Microfluidic Encapsulation of Pickering Oil Microdroplets into Alginate Microgels for Lipophilic Compound Delivery

Alginate microgels are widely used as delivery systems in food, cosmetics, and pharmaceutical industries for encapsulation and sustained release of hydrophilic compounds and cells. However, the encapsulation of lipophilic molecules inside these microgels remains a great challenge because of the complex oil-core matrix required. The present study describes an original two-step approach allowing the easy encapsulation of several oil microdroplets within alginate microgels. In the first step, stable oil microdroplets were formed by preparing an oil-in-water (O/W) Pickering emulsion. To stabilize this emulsion, we used two solid particles, namely the cotton cellulose nanocrystals (CNC) and calcium carbonate (CaCO₃). It was observed that the surface of the oil microdroplets formed was totally covered by a CNC layer, whereas CaCO₃ particles were adsorbed onto the cellulose layer. This solid CNC shell efficiently stabilized the oil microdroplets, preventing them from undesired coalescence. In the second step, oil microdroplets resulting from the Pickering emulsion were encapsulated within alginate microgels using microfluidics. Precisely, the outermost layer of oil microdroplets composed of CaCO₃ particles was used to initiate alginate gelation inside the microfluidic device, following the internal gelation mode. The released Ca²⁺ ions induced the gel formation through physical cross-linking with alginate molecules. This innovative and easy to carry out two-step approach was successfully developed to fabricate monodisperse alginate microgels of 85 μm in diameter containing around 12 oil microdroplets of 15 μm in diameter. These new oil-core alginate microgels represent an attractive system for encapsulation of lipophilic compounds such as vitamins, aroma compounds or anticancer drugs that could be applied in various domains including food, cosmetics, and medical applications.