Spray-Assisted Formation of Micrometer-Sized Emulsions

Poly(N,N-diethylacrylamide)-endowed spontaneous emulsification during the breath figure process and the formation of membranes with hierarchical pores

The spontaneous emulsification for the formation of water-in-oil (W/O) or oil-in-water (O/W) emulsions needs the help of at least one kind of the third component (surfactant or cosolvent) to stabilize the oil-water interface. Herein, with the water/CS2-soluble polymer poly(N,N-diethylacrylamide) (PDEAM) as a surfactant, the spontaneous formation of water-in-PDEAM/CS2 emulsions is reported for the first time. The strong affinity between PDEAM and water or the increase of PDEAM concentration will accelerate the emulsification process with high dispersed phase content. It is demonstrated that the spontaneous emulsification of condensed water droplets into the PDEAM/CS2 solution occurs during the breath figure process, resulting in porous films with two levels of pore sizes (i.e., micron and submicron). The emulsification degree and the amounts of submicron-sized pores increase with PDEAM concentration and solidifying time of the solution. This work brings about incremental interest in spontaneous emulsification that may happen during the breath figure process. The combination of these two simultaneous processes provides us with an option to build hierarchically porous structures with condensed and emulsified water droplets as templates. Such porous membranes may have great potential in fields such as separation, cell culture, and biosensing.

Trifunctional Microgel-Mediated Preparation and Toughening of Printable High-Performance Chitosan Hydrogels for Underwater Communications

Natural and biocompatible chitosan has demonstrated wide applications. However, rapidly fabricating high-performance chitosan hydrogels in one-step controllable processes is still a challenge for some advanced applications. Here, we report a trifunctional microgel-mediated photochemical (TMMP) strategy to achieve the fabrication of printable tough chitosan-based hydrogels (PTCHs) in seconds. Such microgels help the slow release of persulfate anions and their uniform dispersion in an aqueous solution of cationic chitosan. The released persulfates are available for preparing multiple networks of phenolic coupling of modified chitosan and radical polymerization of Pluronic F127 via orthogonal tris(bipyridine)ruthenium(II)-based photochemistry, respectively. Trifunctional microgels have reversible Ca2+-crosslinked networks that further improve the hydrogels' mechanical properties and toughness. The maximum stress and toughness increase by >20 folds compared to the chitosan and F127 hydrogels with single network structures. Moreover, these microgels enable the precursor to have a good shearing-thinning property and benefit the controllable preparation of PTCHs in a short time, as low as ∼4 s under visible light irradiation. It, therefore, is compatible with standard printing techniques to make complex structures. Strain sensors based on structured PTCHs have stable mechanical and responsive properties in the water, which are applied for real-time underwater communications (<0.4 s). It is anticipated that this one-step TMMP strategy opens new horizons for designing advanced chitosan hydrogels.

Recent advances in the microfluidic production of functional microcapsules by multiple-emulsion templating

Multiple-emulsion drops serve as versatile templates to design functional microcapsules due to their core-shell geometry and multiple compartments. Microfluidics has been used for the elaborate production of multiple-emulsion drops with a controlled composition, order, and dimensions, elevating the value of multiple-emulsion templates. Moreover, recent advances in the microfluidic control of the emulsification and parallelization of drop-making junctions significantly enhance the production throughput for practical use. Metastable multiple-emulsion drops are converted into stable microcapsules through the solidification of selected phases, among which solid shells are designed to function in a programmed manner. Functional microcapsules are used for the storage and release of active materials as drug carriers. Beyond their conventional uses, microcapsules can serve as microcompartments responsible for transmembrane communication, which is promising for their application in advanced microreactors, artificial cells, and microsensors. Given that post-processing provides additional control over the composition and construction of multiple-emulsion drops, they are excellent confining geometries to study the self-assembly of colloids and liquid crystals and produce miniaturized photonic devices. This review article presents the recent progress and current state of the art in the microfluidic production of multiple-emulsion drops, functionalization of solid shells, and applications of microcapsules.