Citation: Bujnowski, A.; Osi ´ nski, K.; Przystup, P.; Wtorek, J. Non-Contact Monitoring of ECG in the Home Environment—Selecting Optimal Electrode Configuration. Sensors 2022, 22, 9475. https://doi.org/ 10.3390/s22239475 Academic Editors: Carlos Sánchez and Jan Cornelis Received: 27 October 2022 Accepted: 28 November 2022 Published: 4 December 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sensors Article Non-Contact Monitoring of ECG in the Home Environment—Selecting Optimal Electrode Configuration Adam Bujnowski 1 , Kamil Osi ´ nski 1 , Piotr Przystup 2 and Jerzy Wtorek 1, * 1 Biomedical Engineering Department, Faculty of Electronics Telecommunication and Informatics, Gdansk University of Technology, Narutowicza 11/12, 80-233 Gdansk, Poland 2 Dynamic Precision, ul. Trzy Lipy 3, 80-172 Gdansk, Poland * Correspondence: jerzy.wtorek@pg.edu.pl; Tel.: +48-58-347-13-84 Abstract: Capacitive electrocardiography (cECG) is most often used in wearable or embedded measurement systems. The latter is considered in the paper. An optimal electrocardiographic lead, as an individual feature, was determined based on model studies. It was defined as the possibly highest value of the R-wave amplitude measured on the back of the examined person. The lead configuration was also analyzed in terms of minimizing its susceptibility to creating motion artifacts. It was found that the direction of the optimal lead coincides with the electrical axis of the heart. Moreover, the electrodes should be placed in the areas preserving the greatest voltage and at the same time characterized by the lowest gradient of the potential. Experimental studies were conducted using the developed measurement system on a group of 14 people. The ratio of the R-wave amplitude (as measured on the back and chest, using optimal leads) was less than 1 while the SNR reached at least 20 dB. These parameters allowed for high-quality QRS complex detection with a PPV of 97%. For the “worst” configurations of the leads, the signals measured were practically uninterpretable. Keywords: capacitive ECG; optimal lead configuration; motion artifacts; forward problem 1. Introduction Non-contact heart rate monitoring can be performed in many ways [1–3]. However, two techniques are the most widespread, i.e., the imaging and capacitive ECG methods. The former is based on the detection of changes in skin color caused by the variable blood supply to the tissue. Thus, it is synchronized with the heart activity. The signal can be acquired from images of the face or its fragments [1,4]. The classical approach, with the electrodes applied to the body of the examined person, is widely known as an electrocardiogram. With a non-contact approach, the measurement is performed with the use of electrodes that are not in direct contact with the body of the examined person and are embedded in furniture, e.g., a chair, armchair, bed, bathtub, car seat, or as wearable devices [5–9]. In this case, the electrodes are often described as capacitive. Each mentioned application demands a specific construction of the measurement system [10], e.g., to make the measurement system wearable, the electrodes are typically parts of the clothing. In general, the capacitive electrode for bioelectric potential measurements could be made of a multilayer printed circuit board [11,12] or a few layers of a conductive fabric [13]. The former solution makes it possible to locate the electronic measuring circuit (at least its input part) directly next to the capacitive electrode. Importantly, advanced shielding and processing techniques can also be used [14–17]. The latter solution takes into account the fact of the body’s changing shape at the surface and minimizes the problems of maintaining the value of coupling capacitance as high as possible. The electrode is usually separated from the body surface by some insulating material, e.g., clothes, and also very often by an air gap. Thus, the electrode, air gap, and insulating material together with the body (having relatively high electrical Sensors 2022, 22, 9475. https://doi.org/10.3390/s22239475 https://www.mdpi.com/journal/sensors