Designing Semipermeable Hydrogel Shells with Controlled Thickness through Internal Osmosis in Triple‐Emulsion Droplets

Abstract

Hydrogel shells that compartmentalize the water core from the aqueous surrounding provide molecular selectivity on size and charge in transmembrane transport. It is highly demanding to produce thin hydrogel shells to minimize diffusion length and maximize core volume. Here, internal osmosis in water-in-oil-in-water-in-oil (W/O/W/O) triple-emulsion droplets is used to produce thin hydrogel shells enclosing a large water core. The triple-emulsion droplets are prepared to have an ultrathin middle oil layer using a capillary microfluidic device. The innermost water droplet has a higher osmolarity than the outer water layer containing photopolymerizable hydrogel precursors, which pumps water from the outer layer to the core through the ultrathin oil layer by the osmosis. Therefore, the outer layer gets thinner and hydrogel precursors are enriched while the size of the triple-emulsion droplets remains unchanged. Through photopolymerization of precursors and phase transfer from oil to water, hydrogel shells enclosing water core are produced in the water environment; the oil layer is ruptured for molecular exchange through the shells. The thickness and composition of the hydrogel shells are precisely controllable by the osmotic conditions. The shells show a high permeation rate due to the thinness as well as controlled cut-off threshold of permeation for neutral and charged molecules.

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

“…Given that PEGDA is soluble in water for molecular weights larger than approximately 400 g mol −1 and the crosslinking density is controllable with the fraction of PEGDA in the aqueous solution, it was introduced in the water shell of O/W/O double-emulsion drops or W/O/ W/O triple-emulsion drops. [57][58][59] In the presence of a photoinitiator, PEGDA is crosslinked by ultraviolet irradiation, forming a hydrogel shell. PEGDA with a molecular weight of 250 g mol −1 is poorly soluble in both water and hydrocarbon oils, and thus it was used to make shells from W/O/W and O/O/W double-emulsion drops to encapsulate hydrophilic and hydrophobic cargoes, respectively.…”

Soft and Tough Microcapsules with Double‐Network Hydrogel Shells

“…[72][73][74] The hydrogel of pPEGDA at the concentration of 30 w/w% has a cut-off threshold of dextran permeation at the molecular weight of ≈20 000 g mol −1 . [57,75] Therefore, it is expected that sodium-alginate chains with Mw 120 000-190 000 g mol −1 are immobilized by the homogeneous network of pPEGDA at the expanded coil state. As the water shells are transformed into solid shells, the droplets can be transferred from the oil to the water.…”

“…The glass capillary microfluidic device was prepared by assembling tapered cylindrical capillaries in a square capillary by following the previous report. [29,57] One of the tapered capillaries had a 120 µm orifice, whose inner wall was treated to be hydrophobic and the outer wall was hydrophilic. Another capillary had a 180 µm orifice, whose whole walls were hydrophobic.…”

“…The triple-emulsion droplets were separated from the double-emulsion droplets by orbital shaking, during which the triple-emulsion droplets migrated outward, whereas the double-emulsion droplets were collected in the central area. [57] Production of DN Hydrogel Shells: To produce DN hydrogel, photopolymerization of PEGDA and ionic crosslinking of alginate were consecutively performed. First, the triple-emulsion droplets were irradiated with UV with wavelengths of 315-400 nm and an intensity of 120 W cm −2 (CoolWave UV Curing System, Nordson) for 30 s, which caused the photopolymerization of PEGDA in the water shell.…”

“…Here, we produce mechanically stable DN hydrogel shells composed of poly(ethylene glycol) diacrylate (PEGDA) network and alginate network using water-in-oil-in-water-in-oil (W/O/W/O) triple-emulsion templates. [57,58] With a capillary microfluidic device, monodisperse triple-emulsion droplets are prepared to have calcium-ion-loaded water core and PEGDAand sodium-alginate-loaded water shell, which are separated by an ultrathin oil layer. The PEGDA network is first produced by ultraviolet (UV)-induced radical polymerization.…”