Casein nanogels as effective stabilizers for Pickering high internal phase emulsions

Pickering emulsion stabilized by colloidal particles is attracting increasing attention in recent years, and emulsions using food-grade or biocompatible particle stabilizers will be less restrained in biomedical applications. In this work, we demonstrated that the casein nanogels (CSNs) prepared by crosslinking casein, a natural protein with good biocompatibility, with glutaraldehyde could be used as effective stabilizers for Pickering high internal phase emulsions (HIPEs) without any other additives. The CSNs had an average diameter of 179 nm and a zeta potential of -16.4 mV at pH 7 with spherical morphologies. Emulsions were fabricated using CSNs as stabilizers under various conditions with three main variables including CSN concentration, pH and ionic strength , and characterized by fluorescence microscopy , photography and rheological measurement. It was found that CSNs could effectively adsorb on oil-water interface when their surface potential was close to neutrality under certain pH and ionic strength conditions and thereby assist in forming stable oil-in-water (O/W) Pickering HIPEs, even at a low CSN concentration of 0.5 wt%. To exploit the potential of these Pickering HIPEs in drug delivery, indomethacin (IDM) as a hydrophobic drug model was encapsulated into the oil phase. In vitro release experiment showed that the release behavior of IDM could be tuned by manipulating the CSN concentration. Overall, this work demonstrated that the casein nanogels could efficiently stabilize HIPEs and these HIPEs have a potential application in drug delivery system.