# 3D Printing Applied to Tissue Engineered Vascular grafts
## 3 Three-Dimensional Printing Strategies for TEVG
### Three-Dimensional Printing of Scaffolds
Its main components are polycaprolactone diol and a second oligodiol containing amphiphilic blocks [62]. Biodegradability can be engineered by tethering with hydrolysis-prone segments. Recently, PU was modified into a hydrogel that can undergo thermal gelation and biodegradation [63]. Tubular constructs derived from PU biodegradable hydrogels were achieved via DLP and included complex micro-architectures, such as high porosity and interconnectivity [51]. The group improved the suture tear resistance of the constructs by developing a resin, based on urethane-diacrylate monomers, which exhibited a reduced cross-linking density. The observed mechanical properties were within the range of those of native ovine carotid arteries. _Polytetrahydrofuran diacrylate (PTHF-DA)_  
PTHF-DA is a non-bioresorbable, commercially available resin that can easily be used as a UV ink through the addition of a photo-initiator. The ratio of the photo-initiator to the resin influences its photosensitivity, while the chain length of the pre-polymer influences the viscosity of the resin and the mechanical properties of the 3D model. [PERSON] et al used PTHF-DA with SLA to engineer bifurcating acellular tubes with a diameter of 2 mm [54]. The elastic modulus was reduced from 30 to 6 MPa by increasing the molar mass of the pre-polymer. The biocompatibility investigations of the photo-cross-linked material showed in vitro cytocompatibility with human dermal fibroblasts. In addition, the authors provided evidence that to produce smaller diameters, e.g., capillary vessels, 2 PP is more appropriate as it allows for sub-micrometer resolution [54; 64]. _Alginate_  
Alginate is derived from seaweed and bacteria [65] and is a popular natural polymer used for 3D printing of scaffolds or as a sacrificial material. Its gelation can be controlled by the addition of non-toxic ionic cross-linking agents, such as calcium chloride (CaCl\({}_{2}\)). One critical drawback of ionically cross-linked alginate hydrogels is the release of water-soluble cations into the surrounding liquids. Nevertheless, covalent cross-linking using Poly (ethylene glycol) (PEG), as well as photo cross-linking, using methylates and photoinitiators, allows for more stable hydrogels, and for the tuning of their mechanical properties. Alginate is not biodegradable by mammalian cells, as they lack the enzyme (i.e., alignase) to cleave the polymer chains. In a pioneer investigation, the fabrication of tubular hydrogel structures by drop-on-demand inkjet printing was achieved by modifying a Hewlett Packard standard printer to jet the CaCl\({}_{2}\) solution into an alginate bath with a z-stage that moved the substrate after each layer was printed [55]. Between the layers, SMCs were manually pipetted to ensure their encapsulation in the construct. Because of the difficulty to achieve precise patterns when using ionic CaCl\({}_{2}\) cross-linking, and the fragility of the model, the printing process could not be used to engineer high aspect ratio constructs. The inner diameters, wall thicknesses, and heights of the resulting structures were each 2 mm. Interestingly, the construct was cultured for 18 days and showed vasoreactivity to agonists while remaining mechanically suitable for manipulation. In an innovative approach, named freeform reversible embedding of suspended hydrogels (FRESH), alginate-based hollow tubular structures were created with the use of a sacrificial material. An alginate solution was extruded into a supporting gelatin/CaCl\({}_{2}\) hydrogel bath to print branched coronary artery structures [49]. Gelatin was added to the bath and thermally converted into a gel in order to prevent the alginate construct's collapse during the printing process. By providing a continuous support of the printed model, this approach enables the fabrication of soft structures with mechanical integrity. To remove the construct from the bath, the gelatin was heated above its liquefaction point.