Hydrogels are hydrophilic, consisting mainly (85–95%) of water or an aqueous-alcoholic mixture and the gelling agent. The latter is usually an organic polymeric compound such as polyacrylic acid (Carbopol), sodium carboxy methylcellulose, or nonionic cellulose-ethers. Hydrogels have to be preserved against microbial growth.
After application, hydrogels show a cooling effect caused by evaporation of the solvent. They are easily applicable and humidify instantaneously, but if applied over a long time they desiccate the skin. For that reason, humectants such as glycerol may be added. After evaporation, the polymer residue may cause a sticky or ‘‘tearing’’ feel on the skin if inappropriate thickening agents have been used. Careful selection and testing of the needed adjuvants is therefore recommended.

Source: Handbook of Cosmetic Science and Technology - André O. Barel, Marc Paye, Howard I. Maibach

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