## 1 Introduction
### Titanium Degradation
#### 1.2.1 Titanium Surface Contamination  
The presence of impurities on the surface of an implant affects wettability as these impurities prevent the adhesion and adherence of water molecules. The hydrocarbon contamination of titanium dental implants could occur during machining or surface modifications, sterilization, packaging, and storage prior to clinical use. Even at small quantities, trace compounds such as polycarbonyls or hydrocarbons may alter the implant surface properties. The presence of trace organic impurities or adventitious contaminants on the surface of an implant is unavoidable and is thought to affect the response to protein absorption and cells adjacent to the implant. [PERSON] et al. [25] evaluated the effect of carbon contamination on cell behaviors by regulating the amount of carbon deposited onto the titanium surface using machined oils prior to cell culture. As expected, the specimens treated with acetone and machine oil exhibited a high carbon content and reduced peaks for TiO\({}_{2}\) on the surface [25]. Generally, machined surfaces contained significantly more carbon than the roughened surfaces [17] because the process of machining involves cutting and polishing, in which the implants directly make contact with the machining tools such as organic lubricating fluids [18]. In comparison, the plasma-spraying technique resulted in cleaner surfaces because of the nature of the finishing technique, where the implant surface does not come into contact with machining tools or lubricating fluids and organic contaminants are literally removed during the process due to the high temperature of the plasma spray. Conversely, acid etching either by hydrofluoric or hydrochloric/sulfuric acid dissolves the outermost TiO\({}_{2}\) layers of the implant surface; hence, the hydrocarbon compounds are virtually eliminated alongside the outer layers [35; 46]. Notably, traces of foreign materials such as metals, lubricants, detergents, or other specific chemical compounds attach to the implant surface during processing and act as contaminants [39]. Hydrocarbon contamination on the surface was found to alter the surface zeta potential of the titanium surface to become electronegative. This reaction led to the entrapment of air bubbles and the blocking of the protein receptor, thereby interfering with the interaction between the proteins and cells [1; 47; 48]. Following manufacture, sterilization is one of the final surface preparations performed before packaging to ensure that the implants prepared are free from bacterial contamination. Interestingly, one further issue highlighted by studying cell-surface interactions is the fact that cleaning and sterilization methods may affect the surface energy of implants [49; 50; 51]. Notwithstanding, our effort to reduce bacterial contamination inevitably contributes to non-biological surface contamination of the titanium implants. Thus, autoclaving or ethanol or butanol sterilization creates organic contamination [49; 52]. Hence, sterilization via the hydrothermal method [53; 54], gamma ray [55], or intense UV light exposure [50; 51; 56] is recommended to achieve titanium with high surface energy that can induce cell adhesion [41; 44] as well as improve cellular activity [22; 47] and osseointegration [57; 58]. In addition to processing and cleaning, the surface properties of titanium implants are also affected by the storage medium used. To our knowledge, most commercially used titanium implants are provided in sterile, gas-permeable packaging so that they can be stored up to expiry dates of approximately four years following fabrication. Given the nature of the packaging, plastic casing, and absence of light, the chemisorbed hydroxyl groups on the titanium surface are replaced with oxygen and carbon from air. However, the level of hydrocarbon, not hydrophilicity level, was found to be inversely correlated with protein adsorption and cell attachment [25]. The hydrocarbon formation on the titanium surface can start as early as four weeks after the production.