Pickering Emulsions for the Emulsion Stability and Skin Delivery of Flavonoids using different Oil Types

Introduction A Pickering emulsion (PE) is a particle stabilised emulsion. Due to the amphiphilic structure of some flavonoids, they can form good stable PE. The use of Pickering emulsions serve as a potential useful approach for improving the formulation solubility of flavonoids, as well as reducing skin irritancy for topical formulations by removing emulsifiers from cosmetic formulations. This research in this study is the first (to the authors knowledge) to investigate the skin release kinetics and permeation of the flavonoids incorporated into a Pickering emulsion. Changes to the barrier properties of porcine Stratum corneum (SC) in vivo were also evaluated by investigating lipid morphology changes of the stratum corneum post hoc after the application of the Pickering emulsion and skin permeation studies. Oil in water (O/W) Pickering emulsions were made with three flavonoids differing in structure and physiochemical properties; rutin, isoquercetin and quercetin, each with 20 % w/w of oil. Three types of oil were used to make the Pickering emulsions; paraffin (hydrocarbon oil), almond and coconut (vegetable). Pickering emulsion were made with a jet homogeniser. PEs were evaluated for emulsion structure. Skin permeation release kinetics were established using split thickness porcine skin (intact stratum corneum and epidermis) in a Franz diffusion set up over 24 hours using an infinite dose technique. They were benchmarked against comparison controls, using mixtures of oil and flavonoid (omitting high pressure homogenisation), which did not form PEs. Flavonoids permeating through the skin membrane were identified by Reverse-Phase High Performance Liquid Chromatography (RP-HPLC). Various mathematical models from literature were used to describe the release kinetics of the flavonoids based on the permeation data. The morphology of the lipid chain packing in the SC was evaluated using Fourier Transfer Infrared (FT-IR) spectroscopy and subsequent analysis using a Gaussian curve fitting algorithm. iv Results Flavonoids were found to aggregate at the oil/water interface to form Pickering emulsions. From visual stability observations (low-high phase separation and creaming); rutin > isoquercetin > quercetin, and for oil types this order paraffin > almond ≥ coconut oil. High shear homogenisation is essential for Pickering emulsion formation, and PEs do not form spontaneously. Quercetin did not form a PE with coconut oil. FT-IR results indicated a change in lipid morphology from the CH2 symmetric stretching and the CH2 scissoring bandwidths. A greater disruption in the extracellular matrix lipid packing was observed from the flavonoid suspensions and oil mixtures more than the Pickering emulsions, indicating that when the flavonoids are coating the oil in a Pickering emulsion, it reduces oil exposure to the SC lipids. In addition, a change in lipid morphology was seen between flavonoids; with the effect being in the order rutin > isoquercetin > quercetin. For skin permeation assays, after 7 hours there was no difference between the amount of flavonoids released from the epidermis, regardless of flavonoid structure. At 24 hours there was significantly more rutin delivered from paraffin and almond oil suspension (control) than the corresponding Pickering Emulsion (P < 0.05) and significantly more isoquercetin was delivered from vegetable oils suspensions (control) than the corresponding Pickering Emulsion (P < 0.05). Quercetin from PEs was not released from the membrane, only from the suspension (control). When flavonoids are aggregated at the O/W interface in a PE it changes the release kinetics and SC/epidermal penetration due to flavonoids being held at the interface before emulsion collapse.From the % dose applied, flavonoids were delivered in the order isoquercetin > rutin for the PEs and quercetin > isoquercetin > rutin for non-emulsions. This follows the predicted permeability behaviour due to the physiochemical properties of those specific flavonoids.