Coaxial 3D bioprinting has simplifi

Coaxial 3D bioprinting has simplified the process of directly printing vascular constructs for nutrient delivery. The most commonly used method involves core-shell flows within a coaxial nozzle. In this approach, one or multiple materials in laminar flow can be used in parallel streams. The multiple phase filaments thus include multiple materials fabricated as fiber. These multiple phases have several capillaries connected in a coaxial form. During printing, for example, when two materials have been loaded and dispensed individually from inner and outer capillaries via a coaxial nozzle, the structure can be created by the dispensed materials. Therefore, two-phase filaments are achieved by these two materials in coaxial distribution. The utilization of a coaxial nozzle in extrusion-based bioprinting increases the possibility of producing a hollow structure. The coaxial nozzle is fixed on the axis that moves along a pre-planned path. In this approach, if the calcium chloride solution is dispensed from the inner capillary, whereas the alginate solution is delivered from the outer capillary of the coaxial nozzle, the result is the construction of a hollow fiber. The material used in this method must have a rapid crosslinking mecha-nism to impede collapse within the nozzle (Fig. 3a) (Gao et al., 2015). If the bioink is pumped into the inner capillary and the crosslink agent solution to the outer capillary of the nozzle, a single-phase filament is printed. Furthermore, the size of the hollow fiber can be adjusted by controlling pressure (Colosi et al., 2016). In a different method, the non-viscous Gel MA solution was loaded into the internal needle, and a viscous solution containing sodium alginate to the external needle. Due to a low Reynolds number, these materials created laminar flow in the transparent capillary channel. The crosslinking mechanism was the blue light created by the Gel MA fiber as the standard product (Fig. 3b) (Shao et al., 2019). Microfluidic bioprinting using a coaxial nozzle is another strategy to create micro-fibrous constructs, where Gel MA/alginate is printed through a core/sheath coaxial nozzle. This coaxial nozzle, which is assembled in extrusion bioprinting, is stable and concentric, leading to a continuous generation of hollow microfibers. In this method, alginate can be crosslinked with Ca Cl2 and Gel MA bioink in an alginate sheath with a form of in situ gelation, and photo-crosslinked with UV light. Printing can be improved if the bioink extrusion rate is matched with nozzle speed (Liu WJ et al., 2018). In another approach, bioink is extruded by two coaxial nozzles to print a hollow filament in a rotating rod temple. As bioink from the outer needle contains alginate, a crosslink solution is extruded from the inner needle. The flow rate of both solutions is the same, resulting in a hollow filament twined over a rod. This hollow alginate filament is partially attached to the crosslink-loaded fibroblasts and smooth muscle cells via the use of the coaxial nozzle rolling process. Concurrently, ECs are seeded in the inner wall. In this formation, multilevel fluidic channels with multiple layers of cells are fabricated, whereby smooth muscle