Template-Free Microfluidic Fabrication of Water-in-Water Microcapsules for Controlled Release

Template-free fabrication of water-in-water microcapsules in completely miscible aqueous one-phase systems (AOPS) remains a long-standing challenge due to the lack of a well-defined interface. Current microcapsule fabrication is typically achieved through immiscible two-phase systems, including oil-water systems and aqueous two-phase systems (ATPS). This study proposes a microfluidic strategy that enables complete template-free microcapsule formation entirely within an aqueous one-phase system. Local coacervation is directly induced at the mixing boundary by controlling streams containing oppositely charged polymers and surfactants, forming a continuous shell that encapsulates the inner volume effectively without the need for an immiscible template or subsequent core material removal. The resulting microcapsules, reaching diameters of several hundred micrometers, exhibited homogeneity and long-term stability in surfactant-rich environments without aggregation and disintegration. Under specific conditions, shell formation was reversible, allowing dissolution in alkaline environments while remaining intact for over 4 months without chemical perturbations. Particle encapsulation experiments demonstrated that this method effectively retains particles larger than 500 nm, while smaller solutes diffuse freely through the shell, indicating size-selective permeability. Unlike traditional aqueous two-phase strategies that rely on pre-existing interfaces, our method operates entirely in a completely miscible aqueous system, with shell formation originating from controlled localized coacervation at the mixing boundary. This simple, materially flexible and environmentally friendly strategy is potentially scalable and broadens possibilities for sustainable next-generation microcapsule design, enabling high-throughput controlled-release applications in aqueous formulations such as food, pharmaceutical, and personal care products.