Microcapsules semi-permeable

Enzymes can be immobilized by enclosing them within semi-permeable polymer membranes. The preparation of the microcapsules requires extremely well controlled conditions and it is possible to use different procedures for their preparation ... 

For biological particles, for example, cells which have a semi-permeable membrane and semi-permeable microcapsules, their mechanical integrity can be characterised by exposuring them to media with different osmotic pressures (Van Raamsdonk and Chang, 2001). 

The wall of the coacervate drop can be made permeable, semi-permeable, or impermeable to diffusion of molecules through the microcapsule wall. The rate of release of the contents of the coacervate drop or intake of molecules from outside the drop depends on the nature of the polymer(s) which make up the wall material, the thickness of the microcapsule wall, the pore width of the wall, the molecular weight of permeating materials, and the degree to which the polymeric wall materials are cross-linked (2). 

Enzymes may be immobilized by encapsulation in nonpermanent (e.g., liposomes) or permanent (e.g., nylon) microcapsules. The enzyme is trapped inside by a semi-permeable membrane, where substrates and products are small enough to freely diffuse across the boundary. While nonpermanent microcapsules are useful in biochemical research, only permanent microencapsulations yield analytically useful systems, because of their mechanical stability. 

Another coating technique is microencapsulation (see also Section 4.1.3, Figure 94). The technique coats liquid droplets or solid particles and forms microcapsules with diameters between 1 and 5000/im. The coating consists of natural or synthetic polymers and may be dense, permeable, or semi-permeable. Therefore, this technology allows capsules containing a reactive substance to be produced which can be liberated in a controlled fashion by destruction of the skin or by permeation. It is also possible to carry out reactions within the capsules by permeation of reaction partners from the outside. 

Encapsulation achieves the confinement of biological components by using various semi-permeable membranes. Encapsulation allows for the enzymes to exist freely in solution, which is confined within the small area surrounded by the membrane. Macromolecules cannot cross the membrane barrier, which is permeable for small molecules only (substrates or products). Nylon and cellulose nitrate are the most popular materials used for the production of microcapsules that need to have a chameter between 10 and 100 pm chameteis. Furthermore, biological cells could be used as capsules as it shown in erythrocytes based sensor. Alternatively enzyme solution can be encapsulated in a thin layer, which covers the electrode and confined between the electrode and semi-permeable membrane surface. ... 

Pariot N, Edwards-Levy F, Andry M-C, Levy M-C. Cross-linked P-cyclodextrin microcapsules. II. Retarding effect on drug release through semi-permeable membranes. Int J Pharm. 2002 232(1-2) 175-181. 

For the microcapsule-type entrapment method, a semi-permeable polymer membrane is used to surround the enzymes. As with lattice-type entrapment, control of the conditions is very important as they can have a detrimental effect on the preparation of the enzyme microcapsules. There... 

Enzymes may also be immobilized by microencapsulation. In this technique, which has medical applications, enzymes are enclosed by various types of semi-permeable membrane, e.g. polyamide, polyurethane, polyphenyl esters and phospholipids. Microcapsules of phospholipids are also called liposomes. The micro-encapsulated enzymes and proteins inside the micro-capsule cannot pass the membrane envelope, but low M, substrates can pass into it, and products can leave. Such encapsulated proteins do not elicit an antigenic response, and they are not attacked by proteases outside the microcapsule. They are therefore suitable for the delivery of enzymes for therapeutic purposes. This area of application is still at an early stage of development, but positive results have been reported from animal experiments and clinical studies, e.g. treatment of inherited catalase deficiency with encapsulated catalase. There are various methods of administration intramuscular, subcutaneous or intraperito-neal injection. However, their major area of application is outside the body. For example, microencapsulated urease can be employed as an artificial kidney in hemodiffusion (Rg.2). 

The abundance of natural and man-made polymers provides a wider scope for the choice of shell material, which may be made permeable, semi-permeable or impermeable. Permeable shells are used for release applications, while semi-permeable capsules are usually impermeable to the core material but permeable to low molecular-weight liquids. Thus, these capsules can be used to absorb substances from the environment and to release them again when brought into another medium. The impermeable shell encloses the core material and protects it from the external environment Hence, to release the content of the core material the shell must be ruptured by outside pressure, melted, dried out dissolved in solvent or degraded under the influence of light (see Chapter 7). Release of the core material through the permeable shell is mainly controlled by the thickness of the shell wall and its pore size. The dimension of a microcapsule is an important criterion for industrial applications the following section will focus on spherical core-shell types of microcapsules (Fig. 1.8). 

I. Leveque, K. H. Rhodes and S. Mann, Biomineral-inspired fabrication of semi-permeable calcium-phosphate -polysaccharide microcapsules. /. Mats Chem. 12, 2178-2180 (2002). 

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