ABSTRACT

The skin is the largest human organ and absorption tissue in the body. However, the outerm ost layer of the skin (stratum corneum , SC) is a very effective barrier to the entrance of xenobiotics, and not all kinds of drugs can passively diffuse through this organ. The possibility of diffusion is mainly limited to the physicochemical properties of the molecules, such as molecular weight, partition coefficient, and

ionization degree in the formulation, among others. Therefore, it is possible to modulate the access o f drugs to the deeper skin layers, if the therapeutic target is local, or even achieve plasma concentrations to provide systemic effects. If the intention is a local therapy, drugs should not reach systemic concentrations, as the effects would be regarded as undesired side effects. In this case, m atrix systems can be a good approach to control drug release and to reduce or retard drug diffusion. On the contrary, if the desired effect is systemic and the drug is not absorbed in a sufficient rate and amount, there is a need to enhance penetration. For these purposes, polyester particles have been deeply investigated. Particles can provide additional advantages, as for example, protecting the drug from chemical or light degradation, increasing drug solubility in aqueous solvents or the stability. Recently, studies have been started to focus on hair follicle targeting, as it has been observed that some kinds of nanoparticles can accumulate into the hair follicle and release the drug for a longer period with topical and systemic applications as well. Furthermore, some groups are also investigating the possibility to develop immune responses using nanoparticles that can be phagocytized by skin immune cells. This fact would represent a new very promising approach to develop non-invasive vaccines that would ease the procedures and increase patient's acceptability.