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

Area of Science:

• Colloid and Surface Science
• Materials Science
• Chemical Engineering

Background:

• Double emulsions (DEs) are complex liquid-in-liquid-in-liquid systems with two interfacial layers, enabling co-encapsulation of hydrophilic and lipophilic substances.
• Their stability and functionality depend on interfacial composition, inter-droplet transport, osmotic pressure, and inherent instability mechanisms.

Purpose of the Study:

• To critically review double-emulsion systems designed for stimuli-responsive behavior.
• To differentiate deliberately engineered responses from non-specific destabilization.
• To highlight requirements for robust and reproducible stimuli-responsive DEs.

Main Methods:

• Literature review and critical analysis of existing double-emulsion systems.
• Categorization of systems based on stimulus, responsive element location, mechanism, and observed response.
• Distinction between liquid DEs for sustained compartmentalization and DE-templated carriers for fabrication.

Main Results:

• Many reported structural changes in DEs are not true stimuli-responsive behaviors but result from baseline instability.
• Robust stimuli-responsive DEs necessitate simultaneous control over droplet stability, osmotic balance, interfacial mechanics, and barrier phase transport.
• Successful DEs require careful consideration of preparation methods and stimulus exposure conditions.

Conclusions:

• Distinguishing designed stimuli-responsiveness in DEs demands rigorous evidence beyond simple structural changes.
• Future advancements rely on standardized characterization, quantitative assays, and manufacturing-compatible formulation strategies.
• Achieving reproducible and application-relevant stimuli-responsive DEs is achievable with integrated control over multiple physicochemical parameters.