# 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
To further reduce power usage, the system incorporates hardware such as Nordic's low-power microcontroller and Texas Instruments' low-power analog front-end, resulting in a final average power consumption of 16.5 mW for the system. It is important to emphasize that in the process of achieving low-power consumption, attention must be paid to the relationship between material-based self-powered designs and circuit-level optimization designs, and a rational selection must be made based on application scenarios. In terms of long-term reliability, although the self-powered characteristics of piezoelectric materials (PVDF) can reduce power consumption at the source--as demonstrated by the 28-micrometer-thick flexible substrate-based PVDF sensor designed by [PERSON] et al., which achieves self-powered functionality [61]--prolonged mechanical cycling may lead to the degradation of the piezoelectric coefficient, thereby affecting the stability of signal acquisition. In contrast, circuit-level optimizations, such as application-specific integrated circuit (ASIC) integration and resistor-capacitor (RC) network tuning [63], are not affected by material fatigue; however, the increased hardware complexity may introduce additional leakage currents, thus requiring a trade-off between stability and power consumption. In terms of motion scenario and environmental dependence, the output signal strength of self-powered sensors directly depends on the intensity of human motion and is also affected by environmental factors: sweat during strenuous exercise may corrode the electrodes, and high temperatures will accelerate their oxidation. In static scenarios, in addition to possible signal loss due to insufficient energy harvesting, low temperatures will also weaken the conductivity of flexible materials. For example, the dual-mode system developed by [PERSON] et al. [42] needs to activate the auxiliary energy storage module in low-motion states and also start the temperature compensation mechanism in extremely cold environments. Circuit-level low-power strategies such as time-sharing power supply and clock synchronization [82] are less susceptible to environmental interference, but high-frequency vibrations caused by motion may affect timing stability. The parameter drift of passive components caused by extreme temperature and humidity, combined with mechanical stress generated by motion, will further amplify deviations, requiring additional calibration circuits for compensation, which indirectly increases power consumption. In terms of the complexity of integration with other circuit components, flexible self-powered materials need to achieve molecular-level compatibility with various components in the circuit; otherwise, interface delamination is prone to occur, increasing the difficulty of circuit integration processes [61]. Although the modular design of circuit-level optimizations, such as the hierarchical power supply circuit proposed by [PERSON] et al. [81], reduces the threshold for integration with other circuit components, it may still introduce additional contact resistance, thereby partially offsetting the power consumption optimization effect. In summary, the realization of low-power flexible sensors is mainly reflected in the rational selection of materials and the hardware layout of sensors. In terms of sensor material selection, these new sensors can adopt self-powered materials, such as PVDF and piezoelectric ceramics, endowing them with ultra-thin characteristics and facilitating low-power design. In terms of sensor hardware layout, strategies such as multi-sensor collaboration and unique peripheral synchronization are adopted to minimize power consumption in the noise processing stage. Meanwhile, the system power consumption can be significantly reduced by selecting low-power components and optimizing the sensor working circuit and power management circuit. Future designs need to further synergize material innovation with circuit optimization.