Thermo-induced inversion of water-in-water emulsion stability by bis-hydrophilic microgels

Formation of water-in-water emulsions and microgels in nonionic surfactant + gelatin aqueous mixtures

HYPOTHESIS

Water-in-water (W/W) emulsions can be obtained when two water-soluble components are mutually immiscible. The scientific literature on W/W emulsions focuses on polymer-polymer mixtures, with only a few reports on polymer-salt systems, and no documented cases involving polymer-surfactant mixtures. Our hypothesis was that by lowering the cloud temperature of a surfactant through the addition of a polymer, phase segregation into two immiscible aqueous solutions could enable the formation of W/W emulsions.

EXPERIMENTS

The system composed of an ethoxylated triglyceride surfactant (Kolliphor ELP) and gelatin was selected. The effect of gelatin on the surfactant's cloud temperature was assessed. Water-in-water (W/W) emulsions were prepared by stirring biphasic mixtures at temperatures above gelatin's gelation point. Gelatin microgels were formed by cooling down gelatin-in-Kolliphor W/W emulsions. Mucin particles were incorporated to improve the colloidal stability of these emulsions and genipin, a natural reagent, was added to produce crosslinked microgels.

FINDINGS

The addition of gelatin lowered the cloud temperature of the surfactant, resulting in the formation of two aqueous phases: a surfactant-rich solution and a gelatin-rich solution, which enabled the preparation of W/W emulsions. Chemically crosslinked microgels were successfully obtained by cooling down gelatin-in-surfactant emulsions, crosslinking with genipin and stabilized with mucin microparticles. To the best of our knowledge, this is the first report of W/W emulsions based on polymer-surfactant mixtures, unlike most W/W emulsions reported in the literature, which rely on immiscibility between two polymers in aqueous solutions.

Structure and stabilization of water-in-water emulsions in the presence of two types of microgels

HYPOTHESIS

Water-in-water emulsions can be stabilized by colloidal particles that spontaneously adsorb at the interface. Different types of particles have been shown to exhibited different impact on the microstructure and stability. The combination of two different types of particles is expected to show a synergistic effect on the emulsion stability.

EXPERIMENTS

Synthetic bis-hydrophilic microgels (BIS) and protein microgels (PRO) were studied in emulsions formed by mixing polyethylene oxide (PEO) and Dextran (DEX) as well as in the individual phases. The arrangement of particles at the interface was monitored using confocal laser scanning microscopy and the effect on the microstructure and stability of the emulsions was evaluated during aging.

FINDINGS

The adsorption and arrangement of particles at the interface depended mainly on their affinity for each phase. Formation of a mixed layer significantly increased the stability of the emulsions compared to emulsions with the individual microgels. BIS and PRO co-aggregated in the PEO phase, which affects their arrangement at the interface and the emulsion stability. In some cases, added BIS spontaneously fully replaced PRO that was adsorbed at the interface.

Processable and controllable all-aqueous gels based on high internal phase water-in-water emulsions

Aqueous two-phase systems (ATPSs) have primarily been developed in the form of emulsions to enhance their utilization in green and biocompatible applications. However, numerous challenges have arisen in forming stable and processable water-in-water (W/W) emulsion systems, as well as in fine-tuning the interconnectivity of their internal structure, which can significantly impact their performance. To effectively address these challenges, we elucidate, for the first time, the root cause of the poor stability of W/W emulsions. Leveraging this insight, we successfully stabilize W/W high internal phase emulsions (W/W HIPEs) characterized by an extremely thin continuous phase. This stabilization enables the fine-tuning of interconnectivity between dispersed droplets through photopolymerization of thin continuous phases, resulting in the fabrication of stable and processable all-aqueous gels. This W/W HIPE-based gel fabrication holds promise as a universal technology for a wide range of applications. It facilitates in situ polymerization of the continuous phase of W/W HIPEs, where target molecules are stored in the dispersed phase. Moreover, this method allows easy adjustment of the external release rate or internal transfer rate of target molecules by adjusting the interconnectivity of the internal structures.

Preparation and characterization of PEG-Dex macromolecular schiff base particles and their application on the stabilization of water-in-water emulsion

In this study, polyethylene glycol (PEG) and dextran (Dex) were chemically modified to obtain amino-functionalized PEG (PEG-(NH2)2) and oxidized dextran (ODex). They were subsequently reacted via -NH2 and -CHO groups to synthesize a macromolecular Schiff base particle. The structures, morphologies, and thermal properties of the macromolecular Schiff base particle were characterized using Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and thermogravimetry analysis (TGA). The macromolecular Schiff base particle was then employed as a stabilizer to stabilize Dex/PEG water-in-water (W/W) emulsions, and the effects of stabilizer composition, concentration, and dextran oxidation degree on emulsion phase separation and microstructure were investigated. The results from the laser particle size analyzer indicate that the macromolecular Schiff base stabilizers have particle sizes ranging from 100 to 200 nm and exhibit an interpenetrating network structure, as observed in SEM images. A decrease in emulsion droplet size was observed with increasing mass ratio of PEG-(NH2)2 to ODex, ODex oxidation degree, and stabilizer concentration. Rheological analysis showed that the viscosity of W/W emulsions decreased with increasing shear rate. Contact angle measurements indicated that the macromolecular Schiff base stabilizers preferentially interacted with the continuous phase of PEG, thereby promoting emulsion stability.

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.

Preparation and properties of hydrogel photonic crystals assembled by biodegradable nanogels

Thermally induced physical hydrogels formed through the sol-gel transition of nanogels usually lose structural color above phase transition temperature (Tp). Herein, temperature/pH/redox-responsive nanogels that undergo sol-gel transition still keep structural colors above the Tp have been synthesized and studied. N-isopropylacrylamide (NIPAm) was copolymerized with N-tert-butylacrylamide (TBA) and N-acrylamido-l-phenylalanine (Aphe) to form P(NIPAm/TBA/Aphe) nanogel crosslinked with N,N'-bis(acryloyl)cystine (BISS) (referred to as PNTA-BISS). PNTA-BISS nanogel with a broad range of biodegradable crosslinker BISS content can achieve a reversible sol-gel transition above the Tp, surprisingly, while PNTA nanogels with a comparable content of biodegradable N,N'-Bis(acryloyl)cystam (BAC) crosslinker (referred to as PNTA-BAC) didn't form sol-gel transition. Although BISS and BAC possess same disulfide bonds with redox properties, BISS, unlike BAC, is water-soluble and features two carboxyl groups. The mechanism by which PNTA-BISS nanogels form hydrogel photonic crystals has been deeply explored with temperature-variable NMR. The results showed the introduction of Aphe with both steric hindrance and carboxyl groups greatly slowed down the shrinkage of PNTA-BISS nanogels. Therefore, PNTA-BISS nanogels can form sol-gel transition and further structural color of hydrogel photonic crystals due to carboxyl groups above the Tp. Furthermore, the properties of biodegradable hydrogel photonic crystals above the Tp were investigated for the first time, attributed to the presence of the strong reducing agent 1,4-dithiothreitol (DTT). When loaded with doxorubicin (DOX), PNTA-BISS exhibited favorable degradation properties under the influence of DTT. In summary, the PNTA-BISS nanogel, in addition to its in-situ gelation capabilities, demonstrated degradability, potentially providing a novel nanoplatform for applications in drug delivery, biotechnology, and related fields.

Interaction between stabilized droplets of different phases in the same continuous phase of an aqueous three-phase system

Water-in-water (W/W) emulsions, also called aqueous two-phase systems, are formed by mixing two incompatible polymers in water that phase separate into two distinct phases. They can be stabilized by addition of colloidal particles. Droplets of the dispersed phase can be used to compartmentalize ingredients and induce localized reactions. By mixing more types of incompatible polymers, emulsions containing droplets of different phases can be formed that can potentially capture different ingredients. Here the interaction between dispersed droplets of different types was studied by gently mixing a W/W emulsion containing droplets rich in dextran (DEX) dispersed in a continuous phase rich in polyethylene oxide with an emulsion containing droplets rich in fish gelatin (GEL) dispersed in the same continuous medium. Bis-hydrophilic microgels (MG) consisting of DEX grafted with poly(N-isopropylacrylamide) were added and their effect on the stability of each binary emulsion was investigated. Interestingly, when two very stable emulsions were gently mixed, droplets of different types were observed with confocal scanning laser microscopy to coalesce immediately upon contact. In this manner, Janus-type droplets were formed containing a DEX and a GEL compartment with no MG at the GEL/DEX interface that further associated into strings of alternating droplets. Contact angles between the different phases in emulsions with and without MG were compared and used to determine the effect of the microgels on the interfacial tension between the phases.

Tuning the bis-hydrophilic balance of microgels: A tool to control the stability of water-in-water emulsions

HYPOTHESIS

The stability of purely aqueous emulsions (W/W) formed by mixing incompatible polymers, can be achieved through the Pickering effect of particles adsorption at the interface. However, there is, as yet, no guideline regarding the chemical nature of the particles to predict whether they will stabilize a particular W/W emulsion. Bis-hydrophilic soft microgels, made of copolymerized poly(N-isopropylacrylamide) (pNIPAM) and dextran (Dex), act as very efficient stabilizers for PEO/Dextran emulsions, because the two polymers have an affinity for each polymer phase.

EXPERIMENTS

The ratio between both components of the microgels is varied in order to modulate the bis-hydrophilic balance, the content of Dex compared to pNIPAM varying from 0 to 60 wt%. The partition between the two aqueous phases and the adsorption of microgels at the W/W interface is measured by confocal microscopy. The stability of emulsions is assessed via turbidity measurements and microstructural investigations under sedimentation or compression.

FINDINGS

The adsorption of particles and their partitioning is found to evolve progressively as a function of bis-hydrophilic balance. At room temperature, the stability of the resulting W/W emulsions also depends on the bis-hydrophilic balance with a maximum of stability for the particles containing 50%wt of Dex, for the Dex-in-PEO emulsions, while the PEO-in-Dex become stable above this value. The thermo-responsiveness of the microgels translates into stability inversion of the emulsions below 50 wt% of Dex in the microgels, whereas above 50 wt%, no emulsion is stable. This work paves the way of a guideline to design efficient and responsive W/W stabilizers.

The effect of the contact angle on particle stabilization and bridging in water-in-water emulsions

HYPOTHESIS

Water-in-water (W/W) emulsions formed by mixing incompatible polymers in aqueous solution can in some cases be stabilized by adding particles that adsorb spontaneously at the W/W interface. The importance of the contact angle of the particles with the interface on the stability of W/W emulsions is still an outstanding issue. We hypothesize that if the contact angle with the continuous phase is smaller than 90°, particles can bridge dispersed droplets, which enhances the stability of the emulsion.

EXPERIMENTS

The W/W emulsions consisted of a dispersed poly(ethylene oxide) (PEO) phase in a continuous dextran phase or vice versa. Gelatin microgels were added and their contact angle was varied by varying the pH. The morphology during aging was observed by microscopy.

FINDINGS

The contact angle of the microgels with the PEO phase varied between 110° close to neutral pH and 0° at pH 3 and pH 11. The W/W emulsions were stable only when the contact angle with the continuous phase was smaller than 90°. In this case, microgels could form bridges between dispersed droplets creating a network of droplets.

Effect of charge on the stabilization of water-in-water emulsions by thermosensitive bis-hydrophilic microgels

HYPOTHESIS

Molecular surfactants are not able to stabilize water-in-water (W/W) emulsions, unlike nano or micro-particles, which can achieve this in some cases. However, the effect of electrostatic interactions between particles on the emulsion stability has rarely been investigated. We hypothesize that introducing charges modifies the stabilization capacity of particles and renders it both pH- and ionic strength-dependent.

EXPERIMENTS

Charge was introduced into bis-hydrophilic and thermoresponsive dextran/polyN-isopropylacrylamide microgels by replacing a small fraction of polyN-isopropylacrylamide with acrylic acid groups. The size of the microgels was obtained by dynamic light scattering. The stability and microstructure of dextran/poly(ethyleneoxide)-based W/W emulsions, was studied as a function of pH, NaCl concentration and temperature using confocal microscopy and by analytical centrifugation.

FINDINGS

The swelling degree of charged microgels depends on the pH, ionic strength and the temperature. In the absence of salt, charged microgels do not adsorb at the interface and have little stabilizing effect even after neutralization. However, the interfacial coverage and the stability increase with rising concentration of NaCl. Saltinduced stabilization of these emulsions was also observed at 50 °C. Increasing the temperature strongly influences the emulsion stability at low pH.

Redox-responsive degradable microgel modified with superparamagnetic nanoparticles exhibiting controlled, hyperthermia-enhanced drug release

A novel degradable microgel based on poly(N-isopropylacrylamide) (pNIPA) cross-linked with N,N’-bisacryloylcystine (BISS) and containing superparamagnetic iron oxide nanoparticles (SPION@CA) was synthesized by semi-batch precipitation polymerization and examined as a potential hyperthermia-enhanced drug carrier. The pNIPA provided the microgel with temperature sensitivity, the BISS was responsible for degradation in the presence of glutathione (GSH) (an –S–S–bond reductor naturally present in cells), while the SPION@CA permitted remote control of temperature to improve drug release. The microgels exhibited volume phase transition temperature at ca. 34 °C, which is near the human body temperature, and were stable across a wide range of temperatures and ionic strengths, as well as in the blood plasma at 37 °C. It was found that the presence of SPION@CA in the polymer network of the microgels enabled the temperature to be increased up to 42 °C by an alternating magnetic field, and that increasing the temperature from 37 to 42 °C significantly enhanced the releasing of the anticancer drug doxorubicin (DOX). The highest DOX release (82%) was observed at pH 5, 42 °C, and in the presence of GSH, and the lowest (20%) at pH 7.4, 37 °C, and in the absence of GSH. MTT assay indicated that compared to free doxorubicin, the microgel particles loaded with doxorubicin have comparable cytotoxicity against MCF-7 cancer cells while being significantly less toxic to MCF-10A healthy cells.

Graphical abstract

Water-in-water emulsion stabilized by cellulose nanocrystals and their high enrichment effect on probiotic bacteria

HYPOTHESIS

The effect of the molecular weight and polymer concentration on the partition behavior of aqueous two-phase systems (ATPs) is significant for constructing water-in-water (W/W) emulsions. Hence, a long-term stable W/W emulsion system might be obtained through selecting the appropriate stabilizer and component phases, which could be a possible carrier for probiotics.

EXPERIMENTS

Compared with the reported molecular weight difference between polyethylene oxide (PEO) and dextran (DEX) systems, PEO and dextran with lower molecular weight had been used for constructing the water in water (W/W) emulsion system. The W/W emulsions were stabilized using cellulose nanocrystals (CNCs), and the potential application of the W/W emulsion for the encapsulation of Lactobacillus was explored.

FINDINGS

Emulsion stability exhibited a "dose-effect" relationship with the CNCs concentration and was decreased with the increase of the DEX concentration. The emulsion phase separation rate was increased with increasing ionic strength and temperature. Both Lactobacillus Plantarum and Lactobacillus helveticus were highly inclined to the DEX phase, and the emulsion droplets were deformed and aggregated when the encapsulation amount was increased. This long-term stability would provide a promising approach for designing high-density culture and fermentation of probiotics.

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.

pH sensitive water-in-water emulsions based on the pullulan and poly(N,N-dimethylacrylamide) aqueous two-phase system

A novel aqueous two-phase system based on pullulan and poly(N,N-dimethylacrylamide) is presented. Furthermore, it is used for the formation of pH sensitive water-in-water emulsions.