Classical ECG monitoring employs well-conducting adhesive electrodes made from silver with a thin silverchloride surface coating being in direct contact to the skin. Such electrodes are coupled to the body surface by well-conducting gels and are typically cable-bound, which prohibits regular use in non-clinical scenarios. Electrochemically, such electrodes are termed non-polarizable. The combination of a difficult to dissolve metal and its metal chloride make these electrodes less sensitive to ion concentration changes and hence their polarization is very stable. The electrode mainly behaves like a resistor.
Plain metal electrodes (like stainless steel) are different in that they represent polarizable dry electrodes. While cheap and simple, their polarization depends on the salt concentrations of the surroundings (e.g., skin) and hence their polarization is not stable. Electrically, such electrodes mainly behave like capacitors.
Low conduction or capacitive electrodes have no direct galvanic contact to the body. Typically, capacitive electrodes are separated from the body by cloth or other textile layers. As the measured ECG signals are at least one order of magnitude smaller than with galvanic electrodes, capacitive electrodes typically have an operational amplifier directly at the electrode. The compensation of common mode by an active driven ground is much more important than in the galvanic case.
3.1. Conductive ECG Monitoring
Placing electrodes on the steering wheel has been one of the early research targets. The available literature dates back at least a decade. In an attempt to quantify driver stress levels, in 2007, Jeong and colleagues [20
] from Yonsei University in Korea investigated driver task load and stress response looking at heart rate variability (HRV) obtained with a steering wheel-based pair of dry ECG electrodes.
Similarly, in the same year, Lee and coworkers from Seoul National University in Korea reported about their study using dry electrodes integrated into the steering wheel [21
]. These tape-type dry electrodes were made of copper tape (3M). Three male students were recruited as test drivers and asked to drive a route of 16 km in about 40–50 min. Heart rate variability was assessed using a modified Tompkins algorithm for R-peak detection and computation of R-peak intervals, heart rate, SDNN, RMSSD and pNN50. While just an abstract without providing any details, Hu and colleagues also reported on the integration of ECG into the steering wheel in 2008 [22
In 2010, Shin et al. [23
] and later Jung et al. [24
] from Pukyong National University in Busan, Korea, presented steering wheel electrodes based on conductive fabric. In parallel, a multisensor system allowing to measure SpO2
, temperature and skin temperature at the steering wheel of a Mercedes S-Class vehicle (DAIMLER AG, Mercedesstraße 137, D-70327 Stuttgart, Germany) was published by Heuer and colleagues [25
] from Karlsruhe Institute of Technology, Germany. Around the same time, D’Angelo and colleagues from the Technical University of Munich demonstrated a multisensor system integrated into the steering wheel of a BMW 730d test vehicle (BMW AG, Petuelring 130, D-80788 München, Germany) [26
] featuring SpO2
and conductive ECG sensing as well. The authors used a design similar to [20
], integrating the electrodes into the steering wheel, the shift lever and the left arm rest. However, the precise electrode position was not revealed in this work.
In 2012, Gomez-Clapers and Casanella [28
] from the Universitat Politecnica de Catalunya in Barcelona, Spain, presented a steering wheel-based ECG-measurement demonstrator which was based on dry ECG electrodes and wireless communication. Similarily, in that year, Silva and colleagues [29
] from IT/IST-UTL in Lisbon, Portugal, demonstrated that dry Ag/AgCl electrodes on a steering wheel can produce ECG measurements of similar quality as traditional electrodes.
Another Korean project from Pukyong National University was presented in 2014. Jung and colleagues [24
] proposed a design featuring a steering wheel with two conductive fabric electrodes, similiar to [29
]. This electronic circuit design was based on an MSP 430 microcontroller and a Chipcon CC2420 RF Transceiver (both manufactured by Texas Instruments Inc., P.O. Box 660199, Dallas, TX, USA) for wireless data transmission at 2.4 GHz.
In 2015, Eßers et al. of Takata corporation (Takata corporation, Tokio, Japan) presented a steering wheel with sensors measuring the heart frequency. The actual integration is not well documented in the publication; it can only be assumed that it is similar to Lee [21
] and Silva [29
gives an overview of different electrode placements.
We note that ECG monitoring using steering wheel-based approaches is a feasible option for heart rate tracking but requires both hands to touch two different conductive parts of the wheel. Hence, significant steering movements with changes of the location of gripping and the quite common habit of steering with one hand pose a problem for this technique.
Hybrid ECG Monitoring
A somewhat different research approach was followed by Matsuda and Makikawa from Ritsumeikan University in Shiga, Japan. In 2008, these authors proposed using a conductive steering wheel and combine it with a capacitive electrode located in the driver seat [32
] (see Figure 3
c). This way, the habit of steering with one hand can be better accomodated. This proposal also combines the concepts of conductive measurements with capacitive sensing, which may justify the name hybrid sensing
One year later, the idea of hybrid ECG sensing
using the steering wheel as one electrode and the seat as the second (capacitive) electrode, was also proposed by Baek and coworkers, see [34
], next to redundant sensing and sensor fusion.
In 2013, the idea of a steering wheel electrode measuring against a capacitve electrode in the seating area was again presented by Xu and Ta [33
] from the Central South University, Hunan, China, possibly without knowledge of Matsuda’s work.
3.2. 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 Richardson [35
]. Early work regarding the integration of capacitive ECG measuring systems into objects of daily life (beds) was performed by Ishijima [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, Park and coworkers from Seoul National University in Korea began to explore integration into other objects of daily life. For example, in 2004 Lim 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, Lim et al. [40
] and also Wu and Zhang [41
] reported about integration of dry and capacitive ECG into beds.
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. 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 Lim et al. [39
In June 2008, capactive ECG monitoring reached the automotive domain, when the first paper proposing capacitive ECG sensing while driving a car was published by Leonhardt and Aleksandrowicz [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° 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 EMFiTM
foil (Emfit Ltd., Konttisentie 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, Chamadiya 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 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, Eilebrecht 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.
= 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, Wartzek 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.
In a joint effort by the Karlsruhe Institute of Technology (KIT) and Daimler (DAIMLER AG, Mercedesstraße 137, D-70327 Stuttgart, Germany), textile integration into the car seat was further demonstrated by Chamadiya et al. in the same year [58
]. A two electrode design featuring two textile electrodes placed horizontally and rather lumbar has been presented which provided decent ECG measurements in pilot trials.
Around that time, Schumm and colleagues from ETH Zurich in Switzerland obtained cECG test equipment from the Philips Chair at RWTH Aachen University. Based on the European SEAT project (EU 6th Framework Program, contract number: AST5-CT-2006-030958), the authors investigated if capacitive ECG measurements would be feasible for passenger monitoring in an airplane seat [47
]. In their 2012 publication, Schumm et al. pointed out that the signal quality of unobtrusive sensing during various passenger activities might not always be sufficient for continuous monitoring [47
]. The authors hence introduced a signal class label that reflects the signal quality and called it the quality label
In addition in 2012, Schneider et al. from the FZI Research Center for Information Technology in Karlsruhe, Germany, developed a framework for vital sign measurements inside a vehicle including a textile seat cover [48
]. The design features two textile electrodes located horizontally in the lumbar region of the back and a driven seat electrode above the two active sensing electrodes. The design was demonstrated in an Audi Q5.
In the same year, Jung and coworkers from Pukyong National University presented another capacitive ECG monitoring system with two horizontal electrodes integrated into a car [49
]. In this design, the active measurement electrodes were again planar copper plates located horizontally in the backrest, while the driven right leg electrode was made of conductive fabrics and located on the seat. In the presented trial, the authors investigated HRV analysis on measurements obtained from n
= 5 subjects without any heart disease.
At the 2012 EXPO in Chicago, IL, USA, Plessey Ltd. (Plessey Semiconductors Ltd., Plymouth PL6 7BQ, UK) announced the availability of their capacitive ECG sensor system EPIC™
. About two years later, the EPIC™
system was tested in an office chair [63
]. In addition, a three-electrode system integrated into the car seat was announced in the media [59
In 2014, Leicht et al. demonstrated that a local release of water vapor can dramatically increase the SNR of capacitive ECG [60
]. As an underlying principle, it was shown that the increase of local air humidity can improve the ohmic coupling (hence the coupling will not be exclusively capacitive anymore). At the same time, this allows static charge difference induced by triboelectric effects to flow to ground. It hence reduces the impact of triboelectricity on capacitive monitoring. Leicht et al. proposed using conductive textile electrodes that are permeable for water vapor and to place chambers with silica gel as a source or sink for humidity behind the textile electrodes, but also other concepts for humidification are conceivable [64
]. Figure 2
h presents the arrangement of the electrodes used for prooving the concept.
In 2015, the RWTH Aachen group presented a multi-electrode design integrated into a Ford S-Max commercial test vehicle (Ford Motor Company, Dearborn, MI, USA). While in this design the electrodes remained solid metal, their shape was changed to round and the contract area was reduced as compared to previous designs. Note that the textile driven ground has now been completely integrated into the seat. In the same year, some of these authors started clinical trials with survivors of heart attacks or heart surgery in the Rosenquelle rehabilitation hospital in Aachen. Results from this study have been published in [55
]. In this trial, patients (n
= 10) were asked to drive in a driving simulator equipped with capacitive ECG in the seat and classical ECG monitoring for reference.
In addition in 2015, the approach of electrode humidification was evaluated by other groups. Fong and Chung [50
] described special electrode designs allowing to release some humidity directly at the spot. Weder et al. [51
] presented a textile breast belt incorporating a water reservoir used to moisten the electrodes. However, while highly integrated, these concepts are open-loop and provide no definition of the acting local humidity.
Hence, in 2017, Leicht et al. from RWTH Aachen University disclosed a closed-loop concept for feedback control of local humidity [53
]. As storage and release of water vapor in silica gel is dependent on the local water vapour pressure, but is also a function of temperature, we proposed to establish closed-loop temperature control as a method to release water vapor from the gel and hence to enhance SNR. The corresponding car seat demonstrator is shown in Figure 4
In 2016, Plessey introduced WARDENTM
, a six-electrode seat cover including a driven ground electrode built into the main electronic unit located at the opposite site of the backrest [52
]. It can be used to easily retrofit car seats for contactless ECG recording and connects to in-vehicle data systems via Bluetooth. In late 2017, a six-electrode design featuring round capacitive electrodes in the backrest was presented to the public by the Belgian research organisation IMEC [54
]. The overall design has a similiar look as the design presented in [60
]. However, in this specific seat, radar sensors were integrated as well, which allows sensor fusion (see Section 9
To simplify any comparison of designs, Table 2
compares specific features of capacitive ECG electrode placements for car seats as reported in the literature to date.