## 5. Degradable Devices
The voltage output of this device in response to periodic sinusoidal vibration aligns with both the measured values on the source silicon wafer and the theoretical values. Beyond achieving the circuit structure using biodegradable materials, the injectable hydrogel relying on ultrasonic sensing can be rendered entirely biodegradable via a straightforward approach. [PERSON] used a hydrogel matrix to construct periodic air columns in a 2 \(\times\) 2 \(\times\) 2 mm hydrogel cube, thus forming an acoustic metamaterial [170]. During practical applications, when specific-frequency ultrasonic waves are emitted by external devices, the hydrogel will undergo subtle deformations when the in vivo environment changes. This deformation causes the peak frequency of the ultrasonic reflected wave to shift. When ultrasonic waves with a peak frequency of \(f_{0}\) are emitted, the super hydrogel will reflect these waves at a peak frequency of \(f_{1}\) (Figure 4d). By carrying out stimulus-responsive functionalization operations (such as pressure, temperature, pH response, etc.) on the hydrogel matrix, the hydrogel can sense the changes in different physiological parameters by tracking the peak frequency shift in the reflected ultrasonic waves. This well-designed structure endows the hydrogel with excellent acoustic reflection performance, enabling it to achieve multi-parameter sensing. It is worth noting that since this device is composed only of biodegradable hydrogel and periodic air columns, this sensor can maintain normal sensing functions within 24 days after being implanted in rats and achieve complete degradation after 18 weeks. The commercial standing of degradable medical devices exhibits a dual recovery tendency in both technology and market. Technologically, novel types of degradable medical devices are emerging in an unceasing manner. For instance, polylactic acid (PLA) is extensively applied in domains such as bone fixation devices and surgical sutures owing to its high tensile strength and favorable biocompatibility. The degradation cycle of PLA is controllable. High-molecular-weight PLA takes 2 to 5.6 years to fully degrade within the body, and its performance can be further optimized through copolymerization with other materials. Moreover, injectable PLLA products (Sculptra(r), Sanofi-Aventis, Paris, France) have been approved by the FDA for the treatment of HIV-associated facial fat loss. Poly (lactic-co-glycolic acid) (PLGA) can regulate the degradation rate by adjusting the ratio of PLA to PGA and demonstrates excellent osteoconduction and osteoinduction capabilities in the orthopedic field. Polycaprolactone (PCL), due to its slow degradation rate, is suitable for long-term implants and drug delivery systems. The advantages of new degradable medical devices lie in the avoidance of secondary surgeries and the reduction in long-term risks. For example, Lepu Medical's bioabsorbable stent (NeoVas) and Shandong Huaan's Xinsorb stent have obtained marketing approval in China [171; 172]. These degradable stents are gradually assimilated by the human body after fulfilling the function of blood vessel repair. This process effectively mitigates the risks of infection and inflammation that could potentially be induced by permanent metal implants. Degradable vena cava filters possess both recovery and degradation capabilities, which significantly reduces the complication rate associated with long-term indwelling. In the orthopedic domain, degradable screws and bone plates fabricated from PLGA not only offer stable support but also facilitate bone tissue healing. As a result, the necessity for secondary surgery for removal is eliminated. The maturation and application of these technologies have broadened the market prospects of degradable medical devices and propelled the development of medical devices in a direction that is safer and more efficient.