Ionic interactions arise from electrostatic attraction between two oppositely charged species (Figure ). The strength of an ionic bond in an aqueous environment ranges between 5-10 kcal/mol and scales directly with the charges of the interacting species, while being inversely proportional to the distance between them. Ionically cross-linked hydrogels can be obtained through the interactions between cationic and anionic functional groups of polymers, or via charged functionalities of polymers and oppositely charged ions in solution that act as cross-linkers.