A Non-contact TMR-Based Active Electrode for BCG Recording

Ballistocardiography (BCG) works as an effective diagnostic tool for various cardiac diseases, such as sleep disorder [1], and heart failure [2] by detecting reactionary forces through the body from heartbeats. Moreover, the combination of ECG and BCG recording can be used for continuous cuffless blood pressure measurement [3]. The comparison between ECG and BCG working scenarios is shown in Fig 1. The conventional ECG recording is based on contact sensing and requires extra wires and ground electrodes (for example, the DRL electrode and lead wires). While BCG recording is based on non-contact sensing, and there is no ground electrode needed. However, conventional BCG recording has its limitations. The foam factor of BCG sensors (the optical fiber or the piezoelectric sensor) is huge because of their limited sensitivity. The BCG sensor needs to cover the entire chest area to measure the mechanical movement and requires post-processing for BCG detection. The bulky sensors are usually integrated into the pillow or the mattress, which makes it difficult to use as wearable devices. Although some work proposed a wearable BCG sensor with MEMS accelerators used in ears [4], the work only achieves the J peak detection due to its low signal-tonoise ratio (SNR). To address the aforementioned issues, this paper presents the first-in-literature TMR-based active electrode for BCG recording. The active electrode consists of two custom-designed chips: the TMR chip (Chip 1) and the AFE chip (Chip 2). When placing the TMR-based active BCG electrode on the chest, the heartbeat causes the ballistic vibration, which leads to the geomagnetic change over the TMR sensor, and the signal fluctuation is amplified and digitized by the AFE chip finally.