## 3 Current State of Acellular TEVG
### Electrospinning  
Electrospinning is the most common method for the fabrication of vascular grafts; this is due to its various advantages [16]. This method has a relatively low cost and a short scaffold preparation time. Various natural materials and synthetic materials can be used in the electrospinning technique, such as poly-caprolactone (PCL), polylactide (PLA), polyglycolic acid (PGA); furthermore, multiple kinds of materials can be easily combined in the same scaffold [17]. Electrospinning is an easy fabrication process owing to the simple mechanism of fiber generation from a polymer solution under high voltage. Electrospun scaffolds have good biocompatibility because of their porous structure that mimics the natural extracellular matrix (ECM), which itself is propitious to endothelial cell attachment and proliferation. In addition, through adjusting the parameters of the fabrication process, such as solution concentration, constituent materials, voltage, jetting speed, structure of deposition platform, the various features of the electrospun scaffold can be controlled. The fiber diameter, porous ratio, thickness of the scaffold, material characteristics, as well as other features can be optimized to improve the mechanical properties and biocompatibility of the scaffold. Electrospinning is one of the best methods for generating TEVGs cultured in vitro. However, for the in situ vascular grafts, the simple acellular scaffold cannot be implanted in vivo directly. These  
Figure 1: The fabrication process of acellular vascular grafts. (**a**) The scaffold of acellular tissue-engineered vascular grafts (TEVGs) can be manufactured using different methods: electrospinning, casting, three-dimensional printing (3D printing), and a technique based on a decellularized matrix; (**b**) the inner surface of the acellular TEVG can be modified with vascular endothelial growth factor (VEGF) or heparin to improve the biocompatibility and anticoagulant properties; (**c**) acellular TEVGs can be implanted directly in vivo without seed cells; the native blood vessel can regenerate in situ. electrospun scaffolds are reinforced and treated by surface modification to improve their mechanical properties and biocompatibility. All of the treatments are performed in order to collect endothelial cells in situ while avoiding the common complications associated with vascular grafts. The mechanical properties can be optimized by combining different materials and fabrication methods. [PERSON] and [PERSON] [PERSON] and [PERSON] (2018) proposed a bilayer heparinized vascular graft that combines electrospinningning and freeze-drying (Figure 2). In their study, the inner layer of the graft was made by co-electrospinning synthetic polymers, PCL, and the natural polymer gelatin (Gel). In addition, heparin was loaded by blending it in the gelatin solution and emulsion electrospinningning of the PCL fibers. Adding heparin resulted in better endothelial cell attachment and proliferation, while fewer platelets were attached to the scaffold. The outer layer was fabricated using freeze-drying of a gelatin hydrogel. With an average pore diameter \(>\)200 \(\upmu\)m, large smooth muscle cells were able to proliferate easily along this layer. Mechanical tests demonstrated the improved mechanical properties of the bilayer scaffold in comparison with the individual freeze-dried or electrospun layers. To improve the hydrophilicity, cytocompatibility, and biodegradation of PCL vascular grafts, [PERSON] et al. [PERSON] _et al._ (2019) fabricated a hybrid vascular graft using PCL and gelatin. This hybrid vascular graft was further functionalized by the addition of heparin, which improved the hemocompatibility. The TEVGs were implanted into rats for four weeks. The luminal surfaces were smooth at the end of the implant time. The results of the study showed that the combination of PCL and gelatin was a good method to solve some problems occurring in the TEVG related to the individual materials. [PERSON] et al.