Building functional materials for health care and pharmacy from microfluidic principles and Flow Focusing

Microfluidic strategies for engineering oxygen-releasing biomaterials

In the field of tissue engineering, local hypoxia in large-cell structures (larger than 1 mm3) poses a significant challenge. Oxygen-releasing biomaterials supply an innovative solution through oxygen ⁠ delivery in a sustained and controlled manner. Compared to traditional methods such as emulsion, sonication, and agitation, microfluidic technology offers distinct benefits for oxygen-releasing material production, including controllability, flexibility, and applicability. It holds enormous potential in the production of smart oxygen-releasing materials. This review comprehensively covers the fabrication and application of microfluidic-enabled oxygen-releasing biomaterials. To begin with, the physical mechanism of various microfluidic technologies and their differences in oxygen carrier preparation are explained. Then, the distinctions among diverse oxygen-releasing components in regards for oxygen-releasing mechanism, oxygen-carrying capacity, and duration of oxygen release are presented. Finally, the present obstacles and anticipated development trends are examined together with the application outcomes of oxygen-releasing biomaterials based on microfluidic technology in the biomedical area. STATEMENT OF SIGNIFICANCE: Oxygen is essential for sustaining life, and hypoxia (a condition of low oxygen) is a significant challenge in various diseases. Microfluidic-based oxygen-releasing biomaterials offer precise control and outstanding performance, providing unique advantages over traditional approaches for tissue engineering. However, comprehensive reviews on this topic are currently lacking. In this review, we provide a comprehensive analysis of various microfluidic technologies and their applications for developing oxygen-releasing biomaterials. We compare the characteristics of organic and inorganic oxygen-releasing biomaterials and highlight the latest advancements in microfluidic-enabled oxygen-releasing biomaterials for tissue engineering, wound healing, and drug delivery. This review may hold the potential to make a significant contribution to the field, with a profound impact on the scientific community.

Free-Boundary Microfluidic Platform for Advanced Materials Manufacturing and Applications

Microfluidics, with its remarkable capacity to manipulate fluids and droplets at the microscale, has emerged as a powerful platform in numerous fields. In contrast to conventional closed microchannel microfluidic systems, free-boundary microfluidic manufacturing (FBMM) processes continuous precursor fluids into jets or droplets in a relatively spacious environment. FBMM is highly regarded for its superior flexibility, stability, economy, usability, and versatility in the manufacturing of advanced materials and architectures. In this review, a comprehensive overview of recent advancements in FBMM is provided, encompassing technical principles, advanced material manufacturing, and their applications. FBMM is categorized based on the foundational mechanisms, primarily comprising hydrodynamics, interface effects, acoustics, and electrohydrodynamic. The processes and mechanisms of fluid manipulation are thoroughly discussed. Additionally, the manufacturing of advanced materials in various dimensions ranging from zero-dimensional to three-dimensional, as well as their diverse applications in material science, biomedical engineering, and engineering are presented. Finally, current progress is summarized and future challenges are prospected. Overall, this review highlights the significant potential of FBMM as a powerful tool for advanced materials manufacturing and its wide-ranging applications.

Design and Validation of a Droplet-based Microfluidic System To Study Non-Photochemical Laser-Induced Nucleation of Potassium Chloride Solutions

Non-photochemical laser-induced nucleation (NPLIN) has emerged as a promising primary nucleation control technique offering spatiotemporal control over crystallization with potential for polymorph control. So far, NPLIN was mostly investigated in milliliter vials, through laborious manual counting of the crystallized vials by visual inspection. Microfluidics represents an alternative to acquiring automated and statistically reliable data. Thus we designed a droplet-based microfluidic platform capable of identifying the droplets with crystals emerging upon Nd:YAG laser irradiation using the deep learning method. In our experiments, we used supersaturated solutions of KCl in water, and the effect of laser intensity, wavelength (1064, 532, and 355 nm), solution supersaturation (S), solution filtration, and intentional doping with nanoparticles on the nucleation probability is quantified and compared to control cooling crystallization experiments. Ability of dielectric polarization and the nanoparticle heating mechanisms proposed for NPLIN to explain the acquired results is tested. Solutions with lower supersaturation (S = 1.05) exhibit significantly higher NPLIN probabilities than those in the control experiments for all laser wavelengths above a threshold intensity (50 MW/cm2). At higher supersaturation studied (S = 1.10), irradiation was already effective at lower laser intensities (10 MW/cm2). No significant wavelength effect was observed besides irradiation with 355 nm light at higher laser intensities (≥50 MW/cm2). Solution filtration and intentional doping experiments showed that nanoimpurities might play a significant role in explaining NPLIN phenomena.

Microfluidics: Insights into Intestinal Microorganisms

Microfluidics is a system involving the treatment or manipulation of microscale (10^-9 to 10^-18 L) fluids using microchannels (10 to 100 μm) contained on a microfluidic chip. Among the different methodologies used to study intestinal microorganisms, new methods based on microfluidic technology have been receiving increasing attention in recent years. The intestinal tracts of animals are populated by a vast array of microorganisms that have been established to play diverse functional roles beneficial to host physiology. This review is the first comprehensive coverage of the application of microfluidics technology in intestinal microbial research. In this review, we present a brief history of microfluidics technology and describe its applications in gut microbiome research, with a specific emphasis on the microfluidic technology-based intestine-on-a-chip, and also discuss the advantages and application prospects of microfluidic drug delivery systems in intestinal microbial research.

Microfluidics in drug delivery: review of methods and applications

Microfluidics technology has emerged as a promising methodology for the fabrication of a wide variety of advanced drug delivery systems. Owing to its ability for accurate handling and processing of small quantities of fluidics as well as immense control over physicochemical properties of fabricated micro and nanoparticles (NPs), microfluidic technology has significantly improved the pharmacokinetics and pharmacodynamics of drugs. This emerging technology has offered numerous advantages over the conventional drug delivery methods for fabricating of a variety of micro and nanocarriers for poorly soluble drugs. In addition, a microfluidic system can be designed for targeted drug delivery aiming to increase the local bioavailability of drugs. This review spots the light on the recent advances made in the area of microfluidics including various methods of fabrication of drug carriers, their characterization, and unique features. Furthermore, applications of microfluidic technology for the robust fabrication and development of drug delivery systems, the existing challenges associated with conventional fabrication methodologies as well as the proposed solutions offered by microfluidic technology have been discussed in details.HighlightsMicrofluidic technology has revolutionized fabrication of tunable micro and nanocarriers.Microfluidic platforms offer several advantages over the conventional fabrication methods.Microfluidic devices hold great promise in controlling the physicochemical features of fabricated drug carriers.Micro and nanocarriers with controllable release kinetics and site-targeting efficiency can be fabricated.Drug carriers fabricated by microfluidic technology exhibited improved pharmacokinetic and pharmacodynamic profiles.

Prospection of Microfluidics for Local Drug Delivery

Significant endeavors can be made to develop effective drug delivery systems. Nowadays, many of these novel systems have gained attention as they focus primarily on increasing the bioavailability and bioaccessibility of several drugs to finally minimize the side effects, thus improving the treatment's efficacy. Microfluidics systems are unquestionably a superior technology, which is currently revolutionizing the current chemical and biological studies, providing diminutive chip-scale devices that offer precise dosage, target-precise delivery, and controlled release. Microfluidic systems have emerged as a promising delivery vehicle owing to their potential for defined handling and transporting of small liquid quantities. The latest microfabrication developments have been made for application to several biological systems. Here, we review the fundamentals of microfluidics and their application for local drug delivery.

Microfluidic nanomaterials: From synthesis to biomedical applications

Microfluidic platforms gain popularity in biomedical research due to their attractive inherent features, especially in nanomaterials synthesis. This review critically evaluates the current state of the controlled synthesis of nanomaterials using microfluidic devices. We describe nanomaterials' screening in microfluidics, which is very relevant for automating the synthesis process for biomedical applications. We discuss the latest microfluidics trends to achieve noble metal, silica, biopolymer, quantum dots, iron oxide, carbon-based, rare-earth-based, and other nanomaterials with a specific size, composition, surface modification, and morphology required for particular biomedical application. Screening nanomaterials has become an essential tool to synthesize desired nanomaterials using more automated processes with high speed and repeatability, which can't be neglected in today's microfluidic technology. Moreover, we emphasize biomedical applications of nanomaterials, including imaging, targeting, therapy, and sensing. Before clinical use, nanomaterials have to be evaluated under physiological conditions, which is possible in the microfluidic system as it stimulates chemical gradients, fluid flows, and the ability to control microenvironment and partitioning multi-organs. In this review, we emphasize the clinical evaluation of nanomaterials using microfluidics which was not covered by any other reviews. In the future, the growth of new materials or modification in existing materials using microfluidics platforms and applications in a diversity of biomedical fields by utilizing all the features of microfluidic technology is expected.

Investigation of viscoelastic focusing of particles and cells in a zigzag microchannel

Microfluidic particle focusing has been a vital prerequisite step in sample preparation for downstream particle separation, counting, detection, or analysis, and has attracted broad applications in biomedical and chemical areas. Besides all the active and passive focusing methods in Newtonian fluids, particle focusing in viscoelastic fluids has been attracting increasing interest because of its advantages induced by intrinsic fluid property. However, to achieve a well-defined focusing position, there is a need to extend channel lengths when focusing micrometer-sized or sub-microsized particles, which would result in the size increase of the microfluidic devices. This work investigated the sheathless viscoelastic focusing of particles and cells in a zigzag microfluidic channel. Benefit from the zigzag structure of the channel, the channel length and the footprint of the device can be reduced without sacrificing the focusing performance. In this work, the viscoelastic focusing, including the focusing of 10 μm polystyrene particles, 5 μm polystyrene particles, 5 μm magnetic particles, white blood cells (WBCs), red blood cells (RBCs), and cancer cells, were all demonstrated. Moreover, magnetophoretic separation of magnetic and nonmagnetic particles after viscoelastic pre-focusing was shown. This focusing technique has the potential to be used in a range of biomedical applications.

Blood Particulate Analogue Fluids: A Review

Microfluidics has proven to be an extraordinary working platform to mimic and study blood flow phenomena and the dynamics of components of the human microcirculatory system. However, the use of real blood increases the complexity to perform these kinds of in vitro blood experiments due to diverse problems such as coagulation, sample storage, and handling problems. For this reason, interest in the development of fluids with rheological properties similar to those of real blood has grown over the last years. The inclusion of microparticles in blood analogue fluids is essential to reproduce multiphase effects taking place in a microcirculatory system, such as the cell-free layer (CFL) and Fähraeus-Lindqvist effect. In this review, we summarize the progress made in the last twenty years. Size, shape, mechanical properties, and even biological functionalities of microparticles produced/used to mimic red blood cells (RBCs) are critically exposed and analyzed. The methods developed to fabricate these RBC templates are also shown. The dynamic flow/rheology of blood particulate analogue fluids proposed in the literature (with different particle concentrations, in most of the cases, relatively low) is shown and discussed in-depth. Although there have been many advances, the development of a reliable blood particulate analogue fluid, with around 45% by volume of microparticles, continues to be a big challenge.

Microfluidic Assembly: An Innovative Tool for the Encapsulation, Protection, and Controlled Release of Nutraceuticals

Nutraceuticals have been gradually accepted as food ingredients that can offer health benefits and provide protection against several diseases. It is widely accepted due to potential nutritional benefits, safety, and therapeutic effects. Most nutraceuticals are vulnerable to the changes in the external environment, which leads to poor physical and chemical stability and absorption. Several researchers have designed various encapsulation technologies to promote the use of nutraceuticals. Microfluidic technology is an emerging approach which can be used for nutraceutical delivery with precise control. The delivery systems using microfluidic technology have obtained much interest in recent years. In this review article, we have summarized the recently introduced nutraceutical delivery platforms including emulsions, liposomes, microspheres, microgels, and polymer nanoparticles based on microfluidic techniques. Emphasis has been made to discuss the advantages, preparations, characterizations, and applications of nutraceutical delivery systems. Finally, the challenges, several up-scaling methods, and future expectations are discussed.

Use of droplet-based microfluidic techniques in the preparation of microparticles

Microparticles are widely used in myriad fields such as pharmaceuticals, foods, cosmetics, and other industrial fields. Compared with traditional methods for synthesizing microparticles, microfluidic techniques provide very powerful platforms for creating highly controllable emulsion droplets as templates for fabricating uniform microparticles with advanced structures and functions. Microfluidic techniques can generate emulsion droplets with precisely controlled size, shape, and composition. A more precise preparation process brings an effective tool to control the release profile of the drug and introduces an easily accessible reproducibility. The paper gives information about basic droplet-based set-ups and examples of attainable microparticle types preparable by this method.

Microfluidic preparation of PLGA composite microspheres with mesoporous silica nanoparticles for finely manipulated drug release

The current study explored the feasibility of a microfluidic preparation of PLGA composite microspheres with mesoporous silica nanoparticles (MSNs) to finely manipulate the drug release behaviors of the microspheres. MSNs were synthesized via a hydrothermal method, and PLGA microspheres loaded with MSNs (PLGA-MSNs) were prepared using a capillary-based three-phase microfluidic device. Drug loading and release behaviors using rhodamine B (RB) as a water-soluble model drug were investigated and compared with those of PLGA microspheres. MSNs with an average particle size of 119 nm, a specific surface area of 902.5 cm²/g, and a pore size of approximately 5 nm were obtained. The mean diameter of PLGA-MSNs was 56 μm (CV = 4.91%). A sustained release duration of encapsulated RB from PLGA-MSNs for 4 months was achieved without any observable burst release. PLGA microspheres with monodispersion could also allow for a similar release duration of encapsulated RB but encountered a burst release in the mid-term of the studied duration. PLGA-MSNs had a denser outer PLGA layer and a more centralized hollow hole than PLGA microspheres without MSNs. Hence, the incorporation of MSNs into PLGA microspheres via microfluidics could be an efficient strategy to finely tune the drug release behavior of PLGA microspheres.

Determination of Dynamic Interfacial Tension during the Generation of Tiny Droplets in the Liquid-Liquid Jetting Flow Regime

Liquid-liquid dispersion coupled with droplet formation and mass transfer of surfactants is one of the most typical phenomena in many chemical processes. As in every aspect of this process, the interfacial tension variation caused by the unsaturated adsorption of surfactants on the droplet surface plays an important role. This article focuses on microdroplet formation and the dynamic interfacial behavior of surfactants in the jetting regime. In a capillary embedded step T-junction device, controllable preparation of monodisperse droplets is achieved, and a correlation for predicting droplet sizes is established. A method for measuring the dynamic interfacial tension is provided. Mass transfer coefficients are then calculated for Tween 20 during the droplet formation process by a semiempirical correlation. The results indicate that dynamic interfacial tensions are lower than those obtained when the surfactant is adsorbed to equilibrium. Based on the tip-streaming phenomenon, mass transfer coefficients for Tween 20 can reach up to ∼10^-3 m/s, higher than those obtained in coaxial microfluidic devices. All the preliminary results shed light on the nature of droplet formation and will be of significance for application in industrial apparatuses.

Dripping, jetting and tip streaming

Dripping, jetting and tip streaming have been studied up to a certain point separately by both fluid mechanics and microfluidics communities, the former focusing on fundamental aspects while the latter on applications. Here, we intend to review this field from a global perspective by considering and linking the two sides of the problem. First, we present the theoretical model used to study interfacial flows arising in droplet-based microfluidics, paying attention to three elements commonly present in applications: viscoelasticity, electric fields and surfactants. We review both classical and current results of the stability of jets affected by these elements. Mechanisms leading to the breakup of jets to produce drops are reviewed as well, including some recent advances in this field. We also consider the relatively scarce theoretical studies on the emergence and stability of tip streaming in open systems. Second, we focus on axisymmetric microfluidic configurations which can operate on the dripping and jetting modes either in a direct (standard) way or via tip streaming. We present the dimensionless parameters characterizing these configurations, the scaling laws which allow predicting the size of the resulting droplets and bubbles, as well as those delimiting the parameter windows where tip streaming can be found. Special attention is paid to electrospray and flow focusing, two of the techniques more frequently used in continuous drop production microfluidics. We aim to connect experimental observations described in this section of topics with fundamental and general aspects described in the first part of the review. This work closes with some prospects at both fundamental and practical levels.

Multifunctional Droplet Microfluidic Platform for Rapid Immobilization of Oligonucleotides on Semiconductor Quantum Dots

Quantum dot-DNA oligonucleotide (QD-DNA) conjugates have been used in many fields such as nucleic acid bioassays, intracellular probes, and drug delivery systems. A typical solid-phase method that achieves rapid loading of oligonucleotides on surfaces of QDs involves a two-step reaction and is performed in a batch-based approach. In contrast, droplet microfluidics offers many advantages that are unavailable when using batch processing, providing rapid and dense immobilized DNA oligonucleotides on QDs. The presented droplet microfluidic approach allows high-quality QD-DNA conjugates to be produced using one single device, which is designed to have two droplet generators, one droplet merger, and one mixer. One of the droplet generators coencapsulates QDs and magnetic beads (MBs) into nanoliter-sized droplets for the production of QD-MB conjugates and the other encapsulates oligonucleotides in nanoliter-sized droplets. These two streams of droplets then merge at a one-to-one ratio in a chamber. The merged droplets travel along the mixer, which is a serpentine microchannel with 30 turns, resulting in QD-DNA conjugation structures of high quality. This multifunctional microfluidic device provides advantages such as higher degree of control over the reaction conditions, minimized cross-contamination and impurities, and reduction of reagent consumption while eliminating any need for external vortexing and pipetting. To evaluate the quality of the QD-DNA conjugates, they were used as Forster resonance energy transfer (FRET) probes to quantify oligonucleic targets.

Focusing of sub-micrometer particles in microfluidic devices

Sub-micrometer particles (0.10-1.0 μm) are of great significance to study, e.g., microvesicles and protein aggregates are targets for therapeutic intervention, and sub-micrometer fluorescent polystyrene (PS) particles are used as probes for diagnostic imaging. Focusing of sub-micrometer particles - precisely control over the position of sub-micrometer particles in a tightly focused stream - has a wide range of applications in the field of biology, chemistry and environment, by acting as a prerequisite step for downstream detection, manipulation and quantification. Microfluidic devices have been attracting great attention as desirable tools for sub-micrometer particle focusing, due to their small size, low reagent consumption, fast analysis and low cost. Recent advancements in fundamental knowledge and fabrication technologies have enabled microfluidic focusing of particles at sub-micrometer scale in a continuous, label-free and high-throughput manner. Microfluidic methods for the focusing of sub-micrometer particles can be classified into two main groups depending on whether an external field is applied: 1) passive methods, which utilize intrinsic fluidic properties without the need of external actuation, such as inertial, deterministic lateral displacement (DLD), viscoelastic and hydrophoretic focusing; and 2) active methods, where external fields are used, such as dielectrophoretic, thermophoretic, acoustophoretic and optical focusing. This article mainly reviews the studies on the focusing of sub-micrometer particles in microfluidic devices over the past 10 years. It aims to bridge the gap between the focusing of micrometer and nanometer scale (1.0-100 nm) particles and to improve the understanding of development progress, current advances and future prospects in microfluidic focusing techniques.

An analytic model of microfluidic system triggered by thermal expansion

Microfluidics shows a great importance in fluid control in biomedicine area. Recently, a programmable soft microfluidic system for applications of pharmacology and optogenetics is reported in Cell. Based on the theory of thermal expansion, we established an analytic model to characterize the injection process of the microfluidic systems. Finite element analyses (FEA) were employed to validate this model. The comparison between the analytic model and FEA well demonstrates the practicality of the analytic model. We also make a parametric analysis of sphere radius, central angle and expandable layer thickness on the infusion volume of the system to optimize the design.

Engineering and delivery of nanocolloids of hydrophobic drugs

A lot of efforts have been devoted to engineering the delivery of hydrophobic drugs due to the high demand of chemotherapy against cancer. While early developed liposomes and polymeric nanoparticles did not meet the requirements of high drug loading efficiency, pure drug nanoparticles appeared to meet these together with high stability. Current drug delivery systems demand an improved performance over the whole aspects of stability, loading capacity, and therapeutic effects. As a result, both new techniques based on traditional methods and totally new procedures are under investigation. In this review, we focus on the evaluation of pure drug nanolloids fabricated by different engineering protocols with emphasis on the size and morphology, delivery and controlled release, and therapeutic effects of these drug nanocolloids.

Hydroxyl-Group-Dominated Graphite Dots Reshape Laser Desorption/Ionization Mass Spectrometry for Small Biomolecular Analysis and Imaging

Small molecules play critical roles in life science, yet their facile detection and imaging in physiological or pathological settings remain a challenge. Matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) is a powerful tool for molecular analysis. However, conventional organic matrices (CHCA, DHB, etc.) used in assisting analyte ionization suffer from intensive background noise in the mass region below m/z 700, which hinders MALDI MS applications for small-molecule detection. Here, we report that a hydroxyl-group-dominated graphite dot (GD) matrix overcomes limitations of conventional matrices and allows MALDI MS to be used in fast and high-throughput analysis of small biomolecules. GDs exhibit extremely low background noise and ultrahigh sensitivity (with limit of detection <1 fmol) in MALDI MS. This approach allows identification of complex oligosaccharides, detection of low-molecular-weight components in traditional Chinese herbs, and facile analysis of puerarin and its metabolites in serum without purification. Moreover, we show that the GDs provide an effective matrix for the direct imaging or spatiotemporal mapping of small molecules and their metabolites (m/z < 700) simultaneously at the suborgan tissue level. Density functional theory calculations further provide the mechanistic basis of GDs as an effective MALDI matrix in both the positive-ion and negative-ion modes. Collectively, our work uncovered a useful matrix which reshapes MALDI MS technology for a wide range of applications in biology and medicine.

Biomaterials Meet Microfluidics: From Synthesis Technologies to Biological Applications

Microfluidics is characterized by laminar flow at micro-scale dimension, high surface to volume ratio, and markedly improved heat/mass transfer. In addition, together with advantages of large-scale integration and flexible manipulation, microfluidic technology has been rapidly developed as one of the most important platforms in the field of functional biomaterial synthesis. Compared to biomaterials assisted by conventional strategies, functional biomaterials synthesized by microfluidics are with superior properties and performances, due to their controllable morphology and composition, which have shown great advantages and potential in the field of biomedicine, biosensing, and tissue engineering. Take the significance of microfluidic engineered biomaterials into consideration; this review highlights the microfluidic synthesis technologies and biomedical applications of materials. We divide microfluidic based biomaterials into four kinds. According to the material dimensionality, it includes: 0D (particulate materials), 1D (fibrous materials), 2D (sheet materials), and 3D (construct forms of materials). In particular, micro/nano-particles and micro/nano-fibers are introduced respectively. This classification standard could include all of the microfluidic biomaterials, and we envision introducing a comprehensive and overall evaluation and presentation of microfluidic based biomaterials and their applications.

Reconfigurable microfluidic device with discretized sidewall

Various microfluidic features, such as traps, have been used to manipulate flows, cells, and other particles within microfluidic systems. However, these features often become undesirable in subsequent steps requiring different fluidic configurations. To meet the changing needs of various microfluidic configurations, we developed a reconfigurable microfluidic channel with movable sidewalls using mechanically discretized sidewalls of laterally aligned rectangular pins. The user can deform the channel sidewall at any time after fabrication by sliding the pins. We confirmed that the flow resistance of the straight microchannel could be reversibly adjusted in the range of 10¹-10⁵Pa s/μl by manually displacing one of the pins comprising the microchannel sidewall. The reconfigurable microchannel also made it possible to manipulate flows and cells by creating a segmented patterned culture of COS-7 cells and a coculture of human umbilical vein endothelial cells (HUVECs) and human lung fibroblasts (hLFs) inside the microchannel. The reconfigurable microfluidic device successfully maintained a culture of COS-7 cells in a log phase throughout the entire period of 216 h. Furthermore, we performed a migration assay of cocultured HUVEC and hLF spheroids within one microchannel and observed their migration toward each other.

Monosized dripping mode of axisymmetric flow focusing

We identify and analyze the perfectly regular dripping mode of flow focusing. This mode occurs within narrow intervals of injected flow rates and applied pressure drops and leads to homogeneous-size droplets with diameters similar to or smaller than that of the discharge orifice. The balance between the local acceleration of the fluid particle and the applied pressure drop yields the scaling law for the droplet diameter. This scaling law is validated experimentally with excellent accord.

Nonaxisymmetric disturbances in compound liquid jets falling under gravity

The disintegration of a compound thread of fluid can be utilized in a wide variety of applications including the production of compound droplets or capsules. In this paper we investigate the linear instability of a compound inviscid liquid jet falling under gravity in a surrounding gas with respect to nonaxisymmetric waves. We derive an analytical expression for the dispersion relation, which takes into account the non-uniform nature of the jet, and which we then solve numerically. Particular attention is paid to investigating the effects of the liquid-to-gas density ratio on the growth and development of different wave modes as well as the influence of gravity. Our results show that there exists some nonaxisymmetric long wavelength disturbances that are more unstable than their asymmetric equivalents and that the influence of gravity can alter the behavior of these modes.

Modification of PDMS to fabricate PLGA microparticles by a double emulsion method in a single microfluidic device

We present an easy, durable method to generate a partially hydrophilic/hydrophobic poly(dimethylsiloxane) (PDMS) microfluidic device. The functionality of this device was assessed in a double flow focusing design to fabricate core-shell structured poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs).

Integrated microfluidic system with simultaneous emulsion generation and concentration

Because the size, size distribution, and concentration of emulsions play an important role in most of the applications, controlled emulsion generation and effective concentration are of great interest in fundamental and applied studies. While microfluidics has been demonstrated to be able to produce emulsion drops with controlled size, size distribution, and hierarchical structures, progress of controlled generation of concentrated emulsions is limited. Here, we present an effective microfluidic emulsion generation system integrated with an orifice structure to separate aqueous droplets from the continuous oil phase, resulting in concentrated emulsion drops in situ. Both experimental and simulation results show that the efficiency of separation is determined by a balance between pressure drop and droplet accumulation near the orifice. By manipulating this balance via changing flow rates and microfluidic geometry, we can achieve monodisperse droplets on chip that have a concentration as high as 80,000 drops per microliter (volume fraction of 66%). The present approach thus provides insights to the design of microfluidic device that can be used to concentrate emulsions (drops and bubbles), colloidal particles (drug delivery polymer particles), and biological particles (cells and bacteria) when volume fractions as high as 66% are necessary.

Dynamics of bubble breakup at a T junction

The gas-liquid interfacial dynamics of bubble breakup in a T junction was investigated. Four regimes were observed for a bubble passing through the T junction. It was identified by the stop flow that a critical width of the bubble neck existed: if the minimum width of the bubble neck was less than the critical value, the breakup was irreversible and fast; while if the minimum width of the bubble neck was larger than the critical value, the breakup was reversible and slow. The fast breakup was driven by the surface tension and liquid inertia and is independent of the operating conditions. The minimum width of the bubble neck could be scaled with the remaining time as a power law with an exponent of 0.22 in the beginning and of 0.5 approaching the final fast pinch-off. The slow breakup was driven by the continuous phase and the gas-liquid interface was in the equilibrium stage. Before the appearance of the tunnel, the width of the depression region could be scaled with the time as a power law with an exponent of 0.75; while after that, the width of the depression was a logarithmic function with the time.

Generation of micro-sized PDMS particles by a flow focusing technique for biomicrofluidics applications

Polydimethylsiloxane (PDMS), due to its remarkable properties, is one of the most widely used polymers in many industrial and medical applications. In this work, a technique based on a flow focusing technique is used to produce PDMS spherical particles with sizes of a few microns. PDMS precursor is injected through a hypodermic needle to form a film/reservoir over the needle's outer surface. This film flows towards the needle tip until a liquid ligament is steadily ejected thanks to the action of a coflowing viscous liquid stream. The outcome is a capillary jet which breaks up into PDMS precursor droplets due to the growth of capillary waves producing a micrometer emulsion. The PDMS liquid droplets in the solution are thermally cured into solid microparticles. The size distribution of the particles is analyzed before and after curing, showing an acceptable degree of monodispersity. The PDMS liquid droplets suffer shrinkage while curing. These microparticles can be used in very varied technological fields, such as biomedicine, biotechnology, pharmacy, and industrial engineering.

Convective-to-absolute instability transition in a viscoelastic capillary jet subject to unrelaxed axial elastic tension

The convective-to-absolute instability transition in an Oldroyd-B capillary jet subject to unrelaxed axial stress is examined theoretically. There is a critical Weber number below which the jet is absolutely unstable under axisymmetric perturbations. We analyze the dependence of this critical parameter with respect to the Reynolds and Deborah numbers, as well as the unrelaxed axial stress. For small Deborah numbers, the unrelaxed stress destabilizes the viscoelastic jet, increasing the critical Weber number for which the convective-to-absolute instability transition takes place. If the Deborah number takes higher values, then the transitional Weber number decreases as the unrelaxed stress increases until two solution branches cross each other. The dominant branch for large axial stress leads to a threshold of this quantity above which the viscoelastic jet becomes absolutely unstable independently of the Weber number. The threshold depends on neither the Reynolds nor the Deborah number for sufficiently large values of these parameters.

Bubble Formation in Yield Stress Fluids Using Flow-Focusing and T-Junction Devices

We study the production of bubbles inside yield stress fluids (YSFs) in axisymmetric T-junction and flow-focusing devices. Taking advantage of yield stress over capillary stress, we exhibit a robust break-up mechanism reminiscent of the geometrical operating regime in 2D flow-focusing devices for Newtonian fluids. We report that when the gas is pressure driven, the dynamics is unsteady due to hydrodynamic feedback and YSF deposition on the walls of the channels. However, the present study also identifies pathways for potential steady-state production of bubbly YSFs at large scale.

Microencapsulation of indocyanine green for potential applications in image-guided drug delivery

We present a novel process to encapsulate indocyanine green (ICG) in liposomal droplets at high concentration for potential applications in image-guided drug delivery. The microencapsulation process follows two consecutive steps of droplet formation by liquid-driven coaxial flow focusing (LDCFF) and solvent removal by oil phase dewetting. These biocompatible lipid vesicles may have important applications in drug delivery and fluorescence imaging.

Microfluidics for Advanced Drug Delivery Systems

Considerable efforts have been devoted towards developing effective drug delivery methods. Microfluidic systems, with their capability for precise handling and transport of small liquid quantities, have emerged as a promising platform for designing advanced drug delivery systems. Thus, microfluidic systems have been increasingly used for fabrication of drug carriers or direct drug delivery to a targeted tissue. In this review, the recent advances in these areas are critically reviewed and the shortcomings and opportunities are discussed. In addition, we highlight the efforts towards developing smart drug delivery platforms with integrated sensing and drug delivery components.

A microdevice for producing monodispersed droplets under a jetting flow

A new capillary extended step microchannel for generating monodispersed droplets with a much wider narrowing jetting flow regime was specially designed.

Production of nanoparticle drug delivery systems with microfluidics tools

INTRODUCTION

Nowadays the development of composite nano- and microparticles is an extensively studied area of research. This interest is growing because of the potential use of such particles in drug delivery systems. Indeed they can be used in various medical disciplines depending upon their sizes and their size distribution, which determine their final biomedical applications.

AREAS COVERED

Amongst the different techniques to produce nanoparticles, microfluidic techniques allow preparing particles having a specific size, a narrow size distribution and high encapsulation efficiency with ease. This review covers the general description of microfluidics, its techniques, advantages and disadvantages with focus on the encapsulation of active principles in polymeric nanoparticles as well as on pure drug nanoparticles. Polymeric nanoparticles constitute the majority of the examples reported; however lipid nanoparticulate systems (DNA, SiRNA nanocarriers) are very comparable and their formulation processes are in most cases exactly similar. Accordingly this review focuses also on active ingredient nanoparticles formulated by nanoprecipitation processes in microfluidic devices in general. It also provides detailed description of the different geometries of most common microfluidic devices and the crucial parameters involved in techniques designed to obtain the desired properties.

EXPERT OPINION

Although the classical fabrication of nanoparticles drug delivery systems in batch is extremely well-described and developed, their production with microfluidic tools arises today as an emerging field with much more potential. In this review we present and discuss these new possibilities for biomedical applications through the current emerging developments.