# 3D Printing Applied to Tissue Engineered Vascular grafts
## 3 Three-Dimensional Printing Strategies for TEVG
### Three-Dimensional Printing of Scaffolds  
As summarized in Table 1, the 3D printing approaches for TEVG support the creation of straight or branched tubular structures, using synthetic or natural biomaterials. The choice of the biomaterials is driven by several considerations, including their mechanical properties, biocompatibility, and biodegradation behavior (mechanism, kinetics, metabolites) [42]. One of the advantages of synthetic polymers is the high reproducibility of their properties, independent of batch variability. The molecular weight, crystallinity, internal organization, and degree of cross-linking are important tunable attributes that can be adjusted during the synthesis, or later, as a post-treatment [43; 44; 45; 31; 46]. As a substitute for synthetic biomaterials, natural polymers extracted from animal and plants have been broadly investigated. Depending on their origin and processing, they generally present biological cues suited to promote desirable cell responses, and are favorable for cell attachment. Their mechanical properties can be tuned by chemical modification (i.e., cross-linking) to match the application. Natural and synthetic polymers can be 3D printed in two forms: As solid polymers or as hydrogels. Solid polymers are mostly hydrophobic and do not significantly absorb water, thus are not permeable to water-soluble molecules. This implies that, without the addition of pores, the exchanges of gases (i.e., oxygen), signaling molecules, and metabolites is obstructed. Hydrogels, on the other hand, are cross-linked 3D networks of hydrophilic polymers, already swollen in water, allowing for the diffusion of water-soluble molecules [59; 60]. The creation and implantation of an acellular, vessel-like structure, is based on the assumption that biocompatible scaffolds allow the migration and the engraftment of the host cells within the printed structure, resulting in the formation of a neotissue. The choice to add cells to the printed  
structure prior to implantation remains an open option taking into consideration the balance between increased time and complexity of the graft preparation, and the improved stimulation of vessel regeneration. In order to create cellularized constructs, cells are seeded and grown on or into printed structures. First, a tubular structure is produced; next, a simple pipetting procedure seeds the appropriate cells on or into the printed structures. The constructs are matured over the course of several days to weeks in an incubator, prior to in vivo implantation. A more sophisticated approach, consisting of printing both the cells and the biomaterials, is described in Section 3.2. _Poly (propylene fumarate) (PPF)_  
PPF is a biodegradable polyester, often used in laser-based additive manufacturing as a UV resin, together with diethyl fumarate as the solvent, and bisacrylphosphrine oxide as the photoinitiator [61]. The variation of the solvent to resin ratio allows for the tuning of viscosity before cross-linking, and mechanical strength of the 3D model. Acellular vascular grafts composed of photo-cross-linked PPF were produced by DLP [53]. [PERSON] et al. printed PPF-scaffolds with an inner diameter of 1 mm and a wall thickness of 150 \(\upmu\)m, with adequate long-term mechanical stability and suturability. After 100 flashes of the UV lamp, the 3D-printed PPF samples demonstrated an ultimate tensile strength of 1.48 MPa and an elastic modulus of 8.79 MPa, within the range of native femoral artery and saphenous vein. The acellular grafts were implanted in the venous system of mice for 6 months and maintained an optimal patency throughout this period. ECM remodeling and a confluent endothelialization of the lumen surface were observed, suggesting that the implants could indeed support neotissue formation in vivo. However, an extended inflammation was observed in the newly formed tissue, suggesting a strong foreign body reaction. _Polyurethane (PU)_  
PU is a non-bioresorbable polymer with a high mechanical strength.