## 2 Sensing Method
### Piezolectric Sensing
#### 2.2.1 Applied Technologies  
Polylactic acid (PLLA), a biodegradable polymer that finds extensive applications, also manifests certain piezoelectric properties upon appropriate treatment. This material exhibits shear piezoelectricity, which can be ascribed to the electrode polarity within the carbon-oxygen double bond branching from the polymer's main chain. By fabricating a 3 multi-layer structure, enhanced piezoelectricity can be attained from polylactic acid, and its \"effective\" conversion efficiency is comparable to that of ceramic PZT. Capitalizing on the favorable dielectric characteristics of polylactic acid, [PERSON] developed a biodegradable piezoelectric polylactic acid pressure sensor [40]. The sensor was integrated with a charge amplifier circuit, a wireless near-field communication (NFC) chip, and a commercial antenna on a printed circuit board (PCB). The entire PCB was encapsulated within a polydimethylsiloxane (PDMS) box and implanted subcutaneously on the back of an animal, thus enabling implantable wireless sensing. The abdominal sensor and connecting wires are capable of self-degradation, while the non-degradable PCB can be easily removed through minimally invasive means at the end of the sensor's service life. Although PLLA has demonstrated remarkable potential in the realm of biodegradable piezoelectric materials and implantable sensors, other researchers are also exploring diverse materials to develop similar devices with distinctive advantages. For example, [PERSON] reported a self-assembly method for wafer-scale synthesis of heterogeneous piezoelectric glycine thin films [41]. These films exhibit a PVA-glycine-PVA sandwich structure (Figure 1c). Glycine, the simplest amino acid, has a high piezoelectric coefficient and notable stability. The hydrogen bonds at the interface between PVA and glycine promote the formation and self-alignment of \(\gamma\)-glycine crystals across the entire film. The synthesized films display excellent, stable, and uniform piezoelectric properties, along with remarkable flexibility and biocompatibility. The device can operate stably for 5 days and completely degrade within 10 weeks. This device can also be utilized for implantable respiratory measurement in mice. Nevertheless, the application of glycine films still encounters some challenges owing to their intrinsic properties. As a result, [PERSON] proposed a different material processing strategy to tackle the brittleness and fragility of glycine films [42]. Polycaprolactone (PCL) was integrated into glycine crystals to fabricate biodegradable, flexible, and piezoelectric glycine crystal nanofibers (Figure 1d). The glycine-PCL nanofiber film demonstrates stable piezoelectric characteristics. Specifically, it exhibits a high ultrasonic output of 334 kPa at 0.15 volts root mean square (Vrms), outperforming existing biodegradable transducers. Biological experiments were conducted on mice, and the function of promoting brain drug delivery significantly extended the survival time of mice with orthotopic glioblastoma. Notwithstanding these progressions in material processing and device fabrication, the matter of wireless data transmission for implantable piezoelectric sensors remains a crucial factor to be taken into account for their practical applications. To achieve the wireless data transmission of implantable piezoelectric sensors, researchers at Purdue University designed an ultra-low power 2.4 GHz transmitter chip [43]. This chip was integrated with a patterned ring antenna on a flexible parylene C substrate. The ring antenna serves dual functions, acting as both a radiator and an inductor element within a voltage-controlled power oscillator (VCPO). Through this configuration, it achieves an ultra-high energy efficiency of 7 pJ/bit. The low-power active mode and sleep mode make it possible to implement continuous transmission and aggressive duty cycle data transmission.