**2.1 From hydrophilic monomers**

The hydrogels with good swelling ability are synthesized from water-soluble hydrophilic acrylic monomers such as acrylamide, acrylic acid, acrylates, itaconic acid, etc. using suitable initiators and crosslinkers through radical or photochemical polymerization methods [17]. This will be achieved either by solution or suspension or emulsion or bulk polymerization methods [17]. Free-radical polymerization mechanism is predominantly employed to synthesize hydrogel using olefinic monomers. The initiation of monomers is carried out by the initiators such as peroxides (benzoyl or t-butyl peroxides), azo-compounds (azobisisobutyronitrile) and persulphates. Peroxides and peroxy compounds can facilitate ambient temperature polymerization under the influence of tetramethylene diamine, sodium metabisulfite/ferrous salts, triethylamine, etc. [17, 18]. Benzyl alcohol, ethanol, water, and ethanol-water mixtures are commonly used solvents to achieve solution polymerization. The monomers (**Table 1**), cross-linkers (**Table 2**) and natural polymers (**Table 3**) that are used for hydrogel synthesis are given in the respective Tables.
