## 1 Introduction  
Skin is a complex and the largest organ (accounting for roughly 16%) of the body, involved in protection against physical, chemical, and microbial violence, vitamin synthesis, temperature regulation, and maintaining the body's moisture content. The outermost layer of skin, the stratum corneum (SC), contains \(\sim\)10-20% water and is filled with corneocytes to delay water loss. The loss of skin moisture affects the functions of the skin, and the use of moisturizer aids in restoring the functionality of the skin and improves skin health [1; 2; 3].  
Moisturizers with bioactive compounds provide moisture to the skin and can be used to treat dermatological diseases [4]. They must preserve the optimal balance between water and skin lipids by maintaining the water content in the skin. A typical moisturizer may contain humectants (hygroscopic substances that improve the water-retaining ability of the SC), emollients or occlusive agents (to prevent the evaporation of water from the SC), and lipids [5].  
Cosmetic formulations, especially skincare products with nano-structured lipid carriers (NLCs), are superior moisturizing agents, improving the skin permeation and stability of active compounds. NLCs are lipid nanoparticles suitable for application to the skin. Mixtures of solid lipids and liquid lipids (in ratios of 70:30 to 99.9:0.1) are constitutive of NLCs [5]. Topical formulations with occlusive properties are used to keep the skin adequately hydrated and the SC intact. Customers prefer emulsions with sufficient occlusive and cosmetically acceptable properties. NLCs are preferred due to their ability to stabilize active compounds, their bioavailability, precise occlusion, film formation, skin hydration properties, etc. [6]. Cutanova Nanorepair Q10 cream, an NLC-containing cosmetical product marketed in 2005, provides high skin hydration, a better consistency, and better spreadability on the skin compared to a water-in-oil cream [7].  
Sea buckthorn (SB) (_Elaagnus rhamnoides_ (L.). [PERSON]), previously called _Hippocampahoides_ L., belongs to _Hippocampah_ L. and is cultivated primarily in China and Russia [8; 9]. SB is traditionally used for food and medicine in Asia and Europe. SB is rich in flavonoids, phenolic compounds, anthocyanins, carotenoids, and phytosterol, and its extracts and oil are known for antimicrobial, anti-inflammatory, antioxidant, anti-cancer, hepatoprotective, neuroprotective, antifatigue, and cardioprotective activities [9; 10; 11; 12]. SB was also reported to have skin protection and health properties. SB oil (SBO) improved atopic dermatitis-like skin lesions by inhibiting the activation of chemokines in the skin [13]. SBO could minimize UV-induced skin damage and skin aging [14; 15].  
The bioactivities of SBO attract the developer and cosmetician for use in their formulations. The development of NLC-based skincare products with SBO could increase skin hydration and prevent photo-damage to the skin. To this end, the present study was undertaken to develop, optimize, and characterize an SBO-loaded NLC (SBO-NLC) for skin hydration.  
The response surface methodology (RSM) and central composite design (CCD) have been employed to reduce experimental burdens and optimize the several influencing factors in extraction conditions, fermentation processes, and other optimization studies [16; 17; 18]. Thus, we adapted the RSM and CCD for the experimental design and the optimization of the selected variable factors.