Gels Jellies

Gels are semisolid systems in which a liquid phase is trapped within an interlocking, three-dimensional polymeric matrix of a natural or synthetic gum. A high degree of physical or chemical cross-linking of the polymer is involved. It only takes from 0.5% to 2.0% of the most commonly used gelants to set up the systems. Some of these systems are as transparent as water, an aesthetically pleasing state. Others are turbid, as the polymer is present in colloidal aggregates that disperse light. Clarity of the latter ranges from slightly hazy to a whitish translucence not unlike that observed with petrolatum.

Agarose gels admirably illustrate the properties and to an extent the structural characteristics of most gels. Agarose solutions are water-thin when warm but solidify near room temperature to form systems that are soft to rubbery depending on the source and concentration of the agarose. A three-dimensional structure arises from the entwining of the ends of polymer strands into double helices. Kinks in the polymer mark the terminal points of these windings. Because individual polymer strands branch to form multiple endings, a three-dimensional array of physically cross-linked polymer strands is formed. The process of physical cross-linking is actually a crystallization phenomenon tying polymeric endings together, fixing the strands in place, yielding a stable, yet pliant structure (54). Less extensive structure than that found in agar growth media results in a spreadable semisolid suitable for medical application. The structure should persist to temperatures exceeding body temperature for the gelled systems to be the most useful. It is important to note that gelation is never a result of mere physical entanglement of polymer strands or otherwise the systems would only be highly viscid. The polymers used to prepare pharmaceutical gels include natural gums such as tragacanth, pectin, carrageen, agar, and alginic acid and synthetic and semisynthetic materials such as methylcellulose, hydroxyethylcellulose, carboxymethylcellulose, and carboxypolymethylene (carboxy vinyl polymers sold under the name Carbopol, B. F. Goodrich, Co., Cleveland, Ohio, U.S.).

Gels or jellies are used pharmaceutical^ as lubricants and also as carriers for spermicidal agents to be used intravaginally with diaphragms as an adjunctive means of contraception. Since the fluid phase of a gel does not have to be strictly water, gels offer a wide range of uses for, by blending solvents, it is possible to form films that exhibit a range of evaporation rates, solvency, and other release-determining attributes (55). Gel products containing anti-inflammatory steroids are used to treat inflammations of the scalp because this is an area of the body where creams and ointments are too greasy for patient acceptance.

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