils, contributing valuable insights for choosing the correct formulation for specific droplet-based microfluidics applications.

Droplet Microfluidics Powered Hydrogel Microparticles for Stem Cell-Mediated Biomedical Applications

Stem cell-related therapeutic technologies have garnered significant attention of the research community for their multi-faceted applications. To promote the therapeutic effects of stem cells, the strategies for cell microencapsulation in hydrogel microparticles have been widely explored, as the hydrogel microparticles have the potential to facilitate oxygen diffusion and nutrient transport alongside their ability to promote crucial cell-cell and cell-matrix interactions. Despite their significant promise, there is an acute shortage of automated, standardized, and reproducible platforms to further stem cell-related research. Microfluidics offers an intriguing platform to produce stem cell-laden hydrogel microparticles (SCHMs) owing to its ability to manipulate the fluids at the micrometer scale as well as precisely control the structure and composition of microparticles. In this review, the typical biomaterials and crosslinking methods for microfluidic encapsulation of stem cells as well as the progress in droplet-based microfluidics for the fabrication of SCHMs are outlined. Moreover, the important biomedical applications of SCHMs are highlighted, including regenerative medicine, tissue engineering, scale-up production of stem cells, and microenvironmental simulation for fundamental cell studies. Overall, microfluidics holds tremendous potential for enabling the production of diverse hydrogel microparticles and is worthy for various stem cell-related biomedical applications.

Drop-On-Demand Microdroplet Generation under Charge Injection by Corona Discharge

In printing, microreactors, and bioassays, the precise control of micrometer-scale droplet generation is essential but challenging, often restricted by the equipment and nozzles used in traditional methods. We introduce a needle-plate electrode corona discharge technique that injects charges into an oil layer, enabling the precise manipulation of droplet polarization and splitting. This method allows for meticulous adjustment of microdroplet formation regarding location, size, and quantity by modulating the discharge voltage, discharge time, and electrode positioning. It enables the immediate initiation and cessation of droplet production, thereby facilitating on-demand droplet generation. Our study on the voltage-dependent droplet stretch coefficient shows that as the voltage increases, the droplets transition from controlled splitting to regular Taylor cone-like ejections, eventually reaching the Rayleigh limit and fully breaking apart. These advancements significantly improve microfluidic droplet manipulation, offering considerable benefits for applications in targeted drug delivery, rapid disease diagnostics, and precise environmental monitoring.

High-Throughput Microfluidic Particle Counter Based on Optical Absorption

With the utilization of advanced microfluidic techniques, the microfluidic particle counter demonstrates significant potential due to its high efficiency, precise manipulation, and portability. This work focuses on a photodetection counter based on optical absorption. To achieve precise particle detection, a Christmas tree-like structure was implemented to separate a single particle from a cluster, which was then detected in independent multiple parallel channels. The system exhibits a high degree of reliability, as evidenced by a linear correlation coefficient over 0.99 obtained during testing with gradient-concentrated beads. Furthermore, when the calculated density of NIH 3T3 cells is compared with that of a traditional flow cytometer, the system achieves a substantial agreement percentage ranging from 87.5 to 99.9%. The system's ability to perform high-throughput analysis with a high acquisition rate positions it as a promising tool for real-time point-of-care testing.

Advances in the isolation, cultivation, and identification of gut microbes

The gut microbiome is closely associated with human health and the development of diseases. Isolating, characterizing, and identifying gut microbes are crucial for research on the gut microbiome and essential for advancing our understanding and utilization of it. Although culture-independent approaches have been developed, a pure culture is required for in-depth analysis of disease mechanisms and the development of biotherapy strategies. Currently, microbiome research faces the challenge of expanding the existing database of culturable gut microbiota and rapidly isolating target microorganisms. This review examines the advancements in gut microbe isolation and cultivation techniques, such as culturomics, droplet microfluidics, phenotypic and genomics selection, and membrane diffusion. Furthermore, we evaluate the progress made in technology for identifying gut microbes considering both non-targeted and targeted strategies. The focus of future research in gut microbial culturomics is expected to be on high-throughput, automation, and integration. Advancements in this field may facilitate strain-level investigation into the mechanisms underlying diseases related to gut microbiota.

Uniform sized cancer spheroids production using hydrogel-based droplet microfluidics: a review

Three-dimensional (3D) cell culture models have been extensively utilized in various mechanistic studies as well as for drug development studies as superior in vitro platforms than conventional two-dimensional (2D) cell culture models. This is especially the case in cancer biology, where 3D cancer models, such as spheroids or organoids, have been utilized extensively to understand the mechanisms of cancer development. Recently, many sophisticated 3D models such as organ-on-a-chip models are emerging as advanced in vitro models that can more accurately mimic the in vivo tissue functions. Despite such advancements, spheroids are still considered as a powerful 3D cancer model due to the relatively simple structure and compatibility with existing laboratory instruments, and also can provide orders of magnitude higher throughput than complex in vitro models, an extremely important aspects for drug development. However, creating well-defined spheroids remain challenging, both in terms of throughputs in generation as well as reproducibility in size and shape that can make it challenging for drug testing applications. In the past decades, droplet microfluidics utilizing hydrogels have been highlighted due to their potentials. Importantly, core-shell structured gel droplets can avoid spheroid-to-spheroid adhesion that can cause large variations in assays while also enabling long-term cultivation of spheroids with higher uniformity by protecting the core organoid area from external environment while the outer porous gel layer still allows nutrient exchange. Hence, core-shell gel droplet-based spheroid formation can improve the predictivity and reproducibility of drug screening assays. This review paper will focus on droplet microfluidics-based technologies for cancer spheroid production using various gel materials and structures. In addition, we will discuss emerging technologies that have the potential to advance the production of spheroids, prospects of such technologies, and remaining challenges.

Dissecting the Interplay Mechanism among Process Parameters toward the Biofabrication of High-Quality Shapes in Embedded Bioprinting

Embedded bioprinting overcomes the barriers associated with the conventional extrusion-based bioprinting process as it enables the direct deposition of bioinks in 3D inside a support bath by providing in situ self-support for deposited bioinks during bioprinting to prevent their collapse and deformation. Embedded bioprinting improves the shape quality of bioprinted constructs made up of soft materials and low-viscosity bioinks, leading to a promising strategy for better anatomical mimicry of tissues or organs. Herein, the interplay mechanism among the printing process parameters toward improved shape quality is critically reviewed. The impact of material properties of the support bath and bioink, printing conditions, cross-linking mechanisms, and post-printing treatment methods, on the printing fidelity, stability, and resolution of the structures is meticulously dissected and thoroughly discussed. Further, the potential scope and applications of this technology in the fields of bioprinting and regenerative medicine are presented. Finally, outstanding challenges and opportunities of embedded bioprinting as well as its promise for fabricating functional solid organs in the future are discussed.

Simple Method for On-Demand Droplet Trapping in a Microfluidic Device Based on the Concept of Hydrodynamic Resistance

We demonstrate an innovative method to catch the desired droplets from a train of droplets and immobilize them in traps located in an integrated microfluidic device. To this end, water-in-oil droplets are generated in a flow-focusing junction and then guided to a channel connected to chambers designated for on-demand droplet trapping. Each chamber is connected to a side channel through a batch of microposts. The side channels are also connected to the flexible poly(vinyl chloride) tubes, which can be closed by attaching binder clips. The hydrodynamic resistance of the routes in the device can be changed by opening and closing the binder clips. In this way, droplets are easily guided into individual traps based on the user's demand. A set of numerical simulations was also conducted to investigate the authenticity of the employed idea and to find the optimal geometry for implementing our strategy. This simple method can be easily employed for on-demand droplet trapping without using on-chip valves or complex off-chip actuators proposed in previous studies.

Portable droplet-based real-time monitoring of pancreatic α-amylase in postoperative patients

Postoperative complications after pancreatic surgery are frequent and can be life-threatening. Current clinical diagnostic strategies involve time-consuming quantification of α-amylase activity in abdominal drain fluid, which is performed on the first and third postoperative day. The lack of real-time monitoring may delay adjustment of medical treatment upon complications and worsen prognosis for patients. We report a bedside portable droplet-based millifluidic device enabling real-time sensing of drain α-amylase activity for postoperative monitoring of patients undergoing pancreatic surgery. Here, a tiny amount of drain liquid of patient samples is continuously collected and co-encapsulated with a starch reagent in nanoliter-sized droplets to track the fluorescence intensity released upon reaction with α-amylase. Comparing the α-amylase levels of 32 patients, 97 % of the results of the droplet-based millifluidic system matched the clinical data. Our method reduces the α-amylase assay duration to approximately 3 min with the limit of detection 7 nmol/s·L, enabling amylase activity monitoring at the bedside in clinical real-time. The presented droplet-based platform can be extended for analysis of different body fluids, diseases, and towards a broader range of biomarkers, including lipase, bilirubin, lactate, inflammation, or liquid biopsy markers, paving the way towards new standards in postoperative patient monitoring.

Advances in Culturomics Research on the Human Gut Microbiome: Optimizing Medium Composition and Culture Techniques for Enhanced Microbial Discovery

Despite considerable advancements achieved using next-generation sequencing technologies in exploring microbial diversity, several species of the gut microbiome remain unknown. In this transformative era, culturomics has risen to prominence as a pivotal approach in unveiling realms of microbial diversity that were previously deemed inaccessible. Utilizing innovative strategies to optimize growth and culture medium composition, scientists have successfully cultured hard-to-cultivate microbes. This progress has fostered the discovery and understanding of elusive microbial entities, highlighting their essential role in human health and disease paradigms. In this review, we emphasize the importance of culturomics research on the gut microbiome and provide new theories and insights for expanding microbial diversity via the optimization of cultivation conditions.

A Label-Free Droplet Sorting Platform Integrating Dielectrophoretic Separation for Estimating Bacterial Antimicrobial Resistance

Antimicrobial resistance (AMR) has become a crucial global health issue. Antibiotic-resistant bacteria can survive after antibiotic treatments, lowering drug efficacy and increasing lethal risks. A microfluidic water-in-oil emulsion droplet system can entrap microorganisms and antibiotics within the tiny bioreactor, separate from the surroundings, enabling independent assays that can be performed in a high-throughput manner. This study presents the development of a label-free dielectrophoresis (DEP)-based microfluidic platform to sort droplets that co-encapsulate Escherichia coli (E. coli) and ampicillin (Amp) and droplets that co-encapsulate Amp-resistant (AmpR) E. coli with Amp only based on the conductivity-dependent DEP force (FDEP) without the assistance of optical analyses. The 9.4% low conductivity (LC) Luria-Bertani (LB) broth diluted with 170 mM mannitol can maintain E. coli and AmpR E. coli growth for 3 h and allow Amp to kill almost all E. coli, which can significantly increase the LCLB conductivity by about 100 μS/cm. Therefore, the AmpR E. coli/9.4%LCLB/Amp where no cells are killed and the E. coli/9.4%LCLB/Amp-containing droplets where most of the cells are killed can be sorted based on this conductivity difference at an applied electric field of 2 MHz and 100 Vpp that generates positive FDEP. Moreover, the sorting ratio significantly decreased to about 50% when the population of AmpR E. coli was equal to or higher than 50% in droplets. The conductivity-dependent DEP-based sorting platform exhibits promising potential to probe the ratio of AmpR E. coli in an unknown bacterial sample by using the sorting ratio as an index.

Droplet-based microfluidic platform for detecting agonistic peptides that are self-secreted by yeast expressing a G-protein-coupled receptor

BACKGROUND

Single-cell droplet microfluidics is an important platform for high-throughput analyses and screening because it provides an independent and compartmentalized microenvironment for reaction or cultivation by coencapsulating individual cells with various molecules in monodisperse microdroplets. In combination with microbial biosensors, this technology becomes a potent tool for the screening of mutant strains. In this study, we demonstrated that a genetically engineered yeast strain that can fluorescently sense agonist ligands via the heterologous expression of a human G-protein-coupled receptor (GPCR) and concurrently secrete candidate peptides is highly compatible with single-cell droplet microfluidic technology for the high-throughput screening of new agonistically active peptides.

RESULTS

The water-in-oil microdroplets were generated using a flow-focusing microfluidic chip to encapsulate engineered yeast cells coexpressing a human GPCR [i.e., angiotensin II receptor type 1 (AGTR1)] and a secretory agonistic peptide [i.e., angiotensin II (Ang II)]. The single yeast cells cultured in the droplets were then observed under a microscope and analyzed using image processing incorporating machine learning techniques. The AGTR1-mediated signal transduction elicited by the self-secreted Ang II peptide was successfully detected via the expression of a fluorescent reporter in single-cell yeast droplet cultures. The system could also distinguish Ang II analog peptides with different agonistic activities. Notably, we further demonstrated that the microenvironment of the single-cell droplet culture enabled the detection of rarely existing positive (Ang II-secreting) yeast cells in the model mixed cell library, whereas the conventional batch-culture environment using a shake flask failed to do so. Thus, our approach provided compartmentalized microculture environments, which can prevent the diffusion, dilution, and cross-contamination of peptides secreted from individual single yeast cells for the easy identification of GPCR agonists.

CONCLUSIONS

We established a droplet-based microfluidic platform that integrated an engineered yeast biosensor strain that concurrently expressed GPCR and self-secreted the agonistic peptides. This offers individually isolated microenvironments that allow the culture of single yeast cells secreting these peptides and gaging their signaling activities, for the high-throughput screening of agonistic peptides. Our platform base on yeast GPCR biosensors and droplet microfluidics will be widely applicable to metabolic engineering, environmental engineering, and drug discovery.

Advanced strategies in high-throughput droplet screening for enzyme engineering

Enzymes, as biocatalysts, play a cumulatively important role in environmental purification and industrial production of chemicals and pharmaceuticals. However, natural enzymes are limited by their physiological properties in practice, which need to be modified driven by requirements. Screening and isolating certain enzyme variants or ideal industrial strains with high yielding of target product enzymes is one of the main directions of enzyme engineering research. Droplet-based high-throughput screening (DHTS) technology employs massive monodisperse emulsion droplets as microreactors to achieve single strain encapsulation, as well as continuous monitoring for the inside mutant library. It can effectively sort out strains or enzymes with desired characteristics, offering a throughput of 108 events per hour. Much of the early literature focused on screening various engineered strains or designing signalling sorting strategies based on DHTS technology. However, the field of enzyme engineering lacks a comprehensive overview of advanced methods for microfluidic droplets and their cutting-edge developments in generation and manipulation. This review emphasizes the advanced strategies and frontiers of microfluidic droplet generation and manipulation facilitating enzyme engineering development. We also introduce design for various screening signals that cooperate with DHTS and devote to enzyme engineering.

Droplet Microfluidics for Current Cancer Research: From Single-Cell Analysis to 3D Cell Culture

Cancer is the second leading cause of death worldwide. Differences in drug resistance and treatment response caused by the heterogeneity of cancer cells are the primary reasons for poor cancer therapy outcomes in patients. In addition, current in vitro anticancer drug-screening methods rely on two-dimensional monolayer-cultured cancer cells, which cannot accurately predict drug behavior in vivo. Therefore, a powerful tool to study the heterogeneity of cancer cells and produce effective in vitro tumor models is warranted to leverage cancer research. Droplet microfluidics has become a powerful platform for the single-cell analysis of cancer cells and three-dimensional cell culture of in vitro tumor spheroids. In this review, we discuss the use of droplet microfluidics in cancer research. Droplet microfluidic technologies, including single- or double-emulsion droplet generation and passive- or active-droplet manipulation, are concisely discussed. Recent advances in droplet microfluidics for single-cell analysis of cancer cells, circulating tumor cells, and scaffold-free/based 3D cell culture of tumor spheroids have been systematically introduced. Finally, the challenges that must be overcome for the further application of droplet microfluidics in cancer research are discussed.

Passive microfluidic devices for cell separation

The separation of specific cell populations is instrumental in gaining insights into cellular processes, elucidating disease mechanisms, and advancing applications in tissue engineering, regenerative medicine, diagnostics, and cell therapies. Microfluidic methods for cell separation have propelled the field forward, benefitting from miniaturization, advanced fabrication technologies, a profound understanding of fluid dynamics governing particle separation mechanisms, and a surge in interdisciplinary investigations focused on diverse applications. Cell separation methodologies can be categorized according to their underlying separation mechanisms. Passive microfluidic separation systems rely on channel structures and fluidic rheology, obviating the necessity for external force fields to facilitate label-free cell separation. These passive approaches offer a compelling combination of cost-effectiveness and scalability when compared to active methods that depend on external fields to manipulate cells. This review delves into the extensive utilization of passive microfluidic techniques for cell separation, encompassing various strategies such as filtration, sedimentation, adhesion-based techniques, pinched flow fractionation (PFF), deterministic lateral displacement (DLD), inertial microfluidics, hydrophoresis, viscoelastic microfluidics, and hybrid microfluidics. Besides, the review provides an in-depth discussion concerning cell types, separation markers, and the commercialization of these technologies. Subsequently, it outlines the current challenges faced in the field and presents a forward-looking perspective on potential future developments. This work hopes to aid in facilitating the dissemination of knowledge in cell separation, guiding future research, and informing practical applications across diverse scientific disciplines.

Enhanced mixing efficiency and reduced droplet size with novel droplet generators

Nowadays, droplet microfluidics has become widely utilized for high-throughput assays. Efficient mixing is crucial for initiating biochemical reactions in many applications. Rapid mixing during droplet formation eliminates the need for incorporating micromixers, which can complicate the chip design. Furthermore, immediate mixing of substances upon contact can significantly improve the consistency of chemical reactions and resulting products. This study introduces three innovative designs for droplet generators that achieve efficient mixing and produce small droplets. The T-cross and cross-T geometries combine cross and T junction mixing mechanisms, resulting in improved mixing efficiency. Numerical simulations were conducted to compare these novel geometries with traditional T and cross junctions in terms of mixing index, droplet diameter, and eccentricity. The cross-T geometry exhibited the highest mixing index and produced the smallest droplets. For the flow rate ratio of 0.5, this geometry offered a 10% increase in the mixing index and a decrease in the droplet diameter by 10% compared to the T junction. While the T junction has the best mixing efficiency among traditional droplet generators, it produces larger droplets, which can increase the risk of contamination due to contact with the microchannel walls. Therefore, the cross-T geometry is highly desirable in most applications due to its production of considerably smaller droplets. The asymmetric cross junction offered a 8% increase in mixing index and around 2% decrease in droplet diameter compared to the conventional cross junction in flow rate ratio of 0.5. All novel geometries demonstrated comparable mixing efficiency to the T junction. The cross junction exhibited the lowest mixing efficiency and produced larger droplets compared to the cross-T geometry (around 1%). Thus, the novel geometries, particularly the cross-T geometry, are a favorable choice for applications where both high mixing efficiency and small droplet sizes are important.

Customizable Microfluidic Devices: Progress, Constraints, and Future Advances

The field of microfluidics encompasses the study of fluid behavior within micro-channels and the development of miniature systems featuring internal compartments or passageways tailored for fluid control and manipulation. Microfluidic devices capitalize on the unique chemical and physical properties exhibited by fluids at the microscopic scale. In contrast to their larger counterparts, microfluidic systems offer a multitude of advantages. Their implementation facilitates the investigation and utilization of reduced sample, solvent, and reagent volumes, thus yielding decreased operational expenses. Owing to their compact dimensions, these devices allow for the concurrent execution of multiple procedures, leading to expedited experimental timelines. Over the past two decades, microfluidics has undergone remarkable advancements, evolving into a multifaceted discipline. Subfields such as organ-on-a-chip and paper-based microfluidics have matured into distinct fields of study. Nonetheless, while scientific progress within the microfluidics realm has been notable, its translation into autonomous end-user applications remains a frontier to be fully explored. This paper sets forth the central objective of scrutinizing the present research paradigm, prevailing limitations, and potential prospects of customizable microfluidic devices. Our inquiry revolves around the latest strides achieved, prevailing constraints, and conceivable trajectories for adaptable microfluidic technologies. We meticulously delineate existing iterations of microfluidic systems, elucidate their operational principles, deliberate upon encountered limitations, and provide a visionary outlook toward the future trajectory of microfluidic advancements. In summation, this work endeavors to shed light on the current state of microfluidic systems, underscore their operative intricacies, address incumbent challenges, and unveil promising pathways that chart the course toward the next frontier of microfluidic innovation.

Closed, one-stop intelligent and accurate particle characterization based on micro-Raman spectroscopy and digital microfluidics

Monoclonal antibodies are prone to form protein particles through aggregation, fragmentation, and oxidation under varying stress conditions during the manufacturing, shipping, and storage of parenteral drug products. According to pharmacopeia requirements, sub-visible particle levels need to be controlled throughout the shelf life of the product. Therefore, in addition to determining particle counts, it is crucial to accurately characterize particles in drug product to understand the stress condition of exposure and to implement appropriate mitigation actions for a specific formulation. In this study, we developed a new method for intelligent characterization of protein particles using micro-Raman spectroscopy on a digital microfluidic chip (DMF). Several microliters of protein particle solutions induced by stress degradation were loaded onto a DMF chip to generate multiple droplets for Raman spectroscopy testing. By training multiple machine learning classification models on the obtained Raman spectra of protein particles, eight types of protein particles were successfully characterized and predicted with high classification accuracy (93%-100%). The advantages of the novel particle characterization method proposed in this study include a closed system to prevent particle contamination, one-stop testing of morphological and chemical structure information, low sample volume consumption, reusable particle droplets, and simplified data analysis with high classification accuracy. It provides great potential to determine the probable root cause of the particle source or stress conditions by a single testing, so that an accurate particle control strategy can be developed and ultimately extend the product shelf-life.

Microfluidic synthesis of nanomaterials for biomedical applications

The field of nanomaterials has progressed dramatically over the past decades with important contributions to the biomedical area. The physicochemical properties of nanomaterials, such as the size and structure, can be controlled through manipulation of mass and heat transfer conditions during synthesis. In particular, microfluidic systems with rapid mixing and precise fluid control are ideal platforms for creating appropriate synthesis conditions. One notable example of microfluidics-based synthesis is the development of lipid nanoparticle (LNP)-based mRNA vaccines with accelerated clinical translation and robust efficacy during the COVID-19 pandemic. In addition to LNPs, microfluidic systems have been adopted for the controlled synthesis of a broad range of nanomaterials. In this review, we introduce the fundamental principles of microfluidic technologies including flow field- and multiple field-based methods for fabricating nanoparticles, and discuss their applications in the biomedical field. We conclude this review by outlining several major challenges and future directions in the implementation of microfluidic synthesis of nanomaterials.

Particle Counting Methods Based on Microfluidic Devices

Particle counting serves as a pivotal constituent in diverse analytical domains, encompassing a broad spectrum of entities, ranging from blood cells and bacteria to viruses, droplets, bubbles, wear debris, and magnetic beads. Recent epochs have witnessed remarkable progressions in microfluidic chip technology, culminating in the proliferation and maturation of microfluidic chip-based particle counting methodologies. This paper undertakes a taxonomical elucidation of microfluidic chip-based particle counters based on the physical parameters they detect. These particle counters are classified into three categories: optical-based counters, electrical-based particle counters, and other counters. Within each category, subcategories are established to consider structural differences. Each type of counter is described not only in terms of its working principle but also the methods employed to enhance sensitivity and throughput. Additionally, an analysis of future trends related to each counter type is provided.

Microfluidics for nano-drug delivery systems: From fundamentals to industrialization

In recent years, owing to the miniaturization of the fluidic environment, microfluidic technology offers unique opportunities for the implementation of nano drug delivery systems (NDDSs) production processes. Compared with traditional methods, microfluidics improves the controllability and uniformity of NDDSs. The fast mixing and laminar flow properties achieved in the microchannels can tune the physicochemical properties of NDDSs, including particle size, distribution and morphology, resulting in narrow particle size distribution and high drug-loading capacity. The success of lipid nanoparticles encapsulated mRNA vaccines against coronavirus disease 2019 by microfluidics also confirmed its feasibility for scaling up the preparation of NDDSs via parallelization or numbering-up. In this review, we provide a comprehensive summary of microfluidics-based NDDSs, including the fundamentals of microfluidics, microfluidic synthesis of NDDSs, and their industrialization. The challenges of microfluidics-based NDDSs in the current status and the prospects for future development are also discussed. We believe that this review will provide good guidance for microfluidics-based NDDSs.

Droplet-Based Microfluidics: Applications in Pharmaceuticals

Droplet-based microfluidics offer great opportunities for applications in various fields, such as diagnostics, food sciences, and drug discovery. A droplet provides an isolated environment for performing a single reaction within a microscale-volume sample, allowing for a fast reaction with a high sensitivity, high throughput, and low risk of cross-contamination. Owing to several remarkable features, droplet-based microfluidic techniques have been intensively studied. In this review, we discuss the impact of droplet microfluidics, particularly focusing on drug screening and development. In addition, we surveyed various methods of device fabrication and droplet generation/manipulation. We further highlight some promising studies covering drug synthesis and delivery that were updated within the last 5 years. This review provides researchers with a quick guide that includes the most up-to-date and relevant information on the latest scientific findings on the development of droplet-based microfluidics in the pharmaceutical field.

Technological advances for analyzing the content of organ-on-a-chip by mass spectrometry

Three-dimensional (3D) cell cultures, including organ-on-a-chip (OOC) devices, offer the possibility to mimic human physiology conditions better than 2D models. The organ-on-a-chip devices have a wide range of applications, including mechanical studies, functional validation, and toxicology investigations. Despite many advances in this field, the major challenge with the use of organ-on-a-chips relies on the lack of online analysis methods preventing the real-time observation of cultured cells. Mass spectrometry is a promising analytical technique for real-time analysis of cell excretes from organ-on-a-chip models. This is due to its high sensitivity, selectivity, and ability to tentatively identify a large variety of unknown compounds, ranging from metabolites, lipids, and peptides to proteins. However, the hyphenation of organ-on-a-chip with MS is largely hampered by the nature of the media used, and the presence of nonvolatile buffers. This in turn stalls the straightforward and online connection of organ-on-a-chip outlet to MS. To overcome this challenge, multiple advances have been made to pre-treat samples right after organ-on-a-chip and just before MS. In this review, we summarised these technological advances and exhaustively evaluated their benefits and shortcomings for successful hyphenation of organ-on-a-chip with MS.

Microtrap-assisted microfluidic magnetic separation and concentration for ultrasensitive immunoassays of biomarkers

Precise and accurate quantitation of important biomarkers is significant, especially in early-stage diseases diagnosis. To realized effective biosample preparation and trace-level biomarker detection, a microtrap-assisted microfluidic magnetic immunoassays (μMI) method was developed in this work. A microtrap was fabricated inside the straight microchannel of μMI device to help magnetic separation and concentration of immunocomplexes. These immunocomplexes were enriched in microtrap of μMI device to accomplish selective and sensitive biomarker detection. Horseradish peroxidase-labeled magnetic beads were employed to evaluate assay feasibility and microtrap effect on assay sensitivity. The microtrap-assisted μMI was then applied for model biomarkers detection. The limits of detection of μMI were 0.025 pg/mL for monocyte chemoattractant protein-1 (MCP-1) and 0.021 pg/mL for matrix metalloproteinase-9 (MMP-9), which corresponded up to 2014-fold sensitivity improvement compared to their standard microwell enzyme-linked immunosorbent assay (ELISA) results. In addition, the selectivity and reproducibility of microtrap-assisted μMI were confirmed. In clinical serum sample analysis, recoveries of 91.3%-106.7% with relative standard deviations less than 6.1% were obtained for MCP-1 and MMP-9, and method accuracy was verified by commercial ELISA kit. The developed μMI can accomplish ultratrace biomarker detection offering practical tool for laboratorial and clinical research.

Molecularly Imprinted Polymeric Particles Created Using Droplet-Based Microfluidics: Preparation and Applications

Recent years have witnessed increased attention to the use of droplet-based microfluidics as a tool for the fabrication of microparticles due to this method's ability to exploit fluid mechanics to create materials with a narrow range of sizes. In addition, this approach offers a controllable way to configure the composition of the resulting micro/nanomaterials. To date, molecularly imprinted polymers (MIPs) in particle form have been prepared using various polymerization methods for several applications in biology and chemistry. However, the traditional approach, that is, the production of microparticles through grinding and sieving, generally leads to poor control over particle size and distribution. Droplet-based microfluidics offers an attractive alternative for the fabrication of molecularly imprinted microparticles. This mini-review aims to present recent examples highlighting the application of droplet-based microfluidics to fabricate molecularly imprinted polymeric particles for applications in the chemical and biomedical sciences.

Multiplex Detection of Infectious Diseases on Microfluidic Platforms

Infectious diseases contribute significantly to the global disease burden. Sensitive and accurate screening methods are some of the most effective means of identifying sources of infection and controlling infectivity. Conventional detecting strategies such as quantitative polymerase chain reaction (qPCR), DNA sequencing, and mass spectrometry typically require bulky equipment and well-trained personnel. Therefore, mass screening of a large population using conventional strategies during pandemic periods often requires additional manpower, resources, and time, which cannot be guaranteed in resource-limited settings. Recently, emerging microfluidic technologies have shown the potential to replace conventional methods in performing point-of-care detection because they are automated, miniaturized, and integrated. By exploiting the spatial separation of detection sites, microfluidic platforms can enable the multiplex detection of infectious diseases to reduce the possibility of misdiagnosis and incomplete diagnosis of infectious diseases with similar symptoms. This review presents the recent advances in microfluidic platforms used for multiplex detection of infectious diseases, including microfluidic immunosensors and microfluidic nucleic acid sensors. As representative microfluidic platforms, lateral flow immunoassay (LFIA) platforms, polymer-based chips, paper-based devices, and droplet-based devices will be discussed in detail. In addition, the current challenges, commercialization, and prospects are proposed to promote the application of microfluidic platforms in infectious disease detection.

Cell Membranes from Tumor-Tropic MSCs Screened by a Microfluidic Chip for Drug Nanoparticles Encapsulation and Cancer Targeted Therapy

Nanoparticles (NPs)-based drug carriers are effective in reducing systemic toxicity and drug resistance for chemotherapy, and an emerging trend focuses on integrating cell membranes with nanomedicines for tumor targeting. Mesenchymal stem cells (MSCs) are promising candidates due to their unique tropism toward cancer cells, yet the tumor-tropic abilities can differ for MSCs sourced from different tissues. Here, a multichannel microfluidic chip to screen different sourced MSCs with the greatest tropism toward cervical cancer cells is developed. Based on this, the cell membranes from the chorionic plate-derived MSC are isolated and membrane-camouflaged platinum prodrug composite NPs for cervical cancer treatment are prepared. Results demonstrate that the composite NPs can effectively target tumor sites and have a therapeutic effect both in vitro and in vivo. It is believed that the present microfluidic platform is a powerful tool for cell screening and tumor-on-a-chip studies, and the derived nanodelivery system represents the great value of cell membrane-camouflaged nanomedicine for targeted cancer therapy.

Multiple-Droplet Selective Manipulation Enabled by Laser-Textured Hydrophobic Magnetism-Responsive Slanted Micropillar Arrays with an Ultrafast Reconfiguration Rate

Biomimetic structures based on the magnetic response have attracted ever-increasing attention in droplet manipulation. Till now, most methods for droplet manipulation by a magnetic response are only applicable to a single droplet. It is still a challenge to achieve on-demand and precise control of multiple droplets (≥2). In this paper, a strategy for on-demand manipulation of multiple droplets based on magnetism-responsive slanted micropillar arrays (MSMAs) is proposed. The Glaco-modified superhydrophobic surface is the basis of multiple-droplet manipulation. The droplet's motion mode (pinned, unidirectional, and bidirectional) can be readily fine-tuned by changing the volume of droplets and the speed of the magnetic field. The rapid movement of droplets (10-80 mm/s) in the horizontal direction is realized by the unidirectional waves of the micropillar array driven by a specific magnetic field. The bending angle of micropillars can be rapidly and reversibly adjusted from 0 to 90° under the action of a magnetic field. Meanwhile, the liquid-involved light, electric switch, and biomedical detection can be designed by manipulating the droplets on demand. The superiority of MSMAs in multiple-droplet programmable manipulation opens up an avenue for applications in microfluidic and biomedical engineering.

Understanding the microfluidic generation of double emulsion droplets with alginate shell

The monodisperse double emulsions obtained by microfluidic method can serve as ideal templates for preparing core-shell alginate microcapsules, which have attracted much attention in biological applications, such as drug delivery systems and cell encapsulation, tissue engineering. However, the formation behavior and dynamic analysis of double emulsion with an alginate shell is still unclear due to the complex rheological behavior of alginate solutions. Herein, we employ a dual-coaxial microfluidic device to generate the high-quality double emulsion droplets with alginate shell, focusing on the effects of the fluid properties of alginate solution in the middle phase (viscosity, μ_m) and the fluid flow rate on the droplet formation mechanism. As the viscosity of the middle fluid (μ_m) increased, the size of compound droplets (D₂) increased and the size of inner droplets (D₁) decreased, and the break-up regimes occurred a dripping-to-jetting transition when μ_m = 160 mPa s. The number of encapsulated inner droplets can be predicted and precisely controlled by regulating the generation frequency of inner (f₁) and outer droplets (f₂). The breakup dynamics of the alginate thread are also analyzed by using the volume-of-fluid/continuum-surface-force (VOF/CSF) method. The results show that the pressure and velocity in the neck of pinch-off is lower in the jetting than that in the dripping regime. This study will provide useful guidance for the rational design and controllable preparation of core-shell alginate microcapsules.

Evaluating the reliability of tumour spheroid-on-chip models for replicating intratumoural drug delivery: considering the role of microfluidic parameters

Several tumour spheroid-on-chip models have already been proposed in the literature to conduct high throughput drug screening assays. The microfluidic configurations in these models generally depend on the strategies adopted for spheroid formation and entrapment. However, it is not clear how successful they are to mimic in vivo transport mechanisms. In this study, drug transport in different tumour spheroid-on-chip models is numerically investigated under static and dynamic conditions using porous media theory. Moreover, the treatment of a solid tumour at the initial stage of development is modelled using bolus injection and continuous infusion methods. Then, the results of tumour spheroid-on-chip, including drug concentration, cell viability, as well as pressure and fluid shear stress distributions, are compared with those of the solid tumour, assuming identical transport properties in all models. Finally, a new configuration of the microfluidic device along with the optimal drug concentrations is proposed, which can well imitate a given in vivo situation.

Variable-position centrifugal platform achieves droplet manipulation and logic circuitries on-chip

Taking information as material to realize non-electronic physical computing is a promising idea, which facilitates the integration of technologies in different fields such as chemistry, biology, and mechanical control into a new computing platform. Here, we propose a novel, efficient and robust manipulation platform that drives droplet computing by way of inertial force. Combining this with droplet flow path design, we demonstrated multiple basic functions of droplet manipulation, including storage, dosing, interrupts, controllable release and addressing. These basic functions without external control lay the foundation for the realization of droplet calculation. We developed AND, OR, and XOR logic gates of the "liquid circuit" and combined them into a binary adder, which successfully completed the addition of four-digit binary numbers through droplet movement. Moreover, we attempted to perform algorithmic design for biological information under the control of droplets based on synchronous logical operations, developing the possibility of biological applications. This programmable physical computing system exists independently of electronic computing, aiming to supplement and expand the computing methods outside the field of electronic technology and to open a new method for the algorithmic operation of materials after combining new physical computing technologies such as biological or chemical computing.

Microfluidic Formulation of Topological Hydrogels for Microtissue Engineering

Microfluidics has recently emerged as a powerful tool in generation of submillimeter-sized cell aggregates capable of performing tissue-specific functions, so-called microtissues, for applications in drug testing, regenerative medicine, and cell therapies. In this work, we review the most recent advances in the field, with particular focus on the formulation of cell-encapsulating microgels of small "dimensionalities": "0D" (particles), "1D" (fibers), "2D" (sheets), etc., and with nontrivial internal topologies, typically consisting of multiple compartments loaded with different types of cells and/or biopolymers. Such structures, which we refer to as topological hydrogels or topological microgels (examples including core-shell or Janus microbeads and microfibers, hollow or porous microstructures, or granular hydrogels) can be precisely tailored with high reproducibility and throughput by using microfluidics and used to provide controlled "initial conditions" for cell proliferation and maturation into functional tissue-like microstructures. Microfluidic methods of formulation of topological biomaterials have enabled significant progress in engineering of miniature tissues and organs, such as pancreas, liver, muscle, bone, heart, neural tissue, or vasculature, as well as in fabrication of tailored microenvironments for stem-cell expansion and differentiation, or in cancer modeling, including generation of vascularized tumors for personalized drug testing. We review the available microfluidic fabrication methods by exploiting various cross-linking mechanisms and various routes toward compartmentalization and critically discuss the available tissue-specific applications. Finally, we list the remaining challenges such as simplification of the microfluidic workflow for its widespread use in biomedical research, bench-to-bedside transition including production upscaling, further in vivo validation, generation of more precise organ-like models, as well as incorporation of induced pluripotent stem cells as a step toward clinical applications.

Biomedical Applications of Microfluidic Devices: A Review

Both passive and active microfluidic chips are used in many biomedical and chemical applications to support fluid mixing, particle manipulations, and signal detection. Passive microfluidic devices are geometry-dependent, and their uses are rather limited. Active microfluidic devices include sensors or detectors that transduce chemical, biological, and physical changes into electrical or optical signals. Also, they are transduction devices that detect biological and chemical changes in biomedical applications, and they are highly versatile microfluidic tools for disease diagnosis and organ modeling. This review provides a comprehensive overview of the significant advances that have been made in the development of microfluidics devices. We will discuss the function of microfluidic devices as micromixers or as sorters of cells and substances (e.g., microfiltration, flow or displacement, and trapping). Microfluidic devices are fabricated using a range of techniques, including molding, etching, three-dimensional printing, and nanofabrication. Their broad utility lies in the detection of diagnostic biomarkers and organ-on-chip approaches that permit disease modeling in cancer, as well as uses in neurological, cardiovascular, hepatic, and pulmonary diseases. Biosensor applications allow for point-of-care testing, using assays based on enzymes, nanozymes, antibodies, or nucleic acids (DNA or RNA). An anticipated development in the field includes the optimization of techniques for the fabrication of microfluidic devices using biocompatible materials. These developments will increase biomedical versatility, reduce diagnostic costs, and accelerate diagnosis time of microfluidics technology.

Application of Janus Particles in Point-of-Care Testing

Janus particles (JPs), named after the two-faced Roman god, are asymmetric particles with different chemical properties or polarities. JPs have been widely used in the biomedical field in recent years, including as drug carriers for targeted controlled drug release and as biosensors for biological imaging and biomarker detection, which is crucial in the early detection and treatment of diseases. In this review, we highlight the most recent advancements made with regard to Janus particles in point-of-care testing (POCT). Firstly, we introduce several commonly used methods for preparing Janus particles. Secondly, we present biomarker detection using JPs based on various detection methods to achieve the goal of POCT. Finally, we discuss the challenges and opportunities for developing Janus particles in POCT. This review will facilitate the development of POCT biosensing devices based on the unique properties of Janus particles.

A droplet-based microfluidic approach to isolating functional bacteria from gut microbiota

Metabolic interactions within gut microbiota play a vital role in human health and disease. Targeting metabolically interacting bacteria could provide effective treatments; however, obtaining functional bacteria remains a significant challenge due to the complexity of gut microbiota. Here, we developed a facile droplet-based approach to isolate and enrich functional gut bacteria that could utilize metabolites from an engineered butyrate-producing bacteria (EBPB) of anti-obesity potential. This involves the high throughput formation of single-bacteria droplets, followed by culturing “droplets” on agar plates to form discrete single-cell colonies. This approach eliminates the need for sophisticated s instruments to sort droplets and thus allows the operation hosted in a traditional anaerobic chamber. In comparison to the traditional culture, the droplet-based approach obtained a community of substantially higher diversity and evenness. Using the conditioned plates containing metabolites from the EBPB supernatant, we obtained gut bacteria closely associated or interacting with the EBPB. These include anaerobic Lactobacillus and Bifidobacterium, which are often used as probiotics. The study illustrates the potential of our approach in the search for the associated bacteria within the gut microbiota and retrieving those yet-to-be cultured.

Label-free electrochemical microfluidic biosensors: futuristic point-of-care analytical devices for monitoring diseases

The integration of microfluidics with electrochemical analysis has resulted in the development of single miniaturized detection systems, which allows the precise control of sample volume with multianalyte detection capability in a cost- and time-effective manner. Microfluidic electrochemical sensing devices (MESDs) can potentially serve as precise sensing and monitoring systems for the detection of molecular markers in various detrimental diseases. MESDs offer several advantages, including (i) automated sample preparation and detection, (ii) low sample and reagent requirement, (iii) detection of multianalyte in a single run, (iv) multiplex analysis in a single integrated device, and (v) portability with simplicity in application and disposability. Label-free MESDs can serve an affordable real-time detection with a simple analysis in a short processing time, providing point-of-care diagnosis/detection possibilities in precision medicine, and environmental analysis. In the current review, we elaborate on label-free microfluidic biosensors, provide comprehensive insights into electrochemical detection techniques, and discuss the principles of label-free microfluidic-based sensing approaches.

Fabrication of Polymer Microfluidics: An Overview

Microfluidic platform technology has presented a new strategy to detect and analyze analytes and biological entities thanks to its reduced dimensions, which results in lower reagent consumption, fast reaction, multiplex, simplified procedure, and high portability. In addition, various forces, such as hydrodynamic force, electrokinetic force, and acoustic force, become available to manipulate particles to be focused and aligned, sorted, trapped, patterned, etc. To fabricate microfluidic chips, silicon was the first to be used as a substrate material because its processing is highly correlated to semiconductor fabrication techniques. Nevertheless, other materials, such as glass, polymers, ceramics, and metals, were also adopted during the emergence of microfluidics. Among numerous applications of microfluidics, where repeated short-time monitoring and one-time usage at an affordable price is required, polymer microfluidics has stood out to fulfill demand by making good use of its variety in material properties and processing techniques. In this paper, the primary fabrication techniques for polymer microfluidics were reviewed and classified into two categories, e.g., mold-based and non-mold-based approaches. For the mold-based approaches, micro-embossing, micro-injection molding, and casting were discussed. As for the non-mold-based approaches, CNC micromachining, laser micromachining, and 3D printing were discussed. This review provides researchers and the general audience with an overview of the fabrication techniques of polymer microfluidic devices, which could serve as a reference when one embarks on studies in this field and deals with polymer microfluidics.

Droplet Manipulation under a Magnetic Field: A Review

The magnetic manipulation of droplets is one of the emerging magnetofluidic technologies that integrate multiple disciplines, such as electromagnetics, fluid mechanics and so on. The directly driven droplets are mainly composed of ferrofluid or liquid metal. This kind of magnetically induced droplet manipulation provides a remote, wireless and programmable approach beneficial for research and engineering applications, such as drug synthesis, biochemistry, sample preparation in life sciences, biomedicine, tissue engineering, etc. Based on the significant growth in the study of magneto droplet handling achieved over the past decades, further and more profound explorations in this field gained impetus, raising concentrations on the construction of a comprehensive working mechanism and the commercialization of this technology. Current challenges faced are not limited to the design and fabrication of the magnetic field, the material, the acquisition of precise and stable droplet performance, other constraints in processing speed and so on. The rotational devices or systems could give rise to additional issues on bulky appearance, high cost, low reliability, etc. Various magnetically introduced droplet behaviors, such as deformation, displacement, rotation, levitation, splitting and fusion, are mainly introduced in this work, involving the basic theory, functions and working principles.

Single-cell droplet microfluidics for biomedical applications

This review focuses on the recent advances in the fundamentals of single-cell droplet microfluidics and its applications in biomedicine, providing insights into design and establishment of single-cell microsystems and their further performance.