## 1 Introduction  
Fiber-reinforced composite posts, or fiber post, are widely used to restore endodontically-treated teeth that have insufficient coronal tooth structure to retain a core for the definitive restoration in dentistry [1]. Fiber posts are made of carbon, quartz, or glass fibers embedded in a matrix of either epoxy resin or methacrylate resin [2]. Among them, epoxy resin is often preferred over methacrylate resin as a matrix due to its high degree of conversion and highly cross-linked structures [3]. However, fiber post retention depends on the strength of the chemical and micromechanical interaction between fiber post material and core resin [4] and the absence of chemical interaction between the methacrylate-based core resin composite and epoxy resin matrix of fiber posts represents the primary cause of weakness in post-to-core resin bonds [5]. Hence, numerous studies have attempted to improve the bond strength between epoxy resin-based fiber posts and core resins [6; 7; 8; 9]. A recently highlighted method of improving the bond strength between epoxy resin-based fiber posts and core resins is using plasma technology. [PERSON] et al. evaluated the tensile shear bond strength between fiber post and core resin after plasma treatment using O\({}_{2}\), Ar, N\({}_{2}\), and He/N\({}_{2}\) gases under a vacuum system [10]. [PERSON] et al. studied the push-out shear bond between epoxy resin-based fiber post and core resin after plasma treatment using Ar and ethylenediamine sources under a vacuum system and compared with conventional surface treatment [11]. From these studies, plasma treatment was shown to be an effective means of improving the bond strength between epoxy resin-based fiber posts and core resins. However, all of previous plasma treatment conditions required vacuum chamber and high temperature, which often result in damage of highly sensitive fiber posts biomaterials and not practicable as the surface treatment is only able to be carried out in factories due to the size and requirements of vacuum [12]. Such factory level of chemical surface treatment would result in so called 'aging process' of the device where advantageous chemical functionality on the surface of the device would be lost during the transport and the shelf lost, and therefore cannot be utilized by the time when the device has reached to the patient [13]. Non-thermal atmospheric pressure plasma (NTAPP) is a technology that is used in many aspects of industrial applications such as gas cleaning, ozone generation, surface modification, etching, thin film deposition, and sterilization [14; 15; 16; 17; 18]. Compared to previously mentioned methods of plasma technology, it has major advantages of the processes using relatively low temperature of gas while being operated at atmospheric pressure. Hence, it would be much more simple, cheap, and environmentally safe to be used, while having the advantage of not requiring complex vacuum equipment, allowing the surface chemical treatment to take place just before the clinical application even at the chairside in dental clinics [19]. Also, in recent years, applications of NTAPP on highly sensitive dental materials have shown to be effective and non-damaging [12; 20]. Hence the device would avoid the worry of the aging process of the surface characteristics as it would not damage the surface while surface treatment would occur just before the clinical application [12]. The objective of this study was therefore to evaluate the adhesion between core resin and epoxy resin-based fiber post after the application of NTAPP treatment. Also, the change in the adhesion between the core resin and epoxy resin-based fiber post was linked to the change in wettability and surface chemistry that may have been caused by NTAPP.