pH-Responsive Core–Shell Polymer Capsules via Liquid–Liquid Encapsulation with Decoupled Release Onset and Diffusion Kinetics

Polymer-based encapsulation systems that enable stimuli-responsive and controlled release are essential for next-generation drug delivery systems. In this work, liquid-liquid encapsulation is used to fabricate pH-responsive polymer capsules containing hydrogel carriers loaded with caffeine and riboflavin. Laser-oil droplets laden with fluorescent particles are first employed as a model system, in which dissolution of the Eudragit L100 shell under alkaline conditions results in rapid, complete release of the encapsulated laser oil-fluorescent particle suspension, thereby providing direct visualization of shell rupture and release dynamics. The approach is then extended to caffeine- and riboflavin-loaded hydrogels encapsulated within Eudragit L100 shells. The resulting capsules remain stable under acidic conditions. However, at near-neutral or basic pH, dissolution of the Eudragit L100 shell results in the release of the drug-loaded hydrogel carriers. The release of caffeine and riboflavin proceeds via a two-step mechanism involving Eudragit L100 shell dissolution followed by diffusion-controlled transport of caffeine and riboflavin from the alginate matrix. Quantitative analysis reveals that the presence of the Eudragit L100 shell introduces a distinct delay in the onset of release and shifts the equilibrium release time, both of which decrease systematically with increasing pH. This system exhibits pH-responsive behaviour, in which the Eudragit L100 shell governs the onset of release, while the hydrogel core regulates the subsequent diffusion kinetics. Notably, compared to the complete release observed in liquid-core systems, hydrogel-based capsules exhibit reduced and sustained release due to the additional diffusion resistance of the alginate matrix. These findings establish a strategy for decoupling release onset from the subsequent diffusion-controlled release within a single platform, providing a scalable route toward controlled, pH-responsive drug delivery.