In the past decade,researchers have focused on fabricating nanofiber-based vascular constructs because nanofibers possess similar topographical cues to ECM[82].Using different electrospinning techniques,random,aligned,and core/shell nanofibers have been printed and nanofiber-based matrices studied for tissue wide range of synthetic and natural materials,including polycaprolactone(PCL),poly(L-lactic acid)(PLLA),PLGA, poly(ethylene oxide)(PEO),PVA,chitosan,gelatin,and collagen,have been explored fornano-fabrication applications[83].Synthetic nanofibers show more mechanical stability compared to natural fibers,but hydrophobicity,the absence of cell binding motifs,and poor biodegradability limit their ,natural polymeric materials are hydrophilic,and biocompatible,and possess cell binding motifs,yet poor degradation and mechanical properties are major shortcomings that need to be overcome for address these issues,researchers have explored composites,copolymers,and hybrid biopolymers to fabricate nano- fibers and achieved some remarkable successes in tissue vascularization[84].In some studies,random or aligned fibers were effective in terms of vasculature example,when electrospun random PCL/collagen(rPCL/Col)and aligned PCL/collagen-PEO(aPCL/Col-PEO)nanofibers were implanted in the arterio-venous loop in rats,a larger number of blood vessels and better capillary density and branching hierarchy were seen in the rPCL/Col fiber-implanted group 8 weeks postoperatively compared to the aPCL/Col-PEO ,rPCL/Col fibers facilitated the formation of small pore sized capillaries,whereas the aPCL/Col-PEO scaffold promoted early and evenly distributed blood vessels throughout the scaffold,resulting in a shorter prevascularization time[85].Prevascularized and aligned nanofibers can also promote or aligned poly(ε-caprolactone)/cellulose electrospun nanofibers have been used to fabricate a mesh structure in a layer-by-layer seeded onto the aligned nanofibers promoted prevascularization in vitro compared to randomly oriented nanofibers by forming capillary-like the prevacularized and aligned nanofibers were implanted subcutaneously in rats,host blood vessels penetrated deep into the nano-meshes and integrated with the vascular network[86].In an attempt to prevent thrombosis in a small-diameter blood vessel,graded chitosan/poly ?-caprolactone(CS/PCL)nanofibers fabricated with sequential coelectrospinning were heparinized to immobilize and SMCs were cultured on the top and bottom surfaces of the graded CS/PCL nanofibers,mimicking the lumen and adventitia of blood vessels, graded and heparinized nanofibers demonstrated outstanding anti-thrombogenic ,better HUVEC attachment,proliferation,and monolayer formation have been identified on graded CS/PCL scaffolds compared to uniform CS/PCL scaffolds[87].

of vascular networks with mechanical spacers

The shortcomings related to sacrificial filaments used to form microfluidic networks have led researchers to investigate alternative of mechanical spacers into the 3D scaffold(Fig.7)generates linear microchannels without causing any particular,several studies have used mechanically removable spacers to create microchannels in a hydrogel one study,linear wire arrays ranging from 152 to 787 μm in diameter were used to form microfluidic channels in a silk fibroin the channels often lose uniformity after removal of the mechanical spacers,attempts have been made to introduce hollow tubes into the spacer-generated hollow channels with open wall porosity,as well as silk tubes and porous silk tubes with incorporated channels were formed in the silk scaffolds,after which human arterial endothelial cells(hAECs)were seeded into the hollow channels in vitro with or without the presence of collagen-I or 7 days of seeding,hAECs formed a nearly continuous layer around the spacer-generated hollow channel and ECM protein-loaded silk tubes[88].Because the formation of confluent monolayers of ECs in spacer-generated hollow channels is difficult,researchers have attempted to transfer self-assembled cell layers into the hollow channel to avoid the cell seeding one study,HUVECs were seeded on oligopeptide-adsorbed micrometric gold rods to form self-assembled monolayers(SAMs),and then the SAM-attached gold rods were encapsulated in photocrosslinked SAMs of ECs were transferred into the GelMA by applying an electrical potential,followed by the perfusion of SAM with medium at 2 μL/min.This approach was further explored to transfer double layers of assembled cells from gold rods into GelMA to mimic native blood form double layers of vascular cells,HUVEC-coated gold rods were dipped into GelMA solution containing NIH 3T3 fibroblast cells,and then encapsulated in the GelMA applying an electrical potential,the double layers of assembled cells were transferred into the bulk gel,and then the layers were perfused with media for stabilization[31].Although promising,mechanical spacers have not to date been able to generate complicated and branched structures that resemble native blood vessels.

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