Thermodynamic characterization of selected biomaterial surfaces: Comparison of surface free energy approaches.

The thermodynamic surface properties of biomaterials play a key role in governing interfacial interactions and are commonly characterized by surface free energy (SFE) and its components derived from wetting data. However, different theoretical approaches used for SFE determination may yield substantially different results, particularly for polar and high-energy surfaces. In this study, the total surface free energy and its components were systematically analyzed for a range of clinically relevant biomaterial surfaces, including titanium, gold, cobalt-chromium alloy, nano-hydroxyapatite, and amorphous Teflon, prepared as smooth thin films on glass substrates. Static contact angles with water, glycerol, ethylene glycol, and diiodomethane were measured to assess wettability and to calculate SFE using three commonly applied models: the Owens-Wendt-Rabel-Kaelble (geometric mean) approach, the Lifshitz-van der Waals/acid-base (LW-AB) approach, and the equation of state (EOS) approach. These models estimate total SFE as well as nonpolar, polar, and acid-base components, enabling a detailed comparison of model-dependent surface energetic data. The results show good agreement between the different approaches for low-energy, hydrophobic surfaces, whereas pronounced discrepancies occur for hydrophilic, high-energy materials, particularly in the calculated polar and acid-base contributions. While total SFE values were partly consistent across methods, the relative magnitudes of individual SFE components strongly depended on the applied theoretical model. Overall, this study highlights that SFE should not be considered an intrinsic material property but a model-dependent descriptor. Careful selection of the SFE calculation approach and cautious interpretation of SFE components are essential when using thermodynamic surface analysis to compare biomaterial surfaces or to relate surface energetics to interfacial phenomena such as biofilm formation.