Wearable Sensing of In-Ear Pressure for Heart Rate Monitoring with a Piezoelectric Sensor

Abstract

In this study, we developed a novel heart rate (HR) monitoring approach in which we measure the pressure variance of the surface of the ear canal. A scissor-shaped apparatus equipped with a piezoelectric film sensor and a hardware circuit module was designed for high wearability and to obtain stable measurement. In the proposed device, the film sensor converts in-ear pulse waves (EPW) into electrical current, and the circuit module enhances the EPW and suppresses noise. A real-time algorithm embedded in the circuit module performs morphological conversions to make the EPW more distinct and knowledge-based rules are used to detect EPW peaks. In a clinical experiment conducted using a reference electrocardiogram (ECG) device, EPW and ECG were concurrently recorded from 58 healthy subjects. The EPW intervals between successive peaks and their corresponding ECG intervals were then compared to each other. Promising results were obtained from the samples, specifically, a sensitivity of 97.25%, positive predictive value of 97.17%, and mean absolute difference of 0.62. Thus, highly accurate HR was obtained from in-ear pressure variance. Consequently, we believe that our proposed approach could be used to monitor vital signs and also utilized in diverse applications in the near future.

Comparisons in heart rate readings between the Bioconnected wireless exercise earpiece and a Polar T31-Coded Chest Strap during a GXT

“…HR measurements between ECG and earpiece HR monitor almost completely overlapped each other (Leboeuf, et al, 2014). In addition, Park et al (2015) identified an earpiece HR monitoring device to be VOLUME 13 | ISSUE 3 | 2018 | 533 significantly accurate across various intensities of exercise. However, several limitations may occur with earpiece devices, including skin irritation, difficulties connecting to additional devices, bulky sensors and hygiene (Chan, Estève, Fourniols, Escriba, & Campo, 2012).…”

“…The absence of bone and predominantly compromising of cartilage and blood suggests the ear is suitable for continuous HR monitoring with photoplethysmography (PPG) (Budidha & Kyriacou, 2014;Park, Jang, Park, & Youm, 2015). The length of the ear canal is roughly 2.5cm, with a diameter of approximately 7mm with exceedingly sensitive skin, (Park et al, 2015), with PPG technology, the earpiece sensors detect how much light is reflected from the subcutaneous blood vessels (Budidha & Kyriacou, 2014). PPG uses optoelectronic features to isolate red and infrared wavelengths and light refraction to dictate blood flow (Schafer & Vagedes, 2013;Spierer, et al, 2015).…”

“…Furthermore, the sensors have been reported to become displaced, that HR measurements can be disrupted via noise and low light levels and for wires to cause distress and pain (Aarts et al, 2013). In relation to the accuracy and convenience of earpiece devices (Leboeuf, et al, 2014;Park, et al, 2015)and limitations identified (Aarts, et al, 2013), Spierer et al (2015) suggests further research to validate earpiece HR monitoring devices using PPG technology, is required.…”

Piezoelectric extraction of ECG signal

“…Various applications have been developed to measure vital signs by means of piezoelectric material45678. A piezoelectric based system has been developed to monitor respiration rate7, heart rate456, and seismocardiography8.…”

“…A piezoelectric based system has been developed to monitor respiration rate7, heart rate456, and seismocardiography8. This paper presents a novel technique for the reconstruction of the ECG corresponding signal from a measured piezoelectric output voltage signal.…”