# Research Progress in the Construction and Application of In Vitro Vascular Models
## 2 Construction of In Vitro Vascular Models
### Construction of In Vitro Vascular Model Based on Non-Microfluidics-Based Methods
#### 2.2.2 Construction of Vascular Model In Vitro by 3D Bioprinting  
The use of 3D bioprinting technology has been on the rise because of its potential to create vascularized tissue structures and replicate intricate vascular structures in vitro [2; 89]. This technology offers the advantage of not only precisely printing cells, biomaterials, and cell-loaded biomaterials separately but also combining them to build 3D tissue structures directly [90]. These combined biomaterials are referred to as \"bioinks\". To date, in the field of bioprinting, various techniques are used for 3D bioprinting of perfusable vascular channels in vitro, including extrusion bioprinting, inkjet bioprinting, and light-assisted bioprinting [48]. Extrusion and inkjet bioprinting can be categorized into direct and indirect bioprinting methods [91]. Direct printing involves depositing cell-laden bioinks layer by layer onto hollow structures and requires rapid cross-linking to maintain stable structures [92]. On the other hand, indirect printing, also known as sacrificial bioprinting, involves embedding the printed sacrificial material within a larger cell-laden material. The sacrificial template is then removed to create a hollow channel, which allows for the inoculation of cells and is particularly effective in constructing complex structures [93]. It is important to note that each bioprinting technology has its own advantages and limitations. Furthermore, these technologies differ in resolution, manufacturing time, and design requirements [94]. Extrusion bioprinting is a cost-effective and simple technology that is suitable for most high-viscosity bioinks. It is also the most widely used bioprinting technology for constructing blood vessels [2; 42]. This method uses a pneumatic system, mechanical (piston or screw), or electromagnetic drive system to continuously drip bioink from a needle, accurately depositing cell-laden biomaterials to form a stable vascular structure. As a result, very high cell densities can be achieved using this technology [43]. However, extrusion bioprinting has low resolution and can easily cause bioink blockage at the nozzle, as well as shear-stress-induced reduction in cell viability [44]. For instance, [PERSON] et al. [95] used a coaxial nozzle system to bioprint sodium alginate loaded with human-umbilical-vein smooth muscle cells to form a perfusable vascular channel in vitro. Although shear force had a negative effect on cell viability during printing, the viability of human umbilical vein smooth muscle cells in the channel increased with increasing culture time, even exceeding the viability of the original cells. Therefore, to improve cell viability, bioink materials should closely mimic the natural extracellular matrix. In other words, the mechanical properties and biodegradability of the bioink will affect cell expansion, which limits the use of this printing technology for biological materials [96]. Inkjet bioprinting uses thermal, piezoelectric, or electromagnetic actuation mechanisms to deposit cell suspension droplets through nozzles in a high-throughput manner [46]. Figure 2: Schematic diagrams of in vitro vascular models prepared by template methods. (**a**) The schematic diagram of an in vitro vascular model prepared using the stainless-steel needle one-step method. (**b**) Schematic diagram of an in vitro vascular model prepared using the semi-synthetic method. This method ensures high cell viability (typically 80-90%) and has the advantages of fast printing speed, high resolution, low cost, and compatibility with various bioinks [2, 45]. However, high-viscosity bioink can cause nozzle blockage, so this method requires bioink with lower viscosity and lower cell density [46, 47]. Moreover, inkjet bioprinting is not commonly used for large-scale vascularized tissue construction due to the limited space for bioink in the printhead and ink cartridge [45].