Microfluidic-templating fabrication of milli-capsules with ultrarigid and homogeneous shells for long-term retention and explosive release of water-soluble cargoes

Encapsulation of water-soluble cargoes in millimeter-sized capsules has enabled major advances in various fields, including pharmaceuticals, food, cosmetics, packaging, and materials. However, because of the lack of fabrication precision, low cargo retention, suboptimal mechanical properties, and difficulty in preventing water evaporation, this technique is more challenging than microencapsulation techniques. In this study, we developed a surfactant-free and organic solvent-free water-in-oil-in-air emulsification approach for synthesizing double-layered “milli-capsules” for the precise encapsulation, enhanced retention, and force-triggered burst release of water-soluble bioactive cargoes. In particular, we synthesized milli-capsules with a first shell of poly(ethylene glycol dimethacrylate) for the efficient encapsulation of bioactive cargoes and a second shell of beeswax to prolong the retention of the entrapped bioactive compounds. Unlike traditional milli-capsules, which exhibit poor shape uniformity and mechanical stability, we introduced metallic ions to stabilize the interfacial tension and employed constant rotation to balance the gravity, buoyancy, inertial, and viscous forces imposed on the droplets, resulting in uniform and rigid milli-capsules with narrow rupture forces. Furthermore, additional hydrophobic beeswax coating prevented water volatilization and substantially prolonged the shelf life of the encapsulated compounds from a few days to a few months while maintaining their bioactivities. The proposed milli-capsule system addresses the challenge of precise fabrication of large carriers for water-soluble cargoes, representing a significant step toward the long-term storage and controlled release of bioactive cargoes for various industrial applications.