# 3D Printing Applied to Tissue Engineered Vascular grafts
## 3 Three-Dimensional Printing Strategies for TEVG
### Three-Dimensional Printing of Scaffolds
A structural analysis compared the 3D model with the 3D printed arterial tree and showed high accuracy of the printed structure, with measured variations of less than 15% in the overall length and width, and less than 3\({}^{\circ}\) difference in the angles of the major bifurcations. When perfused with black ink, the arterial tree showed no leakage through the wall, confirming an appropriate fusion of the alginate wall. _Silicone_  
[PERSON] et al. constructed small-diameter blood vessels using a multi-nozzle 3D bioprinting system [50]. A double-layered silicone (SE1700) tubular structure was produced by extrusion and post-processing laser drilling to form micropores within the structure. The mechanical properties of the silicone supporting structure were tuned to reach 244 kPa for a wall thickness of 150 um in order to maintain the vessel structural integrity. The space between the inner and outer layers was filled with human aortic vascular smooth muscle cells (HA-VSMCs) embedded in a decellularized extracellular matrix (dECM) to form the Media. HUVECS were seeded inside of the channel to form the Intima. Human dermal fibroblasts-neonatal (HDF-n) were seeded outside to form the Adventitia. The complete complex process allowed the successful formation of a three-layered structure similar to that of native vessels. _Polycaprolactone (PCL)_  
PCL is an FDA approved biodegradable polyester with a resorption time of two to three years through hydrolysis [66]. Its high elongation at break and low stiffness provide major advantages over other polymers for low-bearing applications with deformations. To tune the mechanical properties of the printed structures, PCL has often been combined with other polymers. Using extrusion, [PERSON] et al. produced various multi-material anatomically shaped tissue constructs [47]. For a vascular branched model, two extrusion heads were used: One printing a PCL structure with large pores, the other depositing a temporary polyvinyl alcohol (PVA) support. Alternatively, the authors extruded a gelatin methacrylate(GelMA)-gellan hydrogel tube that was reinforced with PCL fibers and alginate support structures. The addition of gellan gum improved the shape-fidelity of the GelMA hydrogel without affecting cell viability. The removal of the PCL fibers and dissolution of the alginate supports resulted in a hydrogel tube with an open lumen, into which a mesenchymal stem cell (MSC) -laden gelatin hydrogel was injected. Interestingly, PCL has also been used in mixed procedures combining 3D printing with electrospinning of nanofibers. This combined approach has recently emerged and holds great potential. Nanofiber-based scaffolds have a high surface-area-to-volume ratio and intrinsic porosity, favorable for cell integration,while the control of the fiber orientation enables the fabrication of anisotropic implants [67; 68; 69]. Artificial blood vessels were produced by first electrospinningning nanofibers from a biomaterial prone to cell attachment on a cylinder. Subsequently, the electrospun tubes were wrapped in an extruded, thicker polymer coil in order to improve the mechanical properties of the vessel [56]. [PERSON] et al. tested CTS (chitosan), PCL, and blended CTS/PCL as an electrospun material, combined with wrapping in a PCL coil. Water absorption, and therefore hydrophilicity (which promotes cell adhesion), increased in correlation to the increase in the CTS content of the nanofibers. The addition of the thick strand of PCL around the electrospun tube led to an increase of the elasticity modulus and ultimate tensile strength of the vessel. Using a similar principle, electrospinningning of a heparin-eluding Poly-L-Lactic Acid (PLLA) tube, combined with a thicker PCL coil extruded on the outer wall, was investigated [58]. The low patency rate observed in grafts made of PLLA alone, due to acute graft thrombotic occlusion, was improved with the addition of heparin. The acellular hybrid graft had a stress-strain profile comparable to that of the human thoracic artery.