October_2021_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 2 1 4 3 5 comprise pseudoelastic NiTi. Root canal shaping requires files with high strength, flexibility, and resistance to torsion- al fracture [28] . During the shaping procedure, cyclic stress- es occur along the file length from rotation with bending inside the canal. The fatigue life depends on the degree of curvature of the canal [29] . Heat treatments are widely used to 6 Fig. 2 — Location of stiffness conditions in the laser-processed archwire. Fig. 3 — Experimental tensile test results for straight wire samples: as-received base-metal (condition 1) and laser-processed samples (conditions 2 - 10). enhance the fatigue life of endodontic files [5] ; however, this comes at the expense of torsional strength and cutting pow- er [28,30] . Cyclic fatigue accounts for approximately 50% of file failures compared to 30% failure due to excessive torsional stress by taper lock of the tip [31] . A functionally graded end- odontic file can tailor the local properties to optimize stress/ strain distribution and achieve balanced performance in fa- tigue life, torsional strength, and cutting efficiency. Attempts at functional grading of NiTi files include a dif- ferential heat treatment intended to create a more flexible zone near the distal tip. As in the example above, differential heat treatments are low resolution, meaning that it is diffi- cult to isolate the heat to certain regions with the outcome of more homogeneous structure. The high spatial resolution and high power density of the laser enables tight control over the stiffness of a wire across very short distances. The laser process can be used to locally shift the transformation temperature, both along the length of a file, and across the section. This enables a more flexi- ble stiffness profile to be programmed into a file while maintaining the harder edges necessary for cutting efficiency and torsional strength. NiTi endodontic files were fabri- cated with psuedoelastic base metal wire. The laser process was applied to the fluted core of the file (see Fig. 4) to program differential material prop- erties across the section. The cutting edge is unaffected austenite, while the core material transformation tempera- ture is shifted above body tempera- ture to improve flexibility and fatigue life. Figure 5 describes the change in Fig. 4 — SEM image of laser processed NiTi endodontic file showing location of laser processing along the flute of the file. 7 FEATURE