## 2 Sensing Method  
### Magnetic Field Sensing  
#### 2.1.1 Applied Technologies  
Magnetic field sensors typically achieve sensing based on the Hall effect. The Hall effect is the phenomenon in which, when an electric current passes through a conductor placed in a magnetic field, the charge carriers are influenced by the Lorentz force, leading to the generation of an electromotive force on both sides of the conductor. Implantable Hall sensors accurately utilize this principle to acquire magnetic field information by measuring the Hall electromotive force. Magnetic field sensors exhibit a high degree of sensitivity to variations in the magnetic field and are capable of detecting feeble magnetic fields, thereby demonstrating high sensitivity. Simultaneously, a certain distance exists between the sensor and the object under measurement, enabling non-contact measurement. Nevertheless, this type of device is susceptible to interference from external magnetic fields, which may lead to errors, necessitating calibration. Moreover, magnetic materials are sensitive to temperature fluctuations, and the impact of temperature changes on the measurement results must be taken into account [14]. Based on the Hall effect, Magkoutas developed an extravascular magnetic flux sensing device [15]. This device can continuously monitor arterial blood pressure (ABP), arterial wall diameter (AD), and arterial circumferential strain (ACS). Moreover, it can additionally derive physiological data such as pulse wave velocity, respiratory rate, and the duration of the cardiac systolic phase. This device consists of a Hall effect sensor and a miniature magnet, which are placed on the outer wall of the blood vessel and separated by a certain distance (Figure 1a). As the heart beats, the distance between these two componentschanges periodically. By means of the shift in the magnetic field magnitude on the Hall effect sensor, ABP, AD, and ACS can be calculated. Based on the aortic blood pressure waveform, the respiratory rate and the systolic phase of the cardiac cycle can also be deduced. When multiple identical sensors are implanted at different positions in the human body, the pulse wave velocity (PWV) can be measured by means of the time difference in the signals detected by the sensors. This device can be placed in a housing that is 3D printed with biocompatible materials. The housing has been optimized for different blood vessel diameters to ensure precise installation at the designated site and accurate measurement. Based on the principle that vibration gives rise to a pulsed magnetic field, [PERSON] developed a millimeter-scale, chip-free, and battery-free magnetic implant system [16]. This system wirelessly induces the large-amplitude vibration of the magnetic implant. Sub  
Figure 1: **(a)** Illustration of the hall-based sensing device attachment and a detailed view showing the components comprising the sensing device. The hall-effect sensor and magnet are completely separated, and they are sutured at a predefined distance on the outer arterial wall. **(b)** Millimeter-scale magnetic implants can achieve two-way wireless communication with fully integrated wearable devices, and data collection and processing can be completed on mobile devices. **(c)** The figure shows the crystallization process of the glycine-polyvinyl alcohol interlayer film. During the nucleation stage, glycine molecules exhibit a specific orientation arrangement on the surface of polyvinyl alcohol (PVA), and this arrangement further triggers the ordered arrangement of long-range crystals. **(d)** A glycine-polycaprolactone (PCL) ultrasonic transducer with fully biodegradable properties. The device mainly consists of a piezoelectric glycine-polycaprolactone (PCL) pad, molybdenum electrodes and wires, and a polylactic acid (PLA) encapsulation layer. sequently, the damped vibration of the magnetic implant generates a dynamic magnetic field, which can be detected by the magnetic sensors integrated in wearable devices.