Microcapsules containing hydrophobic

Encapsulation. Immobilization of enzymes by encapsulation within semipermeable structures dates back to the 1970s. There are three fundamental variations of this approach. In coacervation, aqueous microdroplets containing the enzyme are suspended in a water-immiscible solvent containing a polymer, such as cellulose nitrate, polyvinylacetate, or polyethylene. A solid film of polymer can be induced to form at the interface between the two phases, thereby producing a microcapsule containing the enzyme. A second approach involves interfacial polymerization in which an aqueous solution of the enzyme and a monomer are dispersed in an immiscible solvent with the aid of a surfactant. A second (hydrophobic) monomer is then added to the solvent and condensation polymerization is allowed to proceed. This approach has been used extensively with nylons, but is also applicable to polyurethanes, other polyesters, and polyureas. 

For microencapsulation by miniemulsion polymerization, oil can be a monomer and the hydrophobe can be a hquid active agent in the above-mentioned miniemulsion system. The monomer and oil are chosen in such a way that these two components are misdble before the polymerization process. As soon as polymerization takes place, phase separation occurs and at the end of polymerization microcapsules containing active agent are obtained. 

Figure 9.29 Water-repellent behavior of composite coating containing hydrophobic agent-microcapsules. (a) Copper-plating coating, (b) Composite copper-plating coat-...

A variety of stabihzers, including reactive diol containing two primary hydroxyl groups with a long hydrophobic moiety (e.g., PLMA) (Fig. 5.3a), an amphiphilic block copolymer (Fig. 5.3b) and graft copolymer (Fig. 5.3c) have been used in the preparation of polyurethane microspheres or microcapsules in nonaqueous medium [15-17,19-24]. 

Figure 9.5 SEM images of copper coating containing microcapsules prepared from (a) hydrophobic agent (PVA) (b) inhibitor...

The double-emulsion-solvent-evaporation technique is commonly used to prepare biodegradable hydrophobic microspheres containing hydrophilic pharmaceuticals, proteins, and polypeptides for sustained release applications (Schugens et al., 1994 Blanco-Prieto et al., 1997 Couvreur et al., 1997 Maa and Hsu, 1997 Leo et al., 1998 Zambaux et al., 1998). In most cases the micro-spheres are in the range size of 10 to 100 jm. However, Blanco-Prieto et al. managed to reduce the microcapsules sizes to less than 5 pm (Blanco-Prieto et al., 1997). 

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