Microgel-in-Microgel Biopolymer Delivery Systems: Controlled Digestion of Encapsulated Lipid Droplets under Simulated Gastrointestinal Conditions

Structural design principles are increasingly being used to develop colloidal delivery systems for bioactive agents. In this study, oil droplets were encapsulated within microgel-in-microgel systems. Initially, a nanoemulsion was formed that contained small whey protein-coated oil droplets (d(43) = 211 nm). These oil droplets were then loaded into either carrageenan-in-alginate (O/M-C/M-A) or alginate-in-carrageenan (O/M-A/M-C) microgels. A vibrating nozzle encapsulation unit was used to form the smaller inner microgels (d(43) = 170-324 mu m), while a hand-held syringe was used to form the larger outer microgels (d(43) = 2200-3400 mu m). Calcium alginate microgels (O/M-A) were more stable to simulated gastrointestinal tract (GIT) conditions than potassium carrageenan microgels (O/M-C), which was attributed to the stronger cross-links formed by divalent calcium ions than the monovalent potassium ions. As a result, the microgel-in-microgel systems had different gastrointestinal fates depending upon the nature of the external microgel phase; i.e., the O/M-C/M-A system was more resistant to rupture than the O/M-A/M-C system. The rate of lipid digestion under simulated small intestine conditions decreased in the following order: free oil droplets > O/M-C > O/M-A > O/M-A/M-C > O/M-C/M-A. This effect was attributed to differences in the integrity and dimensions of the microgels in the small intestine, because a hydrogel network surrounding the oil droplets inhibits lipid hydrolysis by lipase. The structured microgels developed in this study may have interesting applications for the protection or controlled release of bioactive agents.