TABLE 3.

Summary of indirect 3D bioprinting applications and bioink selection for different tissue vascularization covered in this review.

Category	Sub-category	Sacrificial material	Scaffold material	Cells and cell density	Cell viability	Progress	Limitations	References
Vascular grafts	Arteriole/venule	Gelatin	Fibrin and collagen/fibrin blends	HUVECs (∼107 cells/ml); SMCs (∼106 cells/ml); normal human dermal fibroblasts (—)	∼83%/91% (1d/4d, SMCs)	In vitro model success	Unable to meet human transplantation standards	Schöneberg et al. (2018)
	Branched vascular structure	Pluronic-nanoclay	Alginate	—	—	In vitro non-cell model success	No biological function	Afghah et al. (2020)
Highly vascularized tissue	Heart-like structure	Pluronic F127	Alginate	—	—	Simplified models for conceptual validation	No good method to fabricate complex structures	Zou et al. (2020)
	Valentine-shaped heart	PVA	Alginate and agarose	HUVECs (∼106 cells/mL); H9c2 rat myoblasts (∼106 cells/ml)	∼95%/90% (1d/14d)	A hollow structure containing a network of micro-fluid channels	Difficult to imitate the ultrastructure of capillaries; low degree of simulation	Zou et al. (2020)
	Simplified cardiac scaffolds	PVA	PUU	Primary human cardiac myocytes (∼104 cells/scaffold)	94% (1d)	A perfusable scaffold with mechanical properties similar to cardiac tissue, and good biocompatibility with cardiac myocytes	A geometrically simplified in vitro scaffold mainly for material performance test	Hernández-Córdova et al. (2016)
	Cardiac spheroids	Gelatin	Collagen I and Matrigel	Cardiomyocytes with primary cardiac fibroblasts (∼109 cells/ml in total); HUVECs (∼107 cells/ml)	Enhanced cell viability throughout the bulk tissue compared to nonvascular tissue	A perfusable cardiac tissue that fuses and beats synchronously over a 7-day period with high cellular density	Lack of sufficient microvascular network formation; a modest contractility (∼1% strain) only	Skylar-Scott et al. (2019)
	Gut-like tissue fragments	PVA	Matrigel, gelatin, and fibrin	Caco-2 intestinal epithelial cells; HUVECs (∼107 cells/ml)	Good cell co-culture results	An in vitro gut model capable of sustaining cells long term	A simplified model mainly for conceptual validation	Hu et al. (2018)
	Liver tissue model	Agarose fiber	GelMA	HUVECs (∼105 cells/ml); HepG2/C3A cells (∼106 cells/ml)	>80% (2d)	A vascularized liver tissue model for mimicking in vivo conditions and testing drug diffusion and toxicity	Difficult to imitate the ultrastructure of capillaries	Massa et al. (2017)
	Liver tissue fragments	PVA and PLA	Gelatin	Liver hepatocellular carcinoma (HepG2) cells (∼106–108 cells/ml)	Good HepG2 cell proliferation to a high cell density	A perfusable thick engineered construct with cellular densities of native tissues	A simplified model for conceptual validation; difficult to create channels with diameter <1 mm	Pimentel et al. (2018)
	Renal proximal tubule models	Pluronic F127	Gelatin	Proximal tubule epithelial cells (∼107 cells/ml); glomerular microvascular epithelial cells (—)	Healthy cell phenotype was observed	A 3D vascularized proximal tubule model that can be independently addressed to investigate renal reabsorption	The reabsorptive properties may be improved by reducing the proximal tubule lumen diameter and the separation distance between the proximal tubule and vascular conduits	Lin et al. (2019)
	Kidney-like structure	Pluronic F127	Alginate	—	—	Simplified models for conceptual validation	No good method to fabricate 3D highly vascularized network in thick tissue or organ	Rocca et al. (2018)
Vascularized osteochondral tissue	Cartilage tissues	Pluronic F127	GelMA	Bone marrow derived mesenchymal stem cells (∼107 cells/ml)	Cells remained viable after 24 h	A promising approach for guiding vascularization and implant remodeling during endochondral bone repair	No obvious enhanced overall-level bone formation	Daly et al. (2018)
Vascularized skin	Finger-shaped highly elastic scaffold	PVA	PLCL	Human dermal fibroblasts (∼106 cells/ml)	Considerable collagen and new blood vessels were observed at 4 weeks	A customized scaffold successful in animal experiments and may act as a dermis substitute	A simplified model without hierarchical structure	Im et al. (2018)
	Thermoresponsive ‘stiffness memory’ elastomeric nanohybrid scaffolds	PVA	PUU-POSS	3T3-J2 mouse embryonic dermal fibroblasts (∼104 cells/scaffold)	Good ingrowth of tissue and new blood vessels were observed at 4 weeks	A unique smart elastomer scaffold that can guide the growth of myofibroblasts, collagen fibers, and blood vessels at real 3D scales	Slow ingrowth of host blood capillaries; local inflammatory response	Wu et al. (2019)