## 2 Materials and Methods
### Contact Angle (CA) Measurements and Surface Free Energy (SFE) Calculation  
The contact angle of a drop on a solid surface measurement was performed by using Drop Shape Analyzer 30 (DSA 30, [PERSON], Hamburg, Germany) at room temperature (25 \(\pm\) 0.5 \({}^{\circ}\)C). In the first part of the measurements, the water (HPLC water, VWR International Ltd., Debrecen, Hungary) and diiodomethane (DIM; VWR International Ltd., Debrecen, Hungary) apparent contact angles were determined on clean and dry RBC surfaces. The drops of water and then DIM (5 \(\mu\)L) were deposited on the polished and sandblasted RBCs with the help of an automatic dosing system (0.5 mm diameter needle). The contact angle measurements and evaluation were performed by the same experienced researcher. The baseline was set where the liquid contacted the solid. For image analysis and contact angle calculation, the time frame of 50 s for each measurement was recorded and evaluated. The Advance software (DSA3, version 1.0.3-08, Kruss, Hamburg, Germany) calculated the surface free energy (SFE) of RBCs based on the Owens, Wendt, Rabel, and Kaelble (OWRK) model, which requires polar (water) and non-polar (DIM) liquids with known surface tension. The surface tension of water and DIM were 72.8 mJ/m\({}^{2}\) and 50.8 mJ/m\({}^{2}\) at 25 \({}^{\circ}\)C, respectively. This calculation was the most frequently used method. The mathematical formula of the OWRK model was the following:  
\[\gamma_{sl}=\gamma_{sv}+\gamma_{lv}-2\bigg{(}\sqrt{\gamma_{sv}^{d}\gamma_{lv}^{ d}}+\sqrt{\gamma_{sv}^{p}\gamma_{lv}^{p}}\bigg{)}\]  
where \(\gamma_{sv}^{d}\) and \(\gamma_{lv}^{d}\) were dispersive components, and \(\gamma_{sv}^{p}\) and \(\gamma_{lv}^{p}\) were polar components of solid and liquid surface energies, respectively. After [PERSON]'s equation combination (\(\gamma_{sv}=\gamma_{sl}+\gamma_{lv}\cos\theta_{\gamma}\), where \(\gamma_{sv}\) was solid surface free energy, \(\gamma_{sl}\) was solid/liquid interfacial free energy, \(\gamma_{lv}\) was the liquid/vapor interfacial tension (liquid surface tension), and \(\theta_{\gamma}\) is the contact angle) the OWRK equation was the following:  
\[\gamma_{lv}(1+cos\theta_{\gamma})=2\bigg{[}\sqrt{\gamma_{sv}^{d}\gamma_{lv}^{ d}}+\sqrt{\gamma_{sv}^{p}\gamma_{lv}^{p}}\bigg{]}\]  
where \(\gamma_{sv}^{d}\) and \(\gamma_{sv}^{p}\) were the two unknowns that were polar and dispersed parts of SFE.  
The sample sizes ensured enough large surfaces to implement contact angle measurements with many drop depositions. For the median contact angle calculation, ten individual sessile drop measurements (\(n=10\)) were performed. The liquid drops were deposited on the surface, and the contact angle was calculated based on the drop shape by a connected camera. In the second part of the measurements, the apparent contact angle of dental adhesives was determined. The drops of adhesives (5 \(\mu\)L) were deposited (\(n=10\)) on the polished and sandblasted composite surface with a manual dosing system holding a 1 mL syringe (0.5 mm diameter needle). In all cases, the ellipse contact angle fitting model was used on polished and sandblasted composite surfaces.