## 3 Current State of Acellular TEVG
### Three-Dimensional Printting  
Three-dimensional printing technology has been applied in many different fields. Bio-printing is already commonly used in tissue engineering and is also a method for fabricating TEVG [25]. As introduced previously, support materials mixed with seed cells are the starting materials for fabricating printed TEVGs. After the formation of the tube shape, this type of TEVG is further cultured in vitro. However, herein, we discuss a method for fabricating acellular scaffolds which is similar to traditional additive manufacturing techniques. Electrospinning is commonly used in TEVG because of the high porosity obtained, which increases the surface area for cell attachment and allows cell invasion. This structure and porosity can also be realized by 3D printing. 3D printing has many advantages such as the possibility of complex structure fabrication and applications using multiple types of materials, which is favorable when producing personalized TEVGs. Today, various extrusion-, inkjet-, and photocrosslinking-based additive manufacturing (AM) techniques are being applied for the fabrication of both porous vascularized tissue constructs and self-standing vascular grafts [26]. However, the high fabrication cost is a problem, compared with other methods such as electrospinningning. [PERSON] et al. [27] developed a type of 3D-printed biodegradable polymeric vascular graft. In this study, poly(propylene fumarate) (PPF) was used to construct a functional TEVG through digital light stereolithography (DLP), due to its photocrosslinking capability and its biocompatibility. The scaffold structure could be designed to fit the specific curvature of the patient's anatomy, because images of the native vessel can be obtained by MRI or CT. These printed grafts were implanted into mice for six months. No thrombosis, graft aneurysm, or stenosis was found during the six-month implantation time for all grafts. The 3D printing method can be combined with other fabrication methods such as electrospinningning. [PERSON] et al. [28] developed a kind of triple-layer poly(e-caprolactone) fibrous vascular graft fabricated by combined electro-hydrodynamic jet (E-jet) 3D printing and electrospinningning. This kind of vascular graft consisted of three layers of different structure. The middle layer, made by electrospun fibers, was fabricated first, and then the interior layer, comprising 3D-printed highly aligned fibers, was printed directly onto the middle layer. The bilayer film was wrapped over the mandrel collector, acquiring a tube shape. Finally, the exterior structure composed of mixed fibers was fabricated by co-electrospinning with PCL and polyethylene glycol (PEG). These vascular grafts were implanted into rats for 12 weeks, after which cells grew throughout the entire structure of the grafts. This triple-layer structure is obtained using 3D printing techniques to construct the lumen of the longitudinally aligned fibers of the graft to enhance the strength of the electrospun monolayer and promote cell growth and spread. [PERSON] et al. [29] also combined electrospinningning and 3D printing techniques to fabricate a mechanically robust, bilayered scaffold (Figure 3). The inner layer was composed of an aligned electrospinningning nanofibrous scaffold, which was formed by rolling the sheet scaffold around a rod. The outer layer was directly printed onto the electrospinningning scaffold. The scaffold material was coated with polydopamine (PDA), and VEGF was grafted directly onto the scaffold surface to induce a potent angiogenic activity. The coated PDA layer was evenly deposited on the bare PCL scaffold, which enabled abundant VEGF immobilization with enhanced hydrophilicity. During in vitro and in vivo testing for four weeks in rats, VEGF-immobilized scaffolds elicited markedly enhanced vascular cell proliferation and angiogenic differentiation compared with untreated groups.