Monodisperse water-in-water-in-oil emulsion droplets

Spontaneous Transfer of Droplets across a Microfluidic Liquid-Liquid Interface

The droplet transfer across an interface in a microchannel is extensively utilized in diverse fields; however, it is challenging to drive droplets to penetrate the interface at such a small scale. In this study, a novel flow pattern of droplet transfer is observed and the mechanism is investigated; accordingly, an accurate prediction equation for determining the critical condition of droplet transfer is proposed. Meanwhile, the liquid film entrainment is also observed, which leads to the formation of an oil-in-water-in-water system. This study serves as a valuable reference for further studies on the mechanism of droplet transfer and provides practical guidance for its industrial application.

Aqueous Two-Phase Systems within Selectively Permeable Vesicles

An aqueous two-phase system (ATPS) encapsulated within a vesicle organizes the vesicle core as two coexisting phases that partition encapsulated solutes. Here, we use microfluidic technologies to produce vesicles that efficiently encapsulate mixtures of macromolecules, providing a versatile platform to determine the phase behavior of ATPSs. Moreover, we use compartmentalized vesicles to investigate how membrane permeability affects the dynamics of the encapsulated ATPS. Designing a membrane selectively permeable to one of the components of the ATPS, we show that out-of-equilibrium phase separations formed by a rapid outflow of water can be spontaneously reversed by a slower outflow of the permeating component across the vesicle membrane. This dynamics may be exploited advantageously by cells to separate and connect metabolic and signaling routes within their nucleoplasm or cytoplasm depending on external conditions.

High-throughput selection of cells based on accumulated growth and division using PicoShell particles

Production of high-energy lipids by microalgae may provide a sustainable energy source that can help tackle climate change. However, microalgae engineered to produce more lipids usually grow slowly, leading to reduced overall yields. Unfortunately, culture vessels used to select cells based on growth while maintaining high biomass production, such as well plates, water-in-oil droplet emulsions, and nanowell arrays, do not provide production-relevant environments that cells experience in scaled-up cultures (e.g., bioreactors or outdoor cultivation farms). As a result, strains that are developed in the laboratory may not exhibit the same beneficial phenotypic behavior when transferred to industrial production. Here, we introduce PicoShells, picoliter-scale porous hydrogel compartments, that enable >100,000 individual cells to be compartmentalized, cultured in production-relevant environments, and selected based on growth and bioproduct accumulation traits using standard flow cytometers. PicoShells consist of a hollow inner cavity where cells are encapsulated and a porous outer shell that allows for continuous solution exchange with the external environment. PicoShells allow for cell growth directly in culture environments, such as shaking flasks and bioreactors. We experimentally demonstrate that Chlorella sp., Saccharomyces cerevisiae, and Chinese hamster ovary cells, used for bioproduction, grow to significantly larger colony sizes in PicoShells than in water-in-oil droplet emulsions (P < 0.05). We also demonstrate that PicoShells containing faster dividing and growing Chlorella clonal colonies can be selected using a fluorescence-activated cell sorter and regrown. Using the PicoShell process, we select a Chlorella population that accumulates chlorophyll 8% faster than does an unselected population after a single selection cycle.

Complex emulsions for shape control based on mass transfer and phase separation

Complex emulsions are used to fabricate new morphologies of multiple Janus droplets, evolving from non-engulfing to complete engulfing core/shell configuration. The produced droplets contain an aqueous phase of dextran (DEX) solution and an oil phase, which is mixed with ethoxylated trimethylolpropane triacrylate (ETPTA) and poly(ethylene glycol) diacrylate (PEGDA). The PEGDA in the oil phase is transferred into the aqueous phase to form complex morphologies due to the phase separation of PEGDA and DEX. The effects are investigated including the ratio of oil to aqueous phase, the content of initial PEGDA, DEX and surfactants, and the type of surfactants. DEX/PEGDA-ETPTA core/shell-single phase Janus droplets are formed with an increasing engulfed oil droplet into the aqueous droplet while the ratio of oil to aqueous phase increases or the initial PEGDA content increases. The high DEX content leads to the DEX-PEGDA-ETPTA doublet Janus. The use of surfactants polyglycerol polyricinoleate (PGPR) and Span 80 results in the formation of DEX/PEGDA/ETPTA single core/double shell and DEX/PEGDA-ETPTA core/shell-single phase Janus droplets, respectively. These complex emulsions are utilized to fabricate solid particles of complex shapes. This method contributes to new material design underpinned by mass transfer and phase separation, which can be extended to other complex emulsion systems.

Aqueous Two-Phase System Formation in Small Droplets by Shirasu Porous Glass Membrane Emulsification Followed by Water Extraction

By utilizing water transport phenomena between two different water-in-oil (W/O) emulsion droplets through continuous oil phase, we developed a novel method of aqueous two-phase system (ATPS) formation in small droplets prepared by Shirasu porous glass (SPG) membrane emulsification technique. When we mixed W/O emulsion droplets containing poly(ethylene glycol) (PEG) and dextran (DEX) at concentrations below the threshold of the phase separation, with droplets containing other solutes at high concentrations, water extraction from the droplets containing PEG and DEX to those containing the other solutes occurred, owing to the osmotic pressure difference. This effect increased the concentrations of PEG and DEX in the droplets above the phase separation threshold. We demonstrated the feasibility of the preparation method by varying the pore sizes of the SPG membranes, the solutes, and their concentrations. Only when the concentration of the solute was high enough to extract sufficient amounts of water did the homogeneous disperse phase consisting of PEG and DEX in droplets turn into a PEG-rich phase and DEX-rich phase, showing ATPS. This result was irrespective of the solute itself and pore size of the SPG membrane. In particular, we successfully demonstrated monodisperse ATPS droplets with diameters of approximately 10 μm under a certain condition.

Microfluidic Formation of Hydrogel Microcapsules with a Single Aqueous Core by Spontaneous Cross-Linking in Aqueous Two-Phase System Droplets

We report a simple process to fabricate monodisperse tetra-arm poly(ethylene glycol) (tetra-PEG) hydrogel microcapsules with an aqueous core and a semipermeable hydrogel shell through the formation of aqueous two-phase system (ATPS) droplets consisting of a dextran-rich core and a tetra-PEG macromonomer-rich shell, followed by a spontaneous cross-end coupling reaction of tetra-PEG macromonomers in the shell. Different from conventional techniques, this process enables for the continuous production of hydrogel microcapsules from water-in-oil emulsion droplets under mild conditions in the absence of radical initiators and external stimuli such as heating and ultraviolet light irradiation. We find that rapid cross-end coupling reaction of tetra-PEG macromonomers in ATPS droplets in the range of pH from 7.4 to 7.8 gives hydrogel microcapsules with a kinetically arrested core-shell structure. The diameter and core-shell ratio of the microcapsules can be easily controlled by adjusting flow rates and ATPS compositions. On the other hand, the slow cross-end coupling reaction of tetra-PEG macromonomers in ATPS droplets at pH 7.0 and lower induces structural change from core-shell to Janus during the reaction, which eventually forms hydrogel microparticles with a thermodynamically stable crescent structure. We believe that these hydrogel microparticles with controlled structures can be used in biomedical fields such as cell encapsulation, biosensors, and drug delivery carriers for sensitive biomolecules.

All-Aqueous-Phase Microfluidics for Cell Encapsulation

Cell-laden hydrogel microcarriers are widely used in diverse biomedical applications like three-dimensional (3D) cell culture, cellular therapy, and tissue engineering, where microcarriers were generally produced by oil, which is the common but not optimal choice, as oil may cause cytotoxicity or protein denaturation. Here, an all-aqueous-phase microfluidics is presented to achieve oil-free emulsification of cell-laden microcapsules and 3D cell culture. Aqueous solutions with different concentration gradients are used as an immiscible continuous phase and a dispersed phase, and oscillation from a solenoid valve facilitates the formation of microcapsules at the water-water interface. By adjusting aqueous-phase flow rates and oscillating frequencies, core-shell microcapsules with controllable structures can be stably and continuously generated. In further 3D cell culture, encapsulated cells maintained good viabilities and aggregated together. These features show that the oil-free microfluidic method may have broad prospects in many biomedical applications.

Liquid-liquid phase separation in artificial cells

Liquid-liquid phase separation (LLPS) in biology is a recently appreciated means of intracellular compartmentalization. Because the mechanisms driving phase separations are grounded in physical interactions, they can be recreated within less complex systems consisting of only a few simple components, to serve as artificial microcompartments. Within these simple systems, the effect of compartmentalization and microenvironments upon biological reactions and processes can be studied. This review will explore several approaches to incorporating LLPS as artificial cytoplasms and in artificial cells, including both segregative and associative phase separation.

Recent advances in engineering microparticles and their nascent utilization in biomedical delivery and diagnostic applications

Complex microparticles (MPs) bearing unique characteristics such as well-tailored sizes, various morphologies, and multi-compartments have been attempted to be produced by many researchers in the past decades. However, a conventionally used method of fabricating MPs, emulsion polymerization, has a limitation in achieving the aforementioned characteristics and several approaches such as the microfluidics-assisted (droplet-based microfluidics and flow lithography-based microfluidics), electrohydrodynamics (EHD)-based, centrifugation-based, and template-based methods have been recently suggested to overcome this limitation. The outstanding features of complex MPs engineered through these suggested methods have provided new opportunities for MPs to be applied in a wider range of applications including cell carriers, drug delivery agents, active pigments for display, microsensors, interface stabilizers, and catalyst substrates. Overall, the engineered MPs expose their potential particularly in the field of biomedical engineering as the increased complexity in the engineered MPs fulfills well the requirements of the high-end applications. This review outlines the current trends of newly developed techniques used for engineered MPs fabrication and focuses on the current state of engineered MPs in biomedical applications.

Microcapsules with a permeable hydrogel shell and an aqueous core continuously produced in a 3D microdevice by all-aqueous microfluidics

We report the continuous production of microcapsules composed of an aqueous core and permeable hydrogel shell, made stable by the controlled photo-cross-linking of the shell of an all-aqueous double emulsion.

Microscale Strategies for Generating Cell-Encapsulating Hydrogels

Hydrogels in which cells are encapsulated are of great potential interest for tissue engineering applications. These gels provide a structure inside which cells can spread and proliferate. Such structures benefit from controlled microarchitectures that can affect the behavior of the enclosed cells. Microfabrication-based techniques are emerging as powerful approaches to generate such cell-encapsulating hydrogel structures. In this paper we introduce common hydrogels and their crosslinking methods and review the latest microscale approaches for generation of cell containing gel particles. We specifically focus on microfluidics-based methods and on techniques such as micromolding and electrospinning.

Slow growth of the Rayleigh-Plateau instability in aqueous two phase systems

This paper studies the Rayleigh-Plateau instability for co-flowing immiscible aqueous polymer solutions in a microfluidic channel. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. Experiments for the well-known aqueous two phase system (ATPS, or aqueous biphasic systems) of dextran and polyethylene glycol solutions exhibited a growth rate of 1 s(-1), which was more than an order of magnitude slower than an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were ruled out as explanations for the observed behavior. The results are remarkable because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case. Microfluidic uses of aqueous two phase systems include separation of labile biomolecules but have hitherto be limited because of the difficulty in making droplets. The results of this work teach how to design devices for biological microfluidic ATPS platforms.

Microfluidics with aqueous two-phase systems

An overview is given about research activities in which aqueous two phase systems (ATPSs) are utilized in microfluidic setups. ATPSs consist of two immiscible aqueous phases and have traditionally been used for the separation and purification of biological material such as proteins or cells. Microfluidic implementations of such schemes are usually based on a number of co-flowing streams of immiscible phases in a microchannel, thereby replacing the standard batch by flow-through processes. Some aspects of the stability of such flow patterns and the recovery of the phases at the channel exit are reviewed. Furthermore, the diffusive mass transfer and sample partitioning between the phases are discussed, and corresponding applications are highlighted. When diffusion is superposed by an applied electric field normal to the liquid/liquid interface, the transport processes are accelerated, and under specific conditions the interface acts as a size-selective filter for molecules. Finally, the activities involving droplet microflows of ATPSs are reviewed. By either forming ATPS droplets in an organic phase or a droplet of one aqueous phase inside the other, a range of applications has been demonstrated, extending from separation/purification schemes to the patterning of surfaces covered with cells.