## 4 Discussion
The high oil content may provide an occlusive barrier, preventing trans-epidermal water loss [42]. [PERSON] et al. [43] stated that NLCs with high lipid contents improved skin hydration levels and prevented trans-epidermal water loss in human volunteers. The results of the current study suggested that the SBO-NLCs have a greater occultation effect, which presumably enhances skin hydration by preventing water loss through evaporation [25]. The occultation effect of the SBO-NLCs had a considerable range (Figure 7). Furthermore, the presence of lipids in SBO-NLCs could improve skin moisture content and prevent skin dryness. The initial burst drug release was reported for tretinoin-loaded NLCs [40], lycopene-loaded NLCs [35], and sillybin-loaded NLCs [34]. Some explanations for the rapid initial drug release have been provided in the literature. The crystalline structure of NLCs may be weaker, facilitating the sudden release of the drug. In another way, the drug release rate was high, due to the small size and high surface area and high diffusion coefficient of NLCs [34; 35]. In addition, the in vitro release profile indicated the distribution of SBO in the lipid matrix [35]. The size, morphology, and surfactant of NLCs could affect the release profiles of active compounds [44; 45]. SBO-NLCs could release the drug faster into the skin within 24 h, and the released drug level after 48 h was significantly higher than that for the free SBO (Figure 8). The results suggested that the SBO-NLCs have a satisfactory level of in vitro drug-releasing capability. Previous studies also suggested that NLCs could deliver drugs effectively. For instance, a higher amount of drug permeation was observed for tenoxicam-loaded NLCs in vitro and ex vivo models. Comparatively, the nanoparticle permeation was higher than that of the pure drug [46]. Similarly, the ex vivo skin penetration of idebenoeno-loaded NLCs was significantly higher than the pure drug [47]. The drug permeation of charged NLC molecules was different and depended on the charge of the NLCs. Cationic NLCs could increase the permeability and deposition of a drug (tripretine) more effectively [48]. In vitro and ex vivo Franz cell systems are comparable to human skin. The SBO-NLCs delivered the drug into the inner skin significantly (22.73 \(\pm\) 1.67 \(\mathrm{\SIUnitSymbolMicro g}\)/\(\mathrm{cm}^{2}\) of skin) (Figure 9b). The results of the present study showed the high drug permeation and deposition of SBO in the skin, and the developed NLCs were potent enough to transfer the SBO into the skin. For the following factors, constraints were set to predict the optimum conditions for SBO-NLC development: wax (in the range of 2.5 to 7.5%), surfactant (in the range of 2.5 to 7.5%), PEG400 in surfactant (in the range of 10 to 30%), size (minimize), PDI (minimize), ZP (minimize), and EE (minimize). Accordingly, the experimental design for the optimum SBO-NLCs was predicted. It was predicted that the combination of 2, 5% wax, 7.5% surfactant, and 30% PEG in the surfactant could produce optimum SBO-NLCs(size = 119.20 nm; PDI = 0.26; ZP = \(-\)21.45 mV; EE = 84.45%). The actual values were comparable to the predicted conditions.