**2.3 Ink-based additive manufacturing technology**

DIW technology is fabricated by layer-by-layer deposition, using an ink with shear-thinning properties that is extruded from a nozzle, shown in **Figure 4c**. The ink needs good viscoelasticity and fluidity to ensure smooth extrusion and maintain a good shape without clogging the nozzles. Because of the high accommodating capacity of ink materials, multi-component printing can be realized under the condition of meeting the basic requirements of ink, which can be widely used in many fields. Usually, the resolution of DIW technology is directly limited by the diameter and path of the nozzle. Latest studies have found that by adjusting the printing-related parameter set, it can successfully break through the constraints of line width and shape to achieve higher-resolution printing.

DIW technology has the advantages of simple process and multi-material printing, but there is also a disadvantage. Green bodies printed with DIW usually require longterm low-temperature drying to remove unnecessary solvents, so this step is critical to avoid cracks formed by drying.

### **2.4 Photosensitive resin-based additive manufacturing technology**

SLA technology uses a laser with a specific wavelength to scan the photosensitive resin from point to line and to the surface according to the set path. After the layer is cured, move the worktable and apply a new layer of liquid resin on the surface of the original cured resin, so as to scan and cure the next layer, so that the newly cured layer can be firmly bonded to the previous layer. Repeating until the entire object has been fabricated, shown in **Figure 4d**. The key process parameters for SLA technology include initiator concentration, laser intensity, and scan rate. The initiator concentration and laser intensity directly determine the speed of the photopolymerization reaction, which in turn affects the processing efficiency. The spot size of the laser directly determines the resolution of the SLA [32]. Compared with FDM and SLS/ SLM technologies, SLA shows better surface finish (nanoscale) and precision (micron scale), but the materials used are limited and it is difficult to achieve multi-material printing.

DLP technology is another AM technology that uses UV lamp curing. Its working principle is similar to SLA. The light source is changed from laser to UV lamp, and the introduction of digital micromirror devices (DMD) significantly shortens the construction time. The working principle of DLP technology is that the UV lamp is used as the light source, the three-dimensional CAD model is sliced through the software to form a two-dimensional dynamic mask pattern, and the sliced image is projected onto the surface of the photosensitive resin by DMD and cured, and the curing of one layer is completed, demonstrated in **Figure 4e**. The build plate is coated with a new layer of photosensitive resin, and the process is repeated layer by layer until the overall build is complete. DLP technology is one of the most widely
