Table 1.

Material advances in 3D bioprinting of skin.

Classifications	Materials	Cell Components (Origin)	Printing Methods	Printability (Fidelity) and Scalability	Biological Assessment of Printed Constructs	Advantages (+) Challenges (−)	Ref.
Natural hydrogel	Alg and Gel	AECs (human term placenta) and WJMSCs (human umbilical cord)	Extrusion	-Precision in 151 ± 13.04 μm-grid -Millimeter-sized tube, cylinder, box, nose, and ear are printable.	-Microarray analysis on global gene expression -rtPCR on verification -CCK-8 and live/dead assay on cell viability and proliferation	+ The bi-layered skin-like constructs were fabricated using AECs and WJMSCs. + High printing precision (151 ± 13.04 μm) and structural fidelity were achieved. + The expression of genes relating to re-epithelization and wound healing was significantly increased. − Limited cell adhesion and spreading.	[46]
	CS-genipin-PEG	KCs (human epidermis) and HDFs (human dermis)	Extrusion	Square disk	-MTT assay on the viability -Live/dead assay	+ The low printing pressure (20—40 kPA) induced high cell viability. + The genipin-based crosslinking maintained high cell viability (93%). − Low printing fidelity and cell spreading.	[47]
	PECMA	HNDF (human neonatal foreskin)	Extrusion	Millimeter-sized few-layered lattice.	-Live/dead assay -IF staining on ECM production	+ Dual crosslinking system using UV radiation and calcium-mediated ionic gelation was achieved. + The bioink supported cell growth and de novo deposition of ECM components of the dermal tissue. − Low printing fidelity	[48]
dECM	dECM (goat)	L929 cells (murine connective tissue)	Extrusion	2 × 2 cm construct with micron-sized lattices	-IF staining on cell morphology and alignment	+ H/H NaCl solutions-based decellularization method showed a much high yield and maintained high residual DNA and ECM contents. + The shear thinning property of bioink induced. − Detailed study for trypsin protocol is required.	[49]
	Porcine dECM and fibrinogen	HDFs (human dermis)	Extrusion	Various geometry at millimeter scales	-H & E staining -Live/dead assay	+ The dECM components strengthen the mechanical and shear thinning properties. + The long-term viability of laden FBs was significantly increased by dECM incorporation. − Cell-level biological assessments are required.	[50]
	Microfat clusters (from human) and ColMA	MSCs, ASCs, and EPCs in human microfats	Extrusion	2 cm lattice disk with 1.5 mm line fidelity	-ELISA on wound healing cytokine -AlamarBlue assay on cell metabolic activity	+ The developed fat processing system can process lipoaspirates into microfat clusters comprising highly viable cells preserved in a native niche. + The expression of proinflammatory and anti-inflammatory cytokines suggests the wound healing microenvironment. − Periodic changes of microfats and bandages are required.	[51]
	DSIS slurry (porcine)	FBs (rat normal skin)	Extrusion	80-layered 2 cm-high lattice constructs with micron-sized lattices	-Live/dead assay -Nucleus staining -MTT assay on proliferation -Western blotting and rtPCR on vasculature and ECM production	+ Highly fine lattice structure was built up to 80 layers (~2 cm in height). + Enhanced FBs behaviors and production of ECM proteins including Col I, Col III, and fibronectin. − Hydration is required for cell-laden printing.	[52]
Synthetic hydrogel	PVA, agarose, nanocellulose, and Alg	HFBs and HUVECs (human umbilical cord)	Extrusion	Tissue-scale human face skin	-Live/dead assay -Alamar blue on proliferation -Biochemical analysis of DNA and Col contents -ELISA and IF staining on angiogenic markers	+ The tissue-engineered faces were fabricated with customizable shapes and sizes that can be vascularized. + The collapsing problem of hydrogel bioink in tissue scale constructs could be solved by the addition of PVA sacrificial layers. + Continuous vascular-like structures were developed with vasculatoid phenotypes. − Fast 3D printing is required for large structure fabrication.	[53]
	PVP and Col	HDFs	Droplet printing	Controllable and layered arrays of micron-sized drops	-Live/dead assay -PrestoBlue assay on proliferation and spreading	+ A hierarchical porous Col architecture was recapitulated similar to native skin. + A novel printing method with a controlled number of droplets with layer deposition was accomplished. − Further biological assessments need to be progressed.	[54]
	PCL, SS, CS, and Alg	NHDF	Extrusion	Layered lattice structure	-MTS and live/dead assay on the viability -dsDNA assay on proliferation -CLSM on cell migration -Anti-bacterial property	+ The combination of electrospun nanofibrous matrices and 3D bioprinted constructs mimicked the epidermis-dermis structures. + The nanofiber layer provides antibacterial property while the printed layer confers a moist environment. − Long-term observation is required for full fusion of dermal-epidermal layers.	[55]

Abbreviations: AECs, amniotic epithelial cells; Alg, alginate; ASCs, adipose stromal cells; CCK-8, cell counting kit-8; CLSM, confocal laser scanning microscopy; ColMA, Col methacryloyl; CS, chitosan; DNA, deoxyribonucleic acid; DSIS, decellularized small intestinal submucosa; ELISA, enzyme-linked immunosorbent assay; EPCs, endothelial progenitor cells; Gel, gelatin; HDFs, human dermal fibroblasts; HFBs, human fibroblasts; HNDF, human neonatal dermal fibroblasts; HUVECs, human umbilical vein endothelial cells; H&E, hematoxylin and eosin; IF, immunofluorescence; MSCs, mesenchymal stem cells; NHDFs, normal human dermal fibroblasts; PECMA, pectin methacrylate; PIL, poly(ionic liquid); Pr, printability; PVA, polyvinyl alcohol; PVP, Polyvinylpyrrolidone; QCS, quaternized chitosan; rtPCR, real-time polymerase chain reaction; SS, silk sericin; and WJMSC, Wharton’s jelly derived mesenchymal stem cells.