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 2 (PDL) [15] . In contemporary orthodontics, the available force from an initial archwire targets resolving the malocclusion in smaller teeth first; thus, a progression in archwire size is required to achieve force levels tomove larger teeth. In theo- ry, a functionally graded archwire with stiffness that varies in the mesio-distal direction could produce the ideal available force for each tooth. Previous attempts at fabrication of functionally graded archwires include welding of segmented wires and selec- tive heat treatment. Fabrication of segmented wires using advanced orthodontic alloys is not trivial. Laser welding [16] , impact butt welding [17] , and resistance welding [18] techniques have been attempted; however, the mechanical properties in the weldments were degraded with a structural weakness that caused the wire to break. The direct electric resistance heat treatment (DERHT) process was used to develop con- tinuous functionally graded archwires [19,20] . These archwires were heat treated selectively along the arch to achieve a low stiffness in the anterior section, progressing to a higher stiff- ness in the posterior section with a transition zone between them [21,22] . The process can be expensive and time consum- ing, and difficult to accurately control, causing high varia- tion in the output [23] . Furthermore, due to conduction of heat through the material, there is low property gradient resolu- tion and a limit of only three segments in an archwire is pos- sible. The benefits of the selective laser vaporizationmethod compared to the selective heat-treatment approach out- lined above are that a higher resolution of stress gradients and greater range of force levels are possible. While each approach maintains the ideal load-deflection rate and large activation ranges of pseudoelastic NiTi, the former can vary the forces on a tooth-by-tooth basis, while the latter is limit- ed to a gradually decreasing stiffness from the posterior to anterior segments. A functionally graded archwire (0.016 in. diameter) was fabricated using the selective vaporization laser process to treat a segment between each tooth. Wire segments were produced and tested for force availability in a simulated orthodontic bracket setup. It was found that 10 different wire stiffness conditions deliver target forces at each tooth bracket. The available forces at each bracket for each wire stiffness condition are shown in Table 1, along with the austenitic transformation temperatures for the respec- tive segment. The locations of the segments are shown in Fig. 2. Stress-strain results for each segment, presented in Fig. 3, show how the laser process depresses the pseu- doelastic stress plateau differently for each condition. Re- sults of clinical observations using a laser processed wire suggest that treatment kinetics are accelerated during the initial treatment phase while avoiding potential side effects that can delay tooth movement progress [24,25] . Orthodontic treatment time can be reduced by 50% [26] , or 3 to 4 months [27] during the leveling and aligning phase. Endodontic files: Like orthodontic archwires, the files used for cleaning the root canal during endodontic proce- dures have evolved from stainless steel to predominantly 5 TABLE 1 — SUMMARY OF LASER PARAMETER CONDITIONS, FORCE AVAILABLE PER TOOTH, AND THERMAL ANALYSIS RESULTS PER SEGMENT Arch Segment Condition Available force, N Transformation T, °C A s A p A f Upper Central incisor – Central incisor 1 U1 1.26 6 17 28 Central incisor – Lateral incisor 2 U2 0.94 0 17 39 Lateral incisor - Cuspid 3 U3 1.20 0 15 33 Cuspid – 1st Premolar 4 U4 1.23 0 15 30 1st Premolar – 2nd Premolar 5 U5 1.18 0 20 38 2nd Premolar – 1st Molar 6 U6 1.68 0 14 30 1st Molar – 2nd Molar 1 6 17 28 Lower Central incisor – Central incisor 7 L1 0.81 1 22 37 Central incisor – Lateral incisor 7 L2 0.80 1 22 37 Lateral incisor - Cuspid 8 L3 1.03 1 22 36 Canine – 1st Premolar 9 L4 1.08 0 15 32 1st Premolar – 2nd Premolar 7 L5 1.13 1 22 37 2nd Premolar – 1st Molar 10 L6 1.79 0 14 30 1st Molar – 2nd Molar 1 6 17 28 *Note: A s , A p , and A f are the austenitic transformation start, peak, and finish temperatures, respectively. U1-6 corresponds to the tooth sequence from maxillary central incisor to first molar; the same is applicable for L1-6 in the mandibular arch. 6 6 FEATURE