## 4 Discussion
The output power increased with the degradation of PAB, as shown in Figure 10b. The stress for PAB with different diameters was calculated with Equation (1), and is given in Table 2. The stress and output power increased as the PAB diameter decreased. The variation of the output power gradually increased with increasing stress, which indicated a positive relationship that was consistent with Equations (2) and (3). The changing tendencies shown in Figures 7-9 were similar with the stress range from phase M to N in Figure 1. Hence, the theoretical analysis agreed with the experimental results. This indicated that the MB sensor can be used to wirelessly monitor the degradation behavior of PAB. The Ni and Mo elements of the MB sensor are toxic. However, as shown in Figure 4d, the MB sensor was fully coated with about 10 \(\upmu\)m of the quick adhesive and about 15 mm of PLA. The MB sensor was not in direct touch with body tissues during the degradation process. When PAB in the body was fully degraded, a micro-operation was performed to remove the MB sensor. Many papers have used MB sensors in vitro and in vivo to measure and monitor blood coagulation [34] and the host response (wound healing) to an implant in vivo [35]. The sensor has also been applied to monitor the tensile force on sutured wound sites [22] and implanted in a 6 mm femoral segmental defect model \begin{table}
\begin{tabular}{c c c c} \hline
**Time (days)** & **0** & **15** & **15** \\
**(PAB Condition)** & **(Without Treatment)** & **(In Alkaline Media)** & **(In PBS Media)** \\ \hline Diameter of PAB (mm) & 15 \(\pm\) 0.05 & 14.92 \(\pm\) 0.02 & 14.96 \(\pm\) 0.02 \\ Weight loss (g) & 0 & 0.3 \(\pm\) 0.03 & 0.15 \(\pm\) 0.03 \\ Varying value of output & 0.02 \(\pm\) 0.004 & 0.09 \(\pm\) 0.04 & 0.06 \(\pm\) 0.02 \\ power (dbm) in 100 N & 0.04 \(\pm\) 0.004 & 0.2 \(\pm\) 0.004 & 0.11 \(\pm\) 0.02 \\ Stress (N/mm\({}^{2}\)) & 0.566 & 0.579 & 0.569 \\ \hline \end{tabular}
\end{table} 
Table 2: Relationship between the degradation time (days) and PAB condition under different immersion conditions. of a rat for non-invasive real-time measurement of the stress [36]. Therefore, the MB sensor was not harmful to the body and was suitable for monitoring the degradation behavior of an implant. Traditional methods [11; 12] are not efficient means of monitoring the degradation behavior if PAB is implanted in the defect site to repair a bone defect. The MB sensor presented for the first time in this study to monitor the degradation behavior of PAB showed innovations in power supply, signal transmission, and data collection. PLA was fully coated on the surface of the MB sensor, which means that it could work without external wires or any batteries to supply power. The signal transmission and the final data are collected by the wireless and passive method which is low cost and does not cause disturbance to the patient, making it convenient to monitor the degradation behavior of PAB in vitro. The results showed that the MB sensor could wirelessly monitor the degradation behavior of PAB in vitro. The MB sensor was smaller in volume and lower in cost [37] than traditional methods. Importantly, biological tissue has almost no effect on the MB sensor, so it could potentially be used in vivo in the future.