A micro-or nanocapsule is shortly defined as a small portion of an active substance that is surrounded by an encapsulating agent with dimensions in the micro- or nanometer regime, thus isolating this substance from the external medium. This protection can be permanent or temporal, in which case the core is generally released by diffusion or in response to a trigger, such as shear, pH, or enzyme action, thus enabling their controlled and timed delivery to a targeted site.
Nanocapsules may range from 1 to 1000 nm in size and they have a multitude of different shapes, depending on the materials and methods used to prepare them. The structure of encapsulated ingredients, which largely depends on the selected shell material and nanoencapsulation method, can be classified into two main categories: capsules with (a) a core that is surrounded by a shell of the matrix material or (b) a core that is entrapped within a continuous network of the matrix material.
Variations of these morphologies include capsules with multiple cores or multilayered capsules. However, the most significant feature of nanocapsules is their nanoscopic size that provides a large surface area. The total surface area is inversely proportional to the capsule diameter. This large surface area is appropriate for incorporating recognition species (functionalization with peptides, antibodies, organic polymers, etc.), sites of adsorption and desorption, chemical reactions, and light scattering, among others.
Morphologies of nanocapsules (from left to right): (a) single-core capsule, (b) dispersed core in polymer gel, (c) multilayer capsule, (d) dual-core capsule, and (e) single-core-multi-shell capsule.
Many different materials can be used as encapsulating matrices, which must be selected depending on the critical properties needed for each intended application. The majority of these carriers are proteins (gelatin and albumin), polysaccharides (dextrin, starch, gums), fats, liposomes, biopolymers, co-polymers (poly(lactic-co-glycolic acid)), micelles, organogels, dendrimers, solid nanoparticles (SLN), polymeric nanoparticles, emulsion-based systems, and metal-organic particles.
The use of encapsulation technologies offer an impressive number of advantages and new properties: (i) unstable materials (e.g. pure chemical substances, viruses, etc.) can be protected from the environment and stabilized or separated from other incompatible components; (ii) the properties of encapsulated materials can also be modified (e.g. taste masking, odor masking, etc.); (iii) the industrial processes can be improved or facilitated (e.g. transformation of liquids into solids for easier handling, reduction of toxicity during manipulation, etc.); and (iv) the release of encapsulated active materials can be modified, providing sustained release (maintaining the right concentration), long lasting (and therefore improving effects), target release (improving adhesion, penetration, or recognition of tissues and cells), or triggered release (mainly by environmental changes in pH, temperature, etc.). Encapsulation and release modification also reduces doses, and therefore, potential toxicity of the encapsulated substances, such as drugs.
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