# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 3 ECG Monitoring in a Car
### Capacitive ECG Monitoring
Beneath the seating area, hidden by the commercial textile cover, a large driven ground electrode is located. It acts as an active reference electrode. The backrest electrodes are coated by a protective black acrylic paint to achieve a purely capacitive coupling. Note that a similar design has been proposed in 2006 by [PERSON] et al. [39]. In June 2008, capacitive ECG monitoring reached the automotive domain, when the first paper proposing capacitive ECG sensing while driving a car was published by [PERSON] and [PERSON] [42]. In this particular design, solid copper plate electrodes coated with a protective acrylic paint were placed on the backrest of a car seat at a 45\({}^{\circ}\) angle, with the aim of improving the signal-to-noise ratio (SNR) by simulating a typical heart axis. A _driven right leg_ electrode (referred to as _driven ground plane_) was also placed in the backrest around the electrodes, as in this design the seat was already carrying an EMF\({}^{\text{1 TM}}\) foil (Emfit Ltd., Kontitsentie 8, FI-40800 Vaajakoski, Finland) for simultaneous BCG measurements. This design can be seen as one of the ancestors from which, over the years, a large variety of seats have evolved. Apart from differences in the the design of the electronics, they differ by the number of electrodes, their size, shape, location and material. Most, but not all, designs employ a driven reference electrode. An overview of several designs and their release date is presented in Figure 4.  
Later in 2008, [PERSON] and colleagues from the Karlsruhe Institute of Technology (KIT) presented a concept integrating textile capacitive electrodes into a W221 Mercedes S-Class seat [43]. In this specific design, the electrodes were placed horizontally and rather lateral as the chosen seat  
Figure 4: Overview of several low-contact ECG electrode arrangements and their year of introduction to the public (red: active electrodes, blue: driven ground electrode). (**a**) SMART seat ([PERSON], 2008) [42]; (**b**) Daimler S-Class ([PERSON], 2008) [43]; (**c**) Ford S-Max ([PERSON], 2011) [44]; (**d**) Daimler car ([PERSON], 2011) [58]; (**e**) Audi Q5 ([PERSON], 2012) [48]; (**f**) Car seat ([PERSON], 2012) [49]; (**g**) EPIC System ([PERSON], 2014) [59]; (**h**) Ford S-Max ([PERSON], 2014) [60]; (**i**) Ford car ([PERSON], 2015) [61]; (**j**) WARDEN ([PERSON], 2017) [52]. already carried several other technical devices (ventilator, massage unit). However, this lateral electrode placement seems suboptimal as it may result in reduced body contact areas and subsequent lower coverage. In 2011, [PERSON] and colleagues from RWTH Aachen University published a multi-electrode design with six rectangular metal plate electrodes integrated into the backrest of a Ford S-Max driver seat [44]. This arrangement has the advantage that, when looking at different electrode pairs, selection of the strongest signal is possible. Implicitly, this allows for adapting to different heart axes and also to the torso size of different drivers. With \(n=7\) test drivers, the practical usability of this design was tested on the Ford proving ground in Lommel (Belgium). Overall, ECG coverage during driving was 85% with three layers of clothing and moved up to 93% when limiting the allowed clothing to two layers [44]. Urban driving was found to be more challenging than smooth driving on a German highway. In specific drivers, the nervous reaction to fear (increased heart rate) and dedicated pathologies like extrasystoles were clearly visible. Using this seat, [PERSON] et al. compared the ECG measurement performance and coverage during Highway and city traffic in \(n=5\) test drivers [45]. Later in that year, the same group [46] described in detail that motion artifacts and especially their effective combination with local as well as global triboelectricity play an important role in capacitive monitoring and to date limit the robustness of this technique.