Table 4.

External gelation with on-chip introduction of cross-linkers

Reference		Zhang et al., 2007 [61]	Liu et al., 2019 [29]	Sugaya et al., 2011 [44]	Pittermannová et al., 2016 [34]
Droplet generation	Concentration of Na-alginate	2 wt%	3 wt%	0.025–0.15 wt%	1 w/v%
	Continuous fluid	Soybean oil	Corn oil	Methyl acetate	1-undecanol
	Use of surfactant	Span 80	SY-Glyster CRS-75	Not mentioned	Abil Em 90
	Geometry	Flow-focusing	Flow-focusing	Flow-focusing	Flow-focusing
	Microfluidic material	PDMS chip	Glass chip	PDMS chip	PDMS chip
External gelation by introducing Na-alginate and cross-linkers on chip before collection	Cross-linkers	Ca (CH3COO)2 (2 wt%) in the continuous fluid	Emulsion of CaCl2 in the continuous fluid	CaCl2 (1 M) in the dispersed fluid	Emulsion of CaCl2 (2 wt%) droplets in the continuous fluid
	Geometry	–	Flow-focusing	Cross-flowing	Flow-focusing
	Mixing	During droplet generation	After droplet generation	After droplet generation	After droplet generation