Monodisperse Emulsions Research Articles

3D-printed microfluidic device for monodisperse emulsions preparation

3D-printed microfluidic device for monodisperse emulsions preparation

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.

Stretch, fold, and break: Intensification of emulsification of high viscosity ratio systems by fractal mixers

AbstractDuring emulsification process design, the bulk and interfacial rheology of the target formulation must be carefully considered. Formulations with high viscosity ratios and/or finite interfacial elasticity are particularly challenging to emulsify, as conventional drop‐breakup methods consume significant energy and provide limited control over polydispersity. Here, we develop a two‐stage process that produces monodisperse emulsions from high viscosity ratio constituents. In the first stage, a custom static mixer generates co‐flowing layers of alternate phases, and progressively thins layers until they rupture, thus forming a high‐internal phase emulsion. The interfacial properties and flow conditions that promote stable fractal multiplication are discussed. In the second stage, extensional flow elements refine the polydispersity. We demonstrate the utility of this novel process by producing remarkably monodisperse polyisobutylene‐in‐water emulsions with an energy efficiency that is orders of magnitude higher than classical emulsification methods. The moderate throughputs achieved show promise for upscaling and intensification in industrial applications.

Manipulation of droplets in a microchannel by ultrasonic vibration for the generation of monodisperse emulsions

Manipulation of droplets in a microchannel by ultrasonic vibration for the generation of monodisperse emulsions

Time-resolved investigation of mesoporous silica microsphere formation using in situ heating optical microscopy

A fundamental understanding of the drying behavior of droplets containing solids or solutes is important for various industrial applications. However, droplets are typically highly polydisperse and time-resolved imaging data of the process dynamics are often lacking, which makes it difficult to interpret the effects of different drying parameters. Here, the controlled drying of monodisperse emulsion droplets containing colloidal silica nanoparticles and their subsequent assembly into mesoporous silica microspheres (MSMs) is investigated using an optical microscope outfitted with a heating and vacuum stage. Quantitative imaging results on droplet shrinkage and observed contrast are compared with a theoretical mass-transfer model that is based on the droplet number density, solvent characteristics and temperature. The results presented here provide key insights in the time-resolved formation of MSMs and will enable an optimized direct synthesis of monodisperse MSMs for separation applications and beyond.

Functional microparticles from multiscale regulation of multiphase emulsions for mass-transfer intensification

Mass transfer of pollutants and drugs in microparticles are important for their applications in the fields of water decontamination and drug delivery. Microparticles with controllable structures and advanced functions allow enhancement of mass transfer processes to achieve improved performances for water decontamination and drug delivery. This review summarizes recent progresses on controllable fabrication of functional microparticles from microfluidic multiphase emulsions, with intensified mass-transfer for water decontamination and drug delivery. This is started by introducing the controllable microfluidic production of diverse monodisperse emulsions, and then the template synthesis of functional microparticles from the emulsion templates for water decontamination and drug delivery. Porous microparticles with intensified mass transfer via creation of hierarchical pores and bubble-propelled motion for enhanced water decontamination are introduced. Then, compartmental microparticles with intensified mass transfer via shell permeability adjustment, shell degradation, shell shrinkage, and their combinations for advanced drug delivery are highlighted. The strategies to engineer the functional microparticles for intensified mass transfer via multiscale regulation of their multiphase emulsion templates are emphasized. Finally, perspectives on the future development of microfluidic techniques for developing advanced functional microparticles with intensified mass transfer for enhanced performances are discussed.

Extractive crystallization of cabotegravir in droplet-based microfluidic devices

Monodisperse emulsions of solvent saturated with the active pharmaceutical ingredient (API) cabotegravir were generated by microfluidic devices. In contrast to model oil-water emulsions that provide idealized conditions for obtaining highly monodisperse emulsions, APIs impart strict limitations on solvent choice. To provide a suitable environment in which extractive crystallization could occur, and to enhance reproducibility, co-solvents were introduced. Subsequently, the effect of each solvent component on morphology and size was observed. Through the manipulation of solvent composition, particles of both plate-like and spherical multi-crystal agglomerates were obtained, providing the ability to tune the operating conditions to provide particles with desirable characteristics such as size, shape and flowability. Scale-up of the manufacturing process was addressed through the application of existing microfluidic parallelization techniques, resulting in a 100-fold increase in throughput over a single-channel device. Subsequently, the obtained pure, and excipient-bound, API spherical agglomerates, were subjected to batch isolation and washing.

Double Emulsion Picoreactors for High-Throughput Single-Cell Encapsulation and Phenotyping via FACS

In the past five years, droplet microfluidic techniques have unlocked new opportunities for the high-throughput genome-wide analysis of single cells, transforming our understanding of cellular diversity and function. However, the field lacks an accessible method to screen and sort droplets based on cellular phenotype upstream of genetic analysis, particularly for large and complex cells. To meet this need, we developed Dropception, a robust, easy-to-use workflow for precise single-cell encapsulation into picoliter-scale double emulsion droplets compatible with high-throughput screening via fluorescence-activated cell sorting (FACS). We demonstrate the capabilities of this method by encapsulating five standardized mammalian cell lines of varying sizes and morphologies as well as a heterogeneous cell mixture of a whole dissociated flatworm (5-25 μm in diameter) within highly monodisperse double emulsions (35 μm in diameter). We optimize for preferential encapsulation of single cells with extremely low multiple-cell loading events (<2% of cell-containing droplets), thereby allowing direct linkage of cellular phenotype to genotype. Across all cell lines, cell loading efficiency approaches the theoretical limit with no observable bias by cell size. FACS measurements reveal the ability to discriminate empty droplets from those containing cells with good agreement to single-cell occupancies quantified via microscopy, establishing robust droplet screening at single-cell resolution. High-throughput FACS screening of cellular picoreactors has the potential to shift the landscape of single-cell droplet microfluidics by expanding the repertoire of current nucleic acid droplet assays to include functional phenotyping.

In vivo evaluation of a monodisperse solid-in-oil-in-water miriplatin/ lipiodol emulsion in transcatheter arterial chemoembolization using a rabbit VX2 tumor model.

Transcatheter arterial chemoembolization (TACE) is a standard treatment for unresectable hepatocellular carcinoma; however, it does not always result in tumor control. Nevertheless, treatment outcome can be improved with monodisperse emulsions of anticancer agents. In this study, the distribution of a monodisperse miriplatin-Lipiodol emulsion in the tumor and its safety were evaluated in ten Japanese white rabbits. VX2 tumor was implanted into the left liver lobe. The animals were divided into control and experimental groups (of five animals each) and respectively administered a conventional miriplatin suspension or the emulsion via the left hepatic artery. Computed tomography (CT) was performed before, immediately after, and two days following TACE. All rabbits were sacrificed two days after the procedure. Each tumor was removed and cut in half for assessment of iodine concentration in one half by mass spectroscopy and evaluation of Lipiodol accumulation and adverse events in the other half. Mean Hounsfield unit (HU) values were measured using plain CT images taken before and after TACE. Iodine concentration was higher in the experimental group [1100 (750–1500) ppm, median (range)] than in the control group [840 (660–1800) ppm], although statistically not significant. Additionally, the HU value for the experimental group was higher than that for the control group immediately after [199.6 (134.0–301.7) vs. 165.3 (131.4–280.5)] and two days after [114.2 (56.1–229.8) vs. 58.3 (42.9–132.5)] TACE, although statistically not significant. Cholecystitis was observed in one rabbit in the control group. Ischemic bile duct injury was not observed in any group. The results show that Lipiodol accumulation and retention in VX2 tumor can possibly be improved with a monodisperse emulsion; however, it should be verified with a larger study. Moreover, no significant adverse events are associated with the use of the emulsion.

One-step microdevices for synthesizing morphology-controlled ultraviolet-curable polysiloxane shell particles

Double emulsions have great potential application in many fields. Although many companies and research institutions have developed various strategies for forming double emulsions, production and stability are not straightforward. Meanwhile, none of the well-organized simple device and scalable technique was available to form highly monodispersed double emulsions. Herein, we present a one-step strategy to produce highly monodisperse double emulsion via assembled micro-cross device. This device has a lot of advantages, such as easy assembly/disassembly and high repeatability. High-throughput is also easily achievable for its high-pressure capacity. Moreover, we systematically studied the effects of the three flow rates on the size and structure of the double emulsions. A mathematical model has been developed to predict the inner and outer droplets size of the double emulsion. We prepared the flame-retardant microcapsules with a dimethyl methylphosphonate (DMMP) inner core and an ultraviolet-curable polysiloxane shell. We hope that this assembled micro-cross device can be employed extensively in the area of microfluidics.

Investigations on the generation of oil-in-water (O/W) nanoemulsions through the combination of ultrasound and microchannel

O/W nanoemulsions are isotropic colloidal systems constituted of oil droplets dispersed in continuous aqueous media and stabilised by surfactant molecules. Nanoemulsions hold applications in more widespread technological domains, more crucially in the pharmaceutical industry. Innovative nanoemulsion-based drug delivery system has been suggested as a powerful alternative strategy through the useful means of encapsulating, protecting, and delivering the poorly water-soluble bioactive components. Consequently, there is a need to generate an emulsion with small and consistent droplets. Diverse studies acknowledged that ultrasonic cavitation is a feasible and energy-efficient method in making pharmaceutical-grade nanoemulsions. This method offers more notable improvements in terms of stability with a lower Ostwald ripening rate. Meanwhile, a microstructured reactor, for instance, microchannel, has further been realised as an innovative technology that facilitates combinatorial approaches with the acceleration of reaction, analysis, and measurement. The recent breakthrough that has been achieved is the controlled generation of fine and monodispersed multiple emulsions through microstructured reactors. The small inner dimensions of microchannel display properties such as short diffusion paths and high specific interfacial areas, which increase the mass and heat transfer rates. Hence, the combination of ultrasonic cavitation with microstructures (microchannel) provides process intensification of creating a smaller monodispersed nanoemulsion system. This investigation is vital as it will then facilitate the creation of new nanoemulsion based drug delivery system continuously. Following this, the fabrication of microchannel and setup of its combination with ultrasound was conducted in the generation of O/W nanoemulsion, as well as optimisation to analyse the effect of varied operating parameters on the mean droplet diameter and dispersity of the nanoemulsion generated, besides monitoring the stability of the nanoemulsion. Scanning transmission electron microscopy (STEM) images were also carried out for the droplet size measurements. In short, the outcomes of this study are encouraging, which necessitates further investigations to be carried out to advance a better understanding of coupling microchannel with ultrasound to produce pharmaceutical-grade nanoemulsions.

Nanotechnology for bioactives delivery systems

The properties of functional foods depend largely on their micro- and/or nano-scale structure. Conventionally, the preparation of functional foods is based on large-scale processing. As a consequence, the structure of the products cannot be precisely controlled. Recently, micro-/nano-technology have been developing and increasingly applied to the formulation of functional foods. This paper summarizes some applications of micro-/nano-processing of food materials into dispersions, foreseeing their application as bioactives delivery systems. Nanodispersions containing antioxidant food materials were formulated and evaluated using various top-down techniques or a bottom-up method, particularly the performance of various polyglycerol esters (PGE) of fatty acid in the preparation and characterization of β-carotene nanodispersions during in vitro gastric digestion. MicroChannel (MC) and nanochannel (NC) emulsification for producing monodisperse emulsions were investigated. The development of NC emulsification devices for monodisperse fine emulsions is also presented.

Monodisperse liquid foams via membrane foaming

HypothesisIt is possible to generate fairly monodisperse liquid foams by a dispersion cell, which was originally designed for the generation of fairly monodisperse emulsions. If this is the case, scaling-up the production of monodisperse liquid and solid foams will be no longer a problem. ExperimentsWe used the dispersion cell - a batch process - and examined the influence of stirrer speed, membrane pore diameter and injection rate on the structure of the resulting liquid foams. We used an aqueous surfactant solution as scouting system. Once the experimental conditions were known we generated gelatin-based liquid foams and methacrylate-based foamed emulsions. FindingsWe found that (a) the bubble size of the generated liquid foams can be adjusted by varying the membrane pore diameter, (b) no stirrer should be used to obtain monodisperse foams, and (c) the bubble size is not influenced by the air injection rate. Since (i) the output for all investigated systems is up to two orders of magnitude larger compared to microfluidics and (ii) the membrane technology can very easily be scaled-up if run in a continuous process, the use of membrane foaming is expected to be heavily used for the generation of monodisperse liquid and solid foams, respectively.

Lipid-Stabilized Double Emulsions Generated in Planar Microfluidic Devices

Microemulsions have found a wide range of applications exploiting their chemical and physical properties. Development of microfluidic-based approaches has allowed for the controlled production of highly monodispersed emulsions, including the formation of multiple and hierarchical emulsions. Conventional poly(dimethylsiloxane)-based microfluidic systems require tight spatial control over the surface chemistry when used for double emulsion generation, which can be challenging to achieve on the micrometer scale. Here, we present a two-dimensional device design, which can selectively be surface-treated in a straightforward manner and allows for the formation of uniform water/oil/water double emulsions by combining two distinct hydrophilic and hydrophobic surface properties. These surfaces are sufficiently separated in space to allow for imparting their functionalization without the requirement for lithographic approaches or complex flow control. We demonstrate that a mismatch between the wettability requirements of the continuous phase and the channel wall inherent in this approach can be tolerated over several hundreds of micrometers, opening up the possibility to use simple pressure-driven flows to achieve surface functionalization. The design architecture exhibits robust efficiency in emulsion generation while retaining simple device fabrication. We finally demonstrate the potential of this approach by generating water in oil in water emulsions with lipid molecules acting as surfactants.

Energy velocity of multiply scattered waves in strongly scattering media

The important influence of the relative refractive index of scattering inclusions on energy transport of classical waves through disordered media is clearly demonstrated through ultrasonic experiments on monodisperse emulsions. Our ultrasonic techniques measure both the transmitted average wave field and the multiply scattered diffusive intensity, enabling a full characterization of wave transport through the media and the measurement of both the group and energy velocities over a wide range of frequencies. The emulsions were fabricated using microfluidic techniques that permit accurate control of droplet size and concentration, for droplet inclusions with very different acoustic properties relative to the yield stress fluids in which the droplets were immersed. Thus we have been able to investigate emulsions containing either ``slow'' fluorinated oil droplets (sound speed ${v}_{1}$ less than ${v}_{0}$ of the surrounding fluid) or ``fast'' liquid metallic droplets $({v}_{1}g{v}_{0})$. We find that the energy velocity that describes the transport of energy by the dominant diffusive waves is mainly governed by the sound speed within the scatterers, and can be either much slower or faster than any of the other wave velocities. The possibility that the energy velocity could be faster than any other wave velocity when ${n}_{\text{rel}}={v}_{0}/{v}_{1}l1$ was not anticipated in previous work. These observations are successfully explained by theories that are valid for scalar waves in media containing a low concentration of scatterers, and are directly applicable to our dilute ``all-fluid'' emulsions. The role of droplet resonances on the behavior of the energy velocity is also demonstrated, and the mechanism leading to the large differences in the energy velocity in the two emulsion systems is elucidated through calculations of the energy density inside the droplets relative to the incident energy density.

Streamlined digital bioassays with a 3D printed sample changer.

Droplet-based microfluidics has permeated many areas of life sciences including biochemistry, biology and medicine. Water-in-oil droplets act as independent femto- to nano-liter reservoirs, enabling the parallelization of (bio)chemical reactions with a minimum sample input. Among the range of applications spanned by droplet microfluidics, digital detection of biomolecules, using Poissonian isolation of single molecules in compartments, has gained considerable attention due to the high accuracy, sensitivity and robustness of these methods. However, while the droplet throughput can be very high, the sample throughput of these methods is poor in comparison to well plate-based assays. This limitation comes from the necessity to convert independently each sample into a monodisperse emulsion. In this paper, we report a versatile device that performs the quick sequential partitioning of up to 15 samples using a single microfluidic chip. A 3D printed sample rotor is loaded with all samples and connected to a pressure source. Simple magnetic actuation is then used to inject the samples in the microfluidic chip without pressure disruption. This procedure generates monodisperse droplets with high sample-to-sample consistency. We also describe a fluorescent barcoding strategy that allows all samples to be collected, incubated, imaged and analyzed simultaneously, thus decreasing significantly the time of the assay. As an example of application, we perform a droplet digital PCR assay for the quantification of a DNA amplicon from 8 samples in less than 2 hours. We further validate our approach demonstrating the parallel quantification of 11 microRNAs from a human sample using an isothermal nucleic acid amplification chemistry. As an off-chip device, the sample changer can be connected to a variety of microfluidic geometries and therefore, used for a wide range of applications.

Discrete fluidization of dense monodisperse emulsions in neutral wetting microchannels.

The rheology of pressure-driven flows of two-dimensional dense monodisperse emulsions in neutral wetting microchannels is investigated by means of mesoscopic lattice Boltzmann simulations, capable of handling large collections of droplets, in the order of several hundreds. The simulations reveal that the fluidization of the emulsion proceeds through a sequence of discrete steps, characterized by yielding events whereby layers of droplets start rolling over each other, thus leading to sudden drops of the relative effective viscosity. It is shown that such discrete fluidization is robust against loss of confinement, namely it persists also in the regime of small ratios of the droplet diameter over the microchannel width. We also develop a simple phenomenological model which predicts a linear relation between the relative effective viscosity of the emulsion and the product of the confinement parameter (global size of the device over droplet radius) and the viscosity ratio between the disperse and continuous phases. The model shows excellent agreement with the numerical simulations. The present work offers new insights to enable the design of microfluidic scaffolds for tissue engineering applications and paves the way to detailed rheological studies of soft-glassy materials in complex geometries.

Optimizing trans-arterial drug delivery using mono-disperse emulsion

Hepatocellular carcinoma is one of the most common human cancer and is the 5th cause of mortality in men and the 7th in female. Surgical resection and tumor ablation (radiofrequency or microwave) are curative therapy; however, some patients are not indicated to these treatments, mainly due to large number or size of the lesions. Transcatheter arterial chemoembolization is a standard of care for these patients. In this treatment, mixture of various anticancer agent (Doxorubicin, Cisplatin, Miriplatin) and lipophilic contrast media (Lipiodol) is administered via catheter inserted into tumor feeding arteries, followed by embolization. Tumor control can be obtained in 50-90 % of the patients and there is a room for improvement. One major cause of poor tumor control is uneven large-sized emulsion droplets, which are now used in clinical practice. The aim of this project is to improve accumulation of anticancer agents/lipiodol in the tumor by using even small-sized emulsion (mono-dispersed emulsion) which is obtained by utilizing cutting-edge technology called 'membrane emulsification'. The application can be also widened to metastatic liver cancer.

Capillary-Based Microfluidics-Coflow, Flow-Focusing, Electro-Coflow, Drops, Jets, and Instabilities.

Capillary-based microfluidics is a great technique to produce monodisperse and complex emulsions and particulate suspensions. In this review, the current understanding of drop and jet formation in capillary-based microfluidic devices for two primary flow configurations, coflow and flow-focusing is summarized. The experimental and theoretical description of fluid instabilities is discussed and conditions for controlled drop breakup in different modes of drop generation are provided. Current challenges in drop breakup with low interfacial tension systems and recent progress in overcoming drop size limitations using electro-coflow are addressed. In each scenario, the physical mechanisms for drop breakup are revisited, and simple scaling arguments proposed in the literature are introduced.

Standing wave assisted acoustic droplet vaporization for dual payload release from acoustically responsive scaffolds

Ultrasound standing waves have been utilized for many biomedical applications. Here, we demonstrate how standing waves can enhance drug release using acoustic droplet vaporization (ADV), which is the phase-transitioning of perfluorocarbon (PFC) emulsions via ultrasound. These experiments utilized acoustically-responsive scaffolds (ARSs), which are composite fibrin hydrogels containing payload-carrying, monodispersed PFC emulsions. Single- and bi-layer ARSs were generated with dextran-loaded emulsions (diameter: 6 mm) containing either perfluorohexane, or perfluorooctane in each layer. First, we studied the influence of standing waves on payload release from single-layer ARSs. At 4 MPa peak rarefactional pressure, elevated amplitudes due to constructive superposition in the standing wave field enhanced payload release up to 35% at 2.5 MHz in a seven-day longitudinal study. Second, the effect of standing waves was combined with the frequency-dependent ADV to enhance dual payload release from bi-layer ARSs. We demonstrated the sequential release of two dextran payloads from ARSs, which were, respectively, contained within each emulsion, using temporally staggered ADV at 3 MHz (day 0) and 8.8 MHz (day 4). These results will also be discussed in the context of practical strategies for achieving similar conditions for in vivo applicationsUltrasound standing waves have been utilized for many biomedical applications. Here, we demonstrate how standing waves can enhance drug release using acoustic droplet vaporization (ADV), which is the phase-transitioning of perfluorocarbon (PFC) emulsions via ultrasound. These experiments utilized acoustically-responsive scaffolds (ARSs), which are composite fibrin hydrogels containing payload-carrying, monodispersed PFC emulsions. Single- and bi-layer ARSs were generated with dextran-loaded emulsions (diameter: 6 mm) containing either perfluorohexane, or perfluorooctane in each layer. First, we studied the influence of standing waves on payload release from single-layer ARSs. At 4 MPa peak rarefactional pressure, elevated amplitudes due to constructive superposition in the standing wave field enhanced payload release up to 35% at 2.5 MHz in a seven-day longitudinal study. Second, the effect of standing waves was combined with the frequency-dependent ADV to enhance dual payload release from bi-layer ARSs. ...