## 6 Real-Time In-Vivo and In-Vitro Corrosion Monitoring
### Microdialysis-Based Monitoring System  
This technique is known for investigating the reactions taking place at the implant's interface and tissue [40]. It is basically used for taking samples where the concentration of the corrosion products formed from the implants will be further evaluated. Microdialysis coupled with a variety of detection techniques such as biosensors has been used for the last three decades to combat issues faced by other corrosion monitoring methods [41]. An experiment was performed under ex-vivo settings by [PERSON] et al. [40], where the biocompatibility of pure magnesium was tested by adopting microdialysis in an isolated, perfused bovine under model. Energy dispersive X-ray spectroscopy (EDX) measurements were then conducted to obtain measurements of the ions formed on the magnesium's surface. They observed that the implant's corrosion did not have a response to inflammation upon direct contact with the tissue. However, these results imply that even when pure magnesium degrades rapidly, it still has good biocompatibility at the interface of the tissue and implant. Another type of microdialysis-based monitoring method is the set-up developed by [PERSON] et al. [41] which involves a microdialysis probe with a cotton fabric-based electrochemical device (FED) attached to a potentiostat for conversion of chemical to electrical analysis. One of the main motivations for [PERSON] et al. to combine microdialysis with a magnesium-reactive biosensor for the monitoring of Mg corrosion is that the combination of the two systems created a more effective system, which enabled real-time monitoring of the whole corrosion process. This is mainly due to the fact that microdialysis could overcome the biocompatibility issues faced by biosensors while biosensorscould increase the temporal resolution of the sampling technique. The set-up obtained a pseudo-linear response and interference was lower than 1% demonstrating that the fabric device has high selectivity. Moreover, only 3 \(\upmu\)L of fluid sample was required to complete the measurement. Another advantage is that regardless of the location of the implant, a probe of this size can be implanted beneath the skin therefore, enabling measurement of deeper implants. Although further research is still required to optimize the combination of both systems, the current state of this method has the potential to be suitable for real-time in-vivo corrosion monitoring. In another research from [PERSON] et al. [42], an online-microflow capillary flow (\(\upmu\)FC) is coupled online through a flow injection analyzing system (FIAS) with an inductively-coupled plasma mass spectrometer (ICP-MS) as shown in Figure 12 provided monitoring of the corrosion process at certain intervals with high spatial resolution. This allowed detailed in-situ investigation of corrosion mechanisms and determination of corrosion rates at open circuit potential. Due to ICP-MS's high sensitivity and low detection limits, the alloy ions discharged could be identified at early corrosion stages. However, inaccurate results might be obtained as the bubbles generated from this set-up hinders the contact between the metal surface under exposure and the corrosion media. In addition, the bubbles prevent uniform mixing and dispersal of elements released in the circulating solution within the capillary. This can be solved if a 800 \(\upmu\)m I.D. large capillary is used. Overall, this method is considered useful for non-homogeneous materials and for determining phase specific processes. Further research was conducted by pairing microdialysis with an electrochemical enzymatic biosensor which is well-known for its sensitivity, selectivity, and temporal resolution. The purpose of coupling both tools was to enable in-vivo corrosion monitoring of magnesium in real-time [43]. The fabrication of the biosensor relies on the prerequisite of magnesium ions occurring in enzymatic reactions which enables the required selectivity for this technique.