## 3 Current State of Acellular TEVG
### Decellularized Matrix  
For TEVGs, decellularized scaffolds can be obtained from autografts, such as from the human umbilical artery [30; 31] and internal mammary artery [32], or from xenografts [33]. They are seeded with cells such as stem cells and then cultured in a bioreactor. After the mature vascular graft is obtained (with good mechanical properties), an immune reaction occurs between the graft and the host. Because of this, the decellularized scaffold is developed into an acellular vascular graft, and the endothelial cells are collected in the body, while the tissue regenerates in situ. Here, we do not discuss the decellularized scaffold from real blood vessels but the in-body tissue architecture (iBTA), a kind of cell-free in vivo tissue engineering technology to produce autologous implantable tissue of the desired shape. A kind of tubular mold is implanted in the abdominal cavity or subcutaneously, using the body microenvironment as a bioreactor. Then, the cells in the body grow on the tubular mold to form a cell wrap layer, and this living tissue is further decellularized and used as a vascular graft. In addition, nonvascular tissues, such as amniotic membranes and the small intestinal submuc (SIS), have also been decellularized and used to produce TEVGs [12]. Figure 3: Schematic illustration of the preparation method used to generate hybrid bilayered 3D polycaprolactone (PCL) scaffolds with surface treatment for artificial vascular graft applications. Reprinted with permission from [29], Copyright Elsevier, 2019. This acellular extracellular matrix vascular scaffold is composed of a natural extracellular matrix. It has the structure and mechanical performance of natural tissue ECM, but the mechanical strength and compliance are not ideal for use as a vascular graft scaffold. [PERSON] et al. [34] developed vascular tubular tissues called biotubes, which consist of autologous tissues. Researchers embedded six kinds of polymeric rods as molds in subcutaneous pouches in the dorsal skin of New Zealand white rabbits. After months of implantation, the biotubes were constructed. Differences in the material influenced how long it took for stable biotubes to form, while also influencing the wall thickness of the biotubes. It is hoped that the biotubes can be prepared as vascular grafts in a patient's body using autologous cells and extracellular matrix components. After a decade, [PERSON] et al. [35] used iBTA to develop biotubes of a higher quality compared to those previously fabricated (Figure 4). An inner spiral mandrel and an outer pipe-shaped spiral shell were assembled to be used as spiral molds for long biotube formation. The molds were embedded into subcutaneous pouches in dogs or goats for one or two months. There were slits on the spiral shell so that autologous cells could move to the space between the mandrel and the shell to form the biotube wall (made of collagenous tissues). The biotubes could be used for implantation with no need for luminal modification or mechanical support. The result demonstrated vascular reconstruction within three months of implantation into dogs. Figure 4: Photographs of (**a**) two spiral-shaped molds used for the preparation of long biotubes of 50 cm (left) or 25 cm (right); (**b**) three parts for the molds; (**c**) cross-sectional view of the mold in the horizontal direction, (**d**) and perpendicular direction at the dotted line in \(c\); (**e**) subcutaneous embedding of the mold into a goat, (**f**) in vivo incubation, and (**g**) harvesting of the mold from a goat. The long biotubes formed around the spiral mandrels after embedding 50 cm spiral molds for (**h**) 1 or (**i**) 2 months or (**j**) obtained from 25 cm spiral molds after two months; (**k**) the luminal surface and (**l**) side view. Reprinted with permission from [35], Copyright Elsevier, 2018. [PERSON] et al. [36] developed a heparin/VEGF-functionalized cell-free vascular graft made of SIS.