## 1 Introduction
On the other hand, lipidic vesicles, such as liposomes, ethosomes, and O/W emulsions, containing waxy compounds pose an innate instability, since, in aqueous media, phospholipids and triglycerides can slowly become oxidized or hydrolyzed [13]. One of the probable solutions to this problem is the entrapment of the OBS into a colloidal lipidic matrix (LM) due to the solid-like state of these systems, reducing the oil mobility and its leakage [14]. If this LM does not melt at body temperature, the OBS are released in a controlled fashion by matrix degradation rather than diffusion. The LM protects the oil from degradation by skin enzymes and, thus, serves as a localized OBS delivery in some types of skin ailments [15]. Hot homogenization is the most widely used preparation method for these matrixes, due to the good reproducibility, high concentration of lipid phase, and absence of organic solvents. This method implies heating above the lipid melting point under homogenization or ultrasonication, followed by rapid cooling in order to achieve partial crystallization [16]. The LM enhances skin penetration, hydration, and film forming properties as compared to current emulsions. As a result, an occlusive effect takes place, which in turn is proportional to the degree of crystallinity of the matrix, whereas amorphous materials do not have occlusive properties. Thus, a high crystallinity and, hence, occlusive effect prevents OBS from evaporation allowing for a deeper penetration into the skin [17]. Further, highly crystalline LM might act as physical sunscreens scattering or reflecting UV radiation. LM are able to retain OBS without the need of additives due to the formation of intramolecular (H-bonding and polar-polar interactions) and intermolecular interactions (dipole-dipole interactions) between the constituents stabilizing the structure of the crystalline network [18]. Therefore, the polarity of OBS affects the magnitude of crystal formation and self-assembling, since compounds having low polarities assemble into short and dispersed fibers that grow and become interconnected, whereas highly polar compounds form conglomerates due to the free mobility in the medium. Thus, an apparent balance between hydrogen bonds and hydrophobic interactions is critical easing self-assembling and the development of a defined micro architecture. In fact, the waxy molecules should be partially insoluble with the OBS to encourage crystallization, but, at the same time, should be partially miscible within the OBS to generate polar and hydrogen bond interactions [19]. The incorporation of complex lipids, such as natural waxes into LM, enhances crystal formation and loading capacity as compared to pure triglycerides. These waxes are able to form an isotropic LM with oils, cosolvents, and surfactants due to the low polarity, long chain length, and high melting point of their compounds (Table 1) [20]. They are very stable resisting oxidation and hydrolysis, and contribute to the excellent skin tolerance, lubrication, and cleansing performance of LM [21]. The aim of this study was to evaluate the rheological, thermal, and microstructural properties of lipidic matrixes produced with representative natural waxes and LEO. The topical application of these lipidic matrixes allows for a localized action, reduce the systemic risk of toxic effects, avoid first-pass hepatic metabolism, it is painless, easy, and increase patient compliance.