## 2 Types of Nanomaterials Used in Cosmetic Formulations
### Polymeric Nanoparticles  
Two distinct classical types of polymeric nanoparticles are described, i.e., nanospheres versus nanocapsules [104]. Nanospheres stand for a continuous matrix core in which the drug is dissolved or dispersed and from which it is released by diffusion. Nanocapsules comprise a liquid core (usually an oil dissolving the drug) surrounded by a polymeric layer which controls the release of drug. Polymeric nanoparticles can be obtained with a variety of sizes and morphologies, using natural or synthetic polymers as starting materials (top down production). Among synthetic polymers, the most commonly used are poly D,L-lactic-\(\alpha\)-glycolic acid (PLGA), poly lactic acid (PLA), polycaprolactone, and polyacrylates. Alginate, chitosan, gelatin, starch, and albumin are examples of natural polymers. Polymeric nanoparticles are produced with the purpose of modifying the release profile of the loaded API, for targeted specific delivery and to increase the bioavailability of the payload. They show additional advantages of being physicochemically stable, non-toxic, and their degradation profile can be tailored-made. A variety of APIs (hydrophilic, lipophilic, amphiphilic) can be loaded into polymeric nanoparticles. [PERSON] et al. proposed the formulation of PLGA nanoparticles loading ibuprofen and hydrocortisone into transdermal patches as an approach to reduce the risk of drug crystallization [105]. The drug flux through excised pig skin was also evaluated demonstrating that high drug loadings and high drug flux can be achieved by proper selection of the polymer [105]. Hydrogels containing paraprofen-loaded PLGA nanoparticles have been produced by [PERSON] et al. [106]. The aim of the proposed formulation has been to increase the penetration of the drug into deeper skin layers in order to have effective anti-inflammatory and analgesic activities. PLGA nanoparticles showed sustained release of the payload. Both ex vivo studies in human skin and in vivo studies confirmed the added-value of using nanoparticles into semi-solids for the treatment of oedema, when compared to classical hydrogels [106]. Benzophenone-3 (BP-3) is an effective chemical UV filter of broad spectrum, but is known to trigger adverse reactions in vivo. To mitigate the risk of adverse side effects, its loading into different types of nanoparticles has been proposed [7; 107]. [PERSON] et al. evaluated the percutaneous absorption of BP-3 when administered as lipid versus polymeric nanoparticles. Poly-\(\epsilon\)-caprolactone nanoparticlescould significantly reduce the BP-3 skin permeation while showing the highest SPF [107]. A new sunscreen formulation based on benzophenone-3-poly(epsilon-caprolactone) nanocapsules has been produced by [PERSON] et al. [7]. A non-Newtonian pseudoplastic semi-solid has been produced which showed no signs of irritation in HET-CAM test [7]. PLGA nanoparticles loading a xanthophyll carotenoid (astaxanthin) were tested for anti-photodamage in HaCaT cells [108]. The cytotoxicity assay, ROS scavenging and mitochondrial membrane potential assay highlighted the advantages of nanoparticles in improving anti-wrinkle and anti-oxidation effects of astaxanthin against the non-loaded carotenoid [108]. PLGA nanospheres containing ubiquinone have been developed for cosmetic application [109]. The drug permeation assay using iontophoresis via electric dermal administration on hairless mouse skin was evaluated. The authors reported that after iontophoresis, nanospheres were kept onto the horny layer of the skin, even after washing. Combining both PLGA nanospheres and iontophoresis may be an interesting approach for cosmetic use [109]. Oleanolic and unsolic acids are recognized triterpenoids with anti-cancer properties, and have been used in cosmetic formulations [110]. They have been proposed for the treatment of photoaged skin, as they show the ability to inhibit elastase thereby improving skin elasticity. The triterpenoids also show antioxidant, antimicrobial and anti-inflammatory activities.