## 6 Real-Time In-Vivo and In-Vitro Corrosion Monitoring
### Microsensor-Based Monitoring System
Figure 9: (**a**) A thin filmed H\({}_{2}\) sensor prior to exposure to H\({}_{2}\); (**b**) H\({}_{2}\) detected by the film through the mouse’s skin [35]. processing program known as ImageJ was used to quantify the color changes associated with different levels of H\({}_{2}\) generated from corroding Mg alloy implant (Figure 10). The three-dimensional view is useful for assessing the real-time corrosion of specific sections of larger metal implants visually. Due to the current stage of development, a considerable amount of time is still needed for the color change to occur during cases where there are very low levels of H\({}_{2}\) infiltrating the skin area [35]. [PERSON] et al. [18] has demonstrated another way to perform hydrogen evolution tests by setting up audiometers with an electronic balance. This setup (Figure 11) automatically records the corrosion rate values with a USB data logger. Of all the three different electrolytes used, it was determined that the corrosion rate for both commercially-pure Mg (CP Mg) and Mg-0.8 Ca alloy increases in the following order SBF \(<\) MEM \(<\) NaCl. This test allows for the obtaining of results at various time points over the course of the experiment, ensuring that the changes in the rate of corrosion happening over the entire duration of the experiment can be monitored [37]. Figure 11: Layout of an hydrogen evolution test experiment involving an eudiometer [18]. Figure 10: 3D figure of the change in brightness in the H\({}_{2}\) sensor area [35]. Another set of tools that can be classified as sensor-based are corrosion sensors that are optically based. These were conceived by having the uncoated area of an optical fiber deposited with specific materials (e.g., nickel-phosphorus layer or Fe-C alloy layer). When there is a loss of coating layer, the optical transmission power will drop [29]. However, its application in real-time still needs verification. A study conducted by [PERSON] et al. [38] discussed the concept using biodegradable wireless implants. The idea consists of fabricating a fully biodegradable sensor for in-vivo applications. This concept requires a biomedical implant that can be made of materials that are biodegradable. To further improve the implant from partially biodegradable to completely biodegradable, the antennas/coils used for wireless communication needs to have the ability to fully biodegradable. This may be achieved with the advancement of radiofrequency-driven resistor-inductor-capacitor resonators (RF-driven RLC resonators) constituted from biodegradable metals. These can be constructed by electric discharge machining using biodegradable conductive polymer composites (e.g., polycaprolactone-polypyrrole (PCL-PPy), polylactide-polypyrrole (PLLA-PPy)). These composites are made by compression molding alongside laser-cutting. RLC resonators are frequently used in wireless power and data transmission for short range telemetry. According to Figure 1 of [38], the partially biodegradable implant is composed of a non-biodegradable block for sensing and electronic purposes while the fully biodegradable coil/antenna (10 MHz operation) is used for data or power transmission outside of the body. In comparison with Figure 2 of [38], the whole sensor is completely biodegradable. The biodegradable metals are used to compose electrical circuits with Mg being a potential candidate due to its high conductivity and promising biodegradable characteristics [39] whereas PCL or PLLA could be used as material for packaging due to their significant strength and ability to remain mechanically stable for a long time. Unlike Figure 1 of [38], the fully biodegradable implant does not have a data-processing block to regulate the sensor's measured signal. However, Figure 2 of [38] shows an implant that is composed of an RLC resonator that consist of a sensitive sensing layer towards stimulus. The stimulus will cause damping of the resonant frequency of the RLC resonator.