Table 1.

Hydrogels’ classification and main features based on different parameters.

Criteria	Classification	Features
Crosslinking	Chemical	Polymers are covalently crosslinked by permanent junctions. It can be carried out by the addition of crosslinker molecules, polymer–polymer conjugation, or photoinitiators.
	Physical	Polymers are hold together by chain entanglements and/or hydrogen bonds or hydrophobic or ionic interactions.
Polymeric composition	Homopolymer	Hydrogel derived from a single species of monomer.
	Copolymer	Hydrogel consists of two or more different monomers with at least one hydrophilic component.
	Semi-interpenetrating network	Hydrogel consists of one crosslinked monomer and another non-crosslinked component.
	Interpenetrating network	Hydrogel are made of two independent crosslinked polymeric chains contained in a network form.
Source	Natural	Polysaccharides and proteins are examples of polymers for natural hydrogels. They are biocompatible and biodegradable.
	Synthetic	Synthetic hydrogels have higher strength and can be designed to have specific mechanical and chemical properties.
	Hybrid	Hydrogels consists of a combination of synthetic and natural polymers.
Physical structure	Amorphous	They contain randomly arranged macromolecular chains.
	Crystalline	They possess dense regions of ordered macromolecular chains.
	Semicrystalline	A mixture of amorphous and crystalline phases.
Network electrical charge	Nonionic	They do not present any charged groups and have ultra-durable and permanent connections.
	Ionic	They can be positive or negative and have different behaviors according to the pH.
	Amphoteric	Hydrogels contain both acidic and basic groups.
	Zwitterionic	They present an equal amount of positive and negative charge.