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.

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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.