Encapsulation approaches

Nanostructured silica and ordered mesoporous silicas have been envisaged as small enzyme immobilization supports [196]. The encapsulation approach is required either to further immobilize enzymes adsorbed in the channels by reducing the pore opening by further silylation or by encapsulating the enzyme directly [197]. 

Figure 2 shows the in situ IR spectra of styrene hydroformylation over 1.9 wt% Rh/sol-gel catalyst at 373 K and 0.83 MPa. Phenylpropanals, indicated by the C=0 stretching vibration at 1717 cm-i, formed after 2 min of the reaction in the batch mode. The IR intensity of the C=0 band increased with the reaction time. The sol-gel encapsulated Rh catalyst exhibited high stability at all reaction conditions. No rhodium leakage has been observed for the entire reaction process. However, a higher loading Rh/sol-gel catalyst is difficult to obtain by the sol-gel encapsulation approach due to the limited solubility of the Aliquat 336, [(CgHivlaN eJCf, which is needed to pair with RhCb- 3H 0. 

The encapsulation approach basically involves dissolution of the polymer and the active ingredient in a common solvent followed by spinning of the solution into fibres by suitable techniques. The technique makes it possible to load the fibre with the... 

Figure 11.12 A schematic illustration of the encapsulation approaches (a) loading the content after the synthesis process and then sealing the open ends of the CPCs by a further polymerization (b) entrapping the chemical species by their permeation phenomenon (See colour Plate 8)...

Figure 11.13 A schematic illustration of the encapsulation approach encapsulating the foreign species via a polymerization procedure...

Several works have been published involving CDs and EOs/volatiles. The association of those compounds (either as inclusion complexes or not) often advantageously modifies various physicochemical properties of the encapsulated molecules such as aqueous solubility and stability. Cabral-Marques (2010) has recently reviewed some issues on flavors/fragrances and/or EO and volatiles using CDs encapsulation approach. 

It should be noted that encapsulation approach could allow ... 

However, functions of these smart textile stmctures could still be improved and their role optimized. Bringing the textile/microcapsule systems for controlled release of active agents to a higher level demands a multidisciplinary approach. Considering all aspects of the controlled release systems such as active agent formulations, microcapsule polymer wall composition, encapsulation approach, and embedding loaded microcapsules into textile structures could push the limits of smart textiles even further. 

The delivery of inhibitors can be based on different possible release mechanisms that depend upon the encapsulation approaches. The simplest example is uncontrollable leakage of active ingredients from a polymer capsule ruptured under mechanical stresses. This approach was used by Yang and Van Ooij when triazole inhibitor was encapsulated using plasma polymerization to produce PP-perfluorohexane and PP-pyrrole layers employing RF plasma discharge (Yang and Van Ooij, 2004). The release of the inhibitor from such a capsule is possible only when it is mechanically mptured. 

The HLB system has made it possible to organize a great deal of rather messy information and to plan fairly efficient systematic approaches to the optimiza-tion of emulsion preparation. If pursued too far, however, the system tends to lose itself in complexities [74]. It is not surprising that HLB numbers are not really additive their effective value depends on what particular oil phase is involved and the emulsion depends on volume fraction. Finally, the host of physical characteristics needed to describe an emulsion cannot be encapsulated by a single HLB number (note Ref. 75). 

Classification of the many different encapsulation processes is usehil. Previous schemes employing the categories chemical or physical are unsatisfactory because many so-called chemical processes involve exclusively physical phenomena, whereas so-called physical processes can utilize chemical phenomena. An alternative approach is to classify all encapsulation processes as either Type A or Type B processes. Type A processes are defined as those in which capsule formation occurs entirely in a Hquid-filled stirred tank or tubular reactor. Emulsion and dispersion stabiUty play a key role in determining the success of such processes. Type B processes are processes in which capsule formation occurs because a coating is sprayed or deposited in some manner onto the surface of a Hquid or soHd core material dispersed in a gas phase or vacuum. This category also includes processes in which Hquid droplets containing core material are sprayed into a gas phase and subsequentiy solidified to produce microcapsules. Emulsion and dispersion stabilization can play a key role in the success of Type B processes also. 

Interfacial Polymerization. Many types of polymerization reactions can be made to occur at interfaces or produce polymers that concentrate at interfaces thereby producing microcapsules. Accordingly, this approach to encapsulation has steadily developed into a versatile family of encapsulation processes. Figure 4 schematically illustrates five types of encapsulation processes that utilize these types of reactions. 

A novel approach to the modification of aminoglycoside pharmacokinetics is under investigation (84). Administration of gentamicin encapsulated in egg phosphatidylcholine Hposomes has been found to lead to a longer half-life and much higher spleen and Hver levels for the gentamicin component. This formulation is undergoing clinical study (85). 

Erythrocyte Entrapment of Enzymes. Erythrocytes have been used as carriers for therapeutic enzymes in the treatment of inborn errors (249). Exogenous enzymes encapsulated in erythrocytes may be useful both for dehvery of a given enzyme to the site of its intended function and for the degradation of pathologically elevated, diffusible substances in the plasma. In the use of this approach, it is important to determine that the enzyme is completely internalized without adsorption to the erythrocyte membrane. Since exposed protein on the erythrocyte surface may ehcit an immune response following repeated sensitization with enzyme loaded erythrocytes, an immunologic assessment of each potential system in animal models is required prior to human trials (250). 

PPV and its alkoxy derivatives are /j-type conductors and, as a consequence, hole injection is more facile than electron injection in these materials. Efficient injection of both types of charge is a prerequisite for efficient LED operation. One approach to lowering the barrier for electron injection is the use of a low work function metal such as calcium. Encapsulation is necessary in this instance, however, as calcium is degraded by oxygen and moisture. An alternative approach is to match the LUMO of the polymer to the work function of the cathode. The use of copolymers may serve to redress this issue. 

---