# Flexible Wearable Heart Rate Monitoring System and Low-Power Design: A Review
## 2 Overview of Flexible Wearable Heart Rate Monitoring Systems
### Low-Power Design Methods of Flexible Wearable Heart Rate Monitoring Systems  
To extend the battery life of flexible wearable systems, a low-power design is crucial. In this paper, the low-power design of flexible wearable systems primarily targets the heart rate signal acquisition module, heart rate signal preprocessing module, heart rate signal computational module, and heart rate signal output module. The low-power design of each module can be further divided into hardware design, algorithm design, and hardware-algorithm co-design. From a hardware perspective, the hardware power consumption of each module can be significantly reduced by selecting low-power materials and optimizing circuit design. In terms of materials, organic field-effect transistors (OFETs) can be selected. Based on organic semiconductor materials, OFETs can be processed in a solution, and their power consumption is more than 10 times lower than that of traditional silicon devices. For example, [PERSON] al. [58] reported a glucose biosensor based on OFETs, which has the characteristics of low-power consumption and high sensitivity for glucose detection. Carbon nanotube (CNT) materials have high electron mobility and can replace some CMOS devices, reducing dynamic power consumption. Low-power glucose sensing based on CNT materials [59] has been developed and applied in a series of potential applications. A series of piezoelectric materials, such as PVDF and piezoelectric ceramics [60], can also be selected to achieve self-power supply of the system, thus reducing the power consumption of the system. For instance, [PERSON] et al. [61] reported an electronic tattooS sensor based on PVDF, realizing a self-powered and high-precision sensing module. In terms of circuit optimization, power consumption can be further reduced by simplifying or adjusting the circuit structure and selecting appropriate component parameters. For example, [PERSON] et al. [62] optimized the configuration and physical layout of a resistor-capacitor (RC) network and carefully selected resistor and capacitor values to design an active low-pass filter for PPG signals. This design effectively suppresses noise in PPG signals while minimizing power consumption. Additionally, application-specific integrated circuits (ASICs) [63] with customized architectures can significantly reduce system power consumption. In hardware-level optimization for low-power design of flexible wearable heart rate monitoring systems, beyond innovations in systems and circuits, non-ideal factors such as contact resistance and wiring resistance play crucial roles that cannot be ignored. These factors, which are less prominent in rigid electronics, become critical in flexible systems due to their mechanical adaptability requirements, directly affecting energy efficiency and signal integrity. Contact resistance [64] primarily arises at two key interfaces: between flexible electrodes and the skin, and between heterogeneous materials (e.g., rigid chips bonded to flexible substrates). Excessively high contact resistance causes significant signal attenuation, forcing front-end amplifiers to operate at higher gain or bandwidth to maintain the required Signal-to-Noise Ratio. This can increase power consumption by 20-30% [45]. Optimizing electrode materials (e.g., using conductive hydrogels) and the contact area reduces skin-electrode resistance to below 10 k\(\Omega\), while adopting low-resistance interconnects lowers interlayer bonding resistance to <5 \(\Omega\), minimizing unnecessary power loss in signal transmission [65]. Wiring resistance [66] on flexible substrates (e.g., polyimide or PDMS) is another critical factor, influenced by material conductivity, line dimensions, and mechanical deformation. A 100 um-wide silver nanowire trace, for instance, may exhibit 5-10 \(\Omega\)/cm resistance, increasing by 15-20% under 20% tensile strain [67]. High wiring resistance exacerbates IR drops and signal loss, especially for high-frequency heart rate signals, requiring additional power to drive transmission.