formation with cell sheets:scaffold-free fabrication approach of hMSC-based tissue engineered blood vessel(TEBV),incorporation of human EPC in the vessel lumen,and culture and maturation of TEBV in a perfusion bioreactor(reproduced with permission from[106]).

of blood vessel formation and maturation within a scaffold cultured in a bioreactor:(A)effect of shear stress on ECs and(B)step-wise demonstration of blood vessel formation by ECs in a scaffold.

In vitro tissue prevascularization requires well-connected and porous scaffolds,microfluidic networks,complex bioreactors,and culture media;however,the long-term fate of the capillary bed formed remains address this issue,researchers have paid attention to in vivo prevascularization of engineered this approach,cell/angiogenic factor-loaded tissue constructs are implanted into a temporary site within the host body,and then transplanted into a specific the case of angiogenic factor-embedded scaffolds,angiogenic sprouting takes place due to the controlled release of angiogenic factors demonstrate short half-lives in vivo,autologous cells are incorporated in the scaffold to express VEGF.In particular,scaffolds are initially implanted close to an arteriovenous loop in the host body to promote vasculature[117].When a well-perfused capillary bed is formed within the matrix after a certain time,scaffolds are explanted,seeded with multiple cell types,and then transplanted into the target one study,an FGF-2-enriched Matrigel chamber was implanted around the epigastric pedicle of a diabetic Matrigel was replaced by highly vascularized adipose tissue after 21 days of of pancreatic islets into the prevascularized chambers in diabetic mice significantly reduced the blood glucose levels,indicating the better survival and function of islets in the prevascularized chamber compared to non-prevascularized chambers[118].Likewise,neonatal cardiac cells seeded onto prevascularized cardiac patches and transplanted onto infarcted rat hearts enhanced cardiac function after 28 days[107].

and future research directions

The success of tissue and organ regeneration largely depends on the formation of a mature and well-perfused vascular network within the developing date,significant progress has been achieved in the printing of vascular constructs(Table 1).

In recent decades,technological advancements in 3D bioprinting systems have made it possible to print cell aggregates,tissue strands,or cell/GF-loaded biomaterials layer-by-layer as per a predefined geometry developed by CAD particular,inkjet-and 3D-bioplotting-based techniques have been recurrently used in different studies[119,120].Each technique has advantages and example,3D inkjet printers are cheap and fast but can result in cell damage and clogged nozzles while 3D bioplotting is suitable for printing intricate networks with sacrificial materials but at the expense of printing ,bioplotting with a coaxial nozzle has attracted significant attention due to the ability to print capillary strands with microfluidics,EC monolayers can be developed inside the hollow the last decade,the rapid advancement of laser technology has brought significant changes to 3D scaffold printing in terms of accuracy and LGDW and MAPLE DW methods,as nozzle-free techniques,can handle highviscosity bioink and pattern 3D vascular cells with high printing resolution and ,research in this direction has not advanced far because these techniques are time-consuming and cause significant cell damage during contrast,DLP and LS methods are comparatively biocompatible and capable of printing intricate DLP technique can print 3D capillary networks promptly with high resolution,but the technique is costly,compromises details for a large construct,and causes resin contrast,the LS approach(particularly two-or multi-photon laser systems)is capable of printing complicated 3D vascular patterns with details,although at a lower printing speed compared to the DLP ,the effect of the laser on cell damage remains controversial,and both LS and DLP techniques are only applicable to photo-crosslinkable hydrogels.

Based on an understanding of physiology,researchers have reached a common agreement that micro-scale capillary networks are needed to maintain the viability of large cell populations incorporated in engineered fabrication techniques have been applied to form capillaries and ECs have been seeded in the micro-lumen using microfluidic etching,laser ablation,soft lithography,and replica molding have been frequently used to generate patterned vascular networks on a single layer planar capillary bed-embedded 3D tissue constructs can be constructed by stacking multiple single layers,assembling complexities and the requirements of long processing time limit their ,significant success in 2D capillary formation has been achieved when advanced computational approaches have been applied to manipulate the shear stress,medium circulation,and bio-molecule distribution in micro-lumen seeded with ECs and mural success in 2D has been further translated to 3D microfluidic systems to promote EC monolayer formation on the lumen.To achieve a self-assembled capillary network rather than apredesigned one,EC-coated micromodules containing tissue-specific cells have been loaded into engineered constructs and then perfused with medium in a ,this approach grows vascular networks that are dissimilar to those of native tissue and demonstrate poor tissue recent years,the introduction of omni-directional printing has enabled the fabrication of native tissue-like capillary networks instead of 3D periodic lattice approach needs further improvement as inefficient removal of pluronic acid can significantly reduce EC ,highly branched or fractal-like structures have been generated in plastic materials with high energy electron beam approach is suitable for post EC seeding,while tissuespecific cell incorporation in the plastic material requires further channels with varying diameter and linear 3D patterns have also been generated,with double layers of vascular cells transferred in the hydrogel using mechanical ,the capillary vessels formed by mechanical spacers are not continuous and cannot mimic the native vascular network.

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