Table 2.

Comparison of self-healing efficiency of hydrogels prepared for 3D/4D printing.

Mechanism	Hydrogels	Reaction Condition	Self-Healing Efficacy	Ref.
Disulfide and acylhydrazone exchange	Dithiodipropionic acid dihydrazide functionalized PEO and HAD	48 h at 25 °C with different pH	50%, 48 h	[93]
Acylhydrazone exchange and Schiff base	N-carboxylethyl CA, OSA, and adipic acid dihydrazide	6 h, 25 °C, pH 7	86 ± 3.4%, 12 h and 90 ± 2.7%, 48 h	[94]
Acylhydrazone bonds	PEO and tris[(4-formylphenoxy) methyl] ethane catalyzed by acetic acid.	At ambient RT	7 h	[95]
Schiff base	CA or GCA, and telechelic difunctional PEG	2 h, RT (20 °C)	≈51%, at the start of healing and ≈91% after 10 min of healing	[86]
Schiff base	AMC and OA	1–3 h, 20 °C		[96]
Schiff base	CSMA and SC			[97]
Schiff-base	CMBC-EDTA and CHO-Pectin		23% at pH = 7	[98]
Schiff-base	CA or GCA, and DF-PEG.	15 min, 30 min, 20 °C	≈390 Pa at 0 min healing time and ≈2000 Pa for virgin sample	[99]
Disulfide bonds	HAuCl4/AgNO3 solutions and tetra-arm thiol-terminated PEG homopolymer [(PEGSH)4]		90% mechanical healing	[100]
Oxime bonds	P(DMA0.68–stat–DAA)	25 °C	2 h	[101]
Imine bond	GCA and DF-PEG		Healed Within 15 min	[99]
Imine bond	PLGA-g-PCL		81% within 6 h	[102]
Boronate ester bonds	Borax, PEGDA, DTT and PEG	25 °C	Healed within 30 min	[103]
Diels-Alder	Dex-l-PEG and PEG–DiCMA	37 °C	7 h, 98.7% healing	[104]

Hex-anedioic acid dihydrazide: HAD; Chitosan: CA; glycol chitosan: GCA; and telechelic difunctional poly(ethylene glycol): PEG; oxidized sodium alginate: OSA; 2-acrylami-dophenylboronic acid: ADBA; N,N′-dimethylacrylamide: DMA; poly(vinyl alcohol): PVA; dopamine acrylamide: DOPAA; poly(N,N–dimethylacrylamide-stat-diacetone acrylamide): P(DMA–stat–DAA); poly(ethylene oxide): PEO; dextran-l-poly(ethylene glycol): Dex-l-PEG; dichloromaleic-acid-modified poly(ethylene glycol): PEG-DiCMA, carboxyl methyl bacterial cellulose: CMBC.