## 4 Discussion  
Recent studies into TEVGs have been mainly focused on acellular TEVGs, which lead to in situ blood vessel regeneration. The goal of all these studies was to produce TEVGs which satisfy as much as possible the main requirements of well-matched mechanical properties, blood compatibility, endothelialization, and biodegradability [37]. Until now, all TEVGs have presented the same problem: the scaffold cannot satisfy all of the necessary mechanical properties at the same time. These include elastic (Young's) modulus, radial compliance, burst pressure, and suture retention strength. A mismatch of any single mechanical property between the graft and the native blood vessel of the host leads to low patency for long-term implantation. Low burst pressure causes the TEVG to expand or even rupture, which directly leads to the failure of the TEVG. Insufficient suture retention strength leads to leakage at the pinhole after implantation surgery. The requirements of burst pressure and suture retention strength can commonly be met by TEVG, but compliance remains a problem. Because of the complex environment in vivo and the degradation process of vascular grafts, we cannot determine which radial compliance is most suitable for implantation. Moreover, unsuitable compliance leads to a hemodynamic difference between the environment near the graft and the real environment. This situation can reduce blood flow velocity, which results in thrombosis and intimal hyperplasia at the anastomotic site [6]. To this end, many research groups are working to combine different fabrication methods and material types in order to construct a scaffold with better mechanical properties. Blood compatibility is basically satisfied by most TEVGs through the proper selection of scaffold materials; this mainly requires low cytotoxicity and surface modifications such the addition of heparin to improve hydrophilicity and anticoagulant properties. Rapid endothelialization is still one of the core problems related to the use of TEVGs. The lack of endothelium cells or the lack of the formation of the EC film on the inner surface of the TEVG generally leads to thrombosis. Particularly for acellular TEVGs, the whole endothelium process occurs in vivo; there are no seed cells or in vitro cultures to accelerate the endothelium process. All ECs need to be recruited from the nearby natural blood vessels. Moreover, the proliferation and attachment processes are of great importance. These concern the microstructure and further surface modifications. The high porous ratio obtained, which is beneficial for proliferation and attachment of ECs, is the main reason for choosing the electrospinning method. Surface modifications, which lead to a higher load capacity of VEGF, were used by many research groups in the attempt to improve the rate of endothelialization. Biodegradability is a crucial factor for the scaffold. A mismatch between the scaffold material's degradation rate and the tissue regeneration rate leads to a change in the mechanical properties of TEVGs after implantation, which can cause different kinds of failures such as TEVG bursting or occlusion. Many groups use in vitro testing to simulate the degradation process, such as bioreactors [38]. In real circumstances, however, it is very difficult to calculate the rate of degradation and scaffold regeneration because of the countless disturbance factors. This unsolved problem requires further analysis and experimentation. The final desired result is long-term patency after implantation. All the in vivo testing in animal models aims to provide more positive experimental results and favorable support for clinical experiments. However, for small-diameter vascular grafts, it is difficult to meet all the requirements at the same time. Nearly all studies include in vivo testing only for several weeks or months; long-term(\(>\)6 months) in vivo testing has barely been reported (the results of recent in vivo testing are shown in Table 1).