Crosslinking is the process of joining two or more polymer chains. Both chemical and physical crosslinking exists. In addition, both natural polymers such as proteins or synthetic polymers with a high affinity for water may be used as starting materials when selecting a hydrogel. Different crosslinking methods can be implemented for the design of a hydrogel. By definition, a crosslinked polymer gel is a macromolecule that solvent will not dissolve. Due to the polymeric domains created by crosslinking in the gel microstructure, hydrogels are not homogenous within the selected solvent system. The following sections summarize the chemical and physical methods by which hydrogels are crosslinked.