## 4 Discussion
The sustained stability of the CE EO and NLC EO suggests their potential for extended shelf life and practical applications in real-world settings. However, the release of EO components from both formulations over time indicated a gradual decrease in content, with the CE EO exhibiting a faster release rate than the NLC EO. This differential release pattern suggests that the lipid matrices of NLCs may provide a more sustained release profile for EO components compared to conventional emulsions [23]. The release kinetics of EO components from the CE EO and NLC EO were investigated to understand the mechanism of release over time. Mathematical models, including the zero-order, first-order, Higuchi, and Hixson-Crowell models, were employed to describe the release profiles [28; 29]. The Korsmeyer-Peppas model was not applicable, as it is most suitable for analyzing up to 60% of the released incorporated material [28; 29]. The models that best fitted were zero-order and Higuchi. The zero-order model describes the release of incorporated ingredients over time at a constant rate unaffected by the ingredients' concentrations [26]. The Higuchi model describes the release of incorporated material from an insoluble matrix as the square root of a time-dependent process based on Fickian diffusion [30]. For the CE EO, the release of eucalypt and linalool was best described by the Higuchi and zero-order models, suggesting a diffusion-controlled release mechanism. In contrast, the NLC EO formulation predominantly followed a zero-order release model, likely due to the averaging of release rates from nanodroplets of varying sizes [23]. These findings suggest a diffusion-based mechanism for the release of EO constituents from both formulations. However, the structured lipid matrix of the NLC provided a more sustained release profile compared to the conventional emulsion. This is attributed to the lipid matrix's ability to gradually release the encapsulated EO, forming a more efficient diffusion barrier than the larger microdroplets [23; 31]. In contrast, the CE formulation exhibited a faster release, lacking the controlled-release capability imparted by the lipid matrix. The retention of EO components in the NLC was influenced by the lipophilicity of each compound. After 90 days of storage, more lipophilic compounds such as linalyl acetate and linalool (log P: 3.93 and 2.97, respectively) were more efficiently retained, with percentages released of 70.41 \(\pm\) 1.32% and 79.76 \(\pm\) 5.22%, respectively [32; 33]. In comparison, less lipophilic components like camphor and eucalypt (log P: 2.38 and 2.74, respectively) exhibited higher release rates, with percentages released of 87.99 \(\pm\) 4.13% and 91.20 \(\pm\) 0.44%, respectively [34; 35]. However, in the CE formulation, the release of EO components after 90 days of storage was similar for all constituents (96-100%) and appeared independent of their lipophilicity. This difference may be attributed to the spatial positioning of each component within the lipid droplets. In the NLC, the more lipophilic components are likely localized in the core of the nanodroplets, resulting in slower release, while the less lipophilic ones are positioned near the surface and are released more readily. Conversely, in the CE formulation, the less structured and more homogeneous droplet structure of the dispersed phase does not allow for lipophilicity-dependent positioning, leading to faster release rates with minimal variation between components. This differential release pattern suggests that NLC formulations may offer enhanced control over the release of EO active ingredients, leading to prolonged efficacy and a reduced frequency of application. Finally, the efficacy of the NLC EO as an insect repellent was evaluated through a controlled human volunteer study. The results demonstrated a significant increase in the duration of insect-repellent action compared to that of the control group, with the estimated median protection time calculated at 45.0 \(\pm\) 3.7 min for the NLC EO.