Gels are an excellent formulation for several routes of administration. They are useful as liquid formulations in oral, topical, vaginal, and rectal administration. Gels can be clear formulations when all of the particles completely dissolve in the dispersing medium. But this doesn't occur in all gels, and some are, therefore, turbid. Clear gels are preferred by patients.
Gels are made using substances (called gelling agents) that undergo a high degree of cross-linking or association when hydrated and dispersed in the dispersing medium, or when dissolved in the dispersing medium. This cross-linking or association of the dispersed phase will alter the viscosity of the dispersing medium. The movement of the dispersing medium is restricted by the dispersed phase, and the viscosity is increased.
If the gel contains small discrete particles, the gel is called a two-phase system. If the gel does not appear to have discrete particles, it is called as a one-phase system. Two-phase systems are thixotropic; e.g., they are semisolid on standing but liquefy when shaken. If the particle size in a two-phase system is large, the gel is referred to as a magma.
Examples of two-phase systems include Aluminum Hydroxide Gel and Bentonite Magma. Single-phase systems contain linear or branched polymer macromolecules that dissolve in water and have no apparent boundary with the dispensing medium. These macromolecules are classed as natural polymers (i.e., tragacanth), semisynthetic cellulose derivatives (i.e., methylcellulose), or synthetic polymers (i.e., Carbomer polymers). Single-phase gels made from synthetic or natural macromolecules are called mucilages.