## 2 Materials and Methods  
Ethics Committee of the Faculty of Medicine of the University of Coimbra approved this study under the number CE-086/2019. To objectively evaluate microleakage, nuclear medicine was utilized through the use of radioactive isotopes. Technetium-99m (99 mTc) is an artificial element, obtained by the radioactive decay of molybdenum, which, in turn, is a radioactive metallic element belonging to the transition metals. Technetium occupies position 43 on the periodic table, being the radioactive element with the lowest atomic number. It has a half-life of 6.04 h, and its decay occurs by the isometric transition and emission and 140.5 keV of gamma radiation [24]. Thirty-six intact human molars or premolars (extracted without caries) were manually cureted and stored in 0.9% normal saline at 5 \({}^{\circ}\)C, no more than 4 months after extraction [25]. Subsequently, class V cavities were prepared on the buccal surface of each tooth. A clear resin mold was made to design the cavities on each tooth surface. Each cavity had the following dimensions: 4 mm mesiodistal, 3 mm occluso-gingival and 3 mm deep. An internal line angle of 90 degrees was maintained to create occlusal and gingival margin walls of approximately 3 mm [26]. All margins finished in enamel. Carbide FG C21-314-010-4.4 burs (Coltene, 9450, Altsattinen, Switzerland, LOT C19102) were used in the cavity preparation and were replaced every five preparations. The prepared specimens were randomly divided into each group. Twenty-six specimens were used for the study groups and ten specimens for the control groups, five for positive control group and five for negative control group. One investigator was calibrated by doing three preliminary preparations and restorations, under the scrutiny of three evaluators. After this calibration, the same investigator performed all samples tested. The experimental design defined was then the following:  
Test Groups--groups 1 and 2  
Group 1: Surefill One(tm) (Table 1) (Dentsply-Sirona, 78467, Konstanz, Germany, LOT 2106000297/2106000948) was used to restore class V of 13 specimens. The procedure was performed according to the instructions recommended by the manufacturer. The capsule was activated and immediately placed in a capsule mixer (4200-4600 osc/min) for 10 s. Then, using the Capsule Extruder 2 (Dentsply-Sirona, Konstanz, Germany) the material was dispensed into the deepest part of the cavity in a unique way, without ever removing the material application tip. It was applied in excess and spread towards the margins during the working time (1:30 min). The surface layer was light cured using Bluephase(r) Style (Ivoclar Vivadent, 5 VDC, Liechtenstein, Austria) for 20 s (1100 mW/cm\({}^{2}\)). The light device was calibrated with a radiometer (Bluephase(r) Meter II, Ivoclar Vivadent, Liechtenstein, Austria). Restorations were polished with the Enhance(r) and Enhance(r)PoGo(r) disc rubber system (Dentsply-Sirona, Konstanz, Germany, LOT 00026514/ 00017898). The first system prepared composites surface for their final polish and the second created the final polish with a diamond-impregnated system. Group 2: SpectraTM ST HV nanohybrid composite (Table 1) (Dentsply-Sirona, Konstanz, Germany, LOT 1136) was used to restore class V cavities from 13 specimens. The enamel was selectively etched for 30 s with 37% orthophosphoric acid gel (3M ESPE, N301289, St. Paul, MN, USA, LOT 643417). Then it was washed with an air/water jet for 20 s, followed by light drying with an air jet. This was followed by the application of a universal adhesive system, Prime & Bond ActiveTM (Dentsply-Sirona, Konstanz Germany, LOT1908001303) applied according to the manufacturer's instructions. The solvent was evaporated with a gentle stream of air before being light cured for 20 s (1100 mW/cm\({}^{2}\)) with the BluephaseR Style (Ivoclar Vivadent, 5 VDC, Liechtenstein, Austria). The light device was calibrated with a radiometer (BluephaseR Meter II, Ivoclar Vivadent, Liechtenstein, Austria).