## 2 Sensing Method
### Magnetic Field Sensing
#### 2.1.2 Developing Technologies  
The sensing technology that is currently under development: When conducting the monitoring of magnetic field signals within the human body, the most formidable challenges encountered are the extremely feeble field strength and the broad frequency range. The magnetic field strength spans from \(10^{-10}\) T to \(10^{-15}\) T, while the frequency range extends from a few hertz to several thousand hertz [20]. To address the issue of capturing weak magnetic field signals, researchers approach it from two perspectives: enhancing the sensitivity of magnetometers and developing advanced magnetic shielding materials.  
* Superconducting Quantum Interference Devices (SQUIDs) [21] operate based on the principle of Josephson junctions. This technology is capable of detecting magnetic fields within the range of pT to fT [22; 23]. Nevertheless, it has certain limitations. The most notable drawbacks include high costs, large size, and the necessity for cooling equipment and a shielding room [21; 24; 25]. Prior to system simplification and miniaturization, the application of this technology in implantable devices remains challenging. Despite these challenges, current research indicates that this technology holds great promise.
* Atomic magnetometers (AM): These devices heat atoms to elevated temperatures and utilize the pumping and detection mechanisms of an optical system to detect the magnetic decay of atomic spins [26; 27]. Atomic magnetometers are further classified into two distinct types: optically pumped magnetometers (OPM) and spin-exchange relaxation-free magnetometers (SERF). Under specific conditions, the performance of atomic magnetometers can be on par with that of Superconducting Quantum Interference Device (SQUID) sensors. In particular, the atomic magnetometer operating on the SERF principle can attain a sensitivity of 10 fT/\(\surd\)Hz at 10 Hz [28; 29]. One of the significant advantages of this sensing approach is that it does not necessitate any cooling structure. Consequently, its volume is substantially smaller than that of SQUIDs and can be fabricated to be just a few centimeters in size [30; 31]. It holds the potential for transformation into an implantable device. Nevertheless, atomic magnetometers are not without their limitations. A fundamental drawback is that they can only function in an environment characterized by a near-zero magnetic field [27]. All varieties of atomic magnetometers necessitate magnetic shielding to maintain a near-zero ambient noise level in order to attain ultra-high sensitivity. Another shortcoming is that, owing to theoretical constraints, the bandwidth of atomic magnetometers is extremely narrow [32], which restricts the frequency range of bio-magnetic signalsthat can be detected. Consequently, there remains a substantial distance to cover before atomic magnetometers can be put into practical use.
* Magnetic field shielding technologies (MSRs): Magnetic field shielding technologies play a crucial role in the magnetic field sensing system. The magnetic shielding room, as an essential component thereof, aims to mitigate background noise during the monitoring of bio magnetic fields. It can be classified into two categories: active and passive [30]. At present, research regarding magnetic shielding materials predominantly centers on the selection of materials for shielding rooms. A representative shielding material is the Mu material. This is a shielding material derived by incorporating various other components into a nickel-iron alloy as the base [33; 34; 35]. Moreover, novel materials such as manganese-zinc ferrite are under investigation as promising alternatives [36].