FIGURE 2.

Schematic diagram of common methods for preparing injectable hydrogels. Injectable hydrogels can be roughly divided into two gel methods: physical cross-linking and chemical cross-linking reactions. The difference between them is whether or not covalent bonds are formed. Physical cross-linking is non-covalent bonding via reversible and instantaneous connections, including physicochemical or molecular entanglement interactions (hydrogen bonding, ionic or hydrophobic interactions). Chemical cross-linking forms covalent bonds in various chemical processes, including enzyme-mediated cross-linking, photopolymerization, click chemistry, Michael’s addition, Schiff base chemistry, and cross-linking agents. Adapted with permission of Wu et al. 2(2020).