## 1 Introduction  
Nanotechnology stands for the production and use of materials at the nanoscale, which show physicochemical properties different from their bulk counterparts. Due to special rearrangements of their inner structure, these new materials exhibit a larger surface area, thereby acting differently with biological systems [1]. The loading of active pharmaceutical ingredients (APIs) within nano-sized drug delivery systems is being currently exploited to promote product innovation by developing nanoproducts. Nanoproducts used for the delivery of APIs to the skin (e.g., nanopharmaceuticals, nanocosmeceuticals) have already proven their efficacy as several products are already available on the market for the treatment of skin injuries (e.g., atopic dermatitis [2], skin cancer [3], skin burns [4], wound healing [4; 5]), and protection from ultraviolet (UV) radiations [6; 7; 8; 9; 10]. Some examples of daily contact with nanomaterials are given in Figure 1. A successful example is the formulation of sunscreens into nanoproducts. This approach was found to decrease the adverse side-effects of UV inorganic filters (e.g., titanium dioxide (TiO\({}_{2}\)), zinc oxide (ZnO) [11]) and of chemical filters (benzophenone-3 [7; 12]), improving the safety to consumers [8; 9; 10]. Besides, it has also been demonstrated that the loading of sunscreen into solid nanoparticles can offer a synergistic effect, as nanoparticles can themselves have a sun-blocking effect [13]. In the past years, different types of nanoparticles have been proposed to load sunscreens, antioxidants and vitamins for skin care formulations. The first cosmetic products manufactured using nanotechnology were the liposomes-containing moisturizing creams, launched 40 years ago [14]. Since then, several other types of biocompatible nanomaterials have been proposed, being nanoemulsions, solid lipid nanoparticles (SLN), nanostructured lipid carriers (NLC) of special interest due to their lipid composition. Besides, lipid nanoparticles also improve skin hydration because of the formation of a thin lipidic film onto the surface of the skin, retaining its moisture for longer time [15; 16; 17]). Over the course of the 20 th century, cosmetics raised their popularity as daily products with a substantial marketed increase [18; 19]. Various cosmeceutical formulations for skin, body, and hair applications are being proposed for the treatment of a range of problems, such as hair damage and/or loss, wrinkles, hyperpigmentation, and photo-aging [20]. In the market of personal care products, cosmeceuticals and in particular, nanocomposites, show the greatest growth in the last decades [21]. The main advantages of nanocosmeceuticals include the controlled release of APIs site-specific targeting, occlusive properties with enhanced hydration, which contribute to increase skin permeation to the API [22]. Part of these advantages are also the reason why nanocosmeceuticals should be regulated and subject of clinical trials, a critical step to assure consumer's safety [23; 24]. Nanoparticles can be highly reactive both in vitro and in vivo, resulting in an enhanced risk of toxicologic events [25; 26; 27; 28]. Cosmeceuticals are used for a variety of applications, e.g., to improve skin texture by stimulating collagen growth, as anti-aging formulations as their antioxidants neutralize reactive oxygen species and protect the structure of keratin, which results in a healthier skin [29; 30; 31]. Marketed antiaging nanocosmeceuticals as hair care products (e.g., shampoos, conditioners, hair growth stimulants, coloring and styling products) may be composed of nanotubes, poly(lactic-co-glycolic acid (PLGA) nanospheres, fullerenes, gold nanoparticles, niosomes, microemulsions, nanoemulsions, liposomes, with the purpose of sealing the moisture around the cuticles and optimizing the contact time with the scalp and the follicles, by forming a protective film [32]. Nanocosmeceuticals have also been included in lipsticks, lip balms, lip glosses, and lip volumizers.