# Flexible Wearable Heart Rate Monitoring System and Low-Power Design: A Review
## 3 Design of the Flexible Wearable Heart Rate Signal Acquisition Module
### Low-Power Hardware Design of Flexible Wearable Heart Rate Signal Acquisition Module  
In this subsection, we mainly focus on the hardware low-power design of the heart rate acquisition module. In the acquisition of traditional heart rate signals, different kinds of sensors are often used as hardware. These sensors include bioelectric sensors [76], photoelectric sensors [77], and electromechanical sensors [78]. These rigidly designed sensors pose inconveniences during human body signal collection. Additionally, traditional sensors lack consideration for low-power consumption, resulting in high overall energy usage. As flexible electronics technology has progressed and there is an increasing need for flexible sensors in the field of biomedicine, researchers have made improvements and innovations based on traditional sensors, giving rise to a series of low-power flexible sensors [79], such as those made of piezoelectric materials. For low-power flexible sensors, the rational selection of materials is crucial to achieving low-power consumption. At the same time, measures can also be taken in the hardware layout of sensors, such as adopting multimodal sensing and optimizing hardware design. As reported by [PERSON] et al. [61], as shown in Figure 3a, a heart rate signal acquisition module in a flexible wearable system has been designed; its core component is a low-power sensor based on polyvinylidene fluoride (PVDF) piezoelectric materials. The sensor utilizes 28-micrometer-thick PVDF as the core flexible material. By leveraging the piezoelectric properties of PVDF, the sensor achieves self-powered sensing, directly converting mechanical vibrations into charge signals without the need for an external power supply, thereby reducing energy consumption at the source. Its dual-sensor design suppresses common-mode noise through differential processing of adjacent signals, minimizing power consumption during the post-processing stage. Compared with traditional continuously powered sensors, this self-powered mode and optimized signal processing strategy significantly reduce the overall power consumption. Figure 3: (**a**) Ultra-thin stretchable ECG sensor based on 28-micrometer-thick PVDF material [61]; this image is licensed under CC BY 4.0, copyright 2019, Wiley Online Library. (**b**) Schematic diagram of the structure of an ECG sensor based on graphene materials and electronic tattoo technology [42]; this image is licensed under CC BY 4.0, copyright 2023, Wiley Online Library. (**c**) Schematic architecture framework of the multi-lead ECG acquisition, computation, and transmission system. Reproduced with permission [80], copyright 2022, IEEE. Furthermore, [PERSON] et al. [42] designed a low-power dual-mode signal acquisition sensor module in the flexible wearable heart rate monitoring system by optimizing the hardware layout of the sensors, as shown in Figure 3b. The system adopts a hardware synchronization mechanism and a time-sharing operation strategy, where each ECG sampling triggers SCG sampling through hardware pulse synchronization, thereby avoiding the computational power consumption caused by subsequent algorithm synchronization. In addition, the system uses a high-stability crystal oscillator (32.768 kHz) to unify the clock, reducing the calibration energy consumption caused by independent clock drift. The total power consumption of the system is approximately 3 mW, and compared with traditional ECG sensors, the power consumption is reduced by more than 40%. Meanwhile, [PERSON] et al. [81] also developed a low-power, real-time signal acquisition module in the flexible wearable health monitoring system. This system features a hierarchical power supply circuit design that separately powers low-power and high-power modules. The on/off states of high-power modules are controlled by a boost converter and a CMOS switch array, enabling them to activate only when necessary, thereby minimizing system power consumption.