Capacitive Chest Strap for Heart Rate Monitoring

Hoffmann, Wagner
Kuo, Matthew
Lowe, Andrew
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Master of Engineering
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Auckland University of Technology

This research focuses on the design and evaluation of a wearable capacitive chest strap for heart rate monitoring. Heart health monitoring is essential for diagnosing and preventing heart conditions. The electrocardiogram (ECG) is a well-established and reliable test that provides vital cardiac rhythm information. Conventionally, silver/silver chloride (Ag/AgCl) gel electrode is used for ECG monitoring. However, gel electrodes can cause skin irritation. In contrast, a capacitive electrode offers an advantage over a gel electrode by enabling comfortable long-term monitoring without the need for adhesives, reducing skin irritation.

The research investigates various factors affecting the detection of capacitive electrocardiogram (cECG) signals. A comprehensive design process of a rigid printed circuit board (PCB) electrode was presented, covering the modelling of key parameters, prototyping, and testing. The impact of using different fabric materials as interface between the body and the electrode during stationary condition measurements was investigated. Additionally, the effect of dynamic force or pressure applied to the electrode faces while measuring ECG from the fi nger was assessed using different materials. Cotton presented the best ECG signal among the fabrics. A fully capacitive system using a capacitive driven right leg (DRL) was implemented, and its results were compared to those obtained with gel DRL and gel passive ground electrodes. During stationary condition testing, R-peaks were detectable, achieving 100% accuracy in R-peak detection, including the fully capacitive case and with all fabrics tested as interface body-electrode.

For testing during body movement, fi ve participants were recruited, and ethics approval was obtained. Participants wore the capacitive chest strap while performing four predefi ned body movements. The results showed the suitability of capacitive electrodes for R-peak detection and heart rate monitoring when compared to gel electrodes, allowing instantaneous HR detection. The worst result for mean and standard deviation for the capacitive electrode was 127.0 ± 20.2 bpm, while for the gel electrode, the same test result was 135.8 ± 3.0 bpm. Despite that, 17 out of 20 test results had a mean HR percent error below 1% between capacitive and gel measurements. In addition, 13 out of the 20 test results had 95% limits of agreement within ± 5 bpm bounds.

The research presented a capacitive electrode system that can be used as a platform for further cECG investigations. The sensor limits were mapped experimentally in different scenarios, and the sensor demonstrated performance on par with gel electrodes during stationary conditions.

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