# Incorporation of UV Filters into Oil-in-Water Emulsions--Release and Permeability Characteristics
## 3 Results and Discussion
### Release and Permeation of UV Filters from Oil-in-Water Emulsions
It is noteworthy to highlight, that octyl methoxycinnamate diffuses more easily through the cellulose membrane in comparison to avobenzene. These differences in the release profiles may be related to the presence of a lipophilic barrier of oil droplets in the oil-in-water system that retards the release of Avo in the internal oily phase. Additionally, the solubility of octyl methoxycinnamate is much higher in ethanol (140 mg/mL) than avobenzene (5 mg/L) [52], which could influence the facilitated liberation of the OMC from the formulation to the receptor fluid. Furthermore, different release behaviors between both the UV filters can be associated with their various physicochemical properties. The log P of avobenzene is 4.8, while the calculated log P for OMC is 5.6 [53]. Since octyl methoxycinnamate is more lipophilic than avobenzene (Table 1), different trends in the release rates of both the UV filters can be expected. In other studies, [PERSON] et al. prepared six different emulsions with OMC and Avo and observed a much lower amount of avobenzene released through the cellulose membrane compared to octyl methoxycinnamate regardless of the type of formulation used [22]. The avobenzene was not determined in the receptor medium up to 8 h of the experiment. The results confirmed that Avo was liberated from the formulations with intermediate lipophilicity. It implies that the diffusion of the UV filter can be regulated by choosing appropriate ingredients for the topical product. [PERSON] et al. also proved that the release patterns of avobenzene and octyl methoxycinnamate were different [54]. It was demonstrated that the emulsifier may influence the permeation of the sunscreen. The release of avobenzene from modified dextrin incorporated into the emulsion was also studied by [PERSON] et al. [55]. It was found that the tendency of the UV filter to permeate the skin was significantly reduced due to the modified dextrin compared to the pure emulsion. Another paper presented that Avo was introduced to the oil-in-water emulsion and included in a complex with cyclodextrin. The permeation studies of Avo through rat skin proved that the complexation affects the lag time [56]. In order to understand the mechanism of the UV filter release, the diffusion data were fitted to five different kinetic models (Table 5). The highest correlation coefficient was observed for the Korsmeyer-Peppas model. The \(n\) values were higher than 0.45 for all the samples, indicating the anomalous transport. It means that in these cases, in addition to the UV filter diffusion, other mechanisms contribute to the release of OMC and Avo [57]. In the next step, in vitro permeation experiments of the UV filters were performed through a multilayer Strat-M membrane, which is a non-animal-based model recommended for the transdermal analysis, because it mimics human skin [58]. It is composed of a tight top layer coated with a lipid blend similar to that of the human stratum corneum and a porous layer simulating the epidermis and dermis [59]. It was reported that the Strat-M membrane exhibits equivalency to human skin in permeation studies of different compounds [60]. To ensure the solubility of lipophilic sunscreen agents, the receptor compartment was filled with a mixture of ethanol and phosphate buffer with pH 5.8 in the ratio of 1:1. The results of the UV filters permeation studies are presented in Figure 6B. The highest cumulative amount of UV filters released after 48 h was noticed for E\({}_{2\%\text{OMC}}\) followed by E\({}_{0.5\%\text{OMC}}\). In turn, the permeation of avobenzene for both E\({}_{0.5\%\text{Avo}}\) and E\({}_{2\%\text{Avo}}\) was at a very low level after 2 days. The sensitivity of the UV analytical method did not enable to detect avobenzene in the receptor fluid in the first 2 h of analysis. Similar results were obtained by [PERSON] and [PERSON] [53] who performed the permeation studies of both UV filters through human skin and did not determine avobenzene until 6 h from the beginning of the experiment.