Antibacterial properties of novel 1D nanostructured ZnO nanowire coatings on medical grade 316L stainless steel surfaces

Abstract

Post-operative osteomyelitis attributing to the biofilm formation on implant surface and medical grade 316L stainless steel have been reported to gain a higher rate of infection among other clinically applied metals. It is believed suppressing bacterial adhesion on implant surface at early stages can help prevent biofilm formation. The major challenges of current antibacterial surface treatments include limited biocompatibility, potential development of antibiotic resistant bacteria, short life cycle and high fabrication cost.

In this study, it is aimed to explore the feasibility of an inexpensive and simple surface modification technique to achieve a long-term antibacterial effect on medical grade 316L stainless steel while maintaining its biocompatibility. Thus, a novel 1D nanostructured ZnO nanowire coating that can provide different special topographies and can be easily fabricated by simple hydrothermal method is suggested to coat on stainless steel surfaces. Two kinds of ZnO nanowire coatings, ZnO_5hrs and ZnO_17hrs, are fabricated for further investigation. Relatively well-aligned ZnO nanowires with diameters of ~50 nm were found on ZnO_5hrs samples, while randomly-oriented ZnO nanowires with diameters of ~150 nm were found on ZnO_17hrs samples.

In the antibacterial tests, both ZnO_5hrs and ZnO_17hrs samples exhibited excellent antibacterial effects, which represent over 90% of bacterial reduction among all of the tested bacterial strains including S. aureus, P. aeruginosa and E. coli, with exception to the case of ZnO_17hrs sample with S. aureus. It is confirmed that antibacterial Zn2+ ions are released from the coatings during the test and help against bacterial adhesion. On the other hand, it is suspected that the increase in hydrophilicity and special physical topography are also antibacterial factors of the ZnO nanowire coatings.

The cytocompatibilities in both ZnO_5hrs and ZnO_17hrs samples were not satisfactory. In the cell adhesion test, the GFP-OB cells did not habitually spread and attach on the treated sample surfaces after 6 hours incubation. Cytotoxicity test results further confirm no viable MC3T3 cells were found on the treated sample surfaces. The cytocompatibility of the coating remains to be improved.

In the in-vivo study, the group of rats with ZnO_5hrs rod samples displayed a reduced number of bacterial cells in the implantation site at day 0, as well as a shorter duration (within 8 days) for bacterial termination as compared to that with untreated stainless steel rod samples. The presence of ZnO nanowire coating on medical grade 316L stainless steel rod samples demonstrates the in vivo antibacterial effect.

In short, the novel 1D antibacterial ZnO nanowire coating is successfully fabricated and coated on medical grade 316L stainless steel surfaces by a simple and inexpensive hydrothermal method. However, the biocompatibility of the ZnO nanowire coating remains to be improved. One of the critical issues is to engineer the coating in order to precisely control the Zn2+ ions release rate. For future study, the key is to find out how to manipulate the characteristics of special surface topography, together with a controllable release of Zn2+ ions on the ZnO nanowire coating to maximize the antibacterial effect while maintaining the original biocompatibility of medical grade 316L stainless steel.