1. Gel characteristics

Although the term "gel" has been given numerous definitions in the literature, it is generally accepted, particularly in the "sol-gel community", that:

In the following, we will adhere to this definition. In most cases, covalent chemical bonds form the basis of the gel's network (as in silica gels) [39] , but sometimes the network is based on weaker chemical bonds, such as coordination bonds or hydrogen bonds. The structure of the gel can (and usually does) change...