# 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  
#### 2.2.1 Construction of Vascular Models In Vitro Using Templates  
The template method involves casting materials around a template and then removing it to form a hollow tube, onto which endothelial cells are seeded to form a vascular model [19]. One of the most commonly used methods is to cast materials around small diameter needles or fiber rods, which are then removed after solidification [85]. In 2006, [PERSON] et al. [86] were pioneers of the needle model method and used it to construct a perfused vascular model with an internal diameter of 120 \(\upmu\)m in vitro. The model maintained cell morphological stability for 2-3 weeks, indicating good stability and potential for drug screening and inflammation treatment. In addition to needles, fiber rods can also be used as templates. In a separate study, [PERSON] et al. [87] used nylon wire as a template to construct a skin model with perfusion vessels by removing the template and inoculating endothelial cells on the inner wall. The model showed endothelial function similar to in vivo and permeability similar to human skin and traditional models. Drugs for skin diseases can be developed by measuring the amount of drug that passes through the skin into the bloodstream. The researchers used oxygen plasma treatment to improve the perfusion capacity of the model, which maintained stable perfusion for 1-2 weeks and showed endothelial function and permeability similar to in vivo and traditional models, providing sufficient time for subsequent skin drug and cosmetic development. While the template method is simple and can form a circular channel after extraction of the template, this step is critical and may encounter issues such as difficult template extraction, biomaterial remaining on the template after extraction, or pipeline collapse. These issues may be related to the viscosity of the hydrogel material, the chemical properties of the template surface, temperature, and other parameters. By optimizing these parameters, the success rate of circular pipeline construction can be improved. In addition to the one-step method for constructing circular microchannels, the template method can also be used to construct vascular models by synthesizing two semicircular microchannels and splicing them together to form a complete circular microchannel (Figure 2). In a study by [PERSON] et al. [88], two semicircular microchannels were aligned and cross-linked to form a seamless closed circular microchannel, onto which smooth muscle cells and endothelial cells were co-cultured. Co-culturing the two types of cells promoted the rearrangement of the endothelial cells and made their arrangement direction closer to the actual vascular structure. However, optimizing the concentration of the inoculated cells is necessary to ensure that the cells of the lower layer are completely covered before proceeding with the inoculation of the upper layer. This method also places high demands on the sealing of the channel during splicing, as misalignment or incomplete cross-linking can lead to leakage or even disintegration of the tubing. In another study, [PERSON] et al. [41] used a similar method to prepare a closed circular channel and inoculated endothelial cells into it to form a vascular model. The cells were evenly distributed in a single layer and maintained a normal phenotype and high viability. The researchers tested the stability of the model and found that it did not leak when the perfusion velocity was higher than the average velocity in the natural artery, indicating good stability. The template method allows for the direct formation or splicing of models through a simple approach, offering advantages of low cost and easy operation. The cross-section of the channel made by this method is similar to the cross-section of natural blood vessels in a circular structure, which is closer to the physiological structure of real blood vessels and difficult to achieve by microfluidic technology.