Microfluidic Flow-focusing Device Research Articles

Generalized correlation for predicting the droplet size in a microfluidic flow-focusing device under the effect of surfactant

This paper describes an investigation on the dynamic behavior of droplet formation in a microfluidic flow-focusing device (MFFD) under the effect of surfactant using a phase-field method and the Koterweg stress applied in Navier–Stokes equations. The effects of variously important parameters, such as capillary number (Ca0), water fraction (wf), the viscosity ratio (α), and particularly surfactant concentration (cb), were examined. Consequently, the numerical results match the experimental ones. Additionally, the droplet formation is significantly affected by the surfactant, and the droplet size decreases with increasing cb for the whole range of both wf and Ca0. Based on the extensive study, the phase diagrams with two main modes, namely, dropping and threading, are provided. A mountain shape of the dropping mode was found, and this mode can be extended for higher wf with the presence of surfactant. In particular, new generalized correlations as a function of wf, Ca0, and cb are first proposed for predicting quickly and effectively the droplet size. Furthermore, the droplet formation depends significantly on α. With the presence of surfactant, smaller size of the droplet forms and the threading mode occurs at very high α. The results obtained in this study are very useful for understanding the dynamic behavior of droplet formation in MFFDs, which can be used in potential applications such as biomedical and drug delivery systems.

The interaction of vortical flows with red cells in venous valve mimics.

The motion of cells orthogonal to the direction of main flow is of importance in natural and engineered systems. The lateral movement of red blood cells (RBCs) distal to sudden expansion is considered to influence the formation and progression of thrombosis in venous valves, aortic aneurysms, and blood-circulating devices and is also a determining parameter for cell separation applications in flow-focusing microfluidic devices. Although it is known that the unique geometry of venous valves alters the blood flow patterns and cell distribution in venous valve sinuses, the interactions between fluid flow and RBCs have not been elucidated. Here, using a dilute cell suspension in an in vitro microfluidic model of a venous valve, we quantified the spatial distribution of RBCs by microscopy and image analysis, and using micro-particle image velocimetry and 3D computational fluid dynamics simulations, we analyzed the complex flow patterns. The results show that the local hematocrit in the valve pockets is spatially heterogeneous and is significantly different from the feed hematocrit. Above a threshold shear rate, the inertial separation of streamlines and lift forces contribute to an uneven distribution of RBCs in the vortices, the entrapment of RBCs in the vortices, and non-monotonic wall shear stresses in the valve pockets. Our experimental and computational characterization provides insights into the complex interactions between fluid flow, RBC distribution, and wall shear rates in venous valve mimics, which is of relevance to understanding the pathophysiology of thrombosis and improving cell separation efficiency.

Hydrodynamic control of droplet formation in narrowing jet and tip streaming regime using microfluidic flow-focusing

The formation of water-in-oil (W/O) emulsion droplets in a microfluidic flow-focusing device was investigated under the wide selection of operating parameters together with liquid materials. This is not only to demonstrate the obtained flow regimes and droplet outcomes but also to propose a predictive correlation among droplet size and operating dimensionless numbers. Data were collected from two etched channels of 100 and 190 µm depth. Experimental results of 12 data sets from over 100 experiments in two different-sized channels were assessed. Under the certain range of studied conditions, the stable narrowing jet and tip streaming regime noticeably created monodisperse droplet formation with high throughput. In addition, droplet size and junction size have been found to correlate with the flow rate ratio (Qc/Qd), viscosity ratio (µc/µd) and capillary number (Ca) following the correlation equation proposed in this work. This correlation will be beneficial in delivering predictable and controllable monodisperse droplets in a high flow process regime.

Beating Poisson stochastic particle encapsulation in flow-focusing microfluidic devices using viscoelastic liquids.

The encapsulation and co-encapsulation of particles in microfluidic flows is essential in applications related to single-cell analysis and material synthesis. However, the whole encapsulation process is stochastic in nature, and its efficiency is limited by the so-called Poisson limit. We here demonstrate particle encapsulation in microfluidic devices having flow-focusing geometries with efficiency up to 2-fold larger than the stochastic limit imposed by the Poisson statistics. To this aim, we exploited the recently observed phenomenon of particle train formation in viscoelastic liquids, so that particles could approach the encapsulation area with a constant frequency that was subsequently synchronised to the constant frequency of droplet formation. We also developed a simplified expression based on the experimental results that can guide optimal design of the microfluidic encapsulation system. Finally, we report the first experimental evidence of viscoelastic co-encapsulation of particles coming from different streams.

Bioactive hydrogel microcapsules for guiding stem cell fate decisions by release and reloading of growth factors

Human pluripotent stem cells (hPSC) hold considerable promise as a source of adult cells for treatment of diseases ranging from diabetes to liver failure. Some of the challenges that limit the clinical/translational impact of hPSCs are high cost and difficulty in scaling-up of existing differentiation protocols. In this paper, we sought to address these challenges through the development of bioactive microcapsules. A co-axial flow focusing microfluidic device was used to encapsulate hPSCs in microcapsules comprised of an aqueous core and a hydrogel shell. Importantly, the shell contained heparin moieties for growth factor (GF) binding and release. The aqueous core enabled rapid aggregation of hPSCs into 3D spheroids while the bioactive hydrogel shell was used to load inductive cues driving pluripotency maintenance and endodermal differentiation. Specifically, we demonstrated that one-time, 1 h long loading of pluripotency signals, fibroblast growth factor (FGF)-2 and transforming growth factor (TGF)-β1, into bioactive microcapsules was sufficient to induce and maintain pluripotency of hPSCs over the course of 5 days at levels similar to or better than a standard protocol with soluble GFs. Furthermore, stem cell-carrying microcapsules that previously contained pluripotency signals could be reloaded with an endodermal cue, Nodal, resulting in higher levels of endodermal markers compared to stem cells differentiated in a standard protocol. Overall, bioactive heparin-containing core-shell microcapsules decreased GF usage five-fold while improving stem cell phenotype and are well suited for 3D cultivation of hPSCs.

Effect of surfactant addition and viscosity of the continuous phase on flow fields and kinetics of drop formation in a flow-focusing microfluidic device

Droplet formation in a flow focusing microfluidic device was studied for capillary numbers between 10−4 and 10−1. The addition of a cationic surfactant to the dispersed phase and the effect of the continuous phase viscosity on droplet size, neck kinetics and flow fields inside the drop was investigated. Dropletsize decreased with an increase of the capillary number following a power law with an exponent dependent on drop confinement. At 1–3 ms before pinch-off, the neck thinning rate is dependent onsurfactant concentration and increases closer to pinch-off. Forthe 96 mPa s continuous phase, the two-fluid Stokes regime is observed with an effective interfacial tension smallerthan the equilibrium value, suggesting a negative contribution of Marangoni stresses. During necking, velocities along the neck interface for the surfactant laden cases are smaller due to Marangoni phenomena. During expansion, increased fluid velocities and recirculation are observed.

Fabrication of Microparticles with Front-Back Asymmetric Shapes Using Anisotropic Gelation.

Droplet-based microfluidics is a powerful tool for producing monodispersed micrometer-sized droplets with controlled sizes and shapes; thus, it has been widely applied in diverse fields from fundamental science to industries. Toward a simpler method for fabricating microparticles with front–back asymmetry in their shapes, we studied anisotropic gelation of alginate droplets, which occurs inside a flow-focusing microfluidic device. In the proposed method, sodium alginate (NaAlg) aqueous phase fused with a calcium chloride () emulsion dispersed in the organic phase just before the aqueous phase breaks up into the droplets. The fused droplet with a front–back asymmetric shape was generated, and the asymmetric shape was kept after geometrical confinement by a narrow microchannel was removed. The shape of the fused droplet depended on the size of prefused NaAlg aqueous phase and a emulsion, and the front–back asymmetry appeared in the case of the smaller emulsion size. The analysis of the velocity field inside and around the droplet revealed that the stagnation point at the tip of the aqueous phase also played an important role. The proposed mechanism will be potentially applicable as a novel fabrication technique of microparticles with asymmetric shapes.

Formation of core–shell droplets for the encapsulation of liquid contents

Accurate control of monodisperse core–shell droplets generated in a microfluidic device has a broad range of applications in research and industry. This paper reports the experimental investigation of flow-focusing microfluidic devices capable of producing size-tuneable and monodisperse core–shell droplets. The dimension of the core–shell droplets was controlled passively by the channel geometry and the flow rate of the liquid phases. The results indicate that microchannel geometry is more significant than flow rates. The highly controllable core–shell droplets could be subsequently employed as a template for generating core–shell microparticles with liquid core. Optical, electron scanning microscopy and X-ray computed microtomography showed that the geometry of the core–shell droplets remains unchanged after solidification, drying and collection. The present study also looks at the thermal stability of core–shell particles depending on the particle size. The larger core–shell particles with a thicker shell provide a higher resistance to heating at elevated temperature. The high degree of control with a flow-focusing microfluidic device makes this a promising approach for the encapsulation, storage, and delivery of lipophilic contents.

Integration of a Dielectrophoretic Tapered Aluminum Microelectrode Array with a Flow Focusing Technique.

We present the integration of a flow focusing microfluidic device in a dielectrophoretic application that based on a tapered aluminum microelectrode array (TAMA). The characterization and optimization method of microfluidic geometry performs the hydrodynamic flow focusing on the channel. The sample fluids are hydrodynamically focused into the region of interest (ROI) where the dielectrophoresis force (FDEP) is dominant. The device geometry is designed using 3D CAD software and fabricated using the micro-milling process combined with soft lithography using PDMS. The flow simulation is achieved using COMSOL Multiphysics 5.5 to study the effect of the flow rate ratio between the sample fluids (Q1) and the sheath fluids (Q2) toward the width of flow focusing. Five different flow rate ratios (Q1/Q2) are recorded in this experiment, which are 0.2, 0.4, 0.6, 0.8 and 1.0. The width of flow focusing is increased linearly with the flow rate ratio (Q1/Q2) for both the simulation and the experiment. At the highest flow rate ratio (Q1/Q2 = 1), the width of flow focusing is obtained at 638.66 µm and at the lowest flow rate ratio (Q1/Q2 = 0.2), the width of flow focusing is obtained at 226.03 µm. As a result, the flow focusing effect is able to reduce the dispersion of the particles in the microelectrode from 2000 µm to 226.03 µm toward the ROI. The significance of flow focusing on the separation of particles is studied using 10 and 1 µm polystyrene beads by applying a non-uniform electrical field to the TAMA at 10 VPP, 150 kHz. Ultimately, we are able to manipulate the trajectories of two different types of particles in the channel. For further validation, the focusing of 3.2 µm polystyrene beads within the dominant FDEP results in an enhanced manipulation efficiency from 20% to 80% in the ROI.

Horizon: Microfluidic platform for the production of therapeutic microbubbles and nanobubbles.

Microbubbles (MBs) have a multitude of applications including as contrast agents in ultrasound imaging and as therapeutic drug delivery vehicles, with further scope for combining their diagnostic and therapeutic properties (known as theranostics). MBs used clinically are commonly made by mechanical agitation or sonication methods, which offer little control over population size and dispersity. Furthermore, clinically used MBs are yet to be used therapeutically and further research is needed to develop these theranostic agents. In this paper, we present our MB production instrument "Horizon," which is a robust, portable, and user-friendly instrument, integrating the key components for producing MBs using microfluidic flow-focusing devices. In addition, we present the system design and specifications of Horizon and the optimized protocols that have so far been used to produce MBs with specific properties. These include MBs with tailored size and low dispersity (monodisperse); MBs with a diameter of ∼2 μm, which are more disperse but also produced in higher concentration; nanobubbles with diameters of 100-600nm; and therapeutic MBs with drug payloads for targeted delivery. Multiplexed chips were able to improve production rates up to 16-fold while maintaining production stability. This work shows that Horizon is a versatile instrument with potential for mass production and use across many research facilities, which could begin to bridge the gap between therapeutic MB research and clinical use.

Effect of Electric Field on the Hydrodynamic Assembly of Polydisperse and Entangled Fibrillar Suspensions

Dynamics of colloidal particles can be controlled by the application of electric fields at micrometer-nanometer length scales. Here, an electric field-coupled microfluidic flow-focusing device is designed for investigating the effect of an externally applied alternating current (AC) electric field on the hydrodynamic assembly of cellulose nanofibrils (CNFs). We first discuss how the nanofibrils align parallel to the direction of the applied field without flow. Then, we apply an electric field during hydrodynamic assembly in the microfluidic channel and observe the effects on the mechanical properties of the assembled nanostructures. We further discuss the nanoscale orientational dynamics of the polydisperse and entangled fibrillar suspension of CNFs in the channel. It is shown that electric fields induced with the electrodes locally increase the degree of orientation. However, hydrodynamic alignment is demonstrated to be much more efficient than the electric field for aligning CNFs. The results are useful for understanding the development of the nanostructure when designing high-performance materials with microfluidics in the presence of external stimuli.

Negative Pressure Provides Simple and Stable Droplet Generation in a Flow-Focusing Microfluidic Device.

Droplet microfluidics is an extremely useful and powerful tool for industrial, environmental, and biotechnological applications, due to advantages such as the small volume of reagents required, ultrahigh-throughput, precise control, and independent manipulations of each droplet. For the generation of monodisperse water-in-oil droplets, usually T-junction and flow-focusing microfluidic devices connected to syringe pumps or pressure controllers are used. Here, we investigated droplet-generation regimes in a flow-focusing microfluidic device induced by the negative pressure in the outlet reservoir, generated by a low-cost mini diaphragm vacuum pump. During the study, we compared two ways of adjusting the negative pressure using a compact electro-pneumatic regulator and a manual airflow control valve. The results showed that both types of regulators are suitable for the stable generation of monodisperse droplets for at least 4 h, with variations in diameter less than 1 µm. Droplet diameters at high levels of negative pressure were mainly determined by the hydrodynamic resistances of the inlet microchannels, although the absolute pressure value defined the generation frequency; however, the electro-pneumatic regulator is preferable and convenient for the accurate control of the pressure by an external electric signal, providing more stable pressure, and a wide range of droplet diameters and generation frequencies. The method of droplet generation suggested here is a simple, stable, reliable, and portable way of high-throughput production of relatively large volumes of monodisperse emulsions for biomedical applications.

Induction of dormancy by confinement: An agarose-silica biomaterial for isolating and analyzing dormant cancer cells.

The principal cause of cancer deaths is the residual disease, which eventually results in metastases. Certain metastases are induced by disseminated dormancy-capable single cancer cells that can reside within the body undetected for months to years. Awakening of the dormant cells starts a cascade resulting in the patient's demise. Despite its established clinical significance, dormancy research and its clinical translation have been hindered by lack of in vitro models that can identify, isolate, and analyze dormancy-capable cells. We have previously shown that immobilization of cells in a stiff microenvironment induces dormancy in dormancy-capable cell lines. In this communication, we present a novel biomaterial and an in vitro immobilization method to isolate, analyze, and efficiently recover dormancy-capable cancer cells. MCF-7, MDA-MB-231, and MDA-MB-468 cells were individually coated with agarose using a microfluidic flow-focusing device. Coated cells were then immobilized in a rigid and porous silica gel. Dormancy induction by this process was validated by decreased Ki-67 expression, increased p38/ERK activity ratio, and reduced expression of CDK-2, cyclins D1, and E1. We showed that we can reliably and repeatedly induce dormancy in dormancy-capable MCF-7 cells and enhance the dormancy-capable sub-population in MDA-MB-231 cells. As expected, dormancy-resistant MDA-MB-468 cells did not survive immobilization. The dormant cells could be awakened on demand, by digesting the agarose gel in situ, and efficiently recovered by magnetically separating the silica gel, making the cells available for downstream analysis and testing. The awakened cells were shown to regain motility immediately, proliferating, and migrating normally.

Hydrodynamics of triple emulsion droplet generation in a flow-focusing microfluidic device

A numerical research is conducted to investigate the triple emulsion droplet generation in a three-dimensional axisymmetric flow-focusing device. The simulation predicts three flow regimes: dripping, jetting, and threading. The hydrodynamics underlying the droplet generation and effects of operation conditions on the droplet generation are clarified. An increase in flow rate of one dispersed phase results in the larger droplet enclosing this phase and smaller droplet enclosed by this phase. As the interfacial tension between any two phases increases or viscosity of any phases decrease, the flow regime transits from jetting to dripping, accompanied by a decrease in the droplet size. Increasing the interfacial tension coefficient or decreasing the viscosity under both two regimes can increase the droplet size. Additionally, regime diagrams for single, middle, and triple emulsion droplet generation are compared, and an empirical equation is proposed to predict the flow regime transition.

Mild formation of core–shell hydrogel microcapsules for cell encapsulation

Internal gelation has been an important sol-gel route for the preparation of spherical microgel for drug delivery, cell therapy, or tissue regeneration. Despite high homogeneity and permeability, the internal gelated microgels often result in weak mechanical stability, unregular interface morphology and low cell survival rate. In this work, we have extensively improved the existing internal gelation approach and core–shell hydrogel microcapsules (200–600 μm) with a smooth surface, high mechanical stability and cell survival rate, are successfully prepared by using internal gelation. A coaxial flow-focusing capillary-assembled microfluidic device was developed for the gelation. Rapid gelling behavior of alginate in the internal gelation makes it suitable for producing well-defined and homogenous alginate hydrogel microstructures that serve as the shell of the microcapsules. 2-[4-(2-Hydroxyethyl)piperazin-1-yl]ethanesulfonic acid (HEPES) was used in the shell stream during the internal gelation. Thus, a high concentration of acid in the oil solution can be used for better crosslinking the alginate while maintaining high cell viability. We further demonstrated that the gelation conditions in our approach were mild enough for encapsulating HepG2 cells and 3T3 fibroblasts without losing their viability and functionality in a co-culture environment.

Preparation of monodisperse S/W/O compound droplets with thick liquid film via a dual-cross microfluidic device

Double-layer polymer shells with stringent specifications are always employed as the inertial confinement fusion (ICF) targets, which can be prepared by solidifying the solid-in-water-in-oil (S/W/O) compound droplets. A microfluidic device with a dual-cross structure is proposed to improve the quality of the resulting S/W/O compound droplets, and further, the formation process is extensively investigated. Especially, the flow regimes of multiple-encapsulation can be avoided when compared with the flow-focusing microfluidic device, and the thickness of the polyvinyl alcohol (PVA) liquid film coated onto the solid particle is also improved because of the large interval distance between sequenced solid particles. Moreover, the influences of both phases’ flow rates on the thickness of the liquid layer are also discussed. With the decrease of the flow rate ratio of the continuous phase to the dispersed phase, the thickness of the PVA liquid film rises from 55 to 383 µm, and the coefficient of variation (CV) of the liquid film thickness is about 4%. Increase the diameter of the main channel to obtain a thicker liquid film range from 122 to 648 µm. The obtained results could provide experimental guidance to prepare S/W/O compound droplets precisely and controllably with a specific thickness of the liquid layer.

Microfluidic-assisted nanoprecipitation of biodegradable nanoparticles composed of PTMC/PCL (co)polymers, tannic acid and doxorubicin for cancer treatment

This study was aimed towards the development of a novel microfluidic approach for the preparation of (co)polymeric and hybrid nanoparticles (NPs) composed of (co)polymers/tannic acid (TA) in the microfluidic flow-focusing glass-capillary device. The MiliQ water was used as water phase, whereas the organic phase was composed of poly(ε-caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) homopolymers and (co)polymers with different proportion of comonomers which were prepared via enzymatic polymerization that allows avoiding the usage of potentially toxic catalyst. To prepare hybrid NPs, TA was additionally added to the organic phase. Subsequently, as a result of mixing between these distinct phases in microfluidic channels, the nanoprecipitation in the form of spherical NPs occurs. The size of NPs was tuned over the range of 140–230 nm by controlling phase flow rates and the composition of NPs. Moreover, the release studies of the encapsulated anticancer drug doxorubicin (DOX) demonstrated that the drug release is greatly influenced by the (co)polymers composition, their molecular weight, NPs size, and the presence of TA. The antitumor activities of the (co)polymeric and hybrid NPs toward breast cancer cells (MCF-7) were tested in vitro. Among all tested formulation, the NPs composed of PCL/TA most efficiently inhibit the cell proliferation of MCF-7 cells, most importantly, their efficiency was higher than free DOX. The proposed strategy may provide an efficient alternative for the construction of nanocarriers with great potential in anticancer therapy.

Synthesis and application of hydrogel calcium alginate microparticles as a biomaterial to remove heavy metals from aqueous media

Microfluidic systems are an emerging technology to fabricate uniform spherical microparticles. This study employed a flow-focusing microfluidic device (MFFD) to produce uniform sodium alginate microdroplets by controlling the continuous (soybean oil) and dispersed (sodium alginate solution) flow rates. The resultant hydrogel particles were spherical in shape with diameters ranging from 70 to 100 μ m , and a size distribution under 10%, depending on the experimental conditions. Observations of dried calcium alginate microparticles using scanning electron microscopy showed a surface characterized by the uniform distribution of regular nodules. The resultant biomaterials were further examined and evaluated in terms of the removal of toxic metals (Cu 2 + and Ni 2 + ions); they demonstrated excellent removal performance. The highest adsorption capacities of the prepared calcium alginate microparticles toward Cu 2 + and Ni 2 + ions were 0.36 and 0.81 mg/mg alginate microparticles, respectively. Thus, calcium alginate microparticles prepared by MFFD demonstrated the potential for toxic metal treatment at different concentrations with short duration and stability. This approach may also be controlled to prepare microparticles that are narrow in size distribution and exhibit uniform morphology. • Synthesis of hydrogel calcium alginate microparticles using a microfluidic device. • The toxic metal adsorption behavior of the microparticles was investigated. • Flow rates (ml/h) of 0.1 (alginate) & 6 (soybean oil) yielded uniform particles. • The adsorption capacities (mg/mg) for Cu & Ni ions were 0.36 & 0.81, respectively.

The preparation of peppermint oil/2-hydroxypropyl-β-cyclodextrin/chitosan composite microcapsule and their prolonged retaining ability

In this paper, the peppermint oil/HP-β-CD (2-hydroxypropyl-β-cyclodextrin)/chitosan composite microcapsules were prepared. The release ability of these microcapsules was improved for peppermint oil through the combination of molecular encapsulation by HP-β-CD and physical encapsulation by chitosan shell. First, stable peppermint oil/HP-β-CD pre-emulsion was prepared by stirring, based on which the optimal conditions were determined for producing the pre-emulsions with the highest encapsulation efficiency. Then, the pre-emulsion was moved into the flow-focusing microfluidic device as the inner phase and was cut into droplets by chitosan aqueous solution as the middle phase and the octanol as the outer phase to obtain templated emulsions. They were collected using a Petri dish with n-octane coagulation bath containing glutaraldehyde and 2 wt% span80 to be cured into microcapsules. The crosslinking time and the ratio of HP-β-CD in the templated emulsions were adjusted to investigate their impact on the microcapsule structures and correspondingly the capability of the microcapsules to retain peppermint. It was found out that the extended crosslinking time and the addition of HP-β-CD can improve the capability of the composite microcapsules to retain peppermint oil. It provides a straightforward method to produce composite microcapsules. Besides, the microcapsules with improved retaining ability were obtained by combing molecular and physical encapsulation, thus laying a foundation for the exquisite, efficient and polylaminate protection of the volatile perfume or other physically or chemically active components.

Production of doxorubicin-loaded PCL nanoparticles through a flow-focusing microfluidic device: encapsulation efficacy and drug release.

The present study shows a facile route for producing doxorubicin (DOX)-loaded polycaprolactone (PCL) nanoparticles using a microfluidic device with a flow-focusing platform in a single step. Indeed, the evaluation of the performance of the flow-focusing microfluidic device for the preparation of DOX-loaded PCL (DOX/PCL) nanoparticles with a uniform size distribution and high encapsulation efficiency (EE) by applying the liquid non-solvent precipitation process is very important. Accordingly, the physicochemical characteristics of the DOX/PCL nanoparticles such as their mean size, polydispersity index (PDI), and EE were investigated by studying different parameters such as the flow rate ratio (FRR) and DOX concentration. Also, the release study was carried out at two pH of 5.5 and 7.4. The mean size of DOX/PCL nanoparticles achieved was in the range of 120-320 nm with a PDI ≤ 0.29 and EE between 48% and 87%. Moreover, the release profile of DOX/PCL nanoparticles was sustained for 10 days (≤66%) at pH 7.4. This means that the production process can result in a high EE and low release of the DOX drug.