## 3 Correlation of In-Vitro and In-Vivo Corrosion  
Prior to the analysis of the corrosion behavior of a specimen in its test environment, there are difficulties faced when it comes to mimicking in-vivo conditions in the in-vitro tests. This is because corrosion behavior such as local inflammation and tissue-implant interactions are influenced by a significant amount of factors inside the human body. Such factors are challenging to implement through in-vitro corrosion tests [18]. As performed by [PERSON] et al. [19] when analyzing the corrosion rate of magnesium alloys, the rate of corrosion obtained from in-vivo tests were only smaller in magnitude by four orders compared to those obtained from in-vitro tests. One of the most problematic issues associated with magnesium and its alloys is their unpredictable corrosion behavior in-vivo mainly due to the fact that there are numerous other ions present within the body fluids (e.g., Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), etc.) which makes metallic corrosion prediction a complicated task [20].  
Meanwhile, there are cases where the corrosion rate of a magnesium alloy (XHP Mg) performed in-vitro (immersion tests) compared well with in-vivo studies (implantation into the femurs of rats) as their degradation rate were almost similar. However, the consistency of results of these reported experiments, performed either in-vivo or in-vitro may not be comparable for all alloyed materials because corrosion behavior is influenced not solely on the composition of the electrolyte but also significantly affected by the interactions between the material and the type of electrolyte used [21]. Furthermore, due to the complexity and diversity of the human body internal environment, different evaluation procedures should be carried out for implants located in different areas of the body. To elucidate an example, one study attempted to examine the corrosion behavior and biocompatibility of hydroxyapatite (HAp) and octacalcium phosphate (OCP) coatings on AZ31 alloy by performing an immersion test for a long period of time in a medium (in-vitro) and by implantation test in transgenic mice (in-vivo) [22]. The surfaces of both alloy samples were examined and characterized with the assistance of an optical microscope (OM), backscattered scanning electron microscopy (BSEM), energy dispersive spectroscopy (EDS), X-ray power diffraction (XRD), and grazing incidence X-ray diffraction (GIXD). It was observed that the corrosion in-vitro was macroscopically-localized while microscopically-localized in-vivo. Whereas for the coating composition, in-vitro, the OCP and HAp coatings showed more precipitation of HAp while in-vivo, the HAp crystals dissolved locally. This shows that the test environment either in-vivo or in-vitro should be carefully designed to determine the corrosion behavior of OCP and HAp coatings [22]. At this present stage, it is even acknowledged that completely mimicking the corrosion behavior of an implant in the human body through in-vitro corrosion test is still a challenging task [18].  
In [PERSON] et al.'s study [23], they developed a type of relationship between in-vivo and in-vitro called IVIVC (in vitro and in vivo correlation) to assist in screening new materials and monitor the quality of the devices in-vitro for ethical reasons involving protection of animals and reducing the time-consuming process of in-vivo experiments, which has a duration of months or years. While the experiment outcome proved promising, additional improvements are still required to be carried out. Lastly, the established IVIVC still needs to be corroborated by other researchers. To further emphasize the importance of correlating the two test conditions, [PERSON] et al. [4] has stated that in order for researchers to compare their results easily, a standard method is needed to evaluate how certain parameters exactly influence the in-vitro corrosion of a metal.