*4.1. Ultrasonic Signals*

Doppler ultrasound has been used to track changes in arterial diameter, which can be used to track heart rate. However, commercially available ultrasound monitors are handheld and rigid and therefore not suitable for continuous monitoring. The active layer for ultrasonic transducers is most commonly lead zirconate titanate (PZT) or composites of PZT, as it exhibits high piezoelectric properties and high electromechanical properties [62]. A conformal ultrasonic device was suggested by Wang et al. that could withstand strains of up to 60% [22]. The device utilized a piezoelectric pillar that was hybridized with soft, stretchable components. Liu et al. built on this concept by arranging stretchable ultrasound sensors into a two-dimensional array based on row and column electrodes [63]. The design consisted of PZT blocks, serving as the piezoelectric islands, connected with polyimide (PI)

serpentine hinges, allowing for stretchability between the otherwise rigid blocks. Lee et al. showed that a calcium-modified silk could be used as an interface layer between sensor and skin for ultrasound transducer arrays and that it has a similar acoustic impedance to human skin [64]. Hamelmann et al. also introduced an ultrasound array based on PZTs (Figure 4a) [15]. Other materials for stretchable ultrasound substrates include polyethylene naphthalate (PEN), PDMS, acrylic, and PET [60,64,65].

**Figure 4.** (**a**) Fabrication processes (left) and photo of a fabricated ultrasonic transducer (right). (**b**) Photo of a finger-worn SCG sensor. (**c**) Photo of a skin-like SCG sensor with fibers. (**d**) Flexible strain sensor for heartbeat monitoring.
