Entrapment technique

Apart from the passive encapsulation methods, different active entrapment techniques are described in the literature. Nichols and Deamer (1976) prepared liposomes with a pH gradient across the membrane (acidic interior with respect to the external buffer). These liposomes efficiently incorporated several catecholamines added to the external buffer. The same technique has been used to concentrate doxorubicin (DXR) in pH gradient liposomes (Mayer et al., 1986b). 

Entrapment in polymeric matrices is a variation of noncovalent attachment where the support is instead generated in the presence of the enzyme. A particularly popular entrapment technique is sol-gel encapsulation, where the enzyme is trapped within an Si02 matrix formed by acid- or base-catalysed hydrolysis of tetraalkoxysilanes in the presence of enzyme. ° The technique can be tuned to provide the appropriate microenvironment for each enzyme in much the same way as can be done with other immobihzation methods. ... 

Another modification has been described by Nyberg et al. [30]. In this design primary hepatocytes are entrapped in cylindrical collagen gels inside the lumen of the hollow fibres as shown in Fig. 3. The gel entrapment technique reported by this group enables a large number of hepatocytes to be employed in the bioreactor. Additionally, this technique also provides a three-dimensional frame-... 

With hydrophobic polyurethanes, the only option is adhesion. The polyol and isocyanate environments producing hydrophobic polyurethanes by the prepolymer method are thought to be too severe for hving cells. As noted earher, the weaker adhesion of adsorption to a hydrophobic polyurethane can be an advantage. Sanroman compared die adsorption and entrapment techniques and determined that the adsorption technique was superior based on citric acid productivity and operational stability. 

PREPARATION OF CATALYTIC CA-ALGINATE GEL BEADS BY ENTRAPMENT TECHNIQUE... 

Tkdc followed the free radical distribution in the molten phase as well as in the pre-flame and flame zones of a burning specimen of polypropylene rod by ESR spectroscopy. Using several entrapping techniques for the radicals, he detected free radicals, bi-radicals, and non-radical fragments leaving the molten phase for the gas phase where they may be combined with traces of oxygen on the surface into peroxy radicals or recombined by cyclization as well as by intermolecular reactions. 

The fourth and probably most popular enzyme immobilization technique is the entrapment technique. In this case, monomer or low molecular weight water soluble polymers are crosslinked in the presence of the enzyme to entrapment the enzyme into the polymer matrix. This has been done with a variety of redox polymer (osmium and ferrocene-based), as well as sol-gels and other hydrogels. This technique effectively covalently links the enzyme to the electrode surface and minimizes leaching and most of these polymers are hydrogels with facile transport of substrate/product in and out of the film. However, frequently this crosslinking affects specific activity of the enzyme. 

Figure 10.12 (a) Immobilization of enzyme using the entrapment technique in a matrix and (b) in a fiber. 

Immobilization of Enzyme in Capillary Microreactor by Entrapment Entrapment technique involves the entrapment of enzymes in gel matrix. The enzyme is mixed with gel formation ingredients and upon gel formation, the enzyme remains trapped in the matrix. Another form of entrapment is the formation of membrane around the droplet of enzyme, which is typically in solution. Immobilization by entrapment differs from adsorption and covalent bonding in that enzymes are free in solution but restricted in movement by the lattice structure of a gel. The membrane must be permeable to diffusion of substrate and product molecules... 

Details and specific formulations of these and other entrapment techniques can be found in Reference 2. In principle, there is no reason why these techniques need to be restricted to one enzyme systems, and, indeed, they have been extended to two or three enzymes in a number of instances. 

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