**2.2 Filament-based additive manufacturing technology**

FDM—the most famous AM technology, belongs to filament-based technology, which is also one of the most widely used technologies in rapid prototyping technology. FDM uses a form similar to squeezing toothpaste to extrude material to build a layered structure. By heating the filamentous thermoplastic material, the nozzle extrudes the material along the printing path under computer control and deposits it on the hotbed. After one layer, the hotbed moves up the distance of one layer of material thickness and then prints the second layer; layer-by-layer deposition finally realizes the overall printing, shown in **Figure 4b** [21]. The key to FDM is the temperature at which the nozzle is heated. At higher temperatures, the viscosity of the filamentous material decreases, resulting in reduced accuracy and even collapse and deformation during the printing process; the filamentous material does not melt completely at lower temperatures, which is prone to delamination and even blocks the nozzle [28].

FDM has the characteristics of simple process flow, low cost, and low environmental requirements. It can also realize multi-material printing by switching nozzles. However, FDM also has disadvantages, such as low-printing accuracy, poor mechanical strength, and being prone to the "step effect" [28]. When using FDM to build an object with a certain degree of complexity, the support of supporting materials is

required to ensure the stability of the built part; otherwise, there may be collapse, which directly affects the manufacturing accuracy. In the actual construction process, the support structure can be printed with water-soluble filaments and then removed after post-processing, and the related problems caused by the support structure can be effectively reduced by optimizing the topology structure [29], optimizing the printing direction [30], and decomposing the printing structure to reduce the use of support materials [31].
