## 3 A Taxonomy of Measuring Methods, Techniques, and Sensors in Physiological-Based DMSs
### Bioimpedance Measurement (BIM)
#### 3.4.2 Inductive Bioimpedance Measurement (Magnetic Induction (MI))  
Magnetic induction is a non-contact method for measuring intrathoracic impedance distribution. Its principle is based on electromagnetic coupling between a coil and the torso in its vicinity. When a high-frequency alternating current (typically with a frequency of 6 to 11 MHz [71]) flows through the coil, this current induces a primary field that affects the tissue underneath, resulting in the formation of eddy currents flowing in closed loops perpendicular to the field. The eddy currents are modulated in amplitude, direction, and orientation by changes in blood and air volumes, and displacement of organ boundaries. The eddy currents create a secondary field that carries information about this modulation,dependent on the impedance of the tissue. The penetration depth is estimated to be approximately the diameter of the coil.  
The inductive bioimpedance measurements can be implemented using a single-coil approach with frequency modulation or a multi-coil approach based on a gradiometer. Single-coil systems operate as part of a resonant Colpitts generator circuit. The coil serves as both a transmitter and a receiver and is part of the circuit for frequency setting. When the impedance of the coil changes due to the secondary field affecting the primary, this will cause a change in the generated frequency, which is the sensor's output signal. This change in frequency is correlated with the physiological activity of the subject. In multi-coil systems, one coil is used as the transmitter and one or two are the receiver coils. When only one receiver coil is used, it should be placed in an orthogonal orientation to the transmitting coil to ensure that the primary field does not induce any voltages in the receiving coil. When two receiving coils are used, they are usually connected as gradiometers so that the primary field is almost completely compensated. In 2011, [PERSON] et al. [72] presented the integration of a single-coil sensor in the backrest of the car seat for bioimpedance monitoring. Although the results are preliminary, the usability of inductive bioimpedance measurement for respiratory monitoring while driving was demonstrated. Recently, [PERSON] and [PERSON] [73] evaluated the accuracy in RR monitoring of a similar single-coil sensor in the backrest of the driver seat for different real-world driving conditions and longer driving periods. The presented results demonstrated that motion artefacts significantly influenced the performance. The inductive bioimpedance sensor is sensitive to driver movements, especially during turning, acceleration, and deceleration phases, which creates difficulties in accurately monitoring RR. In 2017, [PERSON] et al. [74] presented the PhysioBelt device, which includes an inductive bioimpedance sensor with a flexible coil sewn into the seat belt (Figure 4b). In the same year, [PERSON] et al. [75] reported an advance in gradiometer technology for automotive integration. The new sensor with one transmitting and two receiving coils forming a vertical gradiometer is integrated in the backrest of the driver seat (Figure 4c). The reported results are preliminary and have not yet been extensively tested, but they demonstrate that both RR and HR can be monitored. In 2023, [PERSON] et al. [76] embedded an MI sensor in a pillow that monitors the vital parameters of a driver. The results confirmed the workability of the method, but some signals in the experiments were disturbed by superimposed spikes.  
Figure 4: BIM method and relevant techniques and sensors for acquisition of the driver’s bioimpedance signal.