Tissue regeneration requires the simultaneous growth of vasculature to facilitate the diffusional mass transfer of nutrients,oxygen,growth factors,biochemical signaling factors,carbon dioxide,and metabolic waste from the surroundings to cells and vice versa[1,2].In particular,the vascular network should reach within 100–200 μm of the tissue to avoid ischemic conditions and cell death[3].Blood vessels with different diameters(～ 4–300μm)spread in a complicated fashion(i.e.,fractal shapes)into tissue to exchange nutrients,gas,and metabolites to a huge cell population[4].Capillaries in the vascular network play a vital role in the mass transfer ,tissue regeneration with scaffolds requires the incorporation of an interconnected capillary network with vessels located every 100–200μm in all directions.

Tissue vascularization is a complex process that develops through vasculogenesis and angiogenesis in vivo[4–6].In vasculogenesis, endothelial progenitor cells(EPCs)migrate to an ischemic site,proliferate,and differentiate to form capillary vessels,while angiogenesis occurs when new blood vessels sprout from existing ones according to a gradient of angiogenic factors[5,7].The blood vessels formed by either vasculogenesis or angiogenesis are eventually remodeled and mature as per the demands of specific tissues through the upregulation of various growth factors.

Taking into account the in vivo vasculature formation mechanism,a number of 3D fabrication approaches have evolved over past decades to mimic the native vascular and indirect bioprinting approaches have proven promising for the fabrication of large 3D tissue constructs with intricate vascular use of coaxial needles in extrusion-based(EB)systems revolutionized these biofabrication techniques and resulted in the ability to print lumen-incorporated biofabrication is convenient when scaffolding biopolymers demonstrate poor printability and manipulation this regard,sacrificial biopolymers are a smart choice of scaffolding material in vascular network addition to EB biofabrication,several potential approaches including micro-pattern fabrication and assembly,laser-based fabrication,nano-scale fabrication,and natural matrix recellularization have evolved to generate vascular 2D micropatterned substrates or micro tissue modules results in the formation of complex interconnected vascular fabrication allows both 2D and 3D fabrication in a layer-by-layer fashion in the presence of a photo mask,donor substrate,photo sensitive polymer,or photo ,an advanced fabrication technique reported in numerous studies[8–11],supports the fabrication of extracellular matrix(ECM)-like nano-scale filaments that enhance the interaction with endothelial cells(ECs).Seeding ECs into decellularized tissue matrix promotes vascularization,with the preparation process affecting the quality of the native matrix.

A review of recent progress with respect to tissue vascularization with available fabrication techniques is necessary to guide future articles have focused on the synergistic effect of cells,biopolymers,angiogenic factors,and fabrication approaches in terms of tissue ,review articles that focus on recent progress in terms of 3D vasculature formation techniques are article provides a brief overview of the 3D biofabrication of vascular networks with EB,laser,electrospinning,stacking of micropattern or modules,and cell sheet techniques;discusses the effect of prevascularization on the vascular network formed by the various fabrication techniques;summarizes the challenges,advantages,and shortcomings of different fabrication approaches;and proposes potential future research directions.

2.Rapid prototyping

vascular networks

Macro-scale tissue constructs require well connected vascular networks to ensure the viability of the large cell population embedded or seeded in the tissue-/organ-specific cells require time to form functional tissues and,during this period,require nutrients,gas,and biomolecules to maintain metabolism,proliferation,and differentiation and enable ,researchers have attempted to form perfusable capillary networks within macro-engineered grafts seeded with different types of ,some researchers have used rapid prototyping(RP)or additive manufacturing techniques to form complex capillary networks with hydrogel-based,fugitive,and sacrificial (e.g.3D bioplotting,and inkjet printing)and laser-based()techniques in particular have achieved outstanding results with respect to printing vascular networks with intricate architecture.

bioprinting

Bioprinting of cells ensures a higher cell density in the scaffold compared to post fabrication cell number of studies conclude that higher cell densities in the scaffold promote tissue generation by secreting numerous ,researchers have emphasized 3D bioprinting to incorporate a huge cell population in the scaffold in a controlled and well-distributed bioprinting mixtures of cells and hydrogels,extrusion-based RP approaches have been explored in many particular,inkjet printers and 3D bioplotters are commonly used for scaffold and vascular network printing due to some attractive inkjet printing,bioink drops are dispensed layer-by-layer,while a bioplotting system extrudes continuous filaments to fabricate a predefined structure based on computer-generated(CAD)digital piezoelectric-based inkjet bioprinters can print a cell-crosslinking ion mixture at high resolution and speed on hydrogel general,hydrogels with rapid gelation properties are used in inkjet systems to handle the high printing ,inkjet printers allow the deposition of multiple cell types in a controlled and organized fashion to mimic the distribution of multiple cells in native inkjet printing studies used alginate or its composites as a bioink and calcium chloride as a crosslinker[12–16],with vascular networks shown to grow after in vitro or in vivo one study,three bioinks containing canine smooth muscle cells,human amniotic fluid-derived stem cells,and bovine aortic endothelial cells,respectively,were used to print an alginate-collagen scaffold layer-by-layer using a thermal inkjet printer;vascularized,mature,and functional tissues grew when the scaffolds were implanted in vivo[17].Likewise,bioink composed of human microvascular endothelial cells(HMVECs)and thrombin solution and dispensed on fibrinogen using an inkjet printer resulted in the alignment and proliferation of the HMVECs and the formation of a capillary-like tubular structure inside the channels[18].Although inkjet printers are economical and have several attractive features,shortcomings including nozzle clogging,cavitation bubbles,selective ink viscosity,and cell damage during dispensing limit their use for the fabrication of vascular networks[19].

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