4. Hydrogels

4.1 Definition

According to the European Pharmacopoeia, gels are semi-solid preparations for cutaneous application composed of a single-phase liquid base gelled with a suitable gelling agent. The active substance(s) is (are) dissolved or dispersed in the base. These preparations are packaged in single- or multi-dose containers. They are therefore thermodynamically stable systems consisting of a phase in which gelling agents control the rheological properties. They are characterized by viscoelastic rheological behavior and a certain degree of rigidity. A liquid is trapped between the meshes of the network. A distinction is made between hydrophilic gels, known as hydrogels, and hydrophobic gels, known as oleogels.

Single-phase hydrophilic gels feature a network...