## 3 Current State of Acellular TEVG
### Casting  
Casting is a traditional method for the production of TEVGs. When the graft is cast with a solution, the polymer is dissolved in a solvent. Porogen is added to the solution, and then the solution is added to a 3D mold. After the solvent evaporates, the porogen is leached, and a porous structure is formed. The pore size and the porosity can be controlled by the size and the concentration of the porogen. Compared with other methods such as electrospinning, casting is a relatively easy and quick method for fabricating scaffolds. The mechanical properties of the casting scaffold are not as good as those of other grafts; the elastic tensile modulus, suture retention force, and burst pressure are relatively low. Therefore, in order to increase the strength of the graft, different fabrication methods are combined with the casting method. The high degradation rate of the casting scaffold and the mechanical support of the outer layer fabricated by other methods make the whole scaffold suitable for vascular grafts. [PERSON] et al. [22] used a fast-degrading elastomer to fabricate a cell-free synthetic graft. The graft had a two-layer structure. The core layer was fabricated with poly(glycerol sebacate) (PGS) using a salt fusion and leaching method. A 1 mm mandrel and a 1.25 mm outer mold were used to form a vessel-like scaffold. To fabricate the outer layer (the PCL sheath), a PCL solution was directly electrospun onto the rotating PGS-salt template. Then, the salt was removed from the PCL-PGS-salt composites through immersion in deionized water. This study emphasized the rapid graft degradation and host remodeling, with the purpose of producing cell-free biodegradable elastomeric grafts capable of degrading rapidly to yield neoarteries nearly free of foreign materials. This design solved the problem of slow graft degradation, which limits cell penetration and causes a prolonged presence of foreign materials. Three months after interposition grafting in rat abdominal aorta, the results showed that the ECM fibers were less dense and less organized, but the majority of the graft resembled native arteries. [PERSON] et al. [23] also combined casting methods with electrospinning, trying to obtain grafts which have the advantages of each individual method. However, the formation process was very different from that reported in the aforementioned study. First, a nonwoven copolymer bio-felt made of poly(glycolic-co-lactic) (PLGA) approximately 0.3 mm thick was fabricated for the main structure of the scaffold. Then, the felt sections were cut into tube shapes with a diameter of 1.4 mm, and the inner lumen was formed using a 21 g stainless steel needle. A sealant solution of poly(DL-caprolactone-co-lactic acid) (P(CL-LA)) was deposited onto the scaffold to saturate the PGLA felt in order to seal and connect the seam of the rolled felt section. The test showed that this graft had good mechanical properties. After two weeks of in vivo testing in a mouse model, no evidence of thrombosis or graft aneurysm was observed. In addition, cell attachment and proliferation on the graft surface were observed, indicating adequate porosity for cellular infiltration and neotissue formation. In order to inhibit the calcification of the graft, [PERSON] et al. [24] also combined the two methods of casting and electrospinning. Two versions of bioresorbable vascular grafts were created: slow-degrading (SD) grafts and fast-degrading (FD) grafts. The inner layers of the scaffold were constructed by pouring a solution of PLCL into a glass tube and then freeze-drying under a vacuum. In order to improve the mechanical properties, these scaffolds were reinforced by electrospinning fibers on the outer surface. The outer layer of the FD grafts was composed of a combination of PLA and PGA nanofibers, while the outer layer of the SD grafts was composed of PLA nanofibers. Both grafts were implanted in female mice for eight weeks.