# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 6 Magnetic Induction  
In general, human tissue is an inhomogeneous and anisotropic electrical medium. Within the thorax of a human, the distribution of electrical impedance is modulated by cardiorespiratory activity due to organ motion and deformation as well as fluid displacement. Magnetic Induction (MI) monitoring is a no-contact method to measure this intrathoracic impedance distribution.  
Basically, the principle is based on electromagnetic coupling between a coil and the thorax in its vicinity as illustrated in Figure 12.  
Figure 12: Principle of magnetic induction monitoring. (**a**) single coil approach with frequency modulation and (**b**) multi-coil approach based on a gradiometer.  
When an alternating current (typically 10-20 MHz) flows through the air coils, this current induces a primary field affecting the tissue underneath, which results in eddy currents flowing on closed pathways perpendicular to the field. The eddy currents are modulated in amplitude, direction and orientation by air and blood volume changes, displacement of organ boundaries, and microscopic processes. As with any current flow, the eddy currents create a secondary field, which carries the information of the modulation. The penetration depth of this active method is estimated to be approximately the diameter of the coil. A brief introduction to the physics of the signal generation can be found in [162].  
Single coil systems typically operate as part of a resonant oscillator circuit. The coil serves as a transmitting as well as a receiving coil and is a frequency-determining part of the oscillatory circuitry. When the coil's reflective impedance changes due to the secondary field affecting the primary one, this will cause a change in the oscillatory frequency. This frequency change then correlates with physiological activity and is utilized as the sensor output signal.  
By contrast, multi-coil approaches feature several coils, typically one transmitter and one or two receiver coils. In multi-coil arrangements, it is necessary to position the receiver coils such that the primary field induction is as small as possible while the secondary field induction should be as large as possible. When only one receiver coil is used, it has to be brought into an orthogonal orientation with the transmitter coil in order to ensure that the primary field does not induce any voltages into the receiver coil. When two receiver coils are used, they are typically connected as gradiometers such that the primary field is (almost) completely compensated. In [163], planar gradiometer arrangements have been proposed, but they have not been integrated into vehicles yet.  
To date, three articles have been published which report on the use of MI monitoring within a car. Figure 13 gives an overview about the different locations where these MI sensors were located.  
In 2011, [PERSON] et al. presented the integration of a single coil MI sensor into the backrest of a vehicle seat [74] and demonstrated its usability for respiration monitoring. While these results were preliminary, this was the first time that the MI technique has been employed inside a vehicle. In 2017, [PERSON] et al. presented the _PhysioBelt_ device [164], which comprised an MI device with a flexible coil sewn into the seat belt. In the same year, [PERSON] et al. [165] reported on progress with gradiometer technology for automotive integration. This new design features a gradiometer approach with one sending and two receiving coils forming a vertical gradiometer (see Figure 14).  
Figure 13: Coil locations of published in-car MI monitoring systems.  
While the former two devices only measured respiration, the latter MI device was also able to record a cardiac signal. While these results are preliminary and have not yet been thoroughly tested in larger studies, they at least demonstrate both ventilation and cardiac contraction can be observed in a MI signal measured in the backrest, with ventilation dominating by one order of magnitude in signal strength.