## 4. Degradable Sensor Materials
### Degradable Metallic Materials  
Currently, the metals frequently employed in implantable devices predominantly encompass traditional metal materials such as magnesium and its alloys, zinc and its alloys, molybdenum metal, and iron. Their degradation rates exhibit variations (for detailed data, refer to Table 5). Among these, magnesium demonstrates excellent biocompatibility and the most rapid degradation rate; zinc possesses moderate mechanical strength and degradation rate, and its decomposition products are non-toxic; molybdenum features high mechanical strength and good corrosion resistance. However, the limitation is that its degradation rate is slow, rendering it inappropriate for transient electronic devices; iron has the lowest cost, good biocompatibility and mechanical properties, and its metabolic pathway within the human body is clearly controllable. At present, medical biodegradable metal materials are primarily applied in domains such as nerve implant devices [119; 120], vascular stents [121; 122], and orthopedic implants [123; 124]. Mg: Magnesium and its alloys have an elastic modulus similar to that of human bone tissue and are widely used in the field of orthopedic repair. In terms of electronic devices, magnesium metal is often used to manufacture key components of sensors due to its high electrical conductivity and ease of processing and decomposition. For example, it is used in power receivers and miniature heaters of implantable medical devices. [PERSON] successfully prepared a magnesium-based radiofrequency degradable micro resonator [PERSON] (2016). This resonator can achieve frequency-selective wireless heating. This achievement marks an important step forward in using magnesium and its alloys as electrical receivers and miniature heaters in biodegradable implantable medical devices. Meanwhile, magnesium is also commonly used in the manufacturing of radio frequency coils and antennas for passive sensors. Wireless information transmission is achieved through the physical field coupling between the coils [PERSON] (2016). Zn: Zinc and its alloys have good biocompatibility and biodegradability. After special treatment, the strength level of zinc alloy implants is almost twice that of magnesium alloys [PERSON] (2016). Moreover, zinc batteries have advantages such as high safety, low cost, and relatively high energy density, making them suitable for the energy storage systems of implantable electronic devices. For example, an implantable zinc-ion battery based on a gelatin/polycaprolactone composite gel electrolyte completely degrades within 8 weeks in rats [PERSON] (2016). Fe: Ferroelectric materials hold significant promise for crucial applications in implantable electronic devices owing to their outstanding piezoelectric properties. [PERSON] et al. developed a biodegradable organic small-molecule ferroelectric crystal that demonstrates a piezoelectric response (with d33 reaching 138 pC/N), comparable to that of inorganic barium titanate ceramics [PERSON] (2016). This material can be utilized to fabricate flexible ferroelectric composite films for the assembly of controllable transient electromechanical devices. Mo: As one of the bioabsorbable metals, molybdenum is widely used in the fabrication of electrodes for implantable electronic devices and energy devices. Molybdenum has good biocompatibility and can gradually degrade in biological fluids, making it suitable for short-term implantable electronic devices. [PERSON] proposed a high-performance, fully biodegradable and bioabsorbable supercapacitor implant using two-dimensional amorphous molybdenum oxide (MoOx) flakes as electrodes [PERSON] (2016). The research team further proposed a high-performance biodegradable energy storage device realized by a MoO\({}_{3}\)-MoS\({}_{2}\) nanosheet array based on a two-dimensional heterostructure [PERSON] (2016). This device exhibits excellent performance in air/liquid environments, capable of providing a stable high output voltage (about 1.6 V) and a relatively long working lifespan.