Surface Modification of Ti6Al4V Bone Implants for Enhanced Osteointegration and Antibacterial Property

Abstract

Titanium and its alloys have gained widespread attention for application in metallic biomaterials, due to their low toxicity, good biocompatiblity, excellent corrosion resistance and mechanical properties. However, titanium based biomaterials face several impediments, such as implant loosening and microbial infections that leads to implant failure. Surface modification is an important technique and plays a vital role in modifying a biomaterial surface. It enhances the interaction of the implant surface with the surrounding tissues, impedes bacterial adhesion and eventually reduces the occurrence of implant failure. Keeping this in mind, the present work aims to improve osseointegration and confer antibacterial properties on Ti6Al4V through surface modification and coatings. In the first phase, TiO2 nanotubes of varied topography were grown on the Ti6Al4V surface through anodic oxidation. Based on the cellular response, it was observed that among the selected processing parameters, nanotubes formed at 40 V, 4 h (TN(40)4), showed the desired nanotubular morphology with ~2 times increment in the rate of proliferation with respect to the polished surface (PTi64), which is attributed to its higher surface roughness, anatase content and wettability. In the second phase, the osteogenic potential of TN(40)4 surface was improved by coating with silk fibroin (SF) protein, isolated from B.mori silk cocoons (TNTSF) by electrophoretic deposition technique (EPD). The osteogenic potential studied by ALP assay showed ~1.5 time increment in cells grown on TNTSF compared to polished Ti6Al4V (PTi64). The osteogenic potential of the modified surface was further confirmed by the expression of specific osteogenic markers (runx2, collagen I, osterix and osteocalcin) in human mesenchymal stem cells (hMSCs). TNTSF showed increased expression of osteocalcin and osterix (late osteogenic markers). Next, TN(40)4 was coated with curcumin (TNTC) by direct dropping method. The antimicrobial activity of the implant with the modified surface was enhanced by reduction of 43% and 39% E.coli and S.aureus population respectively, within 24 h. Finally, EPD was used to coat silk fibroin-curcumin blend on TiO2 nanotubes. The antimicrobial activity against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus showed that the coated surface effectively inhibited bacterial growth, as evident from the reduction in ~31% and ~30% S.aureus and E.coli growth. The osteogenic potential of the silk fibroin-curcumin coated implant was confirmed by the increase in expression of ALP, expression of early and late osteogenic marker proteins runx2 and osterix, compared to the polished surface (PTi64). The level of released osteocalcin was quantitatively estimated by ELISA and it revealed that after 14 days of culture, the osteocalcin level increased by ~3.5 times on surface PTi64 and ~4.5 times on TNTSF in comparison to PTi64 on day 7. Therefore, the developed TiO2 nanotubes coated with a blend of silk fibroin and curcumin as a dual bio-functional coating material, is able to improve the performance of Ti6Al4V implants, in terms of both antimicrobial resistance and osseointegration.