## 2 Materials and Methods
### Experimental Cyclic Fatigue Model  
Dynamic cyclic fatigue tests were conducted using the aforementioned customized device (Utility Model Patent No. ES1219520) [23] at room temperature (20 \({}^{\circ}\)C) to analyze the mechanical behavior of the instruments as per [PERSON] et al. [24]. The structure of the fatigue-testing device was designed via computer-aided design and engineering (CAD/CAE) and 2D/3D software (Midas FX+\({}^{\circledR}\), Brunleys, Milton Keynes, UK), and it was printed with a 3D printer (ProJet\({}^{\circledR}\) 6000 3D Systems\({}^{\circledR}\), Rock Hill, SC, USA) (Figure 3).  
Figure 2: (**A**) EDX micro-analysis of the 25.06 CM Gold wire NiTi alloy endodontic rotary files, 31 mm in length; (**B**) 25.06 CM Gold wire NiTi alloy endodontic rotary files, 25 mm in length; (**C**) 25.06 CM Gold wire NiTi alloy endodontic rotary files, 21 mm in length; and (**D**) 25.06 CM Blue wire NiTi alloy endodontic rotary files, 17 mm in length.  
The custom artificial root canals were performed using CAD/CAE 2D/3D software v1 for inverse engineering technology as per [PERSON]'s measuring technique [21], with a 60\({}^{\circ}\) curvature and 5 mm radius of curvature. In addition, four lengths were configured to match the different lengths of the NiTi alloy endodontic rotary file from each study group: 31 mm, 25 mm, 21 mm, and 17 mm. Electrical discharge machining (EDM) molybdenum wire-cut technology (Cocchiola S.A., Buenos Aires, Argentina) was used to create the artificial root canal from stainless steel. This process ensures that the root canal walls and the NiTi endodontic reciprocating files are in close contact. The artificial root canal was positioned on its support, and a light-dependent resistor (LDR) sensor (Ref.: C000025, Arduino LLC\({}^{\circledR}\), Ivrea, Italy), located at the apex of the artificial canal, was used to when the endodontic rotary instrument failed. The LDR sensor quantified the continuous light source emitted by a high-brightness white LED (20,000 mcd) (Ref.: 12.675/5/b/c/20k, Batuled, Coslada, Spain), which was placed opposite the artificial root canal. An LDR (Ref.: C000025, Arduino LLC\({}^{\circledR}\)) sensor with a frequency of 50 ms was used to detect the light signals that were emitted by this LED sensor so as to determine the exact time of failure.  
The speed and direction of the movement generated by the brushed DC gear motor (Ref.: 1589, Pololu\({}^{\circledR}\) Corporation, Las Vegas, NV, USA) and controlled by the driver (Ref.: DRV8835, Pololu\({}^{\circledR}\) Corporation, Las Vegas, NV, USA) were transferred via a roller bearing system (Ref.: MR104 ZZ, FAG, Schaeffler Herzogenaurach, Germany). A lineal guide (Ref.: HGH35C 10249-1 001 MA, HIWIN Technologies Corp. Taichung, Taiwan) was used to aid in moving the artificial root canal support in an entirely axial motion. All the NiTi endodontic rotary files were used in conjunction with a torque-controlled motor and 6:1 reduction handpiece (X-Smart Plus, Dentsply Maillefer, Baillagues, Switzerland). All of the files were utilized at 300 rpm and 2.3 N/cm torque, as per the manufacturer's instructions.  
All of the NiTi endodontic files were subjected to a total of 60 packing movements per minute within the dynamic cyclic fatigue device, in concordance with the parameters of a previous study [23]. In order to reduce friction between the rotating files and the artificial canal walls, researchers applied a specialized high-flow synthetic oil (Singer All-Purpose Oil; Singer Corp., Barcelona, Spain), designed to lubricate mechanical parts.  
All NiTi endodontic rotary files were used until the point of fracture. The time to failure was observed and recorded.