Stimuli-responsive double emulsions: Formulation, stability, and triggered release mechanisms

Double emulsions (DEs) are hierarchical nested-droplet systems that can compartmentalize hydrophilic and lipophilic compounds within two coupled liquid-liquid interfaces. Their performance is governed by interfacial composition, transport across the intermediate phase, osmotic coupling, and baseline instability pathways. Consequently, a structural change observed after exposure to a chemical, biochemical, or physical cue does not, by itself, demonstrate deliberately designed stimuli-responsiveness. This review critically examines double-emulsion systems in which a responsive motif, a phase transition, or a switchable interfacial component is intentionally incorporated to produce a measurable structural, interfacial, or transport response under a defined stimulus. Particular attention is given to water-in-oil-in-water and oil-in-water-in-oil architectures. Baseline instability mechanisms and preparation routes are first discussed as determinants of droplet architecture, barrier properties, and response reproducibility. Representative systems are then assessed according to the applied stimulus, the location of the responsive element, the immediate physicochemical mechanism, the resulting structural or transport response, and the evidence required to distinguish designed triggering from non-specific destabilization. The review further distinguishes liquid DEs intended to remain compartmentalized during use from DE-templated carriers, for which the double emulsion serves primarily as a fabrication intermediate. Across the available evidence, robust stimuli-responsive behavior requires the simultaneous control of baseline stability, osmotic balance, interfacial mechanics, barrier-phase transport, and stimulus exposure conditions. Future progress will depend on standardized characterization methods, quantitative stimulus-response assays, and formulation strategies compatible with reproducible manufacturing and application-relevant constraints.