Microcapsules for controlled release fabricated via layer-by-layer self-assembly of polyelectrolytes

Layer-by-layer (LbL) self-assembly of oppositely charged polyelectrolytes on different templates by alternating deposition is a versatile method enabling the construction of ultrathin multilayer films with tunable thickness, composition, and functions. The principal driving force for the LbL self-assembly is dominantly the electrostatic attraction between the polyelectrolyte components accompanied with other associative interactions, such as hydrogen-bonding, hydrophobic interaction, charge-transfer and so on. The LbL self-assembly technique has been a powerful tool for micro/nano-encapsulation. Our strategy is to fabricate the nano-multilayer wall for micro- and sub-microcapsules by the LbL of polyelectrolytes, particularly natural polymers chitosan (CHI) and alginate (ALG) for drug controlled release. Our recent work following this strategy is reviewed in the present article. After determining the charge density threshold for the LbL assembly, we immobilised enzymes of urease and superoxide dismutase on polystyrene nanoparticles through the LbL and found the decrease in enzyme bioactivity but an increase in their storage stability. We successfully fabricated the nanocapsules from natural polysaccharides of CHI and ALG multilayers by the LbL for drug release. The LbL self-assembly of CHI and ALG was used directly on indomethacin (IDM) microcrystals to reduce the release rate. We observed that increasing deposition temperature would produce a more perfect multilayer film with higher thickness and reduced the release rate efficiently. Water soluble protein insulin was spontaneously loaded into the LbL CHI/ALG microcapsules due to the electrostatic attraction and a two-temperature loading procedure was suggested to increase the loading capacity and to reduce the release rate. The LbL multilayers have been used to encapsulate the drug-loading microparticles made from solvent evaporation or adsorption with porous CaCO3 microparticles to enhance the loading capacity and suppress the initial burst. Increasing the layer number, raising the deposition temperature, and cross-linking the neighboring layers were confirmed to slow down the enzymatic desorption of polyelectrolyte multilayer films and the release rate of encapsulated drug effectively.