with natural architecture

Tissue-specific acellular matrix contains biological,structural,functional,and topographical cues that promote vascular tissue growth through the upregulation of signaling pathways,phenotype,mechano-transduction,and differentiation and proliferation of the repopulated vascular cells[89,90].Different studies have prepared decellularized vascular tissue with chemical(acid,base,detergent,hypotonic and hypertonic solutions),biological(,dispase,etc.),or physical(,pressure,electroporation,agitation,pressure gradient,etc.)agents in consideration of tissue properties(e.g.size,lipid content,density,thickness,cellularity,etc.).However,such processes often disrupt the ultrastructure of ECM,remove ECM and growth factors,and provoke immune responses[91–93].A number of studies have successfully grown functional macro blood vessels with decellularized example,in one study a recellularized tissue-engineered vessel with autologous EPCs prevented clotting and intimal hyperplasia for 30 days in a porcine model[94].When decellularized rat iliac arteries seeded with ECs were implanted in the abdominal aorta of rats,native vessel-like structures were observed at 3 months[95].Similarly,the inner layer of decellularized pig arteries were repopulated with human ECs and implanted in the pigs as iliac artery substitutes;the inner layer of the recellularized matrix was covered by ECs,and no thrombi formation was reported 70 days postoperatively[96].Apart from EC seeding,the success of decellularized matrix vascularization largely depends on the implementation of physical example,decellularized rat hearts were repopulated with cardiac cells or ECs,and then cultured in a bioreactor maintaining simulated cardiac physiology over 28 days.In both larger and smaller coronary vessels,EC layer formation was identified 7 days after , cell-seeded heart tissue showed contraction and expansion under electrical stimulation 8 days after cell seeding[97].Decellularized cadaveric lungs containing intact internal structures( vascular network,airways,and alveolar geometry)were seeded with ECs and then cultured in a bioreactor simulating developmental physiological such lungs were transplanted and perfused by the recipient's blood circulation in vivo,gas exchange was observed up to 6 h after ,the success of this approach largely depends on the use of progenitor cells,prolonged in vitro and in vivo culture,and an ideal postoperative ventilation regimen for the regenerated lungs[98].

of cell transfer,and vascular network formation mechanism by mechanical spacers:(A)oligopeptide modified gold surface was seeded with HUVECs,(B)HUVECs seeded on gold substrate were transferred to photocrosslinked GelMA hydrogel with or without electrical potential,(C)HUVECs coated micrometric gold rod was placed in a culture chamber and encapsulated in GelMA hydrogel,and the layer of HUVECs was transferred to GelMA by using an electrical the rod was taken out and the hollow lumen was cultured under perfusion,and(D)HUVECs coated gold rod was dipped into 3T3 fibroblast cell loaded GelMA solution to form double-layered microvascular rod was then encapsulated in hydrogel,the double-layer of vascular cells was transferred by an electrical potential,the rod was removed,and the hollow channel was cultured under perfusion(reproduced with permission from[31]).

elucidating the concept of tissue printing using tissue strands as a new bioink(reproduced with permission from[102]).

fabrication

7.1.Cell aggregates as bioink

In vivo,tissues or organs develop due to the self-assembly and self-organization of multiple cell types without the influence of it was established decades ago that tissue regeneration requires engineered constructs,several complexities related to the scaffolding biomaterial have led researchers in recent years to harness the in vivo mechanism of tissue number of studies report that bioprinted cell aggregates form pre-designed tissue constructs through self-organization and tissue fusion[99].Cell aggregates composed of single cell or multiple cell types can be bio-printed as cell pellets,tissue spheroids,or tissue filaments(Fig.8).Tissue spheroids must be prepared in a controlled fashion to avoid clogging and cell damage in the bioprinting process[100].In contrast,bioprinting of cell pellets ensures high cell densities and does not cause nozzle clogging or cell ,bio-dispensing of cell pellets requires a supportive hydrogel in which they can accumulate,organize,and fuse to form tissue[101].Tissue strands can also be formed by injecting cell pellets into tubular molds prepared with a co-axial nozzle one study,injected cell pellets in permeable alginate capsules organized and fused to form tissue strands in an in vitro strands were loaded into a custom-made bioplotter and then extruded layer-by-layer to form a pre-defined 3D structure[102].These cell aggregate approaches can be applied to form tissue with vascular networks.

up cell sheet

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