Recent advances in the production of controllable multiple emulsions using microfabricated devices

• Multiple emulsions of versatile shape and internal morphology can be produced.
• Encapsulation efficiency can reach 100% due to drop-by-drop manufacture approach.
• Non-planar and axisymmetric geometry of drop generators reduces wetting problems.
• Glass capillary devices can generate complex multiple emulsions in a single step.
• Fabricated drops are useful templates for multi-compartment particles and vesicles.

This review focuses on recent developments in the fabrication of multiple emulsions in micro-scale systems such as membranes, microchannel array, and microfluidic emulsification devices. Membrane and microchannel emulsification offer great potential to manufacture multiple emulsions with uniform drop sizes and high encapsulation efficiency of encapsulated active materials. Meanwhile, microfluidic devices enable an unprecedented level of control over the number, size, and type of internal droplets at each hierarchical level but suffer from low production scale. Microfluidic methods can be used to generate high-order multiple emulsions (triple, quadruple, and quintuple), non-spherical (discoidal and rod-like) drops, and asymmetric drops such as Janus and ternary drops with two or three distinct surface regions. Multiple emulsion droplets generated in microfabricated devices can be used as templates for vesicles like polymersomes, liposomes, and colloidosomes with multiple inner compartments for simultaneous encapsulation and release of incompatible active materials or reactants.

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