## 4 Discussion  
Recently, various 3D printing technologies with different materials have been marketed to clinicians and are being used for oral appliance manufacturing. In this study, we compared widely used six types of 3D printing materials, milling material, and conventional acrylic resin, in terms of the water solubility and absorption, color stability, flexural strength, surface hardness, and observed the fractured surfaces to determine if the 3D-printed materials can replace conventional acrylic resin. Figure 8: Preparation samples and SEM image analysis. Figure 7: Mean and standard deviations of the hardness. \({}^{*}\) denotes \(p<0.05/28\) for comparison between the manual and experimental groups. The physical characteristics of polymers are altered by moisture via their plasticizing effects. [PERSON] et al. have reported that moisture causes increases in plasticity, where expansion of these plastic areas triggers cracks, finally leading to resin degradation [20]. In addition, volume changes in oral appliances under high moisture conditions might affect their suitability and precision. Therefore, water absorption and solubility tests should be conducted. The SLA methods showed significantly higher water absorption and solubility values compared to other 3D-printing and manual methods. This result is consistent with previous studies [21]. The mean absorption of the SLA method was 52.31 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{mm}^{3}\), which is about a 2.8-fold increase compared to that of the manual method, which has a value of 18.51 \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{mm}^{3}\). Therefore, the SLA method is not suitable for replacing the conventional method in manufacturing intraoral appliances. The FDM, SLS, DLP, and milling methods were not only acceptable based on the ISO standard, but also exhibited values directly comparable with the manual method. It is the case that 3D-printed material can be discolored by internal factors such as the substrate of the resin, as well as external factors such as food consumed. Discoloration often reduces esthetics, which is relevant to patient satisfaction. Therefore, the materials used in oral appliances need to be evaluated for color stability to ensure long-term esthetics. Color stability is quantitatively expressed using \(\Delta\)E values. If the \(\Delta\)E value is less than 1, the observer cannot typically recognize any differences, and if the \(\Delta\)E value is between 1 and 3.3, a skilled technician can easily recognize any differences; however, if the \(\Delta\)E value is greater than 3.3, it is clinically unacceptable [22]. In all tested methods except for milling, significant color changes were demonstrated, which far exceeded the clinically problematic level of 3.3. This result was consistent with the previous study by [PERSON] et al., which compared the color changes for manual, milling, and 3D-printed materials. They reported that 3D printing materials showed low color stability, despite there being no significant difference between the manual and milling methods [23]. The possible reason for the low color stability of the 3D-printed materials is that there can exist additional layers in the surface microstructure, since 3D printing is based on an additive manufacturing method [24, 25, 26]. Looking at the SEM results, although the surface characteristics of the specimens were different depending on the 3D printing method used, it was possible to observe the microstructure reflected by the pattern structure of the surface (Figure 8). Since the DLP method uses a micro-mirror, a slightly more characteristic pattern appears on the surface, which may cause color stability to decrease. However, the surface roughness for ungrounded milling is higher in the milling specimens than in the 3D printer resin specimens. Therefore, the low color stability of the 3D-printed specimens cannot be explained by the rough surfaces.