## 2 Experimental Details
### Materials  
Hard (STD-clear, TINT, Gunsan, Korea) and soft (Flexible resin, XYZprinting, New Taipei, Taiwan) materials, which had significantly different mechanical properties after photo-polymerization reactions, were used to investigate the changes in the physical properties of the printed structure through selectively switched printing of multiple materials. In order to provide additional functionality, a photochromic catalyst that was able to visualize the differences without significantly changing the mechanical properties of two materials among various powders was mixed. There are two types of photochromic catalysts, dyes and pigments; most of the previous 3D printing studies have used dyes, which are completely soluble in resin, meaning there are no dispersion problems and they are easy to print; however, the dyes are mainly soluble in organic solvents, such as acetone and ethanol, as well as specific polymers (PP, PE, ABS, acrylics, etc.), so there are restrictions on the usable materials and they are very sensitive to the mixing ratio. In addition, they are chemically unstable because they are altered by the hydroxyl group (OH), so there is a limitation in using them conveniently as general-purpose materials. On the other hand, pigments are chemically stable, meaning they do not dissolve or react in the resin and the color develops by itself; however, when printing composite materials containing pigment-type photochromic powder using a general 3D printer, the printing quality is greatly reduced due to problems such as sedimentation or reduced dispersion. On the other hand, the proposed movable separator in this study can produce a continuous forced flow of resin  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Variables** & **Values** \\ \hline Build size & 134.4 mm \(\times\) 75.6 mm \(\times\) 200 mm \\ X-Y pixel resolution & 70 μm \\ Curing power & 3.12 mm/cm\({}^{2}\) \\ Layer thickness & 50 μm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Printing conditions for test specimen fabrication.  
Figure 3: Vat with movable separator: (**a**) schematic diagram; (**b**) photo of the fabricated vat.  
Figure 2: Conceptual diagram of the suggested printing equipment.  
during the printing process, which can solve the above dispersion problems; therefore, a pigment-type photochromic powder with little influence on the material properties and excellent versatility was physically mixed with two types of commercial resins.  
The pigment-type photochromic powder (C-Red and C-Yellow, Nano ENC, Ansumg, Korea) used in this experiment had an average size of about 1 to 5 \(\upmu\)m and color expression at 280 to 382 nm of UV wavelength; therefore, in normal times, it appeared as a single material without distinguishing colors, although when irradiated with light of a specific wavelength the color changed so that the mechanical properties could be seen. For this purpose, as shown in Table 2, composite materials A and B were prepared by mixing 5 wt% of two types of photochromic powder with two types of commercial resin. Both commercial resins used in this experiment were clear-type resins, although the transparency of composite materials A and B was decreased due to the influence of the mixed photochromic pigment. As a result, the two composite materials showed a similar opaque ivory color in an environment without UV, although there was a difference in contrast. In order to determine the curing conditions for the two composite materials, as shown in Table 3, the cured depth was measured while the light irradiation time was changed in units of 1 s. The curing characteristics of the original materials were different, and as the added pigment was discolored by ultraviolet light during the printing process, it affected the photo-polymerization reaction, and thus the curability was lower than that of the original materials. In order to secure sufficient interlayer bonding strength and mechanical properties, the curing times of materials A and B were set to 2 and 3 s, respectively.