**1. Introduction**

Nanostructures appear as either nanofibers, nanocompounds, nanomembranes, nanoparticles, or nanotubes, and have applications in different fields, such as medicine, cosmetics, environment, and nutrition. They can be used in biomedicine for diagnosis or the prevention and treatment of different diseases. They are employed as drug, protein, nucleic acid, and peptide carriers, or as biosensors, as well as for medical imaging [1, 2]. The incorporation of active principles in nano- or micrometric scale devices is called encapsulation. The encapsulated material is covered with a different type of material, which can be a polymer, a lipid, or a macromolecule. In general, encapsulation provides an increase in the stability of the encapsulated material, it also preserves its chemical and therapeutic properties, and it enlarges its average life, since it protects it from the effects of pH, heat, light, oxygen, humidity, and even from enzymatic degradation by nucleases and proteases, for instance. Besides, encapsulation offers the possibility to modify the physicochemical properties of the encapsulated material to facilitate manipulation, it reduces the loss of volatile compounds, it can mask unpleasant flavor and odor, improve bioavailability, and help the controlled release of active substances following a certain stimulus (pH, T, P, etc.) [3].

Regarding medical applications, encapsulated systems offer great possibilities of improving the safety and efficiency of countless drugs. They are capable of traveling across biological barriers, such as the skin, the gastrointestinal or respiratory mucous membranes, and even the blood-brain barrier (BBB). They can reach the target organ, tissue, or cell group where the drug has to act; they can even reach intracellular compartments. The distribution of the active principle, and therefore, its concentration in the target, is influenced by the size and the properties of the nanoparticles. This dependence permits minimization of side effects and an increase in the therapeutic power of the released molecule of interest, for example, in cancer treatment. Administration in the form of nanoparticles allows to orally dispense antitumor drugs, as well as biotechnologically originated molecules (peptides, proteins, plasmids, etc.), which are very sensitive to physicochemical and enzymatic degradation and cannot cross the mucous membranes [3].

Organic nanocarriers include nanoparticles such as solid lipid nanocarriers, liposomes, dendrimers, polymeric nanocarriers, micelles, and viral nanocarriers. Nanoparticles are defined as solid vesicles under 1000 nm, usually between 100 and 500 nm, formed by natural macromolecules, lipids, or synthetic polymers. For therapeutic application they must have a size under 200 nm since the width of the microcapillaries of the body is 200 nm [1]. The active principle is incorporated inside the nanoparticle, which can be a nanocapsule or a nanosphere. Nanocapsules are kind of reservoirs, they are vesicular systems, that is to say, traditional hollow shell structures constituted by a polymeric or lipid membrane and an internal core where the molecules of the drug are dissolved or dispersed. As to nanospheres, they are spherical matrixial systems, and the drug is homogeneously dispersed in the solid polymeric matrix [2, 4]. Core-shell nanoparticles offer great versatility and, depending on their composition, permit the encapsulation of a huge variety of molecules in solid, liquid, and semi-solid state. The nanocapsule shells can be prepared from several materials, such as polymers, lipids, phospholipids, and silica [5]. Different methods are used to build the core-shell structure, the polymers and methods employed are chosen according to the properties of the compound to be encapsulated and its application [6].

Some criteria to be considered before nanoencapsulation are clear definition of the desired objective, assurance that the active principle does not degrade during the fabrication process and that it disperses homogeneously inside the nanocapsule, choice of a suitable polymer or macromolecule, and cost/performance optimization. The encapsulating material must be biodegradable/bioerodible and inert with respect to the encapsulated material both in production and storage, offer the highest protection of the active principle, and be processable in suitable solvents for biomedical application. There is a real influence of the nature of the material and the production methodology on the physicochemical properties of the prepared systems. The main characteristics of nanoparticles are their large specific surface area, their homogenous dispersion in fluids, the capacity to encapsulate small molecules, and an adequate release rate [3].

In this chapter, systems based on natural macromolecules, lipids, or polymeric/ polyelectrolyte nanocapsules and their principal chemical and functional characteristics are described. In addition, a compilation of different methods and materials employed in the preparation of nanocapsules and a recent review of applications of lipid and polymeric nanocapsules have been made, focussing on the encapsulation of drugs.
