## 2 Types of Nanomaterials Used in Cosmetic Formulations
### Solid Lipid Nanoparticles (SLN) and the Nanostructured Lipid Carriers (NLC)  
Among the different types of lipid nanoparticles, in the last decades, solid lipid nanoparticles (SLN) and the nanostructured lipid carriers (NLC) have emerged as promising nanosized delivery systems for skin administration of APIs [16; 17; 52]. Both SLN and NLC have a solid core of lipids stabilized by a suitable surfactant in aqueous dispersion. Their advantages are linked to their biocompatibility as they are composed of physiological lipids and with the capacity to modify the release profile of loaded APIs. While SLN are composed of one single solid lipid in their core, NLC are based on a blend of solid and liquid lipids offering higher loading capacity for lipophilic APIs over storage time. Scientific literature, however, reports an endless number of APIs (lipophilic, hydrophilic, poorly water-soluble drugs) loaded in both types of lipid nanoparticles, using different production techniques. As similar to nanoemulsions, the small size of SLN and NLC (50-1000 nm) increases their specific surface area, thereby providing an extended area in contact with the skin. The formation of a thin lipid film onto the surface with occlusive properties has also been reported to be an advantage for penetration of APIs into deeper skin layers. Together with their biocompatibility, biodegradability, and biotolerability [25; 27; 28], these properties make SLN and NLC interesting carriers for cosmetic ingredients [53]. The solid core of SLN and NLC has been shown to act as a UV blocker, and a synergistic effect can be achieved when loading organic and/or inorganic sunscreens into the particles [9; 10]. Besides, with the increase of the sun protection factor (SPF), the risk of adverse side effects can be reduced by the reduction of the amount of loaded sunscreen [54; 55]. Fragrances and perfumes have also been loaded in lipid nanoparticles for extended release over the day [56]. [PERSON] et al. have developed NLC composed of prickly pear seed oil for topical delivery of retinyl palmitate [57]. To evaluate the permeation of vitamin A, ex vivo studies using rat skin have been run in vertical Franz diffusion cells. The higher concentration of surfactant and the lower viscosity of the tested NLC formulations were shown to improve skin permeation of the active. In an ex vivo study with skin explants, [PERSON] et al. confirmed that SLN loading a hydrophilic peptide with antioxidant capacity could stimulate NADPH quinone oxidoreductase, heme oxygenase-1, and peroxiredoxin-1 genes as an approach to protect cells from UV induced stress, as these are cell protecting enzymes [58]. The skin treated with the developed SLN formulation showed a significant reduction of the DNA damage in comparison to the placebo formulation. Due to their lipophilic character, essential oils of cosmeceutical interest have been proposed as APIs in NLC [59]. [PERSON] et al. have formulated _Rosmarinus officinalis_ L., _Lavandula x intermedia_ \"Sumian\", _Origanum vulgar_ subsp. _hirtum_, and _Thymus capitatus_ essential oils into Softisan-NLC as a new approach to exploit the antibacterial, antifungal, antiviral, antioxidant, anticancer, immune-modulatory, analgesic, and anti-inflammatory activities of the oils [60]. Cell viability, anti-inflammatory, and anti-oxidant activities have been evaluated in vitro, confirming that NLC formulations can be fine-tuned for dermocosmetic applications. These Mediterranean oils can also be used to dissolve drugs in order to increase the loading capacity of NLC [15]. An antifungal NLC formulation, composed of clotrimazole and Lavandula or Rosmarinus as anti-proliferative agents, has been developed with the potential to be used as co-adjuvants in the treatment of non-tumoral proliferative dermal diseases [15].