## 4 Discussion
The bottom-up systems are Admatec (ADMAFLEX2.0, ADMATEC Europe BV, Alkmaar, The Netherlands) and Lithoz (CeraFab 7500, LithaCon 3Y 230, CeraFab System, Vienna, Austria), whereas the top-down system is 3 DCeram (CERAMAKER C900 Flex, 3 DCeram Sinto, Bonnac-la-Cote, France). [PERSON] and [PERSON] tested the bottom-up DLP-based system, Admatec. In this system, the transportation of the slurry is based on the principle of tape casting [27]. A thin slurry layer is transported to the stage. Then, the platform descends slowly, and photocuring takes place for the slurry from the bottom DLP as the light source. Finally, the platform rises. Another bottom-up DLP-based system was developed by Lithoz [28]. The approach used by this system is similar to that of Admatec. However, this system uses a rotating vat in combination with a static wiper blade instead of tape casting to produce thin films, enabling the use of a high-viscosity slurry. Photocuring in the Lithoz system is also realized from the bottom DLP as the light source. The mechanical strength of the parts fabricated by these systems depends on the molding device, molding direction, and measurement method. However, the mechanical strength of zirconia fabricated using bottom-up DLP-based systems is significantly lower than that of subtractive zirconia fabricated via subtractive manufacturing. [PERSON] et al. observed long cracks parallel to the molding direction in zirconia fabricated using bottom-up DLP-based systems [27]. This is thought to be due to peeling between the molded layers. In a bottom-up system, the top surface of the bottom layer, on which the next layer is formed, polymerizes without oxygen inhibition. It is also easily deformed by the stresses applied to peel away from the vat when lifted to form the next layer. As a consequence, cracks are more likely to occur. Additionally, the Lithoz vat method requires the use of a relatively low-viscosity slurry, which may be difficult to densify. The top-down system used in several studies was obtained from 3 DCeram [29; 30]. Although the mechanical strength of the fabricated zirconia is lower than that of subtractive zirconia, strengths above 1000 MPa were observed. [PERSON] et al. utilized the bottom-up system employed by Lithoz and the top-down system by 3 DCeram to fabricate zirconia using their respective 3Y-TZP slurries. They reported that the zirconia fabricated using the 3D system by Ceram had a significantly higher biaxial flexural strength than the zirconia fabricated using the Lithoz system. [PERSON] et al. also made cross-sectional observations using SEM; however, the state of the layer-to-layer joints could not be confirmed [29]. Both top-down and bottom-up systems have several advantages and disadvantages. [PERSON] et al. compared both bottom-up and top-down approaches using the same slurry [16]. They revealed that the top-down system was unable to fabricate a flat surface. Based on their results, in this research, we used a blade to spread the ceramic-resin slurry in a top-down-based system. The use of blades can enable the uniform spreading of a slurry, even an appropriately highly viscous slurry, which means that numerous ceramics can be fabricated using this approach. When the blade spreads subsequent layers of the slurry, it is able to remove trapped air bubbles. In this system, the light beam can directly reach the slurry. However, the top of the resin slurry that is in contact with the air cannot be cured well owing to oxygen inhibition. Thus, when the next slurry layer arrives by spreading the blade, the oxygen inhibition layer mixes with the new layer. This process creates strong layers. In the present study, SEM images of the additively manufactured zirconia post-sintering showed the presence of small holes. We believe that holes can be reduced by modifying the resin composition and increasing the zirconia filling rate.