A gel is a pharmaceutical vehicle which consists of an aqueous phase, with the addition of a gelling agent. Gelling agents are usually long organic polymers which contain many hydrophilic groups. The addition of these polymers increases the viscosity of the mixture because they bond to the liquid medium, and to other gelling agent molecules forming networks(with pockets of the liquid trapped within the gelling agent networks). When the gelling agent is an organic polymer which is lyophilic (soluble in the medium), the system is more easily mixed together and will be more stable. Cellulose derivatives are often used as gelling agents. Rarely, lyophobic(insoluble in the liquid medium) substances will be incorporated into a medium to form a gel like system, these substances are harder to incorporate into the medium and have less of an effect on viscosity upon addition. The majority of a gel is liquid with around 0.5-2% being the gelling agent. The viscosity of a gel varies by the gelling agent used. Gels may also contain preservatives, solvents, and medicinal agents. Gels can thicken if they stand for too long and should be mixed prior to use.

Gels are considered a colloidal dispersion. Colloidal dispersions are different from solutions because of the particle size of the phase that is dispersed within the medium. The particle size is typically between 1nm and 0.5 um, with the particles usually containing aggregates of many molecules, sometimes one large organic molecule. Because of the particle size of the colloidal dispersion the mixture appears cloudy or turbid, while liquid solutions appear clear.

Gels can undergo syneresis, swelling, and imbibition. Swelling is when the gel actively absorbs available water, leading to an increase in volume. Imbibition is the ability of the gel to absorb water without an increase in volume. Syneresis demonstrates instability properties of the gel. Syneresis occurs when the dispersed phase within the medium begins to bond together, removing the liquid phase from the gelling agent networks. This results in a decrease in volume and is often a result of the gel standing for too long.

For these reasons gels are used as cell carriers, for biosensors and for others aspects of biomedical engineering.

Single phase gels are gels in which the large molecules of the gelling agent are fully surrounded by the liquid, and are evenly dispersed throughout the liquid. This occurs when the dispersed particles are highly soluble in the medium.

Two phase gels, or magmas, are gels in which small aggregates of the gelling agent molecules called floccules form throughout the gel. Typically the dispersed particles are less soluble in the medium.

Hydrogels are gel systems in which the dispersed phase has water soluble properties.

Organogels are gel systems in which the dispersed phase is a hydrocarbon, animal fat, soap based grease, vegetable oil, or polar organogel.

Inorganic gels are two phase systems, examples include aluminum hydroxide gel and bentonite magma.

Organic gels are usually single phase systems.

Common gelling agents include carboxymethylcellulose, hydroxypropyl methylcellulose and carbomers.