Gel-forming substances

Many gel-forming substances are available for preparing pharmaceutical gels and jellies. Although these substances share some common physical characteristics, the intended use may require gelling attributes of a certain substance or blend of substances. Table 3 lists the favorable properties of pharmaceutical gels for particular applications. 

Polyuronides may be defined as polysaccharides that contain one or more uronic acid units in their molecular structures. They have a frequent and wide occurrence in nature. Much of the carbohydrate material in plants belongs to the group. It includes all pectic materials and plant gums and many plant mucilages, hemicelluloses and gel-forming substances and some microbial polysaccharides. These substances are to be looked for in water-soluble plant exudates and mucilages, as well as in water and alkaline extracts of most plant materials. This review will be limited to the polyuronides occurring in plants. 

From a scientific point of view, medical textiles are located at the interfaces between technical disciplines and life sciences. On the one hand, the technical aspect concerns textile engineering, material chemistry, process control, testing and certification, etc., which are needed for the manufacture of high-quality medical textile products. On the other hand, life sciences such as medicine, microbiology, and other related subjects are required for the development of functional performances of these products. In practice, the different scientific and technical disciplines interact and overlap with one another, with new developments in any one of these branches able to generate new innovations in others. For example, new superabsorbent and gel-forming substances invented in chemistry have been applied in the development of new baby diapers and adult incontinence products. 

In view of the increase of the copper number it may be concluded that the substance undergoes strong hydrolysis and oxidation in the course of the treatment with alkali. Operations of this kind should therefore be performed with great care, since the over intensive action of these reagents and the appreciable fall in viscosity that ensues cause the gel formed after evaporation of the solvent to become brittle. In recent times therefore stabilization boiling under pressure has been widely applied. 

Starting from an aqueous acidic Al3+ solution (for example an aluminium sulphate solution) precipitation occurs if the pH of the solution is increased above about pH = 3 by addition of a base. The first precipitate is a gel-like substance in which minute crystals of boehmite (A10(0H)) are present. If this is filtered without aging and then calcined at temperatures up to 600°C an X-ray amorphous material is obtained. The material remains amorphous until after firing to temperatures greater than 1100°C. (X-AI2O3 is formed at higher temperatures. 

Several gel-forming cross-linked hydroxylic substances have been synthesized and tested for their molecular-sieve properties. The list includes cross-linked products of locust bean gum (Deuel and Neukom, 1954) polyvinyl alcohol, sorbitol, cellulose, starch, and partially hydrolyzed... 

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