## 2 Types of Nanomaterials Used in Cosmetic Formulations
### Liposomes  
Liposomes are concentric phospholipid bilayers enclosing an aqueous core, making them suitable to load hydrophilic, lipophilic, and amphiphilic compounds [77; 78; 79]. Their use has been described for a set of administration routes, among which the topical delivery of APIs has been successfully exploited [80]. Their lipid composition makes liposomes biocompatible and biodegradable, with low risk of toxicity acting as penetration enhancers for the delivery of APIs into deeper skin layers (e.g., dermis). The risk of undergoing oxidation and/or hydrolysis, or even suffering drug leakage, are pointed out as the main disadvantages of liposomes.  
The main component of liposomes is phosphatidyl choline, but several other lipids (e.g., glycerophospholipids, glycerolipids, phingophospholipids, ceramides, and cholesterol) can also be included in their composition. Cholesterol is commonly used to increase the rigidity of the vesicles and their encapsulation efficiency. Other compounds, e.g., tetracationic gemini Dabo-surfactants, have also been proposed for transdermal drug delivery [81]. By changing the lipid composition, the physicochemical properties of liposomes can be tailor-made to modify the delivery kinetics of APIs to the skin. For topical administration, liposomes can be further formulated into semi-solids such as creams, gels and ointments [82].  
As happens with lipid nanoparticles, when liposomes are applied onto the skin, a thin lipid film is formed which can offer occlusive properties while avoiding water evaporation from the skin, keeping it moist and permeable to APIs. Liposomes have been produced with lipids from stratum corneum for the treatment of atopic dry skin [83]. Several anti-oxidants (e.g., superoxide dismutase, catalase, glutathione, N-acetylcysteine, Coenzyme Q10, curcumin, resveratrol, tocopherols, retinoids, ascorbic acid, carotenoids) have been formulated in liposomes for anti-aging [84]. The antioxidants are also used to increase the physical and chemical stability of the vesicles when dispersed in water [85]. In addition, the antioxidant agents in their composition improve the stability of formulation also prevent the action of free radicals, pollution and oxidative stress, and improves skin health [42]. The liposomal anti-aging and antioxidant creams reduce the water loss of the skin, which reduces wrinkles and lines of expression, restoring the normal balance of the skin. Another clinical study has shown that flexible liposomes help in wrinkle reduction, have decreased the efflorescence in the treatment of acne and increase skin smoothness [86]. Liposomes have been formulated with chemical (benzophenone-3 [12], octyl methoxy cinnamate [87]) and physical (ZnO and TiO2 [88]) sunscreens to offer UV protection in cosmetic formulations. Liposomes are reported for the delivery of vitamins, mainly in anti-aging creams, moisturizing cream, sunscreens, and for the treatment of hair loss [89]. Natural marine lipid-containing polyunsaturated fatty acids have anti-inflammatory properties [90], which is beneficial in inflammatory skin disorders, and promote the hydration and softening of the skin. The combination of plant extracts, and/or isolated natural compounds, with liposomes created a new class of healthcare products, becoming commonly used in the production of sunscreen formulations, anti-aging and moisturizing products, as they have antioxidant, anti-cellulite, and antimicrobial effects [82].