## 6 Real-Time In-Vivo and In-Vitro Corrosion Monitoring
### Microsensor-Based Monitoring System  
A microsensor-based monitoring system was first created by [PERSON] et al. [33]. The experiment was carried out on mice to test the reliability of transdermal H\({}_{2}\) measurements with the assistance of a biosensor that is highly sensitive to hydrogen (Figure 8). The measurements were recorded by ensuring the H\({}_{2}\) microsensor's sensor tip touched the skin above the gas cavity at the implanted area. The sensor measures the current and is converted to regional H\({}_{2}\) concentration using a calibration curve generated from known levels of H\({}_{2}\) saturated water. Further analysis is then carried out using X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS). The limitation of this set-up is its dependency on the high permeability of hydrogen leaving the skin, which is potentially a problem when monitoring the corrosion of Mg implants located in areas further underneath the skin such as implants placed at the muscle area or internal organs [33]. However, rapid measurements seem plausible as readings can be collected at a short response time of 30 s despite low concentrations (30-400 \(\mu\)M) of hydrogen through the skin [34]. Furthermore, H\({}_{2}\) sensing has been acknowledged to be a beneficial, non-invasive way to monitor the corrosion of Mg alloys in-vivo over a large range of corrosion rates. Thus, this places H\({}_{2}\) sensing in a favorable position when compared to traditional methods of monitoring corrosion rates [33]. Figure 7: (**a**) Anodic polarization curves; (**b**) cathodic polarization curves [30]. Another type of sensor detection method is a visual biosensor for the detection of hydrogen. According to [PERSON] et al. [35], \"This method of detection is non-invasive, user-friendly, promising and cheap compared to electrochemical detection\". Their research has reported a usable visual H\({}_{2}\) sensor that allows monitoring of the corrosion process in-vivo. The sensor is constituted of a thin coating on a plastic sheet that changes color when H\({}_{2}\) gas is present (Figure 9a). By placing it on the skin where the implant or center of the visible gas cavity is located, detection for corrosion can commence. The sheet will undergo a change of color from gray to dark blue after exposure to hydrogen (Figure 9b). This is due to a metal oxide reduction reaction when the H\({}_{2}\) is in the presence of a Pd catalyst. The developed materials could change their color reversibly or irreversibly when H\({}_{2}\) gas is present [35]. The indicator of the visual H\({}_{2}\) sensor can be thin films comprising transition metal oxides that are 0.2 to 10 microns in thickness or nano-sized with a very thin, uneven coating of a catalyst to initiate its reaction with H\({}_{2}\). The color sensitive material within the film only responds to gases that are sufficiently permeable through the skin and react with it, making the visual sensor hydrogen-selective, which is ideal [36]. The visual H\({}_{2}\) sensor responds in a quantitative manner to the amount of H\({}_{2}\) exposed. In-vivo measurements were taken by the visual H\({}_{2}\) sensor and a calibrated electrochemical H\({}_{2}\) sensor to calibrate the color response of the visual sensor with the electrochemically-measured H\({}_{2}\) concentrations. However, it is important to take into consideration that the sensor does not absorb the evolved H\({}_{2}\) entirely, as there could be inconsistent barrier between the smooth sensor and the surface of the skin, causing the H\({}_{2}\) that is leaving through the skin to escape into the air. Therefore, this would not be a total representation of the amount of H\({}_{2}\) released by the corro  
Figure 8: Hydrogen microsensor with direct contact between sensor tip and mice; used to detect and measure hydrogen evolving from an allow of Mg implanted in the mice [34].