Nanoengineered Polymer Microcapsules
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The nanoengineering of colloidal surfaces and the design of functional colloid particles are currently interesting topics of applied chemistry and biochemistry in the field engaged in developing new materials with tailored properties. Research on composite colloidal particles (core–shell structures with size ranging from 1 to 1000 nm) has created interest because of various applications expected in the areas of coatings, electronics, photonics, catalysis, biotechnology, sensorics, medicine, ecology, and others. In general, the research on core–shell structure and encapsulation implies the formation of a colloidal core of defined content and size, and the preparation of a shell providing the required stability, permeability, compatibility, release of core material, and catalytic or affinity properties. Tailoring the different components of one particle becomes important to develop these functionalized colloids (i.e., to combine several properties in one core–shell structure). The desired properties may be adjusted to facilitate the interaction of the core with different solvents or cell membranes. The shell may also have magnetic, optical, conductive, or targeting properties for directing and manipulating the core containing bioactive materials.
A major task in the development of advanced drug formulations deals with the elaboration of delivery systems for providing the sustained release of bioactive materials. Mostly, these systems comprise polymer particles in the size range of 50 nm–100 µm. Drug molecules are embedded in polymer matrices, or in core–shell structures. In the latter case, the shell permeability or degradation rate determines the release rate of the bioactive core material. The composition of the shell may additionally provide certain functionalities.
This review is devoted to the literature on recently introduced novel pathways to fabricate nanoengineered core–shell structures, which can employ a great variety of substances as shell constituents and can be incorporated into hollow spheres. The way the shells are assembled on colloids resembles the formation of ultrathin polymer films by layer-by-layer (LbL) adsorption on macroscopic flat support, an idea proposed by Iler in 1966 and later developed by Lee et al. In 1991, Decher and Hong proposed a method of forming polyelectrolyte films by using the alternate adsorption of polycations and polyanions. A crucial factor for polyionic LbL assembly is the change of the sign of the surface charge on polyelectrolyte adsorption. Beginning in 1998, this strategy of LbL assembly of charged species was transferred to coat micron-sized and submicron-sized colloidal particles. The idea emerged to employ the nanoengineered properties of multilayers as shell structures formed on colloidal particles.