Human serum albumin adsorption on TiO2 from single protein solutions and from plasma

In the present work, the adsorption of human serum albumin (HSA) on commercially pure titanium with a titanium oxide layer formed in a H2O2 solution (TiO2 cp) and on TiO2 sputtered on Si (TiO2 sp) was analyzed. Adsorption isotherms, kinetic studies, and work of adhesion determinations were carried out. HSA exchangeability was also evaluated. Surface characterization was performed by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and wettability studies. The two TiO2 surfaces have very distinct roughnesses, the TiO2 sp having a mean R. value 14 times smaller than the one of TiO2 cp. XPS analysis revealed consistent peaks representative of TiO2 on sputtered samples as well as on Ti cp substrate after 48 h of H2O2 immersion. Nitrogen was observed as soon as protein was present, while sulfur, present in disulfide bonds in HSA, was observed for concentrations of protein higher than 0.30 mg/mL. The work of adhesion was determined from contact angle measurements. As expected from the surface free energy values, the work of adhesion of HSA solution is higher for the TiO2 cp substrate, the more hydrophilic one, and lower for the TiO2 sp substrate, the more hydrophobic one. The work of adhesion between plasma and the substrates assumed even higher values for the TiO2 cp surface, indicating a greater interaction between the surface and the complex protein solutions. Adsorption studies by radiolabeling of albumin (I-125-HSA) suggest that rapid HSA adsorption takes place on both surfaces, reaching a maximum value after similar to60 min of incubation. For the higher HSA concentrations in solution, a multilayer coverage was observed on both substrates. After the adsorption step from single HSA solutions, the exchangeability of adsorbed HSA molecules by HSA in solution was evaluated. The HSA molecules adsorbed on TiO2 sp seem to be more easily exchanged by HSA itself than those adsorbed on TiO2 cp after 24 h. In contrast, after 72 h, nearly all the adsorbed albumin molecules effectively exchange with other albumin molecules.