## 2 Background  
The aim of TEVGs is to replace autologous vessels in surgeries; therefore, TEVGs need to imitate real blood vessels as much as possible. The natural human blood vessel wall is composed of three layers: the intima layer with endothelial cells, the media layer with smooth muscle cells, and the external layer. These three layers confer the blood vessel wall good mechanical properties as well as many different functions.  
TEVG fabrication methods can be categorized into two groups. One group consists of self-assembled TEVGs, and the second of scaffold-based TEVGs [6]. Scaffold-based TEVGs include three key elements [7]: the scaffold, the seed cell, and the bioreactor [8; 9]. The scaffold can be prepared using natural polymers, synthetic polymers, hybrid polymers, or a decellularized matrix. Cells, such as mesenchymal stem cells [10], are used as seed cells and are seeded on the surface of the scaffold. The inner surface and the outer surface can be composed of different kinds of cells. The scaffold containing the seed cells is then placed in the bioreactor and cultured under normal conditions for weeks. During this period, cells proliferate and differentiate. As the scaffold materials (natural or synthetic) degrade, the tissue is regenerated. Finally, at this stage, the mature TEVG is obtained and is ready for further analysis.  
In order to adapt to the complicated environment in vivo, the TEVG scaffold should meet the following requirements [11]:  
* Mechanical properties: the TEVG must be able to maintain its shape under a certain pressure;
* Degradability: the scaffold of the TEVG should be degradable, and the rate of degradability should match the rate of tissue regeneration;
* Biocompatibility: the scaffold should not cause an immune reaction or a toxic reaction;
* Surface structure and biological activity: the inner surface of the vascular graft should be favorable to cell adhesion, growth, and proliferation.  
Self-assembled TEVGs are obtained by three main fabrication methods [12]. The first is sheet-based tissue engineering. This method involves winding a 2D cell sheet around a mandrel to form a tube that is matured into a TEVG. The second method is the assembly of microtissues. Microtissues are placed in a tube-shaped mold and combined to form a TEVG. The final method is bioprinting. Cells and supporting materials are mixed and deposited in a layer-by-layer manner, and a 3D tube is constructed.  
There are also certain challenges associated with TEVG fabrication, which include the requirement for a long in vitro preparation time, the difficulty of storing and transporting TEVGs, and especially, the risk of immune reaction [13]. In order to solve these problems, the acellular TEVG method was developed, which does not need an in vitro culture process and can regenerate blood vessels in situ (Figure 1). These scaffold-based TEVGs do not need long-term in vitro cultures, and the scaffold is implanted in vivo directly. After being implanted into the body, the TEVG recruits cells from the environment, and the scaffold gradually transforms into real blood vessels [14].  
Current TEVGs, however, are not as suitable for vascular transplantation as autologous vessels [15]. Furthermore, there are some common complications associated with TEVGs, such as thrombosis, intimal hyperplasia, atherosclerosis, and infection, which influence the short-term and long-term patency of these TEVGs. Thrombosis is mainly caused by the absence of endothelial cells (ECs), which leads to the adherence of blood proteins on the graft lumen and the activation of clotting mechanisms. Intimal hyperplasia is mainly caused by the immigration of smooth muscle cells (SMCs). This can have many causes, mostly associated with the mechanical properties and the implantation quality during surgery. Atherosclerosis of TEVGs occurs through a similar process to that of native vessels. Infection is mainly affected by the degree of disinfection of the synthetic scaffold.