## 2 Sensing Method
### Capacitive Sensing and Inductive Sensing
#### 2.3.2 Developing Technologies  
The dimensions and operating principle of LC resonant sensing impose limitations on its readout distance. As previously stated, relay coils are employed to strengthen magnetic field coupling and thereby extend the readout distance. This includes the enhanced cyclic scanning repeater and the adaptive repeater, as detailed in references [64; 65]. Nevertheless, this approach gives rise to multiple peak frequencies in the spectrum and leads to a reduction in sensitivity. Recently, it has been discovered that the breaking of PT symmetry can contribute to an increase in the readout distance of LC sensors [66]. Moreover, it enables the achievement of a higher Q-factor [67] and simultaneous multi-parameter measurement [68]. However, there remain numerous controversies regarding the inevitable noise and practical sensitivity of exceptional point sensors. The innovative construction strategy of PT sensors still necessitates further in-depth research and exploration [69]. Evidently, there is a substantial journey ahead before this technology can be effectively translated into practical applications. Furthermore, for resonant sensors, another crucial aspect is how to withstand environmental parameters and enhance sensitivity. LC passive sensors are more susceptible to environmental parameters (such as temperature and humidity) owing to the absence of active correction components. Moreover, the impact of noise on the signal is relatively significant [70]. To address this issue, several approaches have been put forward:  
* Employing differential sensors to mitigate the environmental influence on the sensors via the superposition principle [71; 72]. Nevertheless, this method imposes extremely high demands on the symmetry between the two resonators, posing a challenge during the sensor design and manufacturing processes [73].
* Augmenting the electric field surrounding the resonator serves to enhance the sensor's capacity to polarize molecules within the measured material, thereby boosting the sensor's sensitivity. This approach has been implemented in certain passive resonant sensors [74; 75; 76]. Nevertheless, in specific scenarios, noise sources in the environmental space can obscure readings featuring relatively minor frequency shifts. Consequently, the key challenge confronting this type of device remains how to mitigate environmental noise and further elevate the sensor's sensitivity [77]. There is an urgent need for the emergence of novel technologies.  
In addition to the above research work, some companies have already developed commercial devices that can be used in surgeries. The EndoSure Sensor Wireless AAA pressure measurement system (CardioMEMS) (Figure 2d) that monitors blood pressure in an aneurysm sac is safe and useful in guiding endovascular repair of an abdominal aortic aneurysm (AAA) [78]. This research was published in the Journal of Vascular Surgery. Wireless implantable sensors can be used to diagnose the location and type of endoleaks during endovascular aneurysm repair. Compared with using contrast agent injection and X-ray imaging to monitor endoleaks, this method minimizes the damage to the human body during monitoring and avoids misdiagnosis caused by in vivo motion artifacts mimicking endoleaks.