Table 2.

Intra-microgel crosslinking strategties and applications of various microgel assembly driven by chemical reaction, physical reaction, cell-cell interaction and external driving forces.

Driving forces for microgel assembly (inter-microgel crosslinking)	Materials of microgel	Intra-microgel crosslinking	Application	Ref.
1.Chemical reaction
(a)Enzymatic catalysis	PEG	Thiol-ene click reaction	Skin tissue engineering	[19]
	Hyaluronic acid	Thiol-ene click reaction	Carrier for cell culture and cell behavior regulation	[21]
	PEG	Thiol-ene click reaction	Carrier for cell culture and cell behavior regulation	[28]
	Hyaluronic acid	Thiol-ene click reaction	Neuronal tissue engineering	[29]
(b)Photo-initiated radical polymerization	Hyaluronic acid	Thiol-ene click reaction	Carrier for cell culture and cell behavior regulation	[21]
	Poly(MMA/MAA/EGD)-GM	Free-radical chemistry	Intervertebral discs tissue engineering	[30]
	Gelatin	UV crosslinking	Carrier for cell culture and cell behavior regulation	[31]
	Hyaluronic acid	UV crosslinking	Bio-ink for 3D printing	[32]
	Alginate	Ionic crosslinking	Bio-ink for 3D printing	[33]
	Alginate	Ionic crosslinking	Support medium for 3D printing	[34]
	PEG	Click reaction	Neuronal tissue engineering	[35]
	PAMPS	Free-radical chemistry	Bio-ink for 3D printing	[36]
(c)Click chemistry	PEG	UV crosslinking	Carrier for cell culture and cell behavior regulation	[37]
	PEG	UV crosslinking	Bio-ink for 3D printing	[38]
	PEG	Azide-Alkyne Cycloaddition	Carrier for cell culture and cell behavior regulation	[39]
	Alginate	Ionic crosslinking	Carrier for cell culture and cell behavior regulation	[40]
	Hyaluronic acid	UV crosslinking	Carrier for cell culture and cell behavior regulation	[41]
	PEG	Click reaction	Neuronal tissue engineering	[42]
(d)Non-enzymatic amidation reaction	Gelatin, PEG	UV crosslinking	Cartilage tissue engineering	[43]
2.Physical reaction
(a)Host-guest interaction	Hyaluronic acid	UV crosslinking	Cardiac tissue engineering:	[47]
	Polyacrylamide	Free-radical chemistry	Not available	[48]
	Polyrotaxanes, Hydroxyethyl cellulose	Host-guest interaction	Not available	[49]
(b)Electrostatic interaction	Gelatin, Chitosan	UV crosslinking	Neuronal tissue engineering	[9]
	PEG	UV crosslinking	Carrier for cell culture and cell behavior regulation	[50]
	Methacrylic acid, 2-hydroxyethyl methacrylate	Free-radical chemistry	Not available	[51]
	Chitosan	Schiff base	Bone tissue engineering	[52]
	Gelatin	Schiff base	Carrier for cell culture and cell behavior regulation	[53]
(c)Hydrogen bonding (including DNA base complementation, polypeptide self- assembly)	Chitosan, Polyvinyl alcohol	UV crosslinking	Bio-ink for 3D printing	[54]
	PEG	UV crosslinking	Carrier for cell culture and cell behavior regulation	[[55], [57]]
	PEG	UV crosslinking	Not available	[56]
(d)Biotin and streptavidin conjugation	Alginate	Ionic crosslinking	Carrier for cell culture	[40]
3.Cell-cell interaction	Hyaluronic acid, Gelatin	Thiol-Ene click reaction	Cartilage tissue engineering	[10]
	Collagen	Hydrogen bond	Carrier for cell culture and cell behavior regulation	[60]
	Chitosan	Schiff base	Carrier for cell culture and cell behavior regulation	[61]
	Gelatin	Schiff base	Skin tissue engineering	[62]
4.External driving force
(a)Fluidic force	PEG	UV crosslinking	Not available	[[63], [65], [66]]
(b)Surface tension	PEG	UV crosslinking	Carrier for cell culture and cell behavior regulation	[[67], [68]]
(c)Magnetic and acoustic force	PEG, Gelatin	UV crosslinking	Carrier for cell culture and cell behavior regulation	[[69], [70], [71], [72]]