# Flexible Wearable Heart Rate Monitoring System and Low-Power Design: A Review
## 1 Introduction  
Cardiovascular diseases [1, 2] have become the leading causes of death globally. According to the latest WHO reports, they account for about 32% of all deaths in the world. Moreover, heart rate, being a basic physiological sign, is closely related to cardiovascular health. Therefore, it needs to be monitored in real-time and accurately, and abnormal conditions should be detected promptly [3, 4, 5, 6]. Such continuous monitoring can help find diseases early. Consequently, taking timely actions can greatly improve patients' recovery and the quality of their lives. The systems mentioned in this paper all refer to complete systems with signal input, signal preprocessing, signal computation, and signal output. To break through the scope of clinical medical applications and integrate heart rate monitoring with various application scenarios such as daily home care, researchers have proposed wearable heart rate monitoring systems [7, 8]. A wearable heart rate monitoring system is a portable system integratingtechnologies such as biological sensing [9], signal processing, and data transmission. Its core function is to acquire heart rate physiological signals in real time through attachment or wearing on the human surface and perform analysis and processing. The emergence of this system has enhanced the flexibility and portability of heart rate monitoring and expanded its application scenarios. Wearable heart rate monitoring can be divided into rigid-body systems and flexible systems. A rigid-body wearable heart rate monitoring system [10; 11] refers to a heart rate monitoring system in which the shape of components and the relative positions between internal components remain basically unchanged when the system is in motion or under force. Rigid-body wearable heart rate monitoring systems are widely used in clinical medicine, special industrial environments, and other scenarios due to their strong structural stability and anti-interference capabilities. In contrast, flexible wearable heart rate monitoring systems are widely used in home care, sports health, and other scenarios due to their wearing comfort [12] and lightweight characteristics. Given the huge potential and commercial value of flexible wearable heart rate monitoring systems in home care, sports health, and other scenarios, this paper mainly focuses on the study of flexible wearable heart rate monitoring systems. A flexible wearable heart rate monitoring system refers to a system that uses flexible electronic technology [13; 14] as the core, has bendable and stretchable characteristics, can fit the human surface or clothing, and realizes heart rate monitoring. Within the context of this paper, \"flexible systems\" [15] is defined as systems constructed based on flexible substrates or deformable materials, featuring mechanical adaptability (capable of bending, stretching, or folding) while stably performing electronic functions. Its core characteristics lie in the synergy of material selection, structural design, and performance. From the perspective of technical implementation paths, this paper covers two typical forms of flexible systems, with explicit quantitative criteria for differentiation. Fully flexible systems [16] refer to systems where core components such as sensors, transistors, and circuits are entirely fabricated using organic semiconductors such as polythiophene derivatives, flexible inorganic materials such as ultra-thin silicon films, graphene, or elastic conductive materials [17] such as silver nanowire/PDMS composites. The overall structural thickness is \(\leq\)30 \(\upmu\)m, enabling ultrahigh mechanical flexibility--maintaining stable electrical performance under conditions of a bending radius \(\leq\) 5 mm and tensile strain \(\geq\) 20% [18]. For example, flexible sensors based on organic semiconductors can achieve structures as thin as 2 \(\upmu\)m through molecular-scale thin-film fabrication technology, meeting the conformal characteristics required for skin attachment.