Semi-permeable capsules

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). 

We now consider a capsule which consists of liquid surrounded by a closed semi-permeable membrane (figure 2) details are provided in [3,4], Water and salt can pass through the membrane from side 1 (inside the capsule) to side 2 (outside), and vice versa, but large polymer molecules cannot. Trapped inside the capsule are n p polyelectrolyte molecules of valence zp and partial molar volume Tip. The resulting Donnan equilibrium is reviewed in [5, 6], Inside the capsule, electroneutrality requires zpn p + z+n + + Z-ri - = 0. We now assume the salt to be monovalent. At equilibrium there is a jump in electrical potential across the membrane inside the capsule x +xi- r X2+X2- x2 with x = (Q =F zpX p) where... 

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. ... 

Aqueous-based solutions in semi-permeable packaging, and dosage forms sensitive to low humidity, e.g., hard-gelatin capsules, may require testing at low humidity according to the procedure described in this guideline. 

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). 

A reservoir system consists of an active substance and a membrane, and therefore is also known as a membrane coti-trolled system. A membrane or coating can be applied to a whole tablet or capsule, or to a tablet core. Granulates and even crystals can be coated as well, which are then processed into tablets or capsules. Enteric-coated dosage forms have an acid-resistant coating, which dissolves when the pH is increased. An osmotic system may be regarded as a particular reservoir system, because it has a semi-permeable membrane that is provided with holes with an exact diameter. 

Table 2 indicates that the most suitable capsular membranes comprised semi-or non-transparent systems. Generally, the multicomponent blending resulted in smooth capsules with the exception of the alginate/spermine-polymethylene-co-guanidine systems which were either irregularly shaped or mosaic. There was no correlation observed between the capsule turbidity and permeability. 

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