Chapter 2: Introduction to Hydrogel Synthesis and Crosslinking Methods for Developing Bioinks for 3D Bioprinting

Hydrogels are three-dimensional networks formed from crosslinked polymer systems and are mainly used as biomaterials in biomedical fields including 3D bioprinting for tissue engineering applications. These are generally developed from natural and synthetic sources and are classified into various types based on their crosslinking strategies and functional properties. For instance, chemically crosslinked hydrogels and physically crosslinked hydrogels are the two broad categories of hydrogels. Chemical crosslinking methods mainly involve the use of chemical agents (zero and non-zero length crosslinkers), followed by photo-polymerization and other reactions such as Schiff's base reaction, hydrazone bond formation, Diels–Alder reactions, azide–alkyne cycloaddition reactions, enzyme-driven reactions, etc. In physical crosslinking methods, ionic and coordination bonds crosslinking, hydrogen bonding crosslinking, host–guest interactions crosslinking and self-assembling peptide crosslinking are reported for developing hydrogels. The applications of such hydrogel biomaterials as bioinks in 3D bioprinting are discussed and elaborated in this chapter.