## 1 Introduction  
Nickel-titanium (NiTi) alloy is widely employed in endodontics due to its properties of superelasticity and shape memory [1]. Nevertheless, NiTi instruments are subjected to an unexpected separation, due to the cyclic fatigue and torsional fracture, during the clinical use [2; 3]. Because of this, producers have tried to propose new designs, manufacturing processes as heat treatments and protocols to reduce the fracture risk [4]. Heat treatments change the transitional temperature of martensitic transformation, favoring a high percentage of the martensitic phase to increase the flexibility of heat-treated NiTi [5; 6]. Moreover, the surrounding temperature influences the crystalline phase of NiTi alloy [6; 7; 8; 9]. Single-file systems have been presented to facilitate the instrumentation protocols and minimize mechanical stresses [10]. In addition, the shaping phase with a single-file system may be more rapid in comparison with traditional multi-file sequences [11; 12]. F6 SkyTaper (F6 ST; Komet Dental, Gebr. Brasseler, Lemgo, Germany) is an example of a single-file system made by traditional NiTi alloy available in four different sizes, including 25 size. All instruments have a continuous taper of 0.06 and an S-shaped cross-section [13]. Another single-file system is the OneCurve endodontic file (OC; Micro Mega, Besancon, France), made from a NiTi alloy that receives a patent-protected heat treatment (C-Wire) which provides to the file a shape-memory effect. OC files also present a tip size of 25 and a constant taper (6%) with a variable cross-section [12].  
It is suggested that root canal preparation be performed with sodium hypochlorite (NaOCI) due to its antimicrobial activity and lubricity (in liquid or in the most recent gel formulation), which facilitates the mechanical action of endodontic files [14; 15]. Nevertheless, the contact of NiTi files with NaOCI during instrumentation raises doubt about the consequence of such short-term contact on the integrity of the instruments [14]. A previous study evaluated the effects of liquid and paste-type lubricants on torque values during simulated rotary root canal instrumentation, but in that case ethylenediamine tetraacetic acid (EDTA) effects were evaluated [16]. In particular, it was reported that irrigation solutions were generally more efficacious than the paste-type chelator in reducing torque. Nevertheless, the study did not completely mirror a clinical situation; moreover, the results obtained were limited to the type of lubricant used and instrument tested [16]. In addition, the potentially corrosive effect of NaOCI has been investigated performing the cyclic fatigue test after the files' exposure to NaOCI or when the instruments were immersed in the irrigant solution [7; 14; 17]. Nevertheless, neither case allowed an irrigant flow simultaneous to the fatigue test.  
To date, no studies have investigated the effects of different NaOCI formulations on the cyclic fatigue resistance of single-file instruments at body temperature. Thus, the aim of this study was to evaluate the influence of liquid and gel sodium hypochlorite on the cyclic fatigue of F6 SkyTaper and OneCurve single-file systems at body temperature. For this purpose, a new customized testing machine, which allowed us to inject the irrigant solutions (liquid or gel) simultaneously to the fatigue test, was used. The null hypothesis was that there would be no significant differences among the cyclic fatigue resistance of the NiTi files which were exposed to different NaOCI formulations at different temperatures.