Although Titanium (Ti) and Ti alloys with good stability and biocompatibility are widely used in dental and orthopedic implants, the materials do not have intrinsic antibacterial ability. Bacterial infection is one of the most serious complications after surgery leading to not only implant failure, but also complication, morbidity, and mortality. Many surface modification methods have been proposed to endow Ti with antibacterial properties, e. g. antibiotics and antibacterial peptides combination. However, these strategies suffer from drug resistance and safety doubt from the food and drug administration. As a clean modification method, element doping has attracted much attention in antibacterial material design. Nevertheless, the exact antibacterial mechanism, which will give guidance to advanced antibacterial material design has drawn little attention.

In this thesis, combining with element doping, different modification methods are applied to Ti, endowing it with antibacterial properties with the detailed antibacterial process analyzed from both physicochemical and biological perspectives. Excellent antibacterial properties depending on electron transfer is found on both Titania nanotubes loaded with gold nanoparticles (TNT-Au) and Ti embedded with silver nanoparticles (Ti-Ag). To make a step forward, a capacitance-dependent antibacterial platform is designed with the antibacterial process analyzed in detail. Overall results indicate that the electrical interaction between material surface and bacteria can be taken into consideration in designing clean and efficient antibacterial material for implant application.

Before the experimental study is carried out, Ti alloys and their biomedical applications are briefly introduced, with the evolution of antibacterial surfaces reviewed and compared. Besides, studies on antibacterial mechanisms are discussed, which sets guidance for the experimental study in this thesis.

In the first project, TNT-Au is designed as light-independent antibacterial material. The antibacterial mechanism of TNT-Au in the dark environment is studied and a novel type of extracellular electron transfer (EET) between the bacteria and material surface is observed to cause bacteria death. Although the EET-induced bacteria current is similar to the localized surface plasmon resonance (LSPR) related photocurrent, the former takes place without light and no reactive oxygen species (ROS) are produced during the process. The EET is also different from that commonly attributed to microbial fuel cells (MFC) because it is dominated mainly by the materials surface, but not the bacteria, and the environment is aerobic. EET on the TNT-Au surface kills Staphylococcus aureus but if it is combined with special MFC bacteria, the efficiency of MFC may be improved significantly. In addition, the materials exhibit antibacterial effects against S. aureus according to the antibacterial test carried out for a total time of 21 days, which are normally long enough for early stage tissue healing after surgery. Furthermore, adhesion and proliferation of MC3T3-E1 osteoblasts on TNT-Au reveals cytocompatibility comparable to that of TNT. No ROS are detected from either the bacteria or MC3T3-E1 cells cultured on the TNT-Au surface. The absence of ROS, long-term antibacterial properties and cytocompatibility make TNT-Au promising biomaterial in orthopaedic devices and implants.

The second project focuses on another light-independent antibacterial surface modification, Ti embedded with silver nanoparticles (Ti-Ag) prepared by plasma immersion ion implantation (PIII) technology. The antibacterial effects of the Ag nanoparticles (NPs) depend on the conductivity of the substrate revealing the importance of electron transfer in the antibacterial process. In addition, electron transfer between the Ag-NPs and Ti substrate produces bursts of ROS in both the bacteria cells and culture medium. ROS leads to bacteria death by inducing intracellular oxidation, membrane potential variation, and cellular contents release and the antibacterial ability of Ti-Ag is inhibited appreciably after adding ROS scavengers. Even though ROS signals are detected from osteoblasts cultured on Ti, the cell compatibility is not impaired. This electron-transfer-based antibacterial process which produces ROS provides insights into the design of biomaterials with both good antibacterial properties and cytocompatibility.

Based on the above results which highlight the electrical interactions between bacteria and antibacterial materials, an external current is proposed to alter the antibacterial properties of capacitive materials. Here, direct and alternating currents (DC and AC) are applied to capacitive carbon doped titania nanotubes (TNT-C) fabricated by a one-step annealing method to kill bacteria and impede biofilm formation with the antibacterial mechanism investigated. When the TNT-C is charged, both instantaneous and post-charging antibacterial effects are observed and the post-charging antibacterial and anti-biofilm formation effects are proportional to the capacitance. The higher discharging capacity in the DC+ groups leads to a larger capacitance utilization ratio for better antibacterial effects. EET observed during early contact contributes to the surface-dependent post-charging antibacterial process. Physiologically, the electrical interaction deforms the bacteria morphology and elevates the intracellular ROS level without impairing the growth of osteoblasts. The capacitive TNT-C takes advantage of the electron-transfer-related mechanism spurring the design of light-independent antibacterial materials and providing insights into the use of electrical currents to modify biomaterials.
At the end of the thesis, the antibacterial surface modification methods are summarized with the antibacterial processes compared. Future work which can be done based on results in this thesis is also proposed.