Reports: DNI551459-DNI5: Engineering Interfacial Properties of Emulsions to Limit Transport of Free Radicals and Oxygen Across the Interface

Nitin Nitin, PhD, University of California (Davis)

During this annual reporting period we have focused our research on the understanding the role of lipid core and interface in influencing oxidative reactions. The part (a) of this report focuses on the role of lipid core design in limiting oxidation processes and the part (b) of this report focuses on the role of lipid interface design in limiting oxidation processes. The summary outlines the key innovations in material design, novel assays to measure the permeation of free radicals and imaging technologies to characterize the materials. This research project has partly supported training of two postdoctoral fellows, one whom has recently received a faculty position at Drexel University.

(a) Lipid core design and oxidation processes: The oxidative stability of encapsulated product is a critical parameter in many products from petroleum, food to pharmaceutical to cosmetic industries. The overall objective of this study was to correlate differences in the distribution pattern of encapsulated material within solid lipid nanoparticles (SLNs) and nanostructured lipid carriers (NLCs) with the relative susceptibility of these materials to undergo oxidation. The distribution of an encapsulated lipid soluble dye (Nile Red) in SLNs and NLCs was quantitatively measured using fluorescence imaging. The relative susceptibility of the encapsulated material to react with free radicals generated in the aqueous phase and oxygen from the ambient environment was measured using peroxyl radical and oxygen sensitive fluorescent dyes encapsulated in the lipid phase of colloidal particles respectively. Imaging measurements demonstrate a significant exclusion of the encapsulated dye molecules from the lipid core of SLNs as compared to NLCs. Imaging results also showed significant differences in the intraparticle distribution of encapsulated dye between NLCs containing 1 and 10% liquid lipid. On the basis of these differences in distribution, we hypothesized that the relative susceptibility of encapsulated material to peroxyl radicals and oxygen would be in the order SLNs > 1% NLC > 10% NLC. Measurement of relative susceptibility of peroxyl radical sensitive dye encapsulated in SLNs and NLCs to peroxyl radicals generated in the aqueous phase validated the proposed hypotheses. However, the susceptibility of encapsulated oxygen sensitive dye to ambient oxygen was not significantly different between SLNs and NLCs. The results of this study demonstrate that difference in distribution pattern of encapsulated material within colloidal particles can significantly influence the susceptibility of encapsulated material to react with free radicals. Overall, this study demonstrates a comprehensive approach to characterize the susceptibility of encapsulated materials in colloidal particles to oxidation processes.

(b) Lipid interface design and oxidation processes: Oxidation of encapsulated bioactives in lipid-based nanostructures (e.g., emulsions, liposomes and lipid coatings) is a significant challenge that limits functionality and shelf life of products in many industries including food, petroleum, cosmetic, and pharmaceutical industries and in biomedical devices. The objective of this study is to investigate how physical and chemical properties of the lipid interface can be engineered to inhibit permeability to environmental free radicals. A glass-supported lipid membrane system is used to model the water–lipid interface. Peroxyl radicals generated in the aqueous phase permeate the supported membrane structure and react with an embedded lipid peroxidation sensor. By using epifluorescence microscopy to monitor the rate of fluorescence decay of the lipid sensor, various physical and chemical modifications of the supported membrane can be compared quantitatively. Physical properties tested include lipid composition (gel phase vs. fluid phase and addition of cholesterol) and a double membrane structure formed via layer-by-layer assembly. Chemical properties include direct addition of antioxidant molecules to either the lipid membrane or the aqueous phase. Results show that increasing molecular order of lipid membranes (with cholesterol or long-chain phospholipids) or assembling an additional protective membrane reduces the rate of permeation of free radicals by a small amount (approximately two-fold with respect to the reference membrane). The results also demonstrate that localization of chemical antioxidants at the lipid interface is an order of magnitude more effective in suppressing membrane permeation of free radicals than either aqueous phase antioxidants or any of the tested physical modifications.