Grafting of alpha-tocopheryl phosphate on chemically treated Ti-6Al-4V for antibacterial bone implants

Alpha-tocopheryl phosphate (α-TP) is the water-soluble form of the antioxidant and lipophilic molecule alpha-tocopherol. Even if α-TP's antioxidant and antibacterial properties are known, it has never been exploited to prevent implantable materials against bacterial surface contamination, till now. Accordingly, the purpose of this work is to couple a chemically treated Ti6Al4V surface with α-TP conferring a local antibacterial activity preserving orthopaedic and dental prosthetic devices. The grafting of the α-TP molecule is explored both as a thin coating (<0.2 µm thick) and through molecular functionalization. As a result, a homogeneous and continuous coating is obtained by auto-polymerization of the molecule on the treated titanium surface, as shown by reflectance spectroscopy, ζ potential titration curve, and Kelvin Prove assay. The coating exposes the phosphate groups (demonstrated by XPS and FTIR analysis), which give to the surface a hydrophilic (contact angle of 40° with water) and very effective antibacterial behaviour, thus reducing the surface infection from the pathogen Staphylococcus epidermidis of >3 logs (≈ 92%) in comparison to the uncoated controls. Contrarily, the phosphorylated groups were used to create the binding to the surface on the functionalized samples, as shown by the XPS analysis. The functionalized surface results in poor antibacterial properties (≈1 log reduction of viable colonies) and hydrophobic behaviour. Finally, the cytocompatibility evaluation towards human mesenchymal stem cells showed the coating treatment to be more cell's friendly in comparison with the functionalization.