Composition

A solid network spans the volume of a liquid medium. Both by weight and volume, gels are mostly liquid in composition and thus exhibit densities similar to liquids. However, they have the structural coherence of a solid. The network can be composed of a wide variety of materials, including particles, polymers and proteins.

Cationic polymers

Cationic polymers are positively charged polymers. Their positive charges prevent the formation of coiled polymers. This allows them to contribute more to viscosity in their stretched state, because the stretched-out polymer takes up more space than a coiled polymer and thus resists the flow of solvent molecules around it. Cationic polymers are a main functional component of hair gel, because the positive charged polymers also bind the negatively charged amino acids on the surface of the keratin molecules in the hair. More complicated polymer formulas exist, e.g., a copolymer of vinylpyrrolidone, methacrylamide, and hydrogel N-vinylimidazole.

Hydrogels

Hydrogel is a network of polymer chains that are water-insoluble, sometimes found as a colloidal gel in which water is the dispersion medium. Hydrogels are superabsorbent (they can contain over 99% water) natural or synthetic polymers. Hydrogels also possess a degree of flexibility very similar to natural tissue, due to their significant water content.

Common uses for hydrogels include

• currently used as scaffolds in tissue engineering. When used as scaffolds, hydrogels may contain human cells in order to repair tissue.
• environmentally sensitive hydrogels. These hydrogels have the ability to sense changes of pH, temperature, or the concentration of metabolite and release their load as result of such a change.
• as sustained-release delivery systems
• provide absorption, desloughing and debriding capacities of necrotics and fibrotic tissue.
• hydrogels that are responsive to specific molecules, such as glucose or antigens can be used as biosensors as well as in DDS.
• used in disposable diapers where they "capture" urine, or in sanitary napkins
• contact lenses (silicone hydrogels, polyacrylamides)
• medical electrodes using hydrogels composed of cross linked polymers (polyethylene oxide,polyAMPS and polyvinylpyrrolidone)
• Water gel explosives

Other, less common uses include

• breast implants
• granules for holding soil moisture in arid areas
• dressings for healing of burn or other hard-to-heal wounds. Wound GEL are excellent for helping to create or maintain environment.
• reservoirs in topical drug delivery; particularly ionic drugs, delivered by iontophoresis (see ion exchange resin)