## 4. Degradable Sensor Materials
### Silicon-Based Materials  
Degradable silicon-based materials are generally classified into ultra-thin silicon, silica gel, and amorphous silica. These three materials are all considered to be non-toxic and harmless to the human body. Under normal circumstances, silicon is typically considered a material that is inherently resistant to natural degradation, with its degradation process often spanning hundreds of years. Nevertheless, silicon thin films can undergo decomposition within a time frame that, despite appearing relatively slow, remains within an acceptable range. The decomposition rate is approximately 5-90 nanometers per day [136]. Simultaneously, by introducing impurities into silicon through doping and depositing an oxide layer on the exposed surface, the time scale of this dissolution process can be precisely regulated [137]. When silicon dissolves in water, it engages in a chemical reaction with water to form silicic acid. Furthermore, the ultra-thin silicon surface can be fabricated by means of well-established microfabrication techniques. Owing to these merits, it holds significant application potential in the domains of biomedical implants and environmental monitoring. Nevertheless, ultra-thin silicon still presents several drawbacks that remain unresolved for the time being. For instance, ultra-thin silicon is relatively fragile and might cause harm to the human body upon decomposition. Additionally, the production cost of ultra-thin silicon wafers with a thickness below a certain threshold is relatively high, which is unfavorable for large-scale commercialization. Eye Sensor: [PERSON] fabricated a flexible and biocompatible multifunctional smart contact lens (P&T@DG) using flexible transfer printing and micro/nano processing technologies. This lens is based on an ultrathin single-crystalline silicon nanomembrane with a thickness of 200 nm. It can rapidly and accurately capture intraocular pressure (IOP) fluctuations and monitor ocular surface temperature (OST), thereby being applicable for multi-dimensional auxiliary diagnosis of glaucoma [138]. Electroencephalogram Interface: [PERSON] developed a flexible transfer technique for single-crystalline silicon nanomembranes and subsequently established tens of thousands of implantable large-scale flexible electrode arrays with a controllable distribution density across the entire brain dimension (exceeding 32,000 electrodes). These arrays can adhere seamlessly to the cerebral cortex, reducing interface impedance and motion artifacts. They are capable of mapping real-time neural signals with high fidelity, achieving a high spatio-temporal resolution of 1.42 milliseconds and 20 \(\upmu\)m, along with a signal-to-noise ratio of up to 37 decibels [139]. Silicone is a substance that exhibits extremely low decomposition rate over time, rendering it non-degradable within the human body. Nevertheless, its favorable physical and chemical properties, along with remarkable biocompatibility, have led to its extensive utilization in the medical domain. Silicone demonstrates a broad spectrum of hardness levels, outstanding anti-ultraviolet capabilities, excellent heat resistance, and chemical inertness. Moreover, it possesses superior electrical properties and can be effectively sterilized through various methods. Ranging from wound care gels to flexible drainage tubes and semi-rigid implants, silicone is prevalent in medical device applications [140; 141; 142; 143; 144]. Amorphous silicon: Amorphous silicon is a silicon structure characterized by long-range disorder and functions as a direct-band semiconductor. It offers distinct advantages, including straightforward production processes and relatively low costs. Nevertheless, it also presents several notable drawbacks. Specifically, the carrier mobility and minority carrier lifetime of amorphous silicon are significantly lower compared to those of single-crystalline silicon. Moreover, its performance is highly susceptible to degradation under illumination and high-temperature conditions.