## 2 Materials and Methods
### Mechanical Tests  
Bending, buckling, cyclic fatigue, and torsional fracture resistance tests were performed to assess the mechanical properties of the files. Bending resistance testing was performed in accordance with ISO3630-1:2019 using a universal testing machine (UTM: Universal Mechanics Analyzer, IB Systems, Seoul, Korea) (Figure 1A,B) [35], which consisted of a stainless steel chuck in the upper part and a holding device in the lower part. The UTM included a load cell and a torque sensor for measuring force and torque in real time. The file was gripped at a distance of 3 mm from the tip before being bent horizontally at a point 10 mm away from the grip with a metal arm at a speed of 2 rpm (12 degrees/s). The torque magnitude that induced horizontal bending at 45\({}^{\circ}\) was obtained as the bending resistance (N cm) [26]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**NiTi File** & **Manufacturer** & **Tip Size, Taper** & **Heat Treatment** \\ \hline ProFile & Dentsply Maillefer, Ballaigues, Switzerland & \#25, 6\% & Conventional wire (no heat treatment) \\ ProTaper Gold & Dentsply Maillefer, Ballaigues, Switzerland & \#25, variable 8\% & Gold-wire \\ ProfaTaper Gold & Shenzhen Perfect Medical Instruments Co., Ltd., Guangdong, China & \#25, variable 8\% & Gold-wire \\ HyFlex CM & Coltene/Whaledent AG, Altsätten, Switzerland & \#25, 6\% & CM-wire \\ V Taper 2H & SS White Dental, Lakewood, NJ, USA & \#25, variable 6\% & CM-wire \\
2 Shape & Micro-Mega, Besancon, France & \#25, 6\% (TS2) & T-wire \\ K3 XF & Kerr Dental, Orange, CA, USA & \#25, 6\% & R-phase \\ \hline \hline \end{tabular}
\end{table} 
Table 1: NiTi files evaluated in this study. Figure 1: Experimental setup for mechanical testing. (**A**) A universal testing machine (Universal Mechanics Analyzer). Test setup for (**B**) bending, (**C**) buckling, and (**D**) cyclic fatigue resistance tests. (**E**) Schematic of the artificial canal used in the cyclic fatigue resistance test. (**F**) Test setup for the torsional resistance test. The buckling resistance test was performed using the UTM and a ceramic block (IPS e.max CAD; Ivoclar Vivadent, Schaan, Liechtenstein) bearing small dimples. By holding the handle at the upper part of the UTM, the NiTi file tip was brought into contact with the dimple on the ceramic block (Figure 1C). The file was moved downward at a speed of 1.2 mm/s while maintaining its position in the dimple without slippage. The buckling resistance (N) was determined by measuring the stress at the point where the file handle moved downward by 1 mm [36]. The cyclic fatigue resistance test was conducted by rotating the NiTi file inside an artificial stainless steel curved canal, which was fixed to the lower part of the UTM (Figure 1D). The artificial canal was 17 mm long and had an inner canal diameter of 1.5 mm, a curvature of 45\({}^{\circ}\), and an inner radius of 2 mm (Figure 1E). An endodontic engine (Aseptico, Inc., Woodinville, WA, USA) connected to the upper part of the UTM was used to operate the NiTi file (Figure 1D). Because force sharply decreases immediately upon fracture, a sudden decline in force was regarded as an indication of file fracture. The NiTi files were continuously rotated at 300 rpm, as recommended by the manufacturer. Each NiTi file was rotated until the file broke, and the time to fracture was recorded. The number of cycles to failure (NCF) was calculated by multiplying the time to fracture (minute) by the applied rpm. The torsional fracture resistance test was performed according to ISO3630-1:2019 [35]. The shaft of the NiTi file was fixed to the upper part of the UTM. The file tip was then tightly clamped into a chuck connected to the lower part of the UTM (Figure 1F). The file was rotated clockwise at 2 rpm (12\({}^{\circ}\)/s) until fracture. The rotation angle and torque were automatically recorded using a computer connected to the UTM.