Studies on the Biotribological and Biological Behavior of Thermally Oxidized Ti6Al4V for Use in Artificial Cervical Disk Song Wang, Junhui Li, Junzhe Lu, Rajnesh Tyagi, Zhenhua Liao, Pingfa Feng, and Weiqiang Liu (Submitted May 17, 2016; in revised form March 3, 2017) The artificial cervical disk was simplified and considered as a ball-on-socket model with the material configuration of ultra-high molecular weight polyethylene and Ti6Al4V (PE-on-TC4). In order to improve the wear resistance, an optimized thermal oxidation (TO) coating was applied on TC4 component. The long-term wear behavior of the model was assessed in vitro using a wear simulator under 10 million cycles (MC) testing intervals. The biological behavior was investigated by bone marrow-derived mesenchymal stem cells (BMSCs) cell attachment and cell viability/proliferation assays, respectively. The total average wear rate for PE/TC4 pair was found to be 0.81 mg/MC, whereas the same was about 0.96 mg/MC for PE/ TO pair. The wear rate of the metal has been neglected in comparison with that of the mating polymer. PE component was found to suffer severe damage characterized by scratches, fatigue cracks and arc-shaped wear grooves on the edge zone of ball. The dominant wear mechanism was abrasion for metal component while the dominant failure mechanism was a mix of plowing, fatigue and plastic deformation for polymer component. TO coating improved the cell attachment property of TC4, and the cell viability results were also quite good. TO coating protected TC4 from being plowed and avoided the release of toxic metal ions. However, this intensified the wear of PE component. Considering the biotribological and biological behavior in totality, TO coating could still be promising when applied in articulation surfaces. Keywords artificial cervical disk, biological behavior, simulation, thermal oxidation, Ti6Al4V alloy, wear 1. Introduction Titanium and titanium alloys are excellent biomedical mate- rials due to their low density, excellent biocompatibility and mechanical properties (Ref 1, 2). Nowadays, these are widely being used in various industrial sectors including orthopedic prosthesis such as artificial joint (Ref 3). However, the poor tribological behavior has put a limit on their long-term success in orthopedic implants (Ref 4). Surface modification has been used as a technique to improve their wear resistance as a bearing surface used in artificial joints (Ref 5). Among all these, thermal oxidation (TO) is one of the representative techniques. In fact, oxidation is a common natural phenomenon and titanium alloys do get oxidized naturally. According to the literature, the autoxidation layer can be 6-8 nm (Ref 6). However, this thin layer is not enough to bear high load and sustain long-term friction cycles. In order to obtain thick oxidation coating with the thickness on the order of few microns, heat treatment has been developed (Ref 7). Compared with other surface treatment techniques such as ion implanta- tion, physical or chemical vapor deposition, thermal oxidation treatment has been found to be more economical and effective in preparing thick and in situ grown films (Ref 8). During the past several decades, many researchers have studied the oxidation of titanium alloys (Ref 9-16). Kumar et al. (Ref 10, 11) studied the microstructural and electrochemical characterization of thermally oxidized Ti6Al4V alloy (TC4) under different oxidation temperatures and durations. Ashrafi- zadeh et al. (Ref 12) investigated structural features and corrosion behavior of titanium thermally oxidized at 500- 800 °C for 1 h. The effect of TO on corrosion and corrosion- wear behaviors has been analyzed by Guleryuz et al. (Ref 13). Xiong et al. (Ref 14) compared the tribological properties of oxidized titanium alloy with those of immersion ion implanted one. Saldana et al. (Ref 15) studied the osteoblast response to thermally oxidized Ti6Al4V alloy. Although there are already many studies related to TO and the oxidation itself seems to be a simple phenomenon, however, this simple method has not been used widely in artificial joints or other orthopedic implants (Ref 16). The authors believe that one of the main reasons for the limited use is that the process is not so perfect. A critical review of available literature also suggests that the research has Song Wang and Zhenhua Liao, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China and Biomechanics and Biotechnology Lab, Research Institute of Tsinghua University in Shenzhen, Shenzhen, 518057, China; Junhui Li, The spine surgery of Henan Orthopaedic Hospital, Henan Luoyang Bonesetting Hospital, Zhengzhou, 450046, China; Junzhe Lu, Biomechanics and Biotechnology Lab, Research Institute of Tsinghua University in Shenzhen, Shenzhen, 518057, China; Rajnesh Tyagi, Department of Mechanical Engineering, Indian Institute of Technology (BHU) Varanasi, 221005, India; Pingfa Feng, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China and State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China; and Weiqiang Liu, Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, China, Biomechanics and Biotechnology Lab, Research Institute of Tsinghua University in Shenzhen, Shenzhen, 518057, China, and State Key Laboratory of Tribology, Tsinghua University, Beijing, 100084, China. Contact e-mails: weiqliu@hotmail.com and ydyzhqws@126.com. JMEPEG ÓASM International DOI: 10.1007/s11665-017-2649-z 1059-9495/$19.00 Journal of Materials Engineering and Performance