Table 3.

SMHs currently used for 4D bioprinting.

SMH	4D bioprinter	Mechanism for 4D effect	External stimuli	Cells	Cell viability	Structural stability	Application	Reference
AlgMA or HAMA	MEB	Crosslinking degree gradient across the thickness of the hydrogel	Water	Mouse bone marrow stromal cells	>95%	Self-folded tubes remained stable for at least 7 days but showed cracks on the wall of the tubules	Vasculature	[37]
GelMA and Gel–COOH–MA	Inkjet	Swelling difference between GelMA layer and Gel–COOH–MA layer	Water	Human umbilical vein endothelial cells	>90% on day 3	Self-folded microtubes could remain stable while experiencing the contraction force of the cells during 3 days of in vitro culture	Vasculature	[73]
Silk	DLP	Crosslinking difference between the first and second layer	Water	Chondrocytes and turbinate-derived mesenchymal stem cells	__	Transformed bending structure kept stable during 4-week in vitro culture	Cartilage	[70]
PEGDA	__	Swelling difference between the first and second layer	Water	Fibroblasts	__	Self-folded microtubes maintained their shape for 66 days	Vasculature	[131]
Oxidized AlgMA and GelMA	MEB	Swelling difference between oxidized AlgMA layer and GelMA layer	Water	NIH3T3	__	Bilayer structures with a low cell density (≤5 × 107 cell/ml) could not keep maximum curved structure over 21 days, while those with a high cell density (1 × 108 cell/ml) showed steady, increased curvature during 21 days of in vitro culture	Bone	[132]
Jammed micro flake hydrogel made of oxidized AlgMA	MEB	Crosslinking degree gradient across the thickness of hydrogel	Water or pH	Human MSC	__	Bending structures remained stable during 21 days of in vitro culture	Cartilage	[40]
Oxidized AlgMA, GelMA, or 8-arm PEG-acrylate	MEB	Crosslinking degree gradient across the thickness of hydrogel	Water, pH, or chemicals	Human MSC	86%–89% after 3 days of culture	Deformed structures retained similar bending degrees during 21 days of in vitro culture	Bone	[133]
Oxidized AlgMA and GelMA	MEB	Swelling difference between oxidized AlgMA layer and GelMA layer	Water	Human MSC	__	Bilayer structures underwent tunable deformation due to unstable structure	Cartilage	[134]
Tyramine-functionalized hyaluronan (HAT) and alginate/HAT	MEB	Swelling difference between the first and second layer	Water	MSC	∼75%	Curvature of the bilayer scaffolds gradually decreased during four weeks of in vitro culture	Cartilage	[135]
Ferromagnetic NdFeB microparticles, HAMA, and Alg/polyacrylamide	__	Magnetic actuation	Magnetic field	Fibroblasts, epithelial cells	∼90% for 3% (w/v) HAMA, and ∼70% for 5% (w/v) HAMA	Bilayer constructs retained regular folding under a magnetic field for 2 days with 3 h of stimulation each day	Bronchi	[136]
GelMA, poly-l-lysine (PLL)	MEB	Induction of membrane contraction by immersing anionic GelMA in cationic PLL solution	Opposite charge interactions	MSCs	<40%	Contracted thin membranes remained stable for at least two days	Membranous tissues	[128]
PEGDA/GelMA	DLP	Crosslinking degree gradient across the thickness of hydrogel	Water	NIH3T3	73% on day 1 and 90% on day 4	Folded constructs kept stable for up to 14 days	__	[74]