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Construction of large tissue structures with high-density living cells in vitro is one of the biggest challenges in tissue engineering.To fabricate 3D cellular constructs,cell spheroids have been used as building blocks.One of the most critical problems in the spheroids is that the increase in size results in a depletion of oxygen (hypoxia) and causes cell necrosis in the core of large spheroids [1].It is expected that vascular structures formed within the spheroids can efficiently deliver oxygen and would prevent necrosis of cells in the core.In this study,we attempted to develop a new approach to form capillary-embedding spheroids by employing microfabricated agarose gel chambers and sacrificial fibers.The strategy for fabricating spheroids with a vascular-like structure is shown in Fig.1.First,RGD-conjugated Ba-alginate hydrogel fibers (diameter: 50~200 μm) were prepared by using a PDMS mold with linear groove structures.By culturing vascular endothelial cells (HH) with the prepared fibers,cell were attached on the fiber surface (Fig.2 (a,b)).Microfabricated chambers made of agarose hydrogel having circle wells (diameter: 400 μm,depth: 400 μm) and linear structures (width: 50~200 μm,depth: 50~200 μm) were prepared by using soft lithography and molding (Fig.3 (a,b)).After placing the cell-attached fiber inside the chamber,a cell suspension was dropped onto the chambers,and the trapped cells in the well were cultivated for two days.Formation behaviors of spheroids were observed using C2C12 and HepG2 cells.Finally by enzymatically digesting the cell-attached fibers,capillary structures inside spheroids were formed.Live/dead assays were performed to evaluate the cell viabilities in the spheroids with and without embedding capillary structures.Cell spheroids were successfully formed around the cell-attached fibers in the microfabricated chamber (Fig.4 (a,b)).After two days of cultivation,the spheroids were recovered from the chamber (Fig.4 (c)),and a capillary structure inside the spheroid was successfully observed by digesing the alginate fiber (Fig.4 (d)).The diameters of the capillary structure were corresponded with the diameters of the fibers.The vascular endothelial cells,which initially attached to the surfaces of the alginate fibers,were transferred to the inner surface of the capillary.In addition,we visualized the live/dead cells in spheroids with and without capillary structures.Dead cells (red) were rarely observed in the spheroids having capillary structures (Fig.5 (a,b)),clearly demonstrating that the capillary structure contributed to prevent the necrosis of cells inside the spheroids.We fabricated cell spheroids having a capillary structure by employing agarose gel microchambers and cell-attached fibers.The spheroids presented here would be useful as unit structures for constructing large tissues.