Partitioning-dependent conversion of polyelectrolyte assemblies in an aqueous two-phase system

Exploring the potential of all-aqueous immiscible systems for preparing complex biomaterials and cellular constructs

All-aqueous immiscible systems derived from liquid-liquid phase separation of incompatible hydrophilic agents such as polymers and salts have found increasing interest in the biomedical and tissue engineering fields in the last few years. The unique characteristics of aqueous interfaces, namely their low interfacial tension and elevated permeability, as well as the non-toxic environment and high water content of the immiscible phases, confer to these systems optimal qualities for the development of biomaterials such as hydrogels and soft membranes, as well as for the preparation of in vitro tissues derived from cellular assembly. Here, we overview the main properties of these systems and present a critical review of recent strategies that have been used for the development of biomaterials with increased levels of complexity using all-aqueous immiscible phases and interfaces, and their potential as cell-confining environments for micropatterning approaches and the bioengineering of cell-rich structures. Importantly, due to the relatively recent emergence of these areas, several key design considerations are presented, in order to guide researchers in the field. Finally, the main present challenges, future directions, and adaptability to develop advanced materials with increased biomimicry and new potential applications are briefly evaluated.

3D Printing of Aqueous Two-Phase Systems with Linear and Bottlebrush Polyelectrolytes

We formed core-shell-like polyelectrolyte complexes (PECs) from an anionic bottlebrush polymer with poly (acrylic acid) side chains with a cationic linear poly (allylamine hydrochloride). By varying the pH, the number of side chains of the polyanionic BB polymers (Nbb), the charge density of the polyelectrolytes, and the salt concentration, the phase separation behavior and salt resistance of the complexes could be tuned by the conformation of the BBs. By combining the linear/bottlebrush polyelectrolyte complexation with all-liquid 3D printing, flow-through tubular constructs were produced that showed selective transport across the PEC membrane comprising the walls of the tubules. These tubular constructs afford a new platform for flow-through delivery systems.

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.

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.

All-Aqueous Freeform Fabrication of Perfusable Self-Standing Soft Compartments

Compartmentalized structures obtained in all-aqueous settings have shown promising properties as cell encapsulation devices, as well as reactors for trans-membrane chemical reactions. While most approaches focus on the preparation of spherical devices, advances on the production of complex architectures have been enabled by the interfacial stability conferred by emulsion systems, namely mild aqueous two-phase systems (ATPS), or non-equilibrated analogues. However, the application of non-spherical structures has mostly been reported while keeping the fabricated materials at a stable interface, limiting the free-standing character, mobility and transposition of the obtained structures to different setups. Here, the fabrication of self-standing, malleable and perfusable tubular systems through all-aqueous interfacial assembly is shown, culminating in the preparation of independent objects with stability and homogeneity after disruption of the polymer-based aqueous separating system. Those hollow structures can be fabricated with a variety of widths, and rapidly printed as long structures at flow rates of 15 mm s^-1 . The materials are used as compartments for cell culture, showcasing high cytocompatibility, and can be tailored to promote cell adhesion. Such structures may find application in fields that benefit from freeform tubular structures, including the biomedical field with, for example, cell encapsulation, and benchtop preparation of microfluidic devices.

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.

Layer-by-Layer assembled nano-drug delivery systems for cancer treatment

Nano-drug delivery systems (NDDS) are functional drug-loaded nanocarriers widely applied in cancer therapy. Recently, layer-by-layer (LbL) assembled NDDS have been demonstrated as one of the most promising platforms in delivery of anticancer therapeutics. Here, a brief review of the LbL assembled NDDS for cancer treatment is presented. The fundamentals of the LbL assembled NDDS are first interpreted with an emphasis on the formation mechanisms. Afterwards, the tailored encapsulation of anticancer therapeutics in LbL assembled NDDS are summarized. The state-of-art targeted delivery of LbL assembled NDDS, with special attention to the elaborately control over the passive and active targeting delivery, are represented. Then the controlled release of LbL assembled NDDS with various stimulus responsiveness are systematically reviewed. Finally, conclusions and perspectives on further advancing the LbL assembled NDDS toward more powerful and versatile platforms for cancer therapy are discussed.

Controlled Formation of All-Aqueous Janus Droplets by Liquid-Liquid Phase Separation of an Aqueous Three-Phase System

Janus droplets have been demonstrated in a wide range of applications, ranging from drug delivery, to biomedical imaging, to bacterial detection. However, existing fabrication strategies often involve nonaqueous solvents, such as organic solvent or oil, which largely limits their use in fields that require a high degree of biocompatibility. Here, we present a method to achieve all-aqueous Janus droplets by liquid-liquid phase separation of an aqueous three-phase system (A3PS). An aqueous droplet containing two initially miscible polymers is first injected into an aqueous solution of another concentrated polymer, and then it spontaneously phase-separates into a Janus droplet due to the diffusive mass exchange between the drop and bulk phases during equilibration. To achieve continuous generation of the Janus droplets, the A3PS is further integrated with microfluidics and electrospray. The size and shape of the phase-separated Janus droplets can be easily controlled by tuning the operation parameters, such as the flow rate and/or the initial composition of the drop phases. Dumbbell-shaped and snowman-shaped Janus droplets with average sizes between 100 and 400 μm can be generated by both coflow microfluidics and electrospray. In particular, the phase-separated Janus droplets can simultaneously load two different liposomes into each compartment, which are promising carriers for combination drugs. The obtained Janus droplets are superior templates for biocompatible materials, which can serve as building blocks such as high-order droplet patterns for constructing advanced biomaterials.

Self-Assembled chitosan/phospholipid nanoparticles: from fundamentals to preparation for advanced drug delivery

With the development of nanotechnology, self-assembled chitosan/phospholipid nanoparticles (SACPNs) show great promise in a broad range of applications, including therapy, diagnosis, in suit imaging and on-demand drug delivery. Here, a brief review of the SACPNs is presented, and its critical underlying formation mechanisms are interpreted with an emphasis on the intrinsic physicochemical properties. The state-of-art preparation methods of SACPNs are summarized, with particular descriptions about the classic solvent injection method. Then SACPNs microstructures are characterized, revealing the unique spherical core-shell structure and the drug release mechanisms. Afterwards, a comprehensive and in-depth depiction of their emerging applications, with special attention to drug delivery areas, are categorized and reviewed. Finally, conclusions and outlooks on further advancing the SACPNs toward a more powerful and versatile platform for investigations covering from fundamental understanding to developing multi-functional drug delivery systems are discussed.

Oil-mediated high-throughput generation and sorting of water-in-water droplets

Aqueous two-phase system (ATPS) droplets have demonstrated superior compatibility over conventional water-in-oil droplets for various biological assays. However, the ultralow interfacial tension hampers efficient and stable droplet generation, limiting further development and more extensive use of such approaches. Here, we present a simple strategy to employ oil as a transient medium for ATPS droplet generation. Two methods based on passive flow focusing and active pico-injection are demonstrated to generate water-water-oil double emulsions, achieving a high generation frequency of ~2.4 kHz. Through evaporation of the oil to break the double emulsions, the aqueous core can be released to form uniform-sized water-in-water droplets. Moreover, this technique can be used to fabricate aqueous microgels, and the introduction of the oil medium enables integration of droplet sorting to produce single-cell-laden hydrogels with a harvest rate of over 90%. We believe that the demonstrated high-throughput generation and sorting of ATPS droplets represent an important tool to advance droplet-based tissue engineering and single-cell analyses.

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.

Flower-like droplets obtained by self-emulsification of a phase-separating (SEPS) aqueous film

Self-emulsification, referring to the spontaneous formation of droplets of one phase in another immiscible phase, is attracting growing interest because of its simplicity in creating droplets. Existing self-emulsification methods usually rely on phase inversion, temperature cycling, and solvent evaporation. However, achieving spatiotemporal control over the morphology of self-emulsified droplets remains challenging. In this work, a conceptually new approach of creating both simple and complex droplets by self-emulsification of a phase-separating (SEPS) aqueous film, is reported. The aqueous film is formed by depositing a surfactant-laden aqueous droplet onto an aqueous surface, and the fragmentation of the film into droplets is triggered by a wetting transition. Smaller and more uniform droplets can be achieved by introducing liquid-liquid phase separation (LLPS). Moreover, properly modulating quadruple LLPS and film fragmentation enables the creation of highly multicellular droplets such as flower-like droplets stabilized by the interfacial self-assembly of nanoparticles. This work provides a novel strategy to design aqueous droplets by LLPS, and it will inspire a wide range of applications such as membraneless organelle synthesis, cell mimics and delivery.

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.

Freeform, Reconfigurable Embedded Printing of All-Aqueous 3D Architectures

Aqueous microstructures are challenging to create, handle, and preserve since their surfaces tend to shrink into spherical shapes with minimum surface areas. The creation of freeform aqueous architectures will significantly advance the bioprinting of complex tissue-like constructs, such as arteries, urinary catheters, and tracheae. The generation of complex, freeform, three-dimensional (3D) all-liquid architectures using formulated aqueous two-phase systems (ATPSs) is demonstrated. These all-liquid microconstructs are formed by printing aqueous bioinks in an immiscible aqueous environment, which functions as a biocompatible support and pregel solution. By exploiting the hydrogen bonding interaction between polymers in ATPS, the printed aqueous-in-aqueous reconfigurable 3D architectures can be stabilized for weeks by the noncovalent membrane at the interface. Different cells can be separately combined with compartmentalized bioinks and matrices to obtain tailor-designed microconstructs with perfusable vascular networks. The freeform, reconfigurable embedded printing of all-liquid architectures by ATPSs offers unique opportunities and powerful tools since limitless formulations can be designed from among a breadth of natural and synthetic hydrophilic polymers to mimic tissues. This printing approach may be useful to engineer biomimetic, dynamic tissue-like constructs for potential applications in drug screening, in vitro tissue models, and regenerative medicine.

Superwettable colloidal crystal micropatterns on butterfly wing surface for ultrasensitive detection

HYPOTHESIS

Ultrasensitive detections with enrichment approaches based on hydrophilic-hydrophobic patterns have attracted increasing attention in the early diagnosis and treatment of diseases. However, most of these techniques involve complicated micro-fabrications and chemical modifications to achieve their specific pattern substrate wettability. Hence, the development of a simple and effective approach for the construction of new surface wettability techniques for ultrasensitive detection is with great expectations.

EXPERIMENTS

We present a simple approach to fabricate the superwettable colloidal crystal (CC) micropatterns on superhydrophobic Morpho butterfly wing surface for the ultrasensitive detection. The superwettable CC micropatterns were easily obtained by infiltrating and self-assembling monodispersed silica colloidal nanoparticles on the plasma treated butterfly wing patterns. The analytes could be enriched onto the hydrophilic CC area due to the wettability difference between the hydrophilic CC area and the superhydrophobic substrate.

FINDINGS

It was demonstrated that the detection limit of thrombin was down to 1.8 × 10^-13 mol L^-1 based on the fluorophore-labeled aptamer. Moreover, with two-dimensional position codes of these CC micropatterns for different probes, the multiplex detection capability was also demonstrated with great accuracy. As the elimination of complex instruments and chemical modifications, this proposed platform offers a simple strategy for ultrasensitive multiplex detection in practical applications.

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.