Bicontinuous Interfacially Jammed Emulsion Gels (bijels) as Media for Enabling Enzymatic Reactive Separation of a Highly Water Insoluble Substrate

Bicontinuous Interfacially Jammed Emulsion Gels (Bijels): Preparation, Control Strategies, and Derived Porous Materials

Bicontinuous interfacially jammed emulsion gels, also known as Bijels, are a new type of soft condensed matter. Over the last decade, Bijels have attracted considerable attention because of their unique morphology, property, and broad application prospects. In the present review, we summarize the preparation methods and main control strategies of Bijels, focusing on the research progress and application of Bijels as templates for porous materials preparation in recent years. The potential future directions and applications of Bijels are also envisaged.

Super-Macroporous Lightweight Materials Templated from Bicontinuous Intra-Phase Jammed Emulsion Gels Based on Nanochitin

Non-equilibrium multiphase systems are formed by mixing two immiscible nanoparticle dispersions, leading to bicontinuous emulsions that template cryogels with interconnected, tortuous channels. Herein, a renewable, rod-like biocolloid (chitin nanocrystals, ChNC) is used to kinetically arrest bicontinuous morphologies. Specifically, it is found that ChNC stabilizes intra-phase jammed bicontinuous systems at an ultra-low particle concentration (as low as 0.6 wt.%), leading to tailorable morphologies. The synergistic effects of ChNC high aspect ratio, intrinsic stiffness, and interparticle interactions produce hydrogelation and, upon drying, lead to open channels bearing dual characteristic sizes, suitably integrated into robust bicontinuous ultra-lightweight solids. Overall, it demonstrates the successful formation of ChNC-jammed bicontinuous emulsions and a facile emulsion templating route to synthesize chitin cryogels that form unique super-macroporous networks.

Bijel rheology reveals a 2D colloidal glass wrapped in 3D

We present rheological evidence demonstrating the glass-like nature of bicontinuous interfacially jammed emulsion gels (bijels). Under small amplitude oscillatory shear, bijels exhibited rheological signatures akin to α and β relaxation that are also invariable to interfacial tension changes, behaviors which are reminiscent of caged particle dynamics found in colloidal glasses, and well described by a previously reported adaptation of mode-coupling theory for colloidal glass rheology. Guided by their rheological signatures and supported by particle detachment and attraction energy approximations, we rationalize that bijels can be represented as 2-dimensional (2D) colloidal glasses that percolate in 3-dimensional (3D) space, and attractive interactions are not required for their stability. To provide further support for this conjecture, we qualitatively compare the rheology of bijels and a capillary suspension that is stabilized by strong, rigid capillary bridges between the particles, beyond their limit of linear viscoelasticity. Our results demonstrate that the strong adsorption of particles to the continuous interface and the lack of strong attractive interparticle forces enable recovery by interfacial tension into new jammed configurations after shear deformation. These behaviors are qualitatively different from those in the capillary suspension, where the breaking of attractive interparticle bonds results in dramatic changes to the microstructure and rheology over a narrow range of shear amplitudes. Our findings unveil bijels as 2D colloidal glasses weaving in 3D space and establish that interparticle attractions are not required for stability in bijels, and interfacial jamming alone is sufficient to impart viscoelasticity and gel-like rheology to these materials.

Multicompartment Hydrogels

Hydrogels belong to the most promising materials in polymer and materials science at the moment. As they feature soft and tissue-like character as well as high water-content, a broad range of applications are addressed with hydrogels, e.g. tissue engineering and wound dressings but also soft robotics, drug delivery, actuators and catalysis. Ways to tailor hydrogel properties are crosslinking mechanism, hydrogel shape and reinforcement, but new features can be introduced by variation of hydrogel composition as well, e.g. via monomer choice, functionalization or compartmentalization. Especially, multicompartment hydrogels drive progress towards complex and highly functional soft materials. In the present review the latest developments in multicompartment hydrogels are highlighted with a focus on three types of compartments, i.e. micellar/vesicular, droplets or multi-layers including various sub-categories. Furthermore, several morphologies of compartmentalized hydrogels and applications of multicompartment hydrogels will be discussed as well. Finally, an outlook towards future developments of the field will be given. The further development of multicompartment hydrogels is highly relevant for a broad range of applications and will have a significant impact on biomedicine and organic devices. This article is protected by copyright. All rights reserved.

Multiphase Microreactors Based on Liquid-Liquid and Gas-Liquid Dispersions Stabilized by Colloidal Catalytic Particles

Pickering emulsions, foams, bubbles, and marbles are dispersions of two immiscible liquids or of a liquid and a gas stabilized by surface‐active colloidal particles. These systems can be used for engineering liquid–liquid–solid and gas–liquid–solid microreactors for multiphase reactions. They constitute original platforms for reengineering multiphase reactors towards a higher degree of sustainability. This Review provides a systematic overview on the recent progress of liquid–liquid and gas–liquid dispersions stabilized by solid particles as microreactors for engineering eco‐efficient reactions, with emphasis on biobased reagents. Physicochemical driving parameters, challenges, and strategies to (de)stabilize dispersions for product recovery/catalyst recycling are discussed. Advanced concepts such as cascade and continuous flow reactions, compartmentalization of incompatible reagents, and multiscale computational methods for accelerating particle discovery are also addressed.

Phase Inversion-Based Technique for Fabricating Bijels and Bijels-Derived Structures with Tunable Microstructures

Bicontinuous interfacially jammed emulsion gels ("bijels") are a new class of soft matter containing two interpenetrating continuous phases. They have great potential for applications in many areas. However, difficulties in fabricating bijels and controlling structural features of interest have posed severe barriers to their wide applications. In this study, a phase inversion-based technique was developed for fabricating bijels and bijels-derived structures. The effects of varying the composition of casting solutions for the fabrication of bijels on the porosity, oil-to-water percentage, and domain size of bijels were investigated. Composite bijels prepared from two organic monomers were also made, demonstrating the flexibility of the phase inversion-based technique for the fabrication of bijels. Interestingly, the incorporation of a second monomer into the casting solution also affected the porosity and domain size of bijels formed, which may provide a new strategy for the controlled fabrication of bijels. Doxorubicin hydrochloride (DOX, as a model drug)-loaded bijels-derived hybrid hydrogels comprising two continuous phases were successfully made, with one phase being cross-linked alginate that carried the drug. Controlled release of DOX from the bijels-derived structures could be achieved. In vitro degradation study indicated that cross-linking of alginate in bijels-derived hybrid hydrogels controlled alginate degradation, thereby affecting the DOX release behavior. Our current work has provided a facile and reproducible protocol for the controlled fabrication of bijels and bijels-derived structures, which facilitates expanding their applications in the biomedical field.

Fabrication of emulsion gel based on polymer sanxan and its potential as a sustained-release delivery system for β-carotene

Food-grade emulsion gels have attracted increasing attention in food and drug manufacturing, owing to their potential as novel delivery systems for lipophilic bioactive ingredients. Emulsion gels are structurally either a polymeric gel matrix with incorporated emulsion droplets (emulsion-filled gels), or a network of aggregated emulsion droplets (emulsion particulate gels). In this study, a novel emulsion gel was prepared by formulating an oil-in-water (O/W) emulsion stabilized by sanxan alone, followed by heating and cooling treatment, resulting in a structured solid system. Stable O/W type sanxan emulsion gels (SEGs) were obtained at sanxan concentration >0.5% (w/w). Fluorescence microscopy results confirmed the adsorption of sanxan on oil droplet surfaces. The effect of temperature and sanxan/oil concentrations on the rheological and textural properties of the SEGs was evaluated: the SEG containing 1% (w/w) sanxan and 20% (w/w) sunflower oil exhibited excellent rheological and textural properties. Further, the addition of 10 mM Na⁺ or 5 mM Ca²⁺ greatly enhanced the thermostability of the SEG. The potential of SEGs as sustained-release delivery systems for β-carotene was also explored. The findings are of great interest for the development of novel delivery systems based on emulsion gels stabilized by sanxan for the sustained release of lipophilic components.

Fabrication of solvent transfer-induced phase separation bijels with mixtures of hydrophilic and hydrophobic nanoparticles

Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are co-continuous throughout the structure, have potential for applications in separation, catalysis, tissue engineering and energy devices. Among the possible fabrication paths, the solvent transfer-induced phase separation (STRIPS) method has proven to be a powerful approach to produce bijels in a continuous fashion with a broad selection of liquids and nanoparticles. The successful formation of bicontinuous domains requires the use of neutrally wetting particles which was achieved by in situ modification of silica nanoparticles with an oppositely charged surfactant. This approach, however, is not ideal for applications that are adversely affected by the presence of surfactant. In this work, we use a pair of nanoparticles, one hydrophilic, and the other hydrophobic, to stabilize STRIPS bijels without any surfactants and show that the ratio of the hydrophilic to hydrophobic nanoparticles required to form stable bijels changes with the polarity of the oil phase. Highly non-polar oils require a smaller ratio than moderately polar oils. Furthermore, if a sufficiently polar oil is selected, STRIPS bijels can be stabilized using only the hydrophilic nanoparticle. Our results demonstrate the potential to imbue the interface of biphasic liquid mixtures such as bijels with multifunctionality by using two functional nanoparticles of opposite polarity.

Controlling Surfactant Adsorption on Highly Charged Nanoparticles to Stabilize Bijels

Bicontinuous particle-stabilized emulsions (bijels) are networks of interpenetrating oil/water channels with applications in catalysis, tissue engineering, and energy storage. Bijels can be generated by arresting solvent transfer induced phase separation (STrIPS) via interfacial jamming of nanoparticles. However, until now, STrIPS bijels have only been formed with silica nanoparticles of low surface charge densities, limiting their potential applications in catalysis and fluid transport. Here, we show how strongly charged silica nanoparticles can stabilize bijels. To this end, we carry out a systematic study employing dynamic light scattering, zeta potential, acid/base titrations, turbidimetry, surface tension, and confocal microscopy. We find that moderating the adsorption of oppositely charged surfactants on the particles is crucial to facilitate particle dispersibility in the bijel casting mixture and bijel stabilization. Our results potentially introduce a general understanding for bijel fabrication with different inorganic nanoparticle materials of variable charge density.