## 2 Materials and Methods
### Shear Bond Strength between Epoxy Resin and Core Resin  
The prepared epoxy resin as mentioned above (30 mm in diameter and 15 mm in height) was treated by NTAPP for 1 min on the surface where it would be bonded with core resin (top surface with 10 mm in diameter). Then the mold with inner diameter of 10 mm and height of 5 mm was placed and fixed on the NTAPP-treated surface of epoxy resin by utility wax. Core resin was then poured into the mold where excess material was removed from the top and polymerized for 40 s using a light emitting diode curing unit (Elipar S10, 3M ESPE, Maplewood, MN, USA). The prepared sample has been illustrated in Figure 2a. The specimens were stored at 37 \({}^{\circ}\)C for 24 h and tested for shear bond strength by a universal testing machine (UTM) (5942, Instron, Norwood, MA, USA). The cross-head  
Figure 1: The schematic drawings of the non-thermal atmospheric pressure plasma jet system. Gas supplied by nitrogen gas while electrical power by AC high voltage was supplied to inner and outer electrode separated by quartz tube.  
speed was set to 1 mm/min to push the lateral surface of core resin. The test was also carried out for the NTAPP untreated epoxy resin in same way as the control. The bond strength was expressed in megapascals (MPa) by dividing the load value in Newtons (N) with the surface area (mm\({}^{2}\)) of the bonded interface (Figure 2b).  
The change in shear bond strength according to the times passed from the NTAPP treatment on the epoxy resin surface was also evaluated to consider aging process. The epoxy resin with same size as above were treated for 1 min of NTAPP and left for 1, 3, 5, or 7 days in the desiccator (26 \({}^{\circ}\)C and 20% relative humidity). The shear bond strength with core resin was then measured with the same method described above.  
Push-Out Shear Bond Strength between Commercially Available Epoxy Resin-Based Fiber Post and Core Resin  
Commercially available epoxy resin-based fiber post (DT Light post, Bisco Inc., Schaumburg, IL, USA) was purchased, which had diameter of 2.2 mm on top that was tapered to result in a diameter of 1.2 mm on bottom. Original height was 20 mm but it was cut to 3 mm on the point that tapering was not started and therefore diameter was constant at 2.2 mm. Samples were then divided into in five different groups as shown in Table 1.  
Each surface-treated epoxy resin-based fiber post was placed in an upright position with support from specially designed metal molds with 12 mm in diameter that had a hole of 2.2 mm in diameter (same as the diameter of commercially available fiber post used in this study) and 0.5 mm in height. On top of this metal mold, another mold with 12 mm diameter that had a hole of 10 mm in diameter and 2 mm in height was placed. In the hole of the second mold, core resin was filled in so that core resin surrounded the fiber post. The core resin was then polymerized for 40 s with using a light emitting diode curing unit (Elipar S10, 3M ESPE, Maplewood, MN, USA).  
Figure 2: The photographs showing; (**a**) shear bond strength test specimen; (**b**) actual test carried out.  
The prepared specimen was then placed into a mold for push-out shear strength, which had a height of 5 mm and diameter of 10 mm, with a hole of 2.2 mm. The push-out shear bond strength between each of the surface-treated epoxy resin-based fiber posts and core resin was then measured using a universal testing machine (5942, Instron, Norwood, MA, USA) at a cross-head speed of 1 mm/min (Figure 3).