Extrusion

In extrusion-based bioprinting, a melted polymeric filament or a cell supportive gel can be deposited. The term fused deposition modeling (FDM) is usually used to describe the first way in which a group of polymers of polycaprolactone (PCL), polyurethane (PU) and polylactic acid (PLA) can be extruded. Another way of extruding is the deposition of cell-free or cell-laden hydrogels [14]. For successful construction of vascular networks, researchers introduced and designed coaxial nozzles [48].

Extrusion-based bioprinting is regarded as the most practicable method since the vertical configuration is considered. Nevertheless, the induced shear stress during the printing procedure, often leads to cell death [27]. Moreover, increased shear stress can also lead to a loss of structural integrity regarding the used material, and therefore, the necessity of hydrogels which can regain the mechanical integrity is imperative [49].

According to Tabriz et al., the extrusion technique enables the possibility of bioprinting live human cells with an increased post bioprinting cell survival rate. In their study, alginate hydrogels were formulated with tunable mechanical properties to create straight tubular 3D hydrogel structures with diameters from 7.5 to 20 mm [50].

Furthermore, bioprinting of small diameter vascular grafts through coaxial extrusion possess the advantage of the simultaneous deposition of manifold materials in concentric needles [51]. Intricate multilayered 3D perfusable hollow tubes, with reported diameters 0.5 to 1.5 mm have been manufactured via coaxial nozzle [48]. In the Zhang et al. research, a coaxial nozzle system was used to print vasculature conduits with an outer diameter of 1 mm approximately, and with increased mechanical properties and bioprintability [52]. In their work, human umbilical vein smooth muscle cells (HUVSMCs), were encapsulated in sodium alginate, showed an initial low cell proliferation rate, following though an increased cell viability in prolonged in vitro cell culture.
