## 1 Introduction  
Resin based composites (RBCs) have been widely studied and developed in the last decades for improving functional and esthetic performances [1; 2; 3; 4; 5; 6; 7]. The enhanced translucency in conjunction with a powerful initiator system provides a significant increase in the depth of cure of bulk-fill RBCs [8; 9]. Thanks to the bulk-fill technique, cavities with a depth even higher than 4 mm can be restored, and the chair time required to fill a cavity is reduced if compared with the traditional incremental filling techniques [6; 10]. Bulk-fill RBCs can be distinguished according to rheological properties [11]. Low-viscosity RBCs, also known as flowable composites, allows a self-adaption to the cavity walls. This class of RBCs is directly injected into the cavity through a needle. Historically, SDR (Smart Dentin Replacement) bulk-fill (Sirona Dentsply, Konstantz, Germany) represents the first flowable bulk-fill material on the market. However, hardness of low-viscosity bulk-fill composites is lower than that of conventional nanohybrid and microhybrid RBCs. Therefore, to avoid wear, capping with a conventional RBC is recommended [7; 12; 13; 14]. On the other hand, high viscosity bulk-fill RBCs show mechanical properties similar to conventional nanohybrid and microhybrid RBCs. This class of bulk-fill RBCs is applied into the cavity with a traditional spatula as for Tetric EvoCeram (EVC) Bulk-fill (Ivoclar Vivadent, Schaan, Liechtenstein); the exception is SonicFill 2 (Kerr Corporation, Orange, CA, USA) that is applied through a sonic vibration hand piece that reduces viscosity during material placement [8]. Similar to other restorative composites for posterior teeth, SonicFill 2 shows an amount of inorganic filler higher than other bulk-fill resin composites [15]. SDR and other flowable bulk-fill composites, such as Tetric EvoFlow (EVF) Bulk-fill (Ivoclar Vivadent, Schaan, Liechtenstein), have an elastic modulus higher than flowable RBCs; however, mechanical properties are lower than packable nanohybrid or microhybrid RBCs [7]. The viscosity of SDR and EVF is similar to flowable restorative composites (viscosity lower than 1 kPa-s), while the viscosity of EVC and SFL is similar to that of highly filled RBCs (viscosity higher than 100 kPa-s) [11]. An important concern of light cured bulk-fill RBCs is related to temperature rise occurring through the polymerization process, as the heat developed through the radical polymerization process is proportional to the amount of resin to be cured. Temperature increase in the pulp cavity is of great importance, and thermal injury thresholds represent the main limit to the power level which can be safely delivered [2; 16]. Curing of restorative material causes a pulp temperature rise that may result in pulp inflammation [17]. This temperature increase is related to both the exothermic polymerization reaction and to the heat irradiated by the light-curing unit [18]. To prevent tissue damage, temperature rise should be constrained. It is proved that temperature rise in experiments with active pulp fluid circulation is lower than the temperature rise in absence of the pulp fluid [19; 20; 21]. Thermocouple have been used to measure temperature changes in the oral cavity from several decades [22] and they still remain a valuable tool to evaluate temperature variation in vitro [2]. In fact, the small dimension of these temperature sensors allows to measure temperature variation occurring in the bulk of both the composite and dental tissues [23; 24]. [PERSON] et al., using thermocouples, have demonstrated that pulp temperature was significantly increased during photo-polymerization [25]. However, using a similar measurement system, it has been suggested that temperature rise in the pulp chamber after composite light curing is below the critical temperature value above which irreversible pulp damage occurs [26].