# Unobtrusive Vital Sign Monitoring in Automotive Environments--A Review
## 3 ECG Monitoring in a Car
### Capacitive ECG Monitoring  
Unlike the classical conductive electrodes, low-contact electrodes do not rely on a galvanic skin contact, but achieve the signal coupling by means of a high-impedance or even capacitive coupling. In the case of capacitive electrodes, the surface is electrically insulated and remains stable in long-term applications. This concept is of course only of theoretical nature, since a purely capacitive electrode would require an infinitively high ohmic resistance--which no real electrode can provide. Thus, all existing capacitive electrodes are in fact low-contact electrodes. Consequently, they should also be named as such. However, in most of the recent literature, they are entitled as capacitive electrodes. For the sake of consistance, we will also use the term capacitive electrode in that synonymous sense, describing an electrode with a high-impedance coupling to the patient. A timeline of the development of capacitive ECG systems is given in Table 1. Capacitive (insulated) electrodes date back more than 50 years and were first described by [PERSON] [35]. Early work regarding the integration of capacitive ECG measuring systems into objects of daily life (beds) was performed by [PERSON] [36]. In his work, the author used conductive dry textile electrodes acting as an underlay or pillow. Proper ECG detection during sleep was reported. However, due to potential direct skin contact, the patient's coupling may not have been exclusively capacitive. After the millenium, [PERSON] and coworkers from Seoul National University in Korea began to explore integration into other objects of daily life. For example, in 2004 [PERSON] et al. demonstrated the integration of capacitive electrodes into a bathtub [37] and also in 2004 a toilet seat was presented [38]. In 2006, this group published the integration of capacitive ECG electrodes into a chair [39]. Later, [PERSON] et al. [40] and also [PERSON] and [PERSON] [41] reported about integration of dry and capacitive ECG into beds. \begin{table}
\begin{tabular}{c c} \hline
1967 & First description of insulated electrodes ([PERSON], [35]) \\
1993 & cECG in objects of daily life ([PERSON], [36]) \\
2004 & cECG in a bathtub and toilet seat ([PERSON], [37], [PERSON] [38]) \\
2006 & Integration of cECG into a chair ([PERSON], [39]) \\
2007 & cECG integrated into beds ([PERSON], [40]) \\
2008 & First integration of cECG into an automotive seat ([PERSON], [42]) \\
2008 & Textile cECG electrodes in an automotive seat ([PERSON], [43]) \\
2011 & Evaluation of cECG during test drives ([PERSON], [44]) \\
2011 & Measurement performance and coverage rate tested during highway and city traffic ([PERSON], [45]) \\
2011 & Triboelectricity and motion artifacts identified as factor for robustness ([PERSON], [46]) \\
2011 & cECG evaluated as a technology for passenger monitoring in airplanes ([PERSON], [47]) \\
2012 & Introduction of a quality label to describe cECG signal quality ([PERSON], [47]) \\
2012 & Textile seat cover for retrofitting seats with cECG systems ([PERSON], [48]) \\
2012 & cECG used for heart rate variability analysis (HRV) ([PERSON], [49]) \\
2012 & First cECG sensor system commercially available: EPIC system by Plessey Inc. \\
2015 & Active humidification further evaluated ([PERSON] [50], [PERSON] [51]) \\
2016 & WARDEN seat cover based system for retrofitting ([PERSON], [52]) \\
2017 & Closed-loop humidification for artifact suppression ([PERSON], [53]) \\
2017 & IMEC sensor system fusing cECG and radar (IMEC, [54]) \\
2018 & Clinical trial to evalute cECG as a diagnostic tool for heart attack survivors ([PERSON], [55]) \\ \hline \end{tabular}
\end{table} 
Table 1: Timeline of the development of low-contact ECG systems. At RWTH Aachen University in Aachen, Germany, the first applications of capacitive ECG electrodes were published in 2006 [56] and 2007 [57]. The so-called _Aachen Smart chair_ is an office chair equipped with two solid copper plate electrodes integrated into the backrest.