Table 1.

Key factors influencing the printability of extrusion-based printed scaffolds.

Elements affecting printability	Bioinks	Results	Refs.
Bioink-related factors
Flow behavior and bioink composition
	Chitosan, chitosan-collagen, and methylcellulose-hyaluronan	Higher printability of bioinks with higher viscosity.	[91]
	Oxidized alginate	Viscosity in the range of 400–3000 mm2/s has a relatively high printability.	[92]
	Methacrylated hyaluronic acid and methacrylated gelatin	Highly viscous bioinks are not printable; similarly, low viscosity bioinks have poor printability.	[93]
	Alginate-nanocellulose	Nanocellulose can improve the printability of alginate by increasing viscosity.	[94]
	Collagen, gelatin, methacryloyl	Adding collagen can improve printability in terms of fidelity.	[95]
	Alginate-graphene oxide	Adding graphene oxide to alginate to modulate the flow behavior can improve printability.	[96]
	Alginate-gelatin	Addition of gelatin to alginate results in a significant improvement in printability.	[45,97]
	Lithium oxide-based inks	There is a trade-off: on the one hand, printing a highly viscous bioink is challenging due to high-pressure requirements; on the other hand, low viscosity bioinks are plagued with surface wetting problems.	[98]
Physical properties
Surface tension, surface energy, and contact angle	Various materials	Substrates with lower surface energy result in reduced bioink spreading during the printing process. A contact angle of 90° was also reported as optimum; a lower contact angle means more spreading. Additionally, the nozzle type should be carefully selected as nozzles' surface energy can affect printability (high surface energy needles result in a high degree of capillary rise).	[99]
	Polydimethylsiloxane	Printing a relatively small construct can result in poor printability due to the significant effect of surface tension and its subsequent flow resistance.	[69]
Scaffold design-related factors
Pore size
Dimension and grid geometry
Angle and orientation
	Alginate/gelatin	Poor printability for cases with acute angles in the scaffold design compared to obtuse and right angles.	[22]
Printing process-related factors
Pressure
	Alginate/gelatin	Pressure is the most significant element affecting printability. Excessive pressure can cause poor printability.	[22]
Speed
	Highly concentrated silver nanoparticle ink	Considering a constant flow rate, lower nozzle speed leads to poor printability due to the extrusion of more bioink per unit time. A high-speed printing nozzle can also lead to discontinuous filaments.	[100]
Cross-linke
	Alginate-gelatin	A relatively long crosslinking time causes poor printability.	[45]
Nozzle
	Poly(ethylene glycol)-diacrylate-alginate-	The smaller the nozzle diameter, the higher the resolution and printability.	[101]
	Alginate, photo-crosslinkable polyethylene-glycol diacrylate, gelatin	Not all nozzles with smaller diameters lead to improvements in printability. Nozzles with smaller diameters sometimes require higher pressures to extrude bioinks with lower printability costs.	[102]
	Polyelectrolyte inks	Printability directly corresponds to nozzle size.	[103,104]
Offset (distance between needle and substrate)
	Alginate	The shorter the distance between the nozzle and a crosslinking agent, the higher the printability.	[105]