All-Aqueous Assemblies via Interfacial Complexation: Toward Artificial Cell and Microniche Development

Biomimetic Materials to Fabricate Artificial Cells

As the foundation of life, a cell is generally considered an advanced microreactor with a complicated structure and function. Undeniably, this fascinating complexity motivates scientists to try to extricate themselves from natural living matter and work toward rebuilding artificial cells in vitro. Driven by synthetic biology and bionic technology, the research of artificial cells has gradually become a subclass. It is not only held import in many disciplines but also of great interest in its synthesis. Therefore, in this review, we have reviewed the development of cell and bionic strategies and focused on the efforts of bottom-up strategies in artificial cell construction. Different from starting with existing living organisms, we have also discussed the construction of artificial cells based on biomimetic materials, from simple cell scaffolds to multiple compartment systems, from the construction of functional modules to the simulation of crucial metabolism behaviors, or even to the biomimetic of communication networks. All of them could represent an exciting advance in the field. In addition, we will make a rough analysis of the bottlenecks in this field. Meanwhile, the future development of this field has been prospecting. This review may bridge the gap between materials engineering and life sciences, forming a theoretical basis for developing various life-inspired assembly materials.

Interfacial Membranization of Regenerated Cellulose Nanoparticles and a Protein Renders Stable Water-in-Water Emulsion

Pickering water-in-water (W/W) emulsions stabilized by biobased colloids are pertinent to engineering biomaterials with hierarchical and confined architectures. In this study, stable W/W emulsions are developed through membranization utilizing biopolymer structures formed by the adsorption of cellulose II nanospheres and a globular protein, bovine serum albumin (BSA), at droplet surfaces. The produced cellulose II nanospheres (NPcat, 63 nm diameter) bearing a soft and highly accessible shell, endow rapid and significant binding (16 mg cm- 2) with BSA. NPcat and BSA formed complexes that spontaneously stabilized liquid droplets, resulting in stable W/W emulsions. It is proposed that such a system is a versatile all-aqueous platform for encapsulation, (bio)catalysis, delivery, and synthetic cell mimetics.

Marangoni Droplets of Dextran in PEG Solution and Its Motile Change Due to Coil-Globule Transition of Coexisting DNA

Motile droplets using Marangoni convection are attracting attention for their potential as cell-mimicking small robots. However, the motion of droplets relative to the internal and external environments that generate Marangoni convection has not been quantitatively described. In this study, we used an aqueous two-phase system [poly(ethylene glycol) (PEG) and dextran] in an elongated chamber to generate motile dextran droplets in a constant PEG concentration gradient. We demonstrated that dextran droplets move by Marangoni convection, resulting from the PEG concentration gradient and the active transport of PEG and dextran into and out of the motile dextran droplet. Furthermore, by spontaneously incorporating long DNA into the dextran droplets, we achieved cell-like motility changes controlled by coexisting environment-sensing molecules. The DNA changes its position within the droplet and motile speed in response to external conditions. In the presence of Mg2+, the coil-globule transition of DNA inside the droplet accelerates the motile speed due to the decrease in the droplet's dynamic viscosity. Globule DNA condenses at the rear part of the droplet along the convection, while coil DNA moves away from the droplet's central axis, separating the dipole convections. These results provide a blueprint for designing autonomous small robots using phase-separated droplets, which change the mobility and molecular distribution within the droplet in reaction with the environment. It will also open unexplored areas of self-assembly mechanisms through phase separation under convections, such as intracellular phase separation.

Synthetic-Cell-Based Multi-Compartmentalized Hierarchical Systems

In the extant lifeforms, the self-sustaining behaviors refer to various well-organized biochemical reactions in spatial confinement, which rely on compartmentalization to integrate and coordinate the molecularly crowded intracellular environment and complicated reaction networks in living/synthetic cells. Therefore, the biological phenomenon of compartmentalization has become an essential theme in the field of synthetic cell engineering. Recent progress in the state-of-the-art of synthetic cells has indicated that multi-compartmentalized synthetic cells should be developed to obtain more advanced structures and functions. Herein, two ways of developing multi-compartmentalized hierarchical systems, namely interior compartmentalization of synthetic cells (organelles) and integration of synthetic cell communities (synthetic tissues), are summarized. Examples are provided for different construction strategies employed in the above-mentioned engineering ways, including spontaneous compartmentalization in vesicles, host-guest nesting, phase separation mediated multiphase, adhesion-mediated assembly, programmed arrays, and 3D printing. Apart from exhibiting advanced structures and functions, synthetic cells are also applied as biomimetic materials. Finally, key challenges and future directions regarding the development of multi-compartmentalized hierarchical systems are summarized; these are expected to lay the foundation for the creation of a "living" synthetic cell as well as provide a larger platform for developing new biomimetic materials in the future.

Multimodal peptide ligand extracts parvovirus from interface in affinity aqueous two-phase system

Aqueous two-phase systems (ATPS) have found various applications in bioseparations and microencapsulation. The primary goal of this technique is to partition target biomolecules in a preferred phase, rich in one of the phase-forming components. However, there is a lack of understanding of biomolecule behavior at the interface between the two phases. Biomolecule partitioning behavior is studied using tie-lines (TL), where each TL is a group of systems at thermodynamic equilibrium. Across a TL, a system can either have a bulk PEG-rich phase with citrate-rich droplets, or the opposite can occur. We found that porcine parvovirus (PPV) was recovered at a higher amount when PEG was the bulk phase and citrate was in droplets and that the salt and PEG concentrations are high. To improve the recovery, A PEG 10 kDa-peptide conjugate was formed using the multimodal WRW ligand. When WRW was present, less PPV was caught at the interface of the two-phase system, and more was recovered in the PEG-rich phase. While WRW did not significantly increase the PPV recovery in the high TL system, which was found earlier to be optimal for PPV recovery, the peptide did greatly enhance recovery at a lower TL. This lower TL has a lower viscosity and overall system PEG and citrate concentration. The results provide both a method to increase virus recovery in a lower viscosity system, as well as provide interesting thoughts into the interfacial phenomenon and how to recover virus in a phase and not at the interface.

Flows of a nonequilibrated aqueous two-phase system in a microchannel

Liquid-liquid phase separation is a rich and dynamic process, which recently has gained new interest, especially in biology and for material synthesis. In this work, we experimentally show that co-flow of a nonequilibrated aqueous two-phase system within a planar flow-focusing microfluidic device results in a three-dimensional flow, as the two nonequilibrated solutions move downstream along the length of the microchannel. After the system reaches steady-state, invasion fronts from the outer stream are formed along the top and bottom walls of the microfluidic device. The invasion fronts advance towards the center of the channel, until they merge. We first show by tuning the concentration of polymer species within the system that the formation of these fronts is due to liquid-liquid phase separation. Moreover, the rate of invasion from the outer stream increases with increasing polymer concentrations in the streams. We hypothesize the invasion front formation and growth is driven by Marangoni flow induced by the polymer concentration gradient along the width of the channel, as the system is undergoing phase separation. In addition, we show how at various downstream positions the system reaches its steady-state configuration once the two fluid streams flow side-by-side in the channel.

In Situ Endothelialization of Free-Form 3D Network of Interconnected Tubular Channels via Interfacial Coacervation by Aqueous-in-Aqueous Embedded Bioprinting

The challenge of bioprinting vascularized tissues is structure retention and in situ endothelialization. The issue is addressed by adopting an aqueous-in-aqueous 3D embedded bioprinting strategy, in which the interfacial coacervation of the cyto-mimic aqueous two-phase systems (ATPS) are employed for maintaining the suspending liquid architectures, and serving as filamentous scaffolds for cell attachment and growth. By incorporating endothelial cells in the ink phase of ATPS, tubular lumens enclosed by coacervated complexes of polylysine (PLL) and oxidized bacteria celluloses (oxBC) can be cellularized with a confluent endothelial layer, without any help of adhesive peptides. By applying PLL/oxBC ATPS for embedded bioprinting, free-form 3D vascular networks with in situ endothelialization of interconnected tubular lumens are achieved. This simple approach is a one-step process without any sacrificed templates and post-treatments. The resultant functional vessel networks with arbitrary complexity are suspended in liquid medium and can be conveniently handled, opening new routes for the in vitro production of thick vascularized tissues for pathological research, regeneration therapy and animal-free drug development.

Ionic Strength-Dependent Assembly of Polyelectrolyte-Nanoparticle Membranes via Interfacial Complexation at a Water-Water Interface

Complexation between oppositely charged nanoparticles (NPs) and polyelectrolytes (PEs) is a scalable approach to assemble functional, stimuli-responsive membranes. Complexation at interfaces of aqueous two-phase systems (ATPSs) has emerged as a powerful method to assemble these functional structures. Membranes formed at these interfaces can grow continuously to thicknesses approaching several millimeters and display a high degree of tunability via modification of solution properties such as ionic strength. To identify the membrane assembly mechanism, we study interfacial assembly in a prototypical dextran/PEG ATPS, in which silica (SiO₂) NPs suspended in the PEG phase undergo interfacial complexation with poly(diallyldimethylammonium chloride) (PDADMAC) supplied in the dextran phase. Using a microfluidic device that facilitates sequential insertion of fluorescent and nonfluorescent PDADMAC, we observe a transition in the membrane growth mechanism with ionic strength. In the absence of added salt ([NaCl] = 0 mM) PDADMAC chains permeate through the existing membrane to complex with NPs on the PEG side of the membrane, leading to the formation of well-stratified structures. At elevated ionic strength ([NaCl] = 500 mM), this permeation mechanism is lost. Rather, the complexing species incorporate uniformly across the membrane. We attribute this transition to a rapid exchange of PE-counterion, NP-counterion, and PE/NP binding sites facilitated by an increase in extrinsically compensated charged groups on the NPs and PEs at high salinity. These PDADMAC/SiO₂ NP membranes have tremendous potential for the formation of functional membranes, offering control over the internal structure and serving as an ideal system for the generation of targeted release systems.

Water-in-water droplet microfluidics: A design manual

Droplet microfluidics is utilized in a wide range of applications in biomedicine and biology. Applications include rapid biochemical analysis, materials generation, biochemical assays, and point-of-care medicine. The integration of aqueous two-phase systems (ATPSs) into droplet microfluidic platforms has potential utility in oil-free biological and biomedical applications, namely, reducing cytotoxicity and preserving the native form and function of costly biomolecular reagents. In this review, we present a design manual for the chemist, biologist, and engineer to design experiments in the context of their biological applications using all-in-water droplet microfluidic systems. We describe the studies achievable using these systems and the corresponding fabrication and stabilization methods. With this information, readers may apply the fundamental principles and recent advancements in ATPS droplet microfluidics to their research. Finally, we propose a development roadmap of opportunities to utilize ATPS droplet microfluidics in applications that remain underexplored.

Aqueous Two-Phase Interfacial Assembly of COF Membranes for Water Desalination

Aqueous two-phase system features with ultralow interfacial tension and thick interfacial region, affording unique confined space for membrane assembly. Here, for the first time, an aqueous two-phase interfacial assembly method is proposed to fabricate covalent organic framework (COF) membranes. The aqueous solution containing polyethylene glycol and dextran undergoes segregated phase separation into two water-rich phases. By respectively distributing aldehyde and amine monomers into two aqueous phases, a series of COF membranes are fabricated at water-water interface. The resultant membranes exhibit high NaCl rejection of 93.0-93.6% and water permeance reaching 1.7-3.7 L m^-2 h^-1 bar^-1, superior to most water desalination membranes. Interestingly, the interfacial tension is found to have pronounced effect on membrane structures. The appropriate interfacial tension range (0.1-1.0 mN m^-1) leads to the tight and intact COF membranes. Furthermore, the method is extended to the fabrication of other COF and metal-organic polymer membranes. This work is the first exploitation of fabricating membranes in all-aqueous system, confering a green and generic method for advanced membrane manufacturing.

Recent progress in the synthesis of all-aqueous two-phase droplets using microfluidic approaches

An aqueous two-phase system (ATPS) is a system with liquid-liquid phase separation and shows great potential for the extraction, separation, purification, and enrichment of proteins, membranes, viruses, enzymes, nucleic acids, and other biomolecules because of its simplicity, biocompatibility, and wide applicability [1-4]. The clear aqueous-aqueous interface of ATPSs is highly advantageous for their implementation, therefore making ATPSs a green alternative approach to replace conventional emulsion systems, such as water-in-oil droplets. All aqueous emulsions (water-in-water, w-in-w) hold great promise in the biomedical field as glucose sensors [5] and promising carriers for the encapsulation and release of various biomolecules and nonbiomolecules [6-10]. However, the ultralow interfacial tension between the two phases is a hurdle in generating w-in-w emulsion droplets. In the past, bulk emulsification and electrospray techniques were employed for the generation of w-in-w emulsion droplets and the fabrication of microparticles and microcapsules in the later stage. Bulk emulsification is a simple and low-cost technique; however, it generates polydisperse w-in-w emulsion droplets. Another technique, electrospray, involves easy experimental setups that can generate monodisperse but nonspherical w-in-w emulsion droplets. In comparison, microfluidic platforms provide monodisperse w-in-w emulsion droplets with spherical shapes, deal with the small volumes of solutions and short reaction times and achieve portability and versatility in their design through rapid prototyping. Owing to several advantages, microfluidic approaches have recently been introduced. To date, several different strategies have been explored to generate w-in-w emulsions and multiple w-in-w emulsions and to fabricate microparticles and microcapsules using conventional microfluidic devices. Although a few review articles on ATPSs emulsions have been published in the past, to date, few reviews have exclusively focused on the evolution of microfluidic-based ATPS droplets. The present review begins with a brief discussion of the history of ATPSs and their fundamentals, which is followed by an account chronicling the integration of microfluidic devices with ATPSs to generate w-in-w emulsion droplets. Furthermore, the stabilization strategies of w-in-w emulsion droplets and microfluidic fabrication of microparticles and microcapsules for modern applications, such as biomolecule encapsulation and spheroid construction, are discussed in detail in this review. We believe that the present review will provide useful information to not only new entrants in the microfluidic community wanting to appreciate the findings of the field but also existing researchers wanting to keep themselves updated on progress in the field.

Progress in all-aqueous droplets generation with microfluidics: Mechanisms of formation and stability improvements

All-aqueous systems have attracted intensive attention as a promising platform for applications in cell separation, protein partitioning, and DNA extraction, due to their selective separation capability, rapid mass transfer, and good biocompatibility. Reliable generation of all-aqueous droplets with accurate control over their size and size distribution is vital to meet the increasingly growing demands in emulsion-based applications. However, the ultra-low interfacial tension and large effective interfacial thickness of the water-water interface pose challenges for the generation and stabilization of uniform all-aqueous droplets, respectively. Microfluidics technology has emerged as a versatile platform for the precision generation of all-aqueous droplets with improved stability. This review aims to systematize the controllable generation of all-aqueous droplets and summarize various strategies to improve their stability with microfluidics. We first provide a comprehensive review on the recent progress of all-aqueous droplets generation with microfluidics by detailing the properties of all-aqueous systems, mechanisms of droplet formation, active and passive methods for droplet generation, and the property of droplets. We then review the various strategies used to improve the stability of all-aqueous droplets and discuss the fabrication of biomaterials using all-aqueous droplets as liquid templates. We envision that this review will benefit the future development of all-aqueous droplet generation and its applications in developing biomaterials, which will be useful for researchers working in the field of all-aqueous systems and those who are new and interested in the field.

Pickering emulsion droplet-based biomimetic microreactors for continuous flow cascade reactions

A continuous flow cascade of multi-step catalytic reactions is a cutting-edge concept to revolutionize stepwise catalytic synthesis yet is still challenging in practical applications. Herein, a method for practical one-pot cascade catalysis is developed by combining Pickering emulsions with continuous flow. Our method involves co-localization of different catalytically active sub-compartments within droplets of a Pickering emulsion yielding cell-like microreactors, which can be packed in a column reactor for continuous flow cascade catalysis. As exemplified by two chemo-enzymatic cascade reactions for the synthesis of chiral cyanohydrins and chiral ester, 5 − 420 fold enhancement in the catalysis efficiency and as high as 99% enantioselectivity were obtained even over a period of 80 − 240 h. The compartmentalization effect and enriching-reactant properties arising from the biomimetic microreactor are theoretically and experimentally identified as the key factors for boosting the catalysis efficiency and for regulating the kinetics of cascade catalysis.

A continuous flow cascade of multi-step catalytic reactions would provide significant advantages in faster reaction times, waste reduction, and lowered step-count of syntheses, yet this ideal remains challenging in practical applications. Here the authors describe continuous flow cascade catalysis through co-localization of two catalytically active subcompartments within Pickering emulsion droplets.

Higher-order structure of DNA determines its positioning in cell-size droplets under crowded conditions

Background

It is becoming clearer that living cells use water/water (w/w) phase separation to form membraneless organelles that exhibit various important biological functions. Currently, it is believed that the specific localization of biomacromolecules, including DNA, RNA and proteins in w/w microdroplets is closely related to their bio-activity. Despite the importance of this possible role of micro segregation, our understanding of the underlying physico-chemical mechanism is still unrefined. Further research to unveil the underlying mechanism of the localization of macromolecules in relation to their steric conformation in w/w microdroplets is needed.

Principal findings

Single-DNA observation of genome-size DNA (T4 GT7 bacteriophage DNA; 166kbp) by fluorescence microscopy revealed that DNAs are spontaneously incorporated into w/w microdroplets generated in a binary aqueous polymer solution with polyethylene glycol (PEG) and dextran (DEX). Interestingly, DNAs with elongated coil and shrunken conformations exhibit Brownian fluctuation inside the droplet. On the other hand, tightly packed compact globules, as well as assemblies of multiple condensed DNAs, tend to be located near the interface in the droplet.

Conclusion and significance

The specific localization of DNA molecules depending on their higher-order structure occurs in w/w microdroplet phase-separation solution under a binary aqueous polymer solution. Such an aqueous solution with polymers mimics the crowded conditions in living cells, where aqueous macromolecules exist at a level of 30–40 weight %. The specific positioning of DNA depending on its higher-order structure in w/w microdroplets is expected to provide novel insights into the mechanism and function of membraneless organelles and micro-segregated particles in living cells.

Membranization of Coacervates into Artificial Phagocytes with Predation toward Bacteria

Coacervate-based membraneless organelles with diverse functionality as well as the capability of mimicking intracellular physiological environments are attracting researchers' great interest. However, the further studies focusing on functionalized membranization of coacervate as a step toward an advanced membrane-bound protocell are still rare. In this study, we develop a way to compartmentalize coacervate based on reconstitution with a natural cellular wall, which could then serve as a promising chassis for the development of protocells with selective sequestration of various biomacromolecules. Significantly, the compartmentalized protocell could behave like a phagocyte and selectively capture, engulf, and then kill Escherichia coli efficiently. Taken together, our studies present a strategy for advancing coacervate-based protocell design as well as the development of smart materials with on-demand functionalization.

A new method to prepare microparticles based on an Aqueous Two-Phase system (ATPS), without organic solvents

HYPOTHESIS

Aqueous Two-Phase Systems (ATPS) are aqueous droplets dispersed in an aqueous phase. This specific behavior arises from interactions between at least two water-soluble entities, such as thermodynamically incompatible polymers. A simple, fast, and "green" process to produce ATPS with an aqueous core would be of high interest to the pharmaceutical field for drug delivery. However, to date, rapid destabilization of ATPS represents the main hurdle for their use. Herein we present a novel process to achieve a stabilized microparticle-ATPS, without the use of organic solvents.

EXPERIMENTS

ATPS composed of dextran and polyethylene oxide were prepared. A Pickering-like emulsion technique was used to stabilize the ATPS by adsorbing semi-solid particles (chitosan-grafted lipid nanocapsules) at the interface between the two aqueous phases. Finally, microparticles were formed by a polyelectrolyte complexation and gelation. The structure and stability of ATPS were characterized using microscopy and Turbiscan analysis.

FINDINGS

Adding chitosan-grafted lipid nanocapsules induced ATPS stabilization. Adding a polyelectrolyte such as sodium alginate allowed the formation of microparticles with a gelled shell that strengthened the formulation against shear stress and improved long-term stability, thus demonstrating that is possible to use ATPS to form delivery systems to encapsulate hydrophilic molecules.

Liquid/liquid interface in periodic boundary condition

We study how surface phenomena can change the interface geometry in liquid-liquid two-phase systems with periodic boundary conditions. Without any curvature effect on surface tension, planar (slab), cylindrical, and spherical structures are successively obtained as a function of the total composition and elongation of the box, in accordance with molecular dynamics simulations for a water/heptane system. The curvature effects described by Tolman relationship desymmetrize the phase diagram by stabilizing a concavity but it leads to inconsistencies with high curvature. Helfrich model partially resolves this and predicts the possible presence of shells reflecting a frustrated system.

Photosynthetic hydrogen production by droplet-based microbial micro-reactors under aerobic conditions

The spontaneous self-assembly of multicellular ensembles into living materials with synergistic structure and function remains a considerable challenge in biotechnology and synthetic biology. Here, we exploit the aqueous two-phase separation of dextran-in-PEG emulsion micro-droplets for the capture, spatial organization and immobilization of algal cells or algal/bacterial cell communities to produce discrete multicellular spheroids capable of both aerobic (oxygen producing) and hypoxic (hydrogen producing) photosynthesis in daylight under air. We show that localized oxygen depletion results in hydrogen production from the core of the algal microscale reactor, and demonstrate that enhanced levels of hydrogen evolution can be achieved synergistically by spontaneously enclosing the photosynthetic cells within a shell of bacterial cells undergoing aerobic respiration. Our results highlight a promising droplet-based environmentally benign approach to dispersible photosynthetic microbial micro-reactors comprising segregated cellular micro-niches with dual functionality, and provide a step towards photobiological hydrogen production under aerobic conditions.

The development of techniques capable of orchestrating the assembly of living cells into multicellular ensembles with synergistic and function is challenge. Here, the authors construct algal or algal/bacterial cells-based core shell-like structure based on aqueous two-phase system for synergic photosynthetic H₂ production.

Magnetic polyelectrolyte microcapsules via water-in-water droplet microfluidics

Polyelectrolyte microcapsules (PEMCs) have biocompatible microcompartments. Therefore, PEMCs are useful for applications in cosmetics, food, pharmaceutics, and other industries. The fabrication of PEMCs often involves the use of harsh chemicals or cytotoxic organic phases that make biomedical applications of the microcapsules challenging. In this report, we present an all-aqueous droplet microfluidics platform for the generation of magnetic PEMCs. In the platform, we use an aqueous-two-phase system (ATPS) of polyethylene glycol (PEG) and dextran (Dex), to generate water-in-water droplets, which are magnetically functionalized with ferrofluid. Strong polyelectrolytes (PEs) with opposite charges are used in each ATPS phase. We make emulsion templates of magnetic Dex, containing the polycations, in a continuous phase of PEG. We then apply a magnetic field to move the magnetic droplets to a second PEG phase, which contains the polyanions. By careful tuning of the fluxes of the two PEs in their respective phases, we trigger the formation of a shell at the droplet interface. Owing to the presence of the ferrofluid, the resulting microcapsules are magnetically responsive. We show that the magnetic PEMCs are capable of passive release of large pseudo-drugs as well as triggered release using external stimuli such as osmotic shock and pH change. We expect that magnetic PEMCs from this biocompatible all-aqueous platform will find utility in the fabrication of functionalized drug carriers for targeted drug delivery.

Bioinspired Protein-Based Assembling: Toward Advanced Life-Like Behaviors

The ability of living organisms to perform structure, energy, and information-related processes for molecular self-assembly through compartmentalization and chemical transformation can possibly be mimicked via artificial cell models. Recent progress in the development of various types of functional microcompartmentalized ensembles that can imitate rudimentary aspects of living cells has refocused attention on the important question of how inanimate systems can transition into living matter. Hence, herein, the most recent advances in the construction of protein-bounded microcompartments (proteinosomes), which have been exploited as a versatile synthetic chassis for integrating a wide range of functional components and biochemical machineries, are critically summarized. The techniques developed for fabricating various types of proteinosomes are discussed, focusing on the significance of how chemical information, substance transportation, enzymatic-reaction-based metabolism, and self-organization can be integrated and recursively exploited in constructed ensembles. Therefore, proteinosomes capable of exhibiting gene-directed protein synthesis, modulated membrane permeability, spatially confined membrane-gated catalytic reaction, internalized cytoskeletal-like matrix assembly, on-demand compartmentalization, and predatory-like chemical communication in artificial cell communities are specially highlighted. These developments are expected to bridge the gap between materials science and life science, and offer a theoretical foundation for developing life-inspired assembled materials toward various applications.

Cell-Inspired All-Aqueous Microfluidics: From Intracellular Liquid-Liquid Phase Separation toward Advanced Biomaterials

Living cells have evolved over billions of years to develop structural and functional complexity with numerous intracellular compartments that are formed due to liquid-liquid phase separation (LLPS). Discovery of the amazing and vital roles of cells in life has sparked tremendous efforts to investigate and replicate the intracellular LLPS. Among them, all-aqueous emulsions are a minimalistic liquid model that recapitulates the structural and functional features of membraneless organelles and protocells. Here, an emerging all-aqueous microfluidic technology derived from micrometer-scaled manipulation of LLPS is presented; the technology enables the state-of-art design of advanced biomaterials with exquisite structural proficiency and diversified biological functions. Moreover, a variety of emerging biomedical applications, including encapsulation and delivery of bioactive gradients, fabrication of artificial membraneless organelles, as well as printing and assembly of predesigned cell patterns and living tissues, are inspired by their cellular counterparts. Finally, the challenges and perspectives for further advancing the cell-inspired all-aqueous microfluidics toward a more powerful and versatile platform are discussed, particularly regarding new opportunities in multidisciplinary fundamental research and biomedical applications.

Emerging aqueous two-phase systems: from fundamentals of interfaces to biomedical applications

This review summarizes recent advances of aqueous two-phase systems (ATPSs), particularly their interfaces, with a focus on biomedical applications.

Spatial Organization in Proteinaceous Membrane-Stabilized Coacervate Protocells

As a model protocell, the membrane-free coacervate microdroplet is widely utilized in functional studies to provide insights into the physicochemical properties of the cell and to engineer cytomimetic soft technologies; however, the lack of a discrete membrane contributes to its instability and limits further application. Herein, a strategy is developed to fabricate a hybrid protocell based on the self-assembly of a proteinaceous membrane at the surface of coacervate microdroplets driven by a combination of electrostatic adhesion and steric/hydrophilic surface buoyancy. The semipermeable proteinaceous membrane can enhance coacervate stability obviously without compromising sequestration behavior. Significantly, such hybrid protocells demonstrate spatial organization whereby various functional enzymes can be located in discrete regions, which facilitates an on/off modulation for a cascade enzymatic reaction along with enhanced chemical communication between subpopulations.

Development of dual-component protein microparticles in all-aqueous systems for biomedical applications

Protein microparticles assisted by an emulsion droplet template have shown great promise in drug/cell delivery and tissue engineering, as well as diagnosis and treatment of diseases.

Microfluidic fabrication of microparticles for biomedical applications

Droplet microfluidics offers exquisite control over the flows of multiple fluids in microscale, enabling fabrication of advanced microparticles with precisely tunable structures and compositions in a high throughput manner. The combination of these remarkable features with proper materials and fabrication methods has enabled high efficiency, direct encapsulation of actives in microparticles whose features and functionalities can be well controlled. These microparticles have great potential in a wide range of bio-related applications including drug delivery, cell-laden matrices, biosensors and even as artificial cells. In this review, we briefly summarize the materials, fabrication methods, and microparticle structures produced with droplet microfluidics. We also provide a comprehensive overview of their recent uses in biomedical applications. Finally, we discuss the existing challenges and perspectives to promote the future development of these engineered microparticles.

Colloidal Particle Adsorption at Water-Water Interfaces with Ultralow Interfacial Tension

Using fluorescence confocal microscopy we study the adsorption of single latex microparticles at a water-water interface between demixing aqueous solutions of polymers, generally known as a water-in-water emulsion. Similar microparticles at the interface between molecular liquids have exhibited an extremely slow relaxation preventing the observation of expected equilibrium states. This phenomenon has been attributed to "long-lived" metastable states caused by significant energy barriers ΔF∼γA_{d}≫k_{B}T induced by high interfacial tension (γ∼10^{-2}  N/m) and nanoscale surface defects with characteristic areas A_{d}≃10-30  nm^{2}. For the studied water-water interface with ultralow surface tension (γ∼10^{-4}  N/m) we are able to characterize the entire adsorption process and observe equilibrium states prescribed by a single equilibrium contact angle independent of the particle size. Notably, we observe crossovers from fast initial dynamics to slower kinetic regimes analytically predicted for large surface defects (A_{d}≃500  nm^{2}). Moreover, particle trajectories reveal a position-independent damping coefficient that is unexpected given the large viscosity contrast between phases. These observations are attributed to the remarkably diffuse nature of the water-water interface and the adsorption and entanglement of polymer chains in the semidilute solutions. This work offers some first insights on the adsorption dynamics or kinetics of microparticles at water-water interfaces in biocolloidal systems.