Table 2.

Characteristics of the most commonly used bio-ink constituents.

	type	cross-linker	pros	cons	references
agarose	natural	thermoresponsive	cheap, good printability	non-adherent and bioinert	[47]
alginate	natural	ionic, (Ca2+, Sr2+, Ba2+)	good printability, tunable characteristics, fast cross-linking	difficult to control shrinking during cross-linking, bioinert	[48–51]
chitosan	natural	pH neutralization		pH changes required	[47]
collagen	natural	thermoresponsive, pH	biologically relevant, adherent, reasonable printability	pH changes required or cold bed for thermal gelling, characteristic of fibrosis, must be sourced from humans for clinical use	[52]
fibrin	natural	enzymatic (thrombin)	biologically relevant and adherent	poor printability	[24,53]
gelatin	natural	thermoresponsive, or UV if methacryloyl	cheap, good printability, adherent and bioactive	cold bed or UV exposure required to cross-link, must be sourced from humans for clinical use, poorly defined	[13,54]
gellan gum	natural	ionic (Ca2+)	cheap, reasonable printability, tuneable with peptide motifs	low mechanical properties, non-adherent and bioinert	[22,55]
hyaluronic acid	natural	dependent on modification	reasonable to print, biologically active and relevant	low mechanical properties, must be human sourced for clinical use, cross-linker can be harmful (H2O2)	[18,56]
pluronic	synthetic	thermoresponsive	good printability, highly tunable viscosity, sacrificial	mainly used for sacrificial inks, requires cold printing bed to maintain structure	[24]
poly(ethylene glycol) (PEG)	synthetic	UV exposure	good printability, tunable, well-defined polymer	potentially harmful UV and photo-initiator exposure	[57]
poly(caprolactone) (PCL)	synthetic	thermoresponsive (high temp)	mechanically strong, bioinert	not suitable for cell printing due to high melting point	[24,58]