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Soft and Tough Microcapsules with Double‐Network Hydrogel Shells

Hydrogel‐shelled microcapsules allow communication with the surrounding through molecule‐selective exchanges, serving as microcarriers for cells, drugs, catalysts, and nanoparticle sensors. However, the low mechanical stability of hydrogels has restricted their use in strong shear flows or compressive fields. Here, microcapsules are designed composed of water core and double‐network (DN) hydrogel shell using water‐in‐oil‐in‐water‐in‐oil triple‐emulsion templates to secure high mechanical stability as well as molecular size‐selective permeation. Monodisperse triple‐emulsion droplets are prepared using microfluidic devices. The core water contains divalent ions and the outer water layer contains poly(ethylene glycol)diacrylate (PEGDA) and sodium alginate. The PEGDA is first photocrosslinked by ultraviolet irradiation and the alginate is second ionically crosslinked with an infusion of divalent ions from the core by rupturing the middle oil layer. The resulting DN shells show excellent mechanical stability in comparison with single‐network (SN) shells made of PEGDA only. Enhanced elasticity of the DN enables the microcapsules to elastically deform and fully recover the original state for wide ranges of strain and strain rate. Moreover, the DN shells show a threshold force for the shell rupture almost one order of magnitude larger than the SN and maintain their integrity under violent shear flows.