An Exploration of Behind-the-Ear ECG Signals From a Single Ear Using Inkjet Printed Conformal Tattoo Electrodes

Area of Science:

• Biomedical Engineering
• Wearable Technology
• Physiological Monitoring

Background:

• Wearable sensors behind-the-ear offer unobtrusive, long-term physiological monitoring.
• Previous research explored ear-based electrocardiography (ECG) with external electrodes.
• Hearing aid form factors enable socially acceptable sensor integration.

Purpose of the Study:

• To investigate the feasibility of measuring ECG components from behind a single ear using tattoo electrodes.
• To assess the reliability of behind-the-ear ECG for heart rate monitoring compared to photoplethysmography (PPG).

Main Methods:

• Utilized novel tattoo electrodes placed behind a single ear to record ECG signals.
• Compared behind-the-ear ECG data with a reference photoplethysmography (PPG) device.
• Analyzed the presence of fundamental heart beat frequency and its harmonics in the ECG signals.

Main Results:

• The fundamental heart beat frequency was detected in only 50% of behind-the-ear ECG recordings.
• The second harmonic of the heart beat frequency was consistently present in all recordings.
• Accurate heart rate extraction within a few beats-per-minute was demonstrated using the second harmonic.

Conclusions:

• Behind-the-ear ECG using single-ear tattoo electrodes is a viable principle for heart rate monitoring.
• The second harmonic offers a robust marker for heart rate extraction, despite variability in the fundamental frequency.
• Further signal processing is needed for automated extraction, especially with short data windows.