*4.2. Mechanoelectric Signals*

' Both ballistocardiography (BCG) and seismocardiography (SCG) have been used to measure cardiac activity based on the heart's displacement, velocity, and accelerations. BCG measures entire body movement due to cardiac ejection, whereas SCG is a local chest measurement that registers cardiac-induced vibrations. Both are measured in terms of acceleration [65]. Since SCG is typically measured on the body, while BCG is measured using non-contact sensors, SCG has been more commonly used in wearable platforms. While the relationship between SCG and cardiac events is still being studied, the literature has estimated the correlation between certain SCG waves and event timing. For example, a low-frequency acceleration wave can be seen at the start of atrial systole [66]. However, the rigid mismatch between current accelerometers and human skin can introduce whole-body inertia measurements into the SCG signal. Skin-compatible SCG sensors have attempted to address this problem. Ha et al. showed a stretchable e-tattoo SCG based on polyvinylidene fluoride (PVDF) [16]. The sensor also showed a great correlation between the systolic time interval and blood-pressure measurements, and it can simultaneously record ECG signals. Other studies have used PVDF for the SCG sensor while incorporating elements essential for long-term monitoring, like wireless charging and communication (Figure 4b,c) [61,67–69]. Wearable phonocardiography (PCG) sensors have also been used to assess the heart. Accelerometer-based mechano-acoustic sensors function similarly to SCG and can pick up auditory frequencies that the human ear cannot hear with a traditional stethoscope. Flexible, wearable stethoscopes based on accelerometry have been developed with flexible substrates and electrodes [67,68]. Kwak et al. also demonstrated a straingauge-based heart-rate sensor that can detect not only the timing of the heart pulsation but also the amplitude and shape of the pulse (Figure 4d) [69].
