## 4 Discussion  
This study was conducted to examine the characteristics of the surface according to the various 3D printing washing conditions and to determine the clinically optimal washing time and washing solution. The experimental results showed no statistically significant difference in terms of surface roughness and flexural strength, but the evaluation on the contact angle and the DOC showed the difference. Therefore, the null hypothesis of this study was partially rejected. When the contact angle was compared, the surface of the TPM-10 group had the highest wettability and the highest surface energy compared to the other three groups (IPA-D, IPA-1, IPA-10). There is still much debate about wetting and bacterial adhesion [19,20]. However, several studies have shown that hydrophobic bacteria colonize on hydrophobic surfaces [19,21]. Streptococcus mutans, which is most commonly found in the oral cavity, is a representative hydrophobic bacterium, and the TPM-10 group conducted in the above study is likely to be more resistant to bacterial adhesion [22]. In the case of amount of residual monomers, for long washing times, as for the IPA-10 and TPM-10 groups, the DOC was high because of the small amount of residual monomer. This resulted in a relatively low DOC. In addition, the IPA-D and IPA-1 groups showed relatively high flexural strength, but this difference was determined to be not statistically significant. The surface roughness of dental prostheses has a significant impact on clinical practice. A rough surface can become easily colonized by bacteria in the oral cavity, and it also increases the wear of the opposing teeth [23]. Moreover, prostheses with rough surfaces are not considered to be aesthetically pleasing [24]. In addition, the fit of a prosthesis may be  
Figure 6: Degree of conversion according to different washing conditions: * significant difference at \(p<0.05\). Figure 7: Flexural strength according to different washing conditions. poor due to its rough surface, which is the most important factor in clinical situations [25]. This is because if the rough surface of the prosthesis is placed in the mouth, it may act as an undercut and the possibility of an inaccurate prosthesis margin increases. Because 3D-printed dental prostheses cannot be polished inside, a smooth inner surface is essential for restoration. However, according to the results of this experiment, there was no significant difference in surface roughness according to the washing solution or time. From the SEM image, residual suspended solids were observed in the IPA-D group, and relatively clear surfaces were observed in the IPA-10 and TPM-10 groups. The clear surface indicates an exposed layer which is unique in 3D printouts. In other words, it was speculated that the well-washed printed surface rather offset the degree of roughness due to residual suspended solids observed in the IPA-D group because the printed layer was exposed. The SEM images showed that the IPA-D and IPA-1 groups, which lacked washing time, showed more residual suspended solids than the IPA-10 and TPM-10 groups. In contrast, if washing is insufficient, such as for the IPA-D group and the IPA-1 group, the residual monomers will be cured naturally, which may lead to over-curing, which may have made the surface lubricated. In the case of the TPM group, the result showed a rougher surface, but visually from SEM image, it showed a smooth state, and there was no statistically significant difference between other three groups (IPA-D, IPA-1, IPA-10). As such, the aspect of surface roughness due to washing should be studied further in the future. Many studies on residual monomers of dental resins have been reported [26; 27]. In particular, in terms of strength and cytotoxicity, many clinicians are concerned about residual monomers, and studies are being conducted on the importance of washing along with efforts to develop substances that are harmless to the human body [28; 29; 30].