Nanoencapsulation of Bioactive Substances
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Nanoencapsulation is one of the most important subcategories of controlled-release bionanotechnology. Normally, active substances are encapsulated in submicrometer-sized devices made of barrier materials. These materials are designed to control the rate of release. This concept has been largely inspired by spontaneous assembling of the phospholipid liposomes as a model of biological membranes. As in Nature, one has to develop preparations of nanovehicles that allow precise control over their structure and morphology. In this context, the self-assembled superstructures of surfactants (micelles, liposomes) and/or polymers (nanoparticles) have proven to be valuable tools.
Nanoparticles may be defined as being submicrometer (from 10 to 1000 nm) colloidal systems generally, but not necessarily, made of polymers (biodegradable or not). Depending on the process used for their preparation, two different types of nanoparticles can be obtained, namely, nanospheres and nanocapsules. Unlike nanospheres (matrix systems where the bioactive substance is dispersed throughout the particles), nanocapsules exhibit a membrane-wall structure with an aqueous or oily core containing the bioactive substance. Thus, nanocapsules may be considered as a “reservoir” or “envelop” system. Because nanoparticles have very high surface areas, the active substance may also be adsorbed or conjugated onto the surface. At present, micellar/liposomal systems have also been included under the term “nanoparticles.”
Another type of nanometer-sized carriers is an inclusion complex or clathrate, which can be assembled by inclusion of bioactive substances into molecular cavities of the so-called cavitands, or dendrimers. Natural examples of such internal-cavity-containing molecules are cyclodextrins, which are used widely for preparation of various drug formulations. The outer diameter of molecular nanocapsules is in the range of 3–50 nm. The nanometer size ranges of liposomes, nanoparticles, and clathrates offer certain distinct advantages for drug delivery. Because of their subcellular size, they can penetrate deep into tissues through fine capillaries, cross the fenestration present in the epithelial lining (e.g., liver), and generally are taken up efficiently by the cells to perform the so-called intracellular trafficking. This allows direct delivery of therapeutic agents to target sites in the body followed by the controlled release.