Nanoengineered Capsules with Specific Layer Structures
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Encapsulation techniques gained great importance during the last decades to protect, store or release materials in well-defined ways. As in other technical fields, the miniaturization of capsules progressed parallel to the development of new materials and techniques. At the present time, microcapsules are already used in many technical applications in the pharmaceutical, cosmetic, food, textile, adhesive, printing, and agricultural industries.
Several approaches are used to fabricate nanocapsules: first, one can use aggregates of lipid molecules in spherically closed bilayer structures, the so-called vesicles or liposomes. These relatively unstable structures can be stabilized by cross-linking. In a similar fashion, amphiphilic block copolymers in aqueous solution can also aggregate to vesicular structures.
Other approaches to prepare capsules concern suspension and emulsion polymerization techniques around latex particles, dendrimers or hyperbranched polymers. Finally, one can cover a sacrificial template core with a membrane permeable for the products of core dissolution. Based on this approach, a new type of microcapsule prepared from polyelectrolytes (PE) was developed 5 years ago using the layer-by-layer (LbL) technology for encapsulating dissolvable templates in the micrometer and nanometer range. The LbL technology enables the nanometer-precise assembling of multilayers. A wide variety of materials can be combined yielding capsules, which can have simultaneously several functions in the capsule wall as well as an adjustable semipermeability. Furthermore, the inner and outer capsule surface can be easily functionalized by biological or chemical binding sites, and the interior can be filled with macromolecules or with precipitates of low molecular weight agents. Monodispersity of the capsules can be achieved by using monodisperse templates.