## 2 Materials and Methods  
In this study, one hundred and thirty-five freshly extracted caries-free human third molars were included. Before the experimental procedure, the teeth were stored in saline solution (0.9% NaCl, B. Braun Meisungen AG, Meisungen, Germany) at room temperature. The preparation of the specimens of the teeth occurred in a manner [PERSON] et al. described in 2008 [38]. This technique allows dentin perfusion based on the simulation of intra-pulpal pressure. Under constant water cooling, the roots were ground down using a grinding machine (Rotopol-35, Struers GmbH, Willich, Germany) until the coronal pulp chamber was disclosed. After the removal of the pulpal tissue, the specimens were reduced from the occlusal part until the distance between the occlusal plateau and the pulp chamber was adjusted to 2.0 mm (\(\pm\)0.2 mm). Afterwards, the apical area needed to be parallelized to the occlusal plateau and reduced to a total sample thickness of 4.0 mm (\(\pm\)0.2 mm). These prepared samples (Figure 1) were stored in saline solution (0.9% NaCl) once again and randomly divided into three main groups. The first group received no further treatment; the specimens of the other two groups were irradiated in cooperation with a radiation physicist from the Department of Radiotherapy using a linear accelerator (ONCOR Impression IMRT Plus, Siemens AG, Munich, Germany) (Figure 2). Half of the irradiated samples underwent one single dose of 5 Gy (Gray), and for the other half irradiation was applied in a fractionated schedule with single doses of 2 Gy per day. For those samples it resulted in a total dose of 60 Gy, since they were fractionally irradiated 5 days per week over a period of 6 weeks, according to conventional radiotherapy of head and neck cancer [13; 39; 40]. Figure 3 shows a schematic illustration of the set-up of the test specimens during irradiation: For the purpose of homogeneity, the specimens were stored in daily renewed saline [40], arranged on one plane level in the zone of the dose maximum during the radiation treatment and placed on a PMMA (polymethyl-methacrylate) scattering body to minimize the scattered radiation. The forty-five samples in each group were randomly divided into three subgroups for the reason that three different universal bonding agents were tested in self-etch mode. Finally, nine subgroups emerged, each consisting of fifteen samples (Table 1). For the purpose of the experimental procedure, the samples were fastened to a special experimental apparatus in a universal testing machine (Zwick _Z_wick _Z_wick _G_mph _& Co., Ltd., KG, Ulm, Germany). Pressure of 30 cm H\({}_{2}\)O was generated via saline solution, thereby simulating intrapulpal pressure and thus dentin perfusion throughout the entire duration of the experiment. The universal bonding agents were applied according to the manufacturers' recommendations in self-etch application mode, which did not differ for the three materials tested. One layer adhesive was applied with a brush-tip and scrubbed into the dentin surface for 20 s. Subsequently, oil-free compressed air was used in order to disperse the adhesive plus remove any solvent, and the adhesive was light-cured for 10 s. The following three universal adhesives were used:  
F: Futurabond\({}^{\text{\textregistered}}\) U (Voco GmbH, Cuxhaven, Germany);  
A: AdheSE\({}^{\text{\textregistered}}\) Universal (Ivoclar Vivadent AG, Schaan, Liechtenstein);  
X: Xeno\({}^{\text{\textregistered}}\) Select (Dentsply Sirona Inc., Charlotte, NC, USA). Table 2 lists the main components of the adhesive materials used according to the safety data sheets provided by the corresponding manufacturers. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **0 Gy** & **5 Gy** & **60 Gy** \\ \hline
**Futurabond\({}^{\text{\textregistered}}\) U** & 0 Fse 1  & 5 Fse & 60 Fse \\
**AdheSE\({}^{\text{\textregistered}}\) Universal** & 0 Ase & 5 Ase & 60 Ase \\
**Xeno\({}^{\text{\textregistered}}\) Select** & 0 Xse & 5 Xse & 60 Xse \\ \hline \hline \end{tabular}
\end{table}