S 2400

Root canals of access cavities in 20 extracted premolars were prepared by the crown-down technique with rotary nickel-titanium files (K3^®; SybronEndo Corporation, Orange, USA) and irrigated with 5 wt% sodium hypochlorite by a 25-gauge blunt-end needle of syringe. The anatomic diameter was determined by introducing successively larger K-files to the working length until resistance was felt upon removal of the file. To investigate the smear layer removal effect of the final irrigation, 20 canals were randomly divided into five groups (N=4) and irrigated with 15 wt% EDTA, 10 wt% citric acids, 0.04 wt% of FSC and ASP solutions, and control (normal saline) for 1 min. All irrigated canals received a final rinse with distilled water for 5 min to halt any chemical activity. The crowns were removed at the cementum–enamel junction. The roots were then split longitudinally with a chisel and a hammer. Finally, specimens were gold-coated and examined by scanning electron microscopy (SEM) (Hitachi S2400, Tokyo, Japan) under an accelerating voltage of 15–20 kV.

All samples were sputter-coated with an Ion Sputter (E−1010, Hitachi, Japan), and the crystal morphology was observed by scanning electron microscopy (SEM) (S2400, Hitachi, Japan) at 15 kV at different magnification. For the 3D printed scaffold, the spacing (pore size) between the scaffold printing wires was obtained by measuring the average value of 20 pore diameters in the SEM image by ImageJ software. EDX energy spectrum obtained by SEM EDX.

Instruments from each system (n = 6) were randomly selected and examined at ×3.4 and ×13.6 magnifications under stereomicroscopy (Opmi Pico, Carl Zeiss Surgical, Jena, Germany) to evaluate (a) the number of active blades (in units), (b) the helical angle by calculating the average angle of the 6 most coronal spirals assessed in triplicate, and (c) the distance (in mm) from the 2 measuring lines (20 and 22 mm) to the instruments’ tip using a digital caliper with a 0.01 mm resolution (Mitutoyo, Aurora, IL, USA). Measurements were made in triplicate and averaged with values higher than 0.1 mm from the reference line position considered significant and (d) presence of major defects or deformations (missed, twisted, or distorted blades). These same instruments were then evaluated under scanning electron microscopy (SEM) (S-2400, Hitachi, Tokyo, Japan) at ×100 and ×500 magnification regarding the symmetry of the spirals (symmetrical or asymmetrical), the geometry of the tip (active or non-active), the cross-sectional shape, and the presence of surface marks, deformations, or defects produced by the machining process.

The metallurgical characteristics of the instruments and their semi-quantitative elemental constitution were evaluated by using differential scanning calorimetry (DSC) (DSC 204 F1 Phoenix; Netzsch-Gerätebau GmbH, Selb, Germany) and energy-dispersive X-ray spectroscopy (Bruker Quantax, Bruker Corporation, Billerica, MA, USA) with scanning-electron microscopy (S-2400, Hitachi) (EDS/SEM), respectively. Fragments acquired from the coronal active portion of 2 instruments (3 to 5 mm in length) from each system, weighing 7 to 10 mg, were evaluated in the DSC test according to the American Society for Testing and Materials guidelines [9 ]. For 2 min, each sample was exposed to a chemical bath composed of a mixture of 45% nitric acid, 30% distilled water, and 25% hydrofluoric and then mounted in an aluminum pan, with an empty pan serving as control. In each group, DSC test was performed twice to confirm the results. Thermal cycles were performed from 150 °C to −150 °C (cooling/heating rate: 10 K/min), under a gaseous nitrogen (N₂) atmosphere, and transformation temperature charts created with dedicated software (Netzsch Proteus Thermal Analysis; Netzsch-Gerätebau GmbH, Selb, Germany). EDS/SEM analysis was performed on the surface (400 µm²) of 3 instruments of each type at a 25 mm distance (20 kV and 3.1 A) using software with ZAF correction (Systat Software Inc., San Jose, CA, USA).

Film thickness was measured with a Bruker's Dektak® 3.21 Profilometer: a cut in the film was made with a scalpel until reaching the glass substrate and the height of the cut (film thickness) was measured upon surface scanning perpendicularly to the cut. The morphology of the cross-linked PEDOT:PSS films was evaluated using scanning electron microscopy (SEM Hitachi S-2400, Hitachi) at 15 kV, after coating with a thin layer of gold/palladium. Elemental analysis was carried using an EDS Bruker SDD light elements detector.