In this comprehensive review, we meticulously delineate the cutting-edge advancements and technological merits of contemporary wearable physical sensors, wearable imaging technologies, and flexible biochemical devices tailored for cardiovascular health. We undertake a rigorous comparison of the strengths and limitations of each sensor, employing diverse metrics pertinent to cardiovascular health assessment. Furthermore, we delve into the evolution of early warning systems and explore the development of closed-loop control systems, offering insights that resonate with the latest trends in this dynamic field. Ultimately, we delineate the existing challenges, delve into potential trajectories, and sketch out the horizons for forthcoming research endeavors.

Pulse wave measurement is a key diagnostic tool for assessing cardiovascular health by analyzing pulse wave propagation ref. . Pulse wave velocity (PWV) is a critical indicator of vascular stiffness ref. , with elevated PWV linked to cardiovascular diseases, atherosclerosis, and other conditions ref. . Regular PWV measurement can aid in early detection of cardiovascular risks and enhance disease prevention. Additionally, Pulse wave analysis (PWA) provides insights into heart rate variability (HRV), reflecting autonomic nervous system function ref. . Higher HRV typically indicates better cardiovascular health, while lower HRV is associated with conditions such as anxiety, depression, and heart disease ref. . In sports medicine, pulse wave monitoring helps optimize training, reduce injury risk, and improve performance ref. . Wearable pulse wave monitoring technologies primarily include optical sensors and pressure sensors.

PPG is a widely used optical technique for biomedical monitoring, typically comprising a light-emitting diode (LED) and a photodetector (PD). The LED emits light (e.g., green, red, or near-infrared) that penetrates the skin and interacts with blood. Changes in arterial blood volume during the cardiac cycle modulate light absorption, and the PD detects the reflected or transmitted signal, converting it into an electrical waveform synchronized with blood flow dynamics. This signal is used to extract physiological parameters such as heart rate, pulse waveform, and oxygen saturation ~. As shown in Fig. 2a, a typical PPG waveform contains a slowly varying direct current (DC) component, related to tissue structure and average blood volume, and a pulsatile alternating current (AC) component, reflecting cardiac-induced blood volume changes. The AC frequency corresponds to heart rate and is superimposed on the DC baseline, which can also vary with respiration ref. . Based on light path, wearable PPG sensors are classified into transmission- and reflection-based configurations. Clinically deployed devices (finger/ear clips) typically use transmission-mode PPG, which provides higher SNR and superior accuracy and repeatability for SpO2 and heart-rate measurements, and remains more reliable under low perfusion in quiet or monitored settings. However, these probes are often wired and conspicuous, limiting subject mobility and making them unsuitable for continuous, all‑day monitoring. By contrast, reflective PPG has somewhat lower SNR and signal fidelity, but this can be partly mitigated by algorithmic optimization and multi‑channel fusion. Reflective sensors can be worn at the wrist, chest or other sites and integrated into watches and wearables, enabling long‑term, wireless, around‑the‑clock monitoring and broadening their prospects for everyday health surveillance.

Transmissive PPG sensors utilize light passing through the skin to provide a clearer representation of blood circulation, offering improved accuracy in cardiac rhythm and oxygen saturation measurement. The light penetration reduces interference from ambient light and surface reflections. However, to achieve deeper tissue penetration, transmissive PPG sensors require light sources with higher power, increasing energy consumption, a major current research direction is to boost detector responsivity (output current per incident optical power) through materials development, thereby reducing overall power consumption. Commercial PPG detectors have largely relied on single-crystal inorganic materials (Si, Ge, GaInAs), which are costly, mechanically rigid, and temperature-sensitive. In contrast, organic semiconductors are cheaper, solution-processable, tunable, and mechanically flexible, attracting attention for integrated photodetection. Huang et al. developed a PD using a novel ultranarrow-bandgap nonfullerene acceptor, achieving over 0.5 A/W responsivity in the NIR region (920-960 nm), the highest reported for organic PDs. These devices also exhibited specific detectivity up to 1012 Jones, comparable to commercial silicon photodiodes. Using this device, a volunteer's heart rate was measured at rest and after exercise. In both conditions, the PPG waveform showed the characteristic systolic and diastolic peaks. Heart rates, computed as 60 divided by the averaged interbeat interval, were 67 and 106 bpm at rest and post-exercise, respectively. (Fig. 2b) ref. .

Reflective PPG sensors emit light from LEDs onto the skin, where it is modulated by capillaries and reflected back to PDs, providing pulse wave data. Unlike transmissive sensors, reflective PPG sensors do not require placement on areas like the ear or finger, making them suitable for use on sites such as the wrist or other skin surfaces for prolonged wear. These sensors are also easier to integrate into commercial wearable devices, such as bracelets and watches ref. . Ultra-thin, flexible PPG sensors are crucial for long-term health monitoring. Optimizing sensor flexibility and thickness improves skin adhesion, reducing interference from ambient light and motion while enhancing signal quality. Yokota et al. demonstrated a highly efficient, ultraflexible three-color polymer light-emitting diode and organic photodetector system, which unobtrusively measured blood oxygen levels when placed on the finger. The total device thickness was just 3 mm, thinner than the epidermis ref. . Most sites suitable for long-term wear are three-dimensionally curved. To maintain comfort, sensors should conform as closely as possible to these surfaces, applying minimal pressure to the skin. Wu et al. developed a patch-type system integrating flexible perovskite PDs and inorganic LEDs for real-time PPG monitoring. The use of three-dimensional (3D) wrinkled-serpentine interconnections improved the device's adaptability to curved surfaces, maintaining excellent functionality even at significant bending angles (60°) (Fig. 2c) ref. . However, reflection-mode PPG is generally less precise than transmission-mode PPG for detecting small microvascular changes. Reflection-mode relies on skin-adjacent diffuse backscatter, making the signal more vulnerable to ambient light and noise. Besides, green illumination common in reflection-mode is more strongly absorbed by melanin, reducing accuracy for darker skin and often requiring extra calibration. The detected light in reflection-mode mainly comes from superficial epidermal/dermal layers, where melanin content strongly modulates absorption and thus the pulsatile amplitude and SNR. By contrast, transmission-mode light traverses thicker tissue to sample deeper vessels, so skin-color effects along the path are averaged and pulsatile interference is smaller, though total intensity can still vary with skin color ref. . Reflective sensors are more suitable for areas like the wrist, forearm, and abdomen, while regions with thinner tissue and higher capillary density, such as the earlobes and fingers, are better for transmissive sensors (Fig. 2d) ref. .

PPG sensors with a single light source are valued for their simple design, cost-effectiveness, and compactness but suffer from limited sensitivity, susceptibility to interference, and shallow data acquisition ref. . To address these limitations, multi-light-source PPG sensors have gained attention for their ability to improve the signal-to-noise ratio, reduce motion artifacts, and capture vascular information at various depths. These sensors facilitate the precise computation of cardiovascular metrics and are essential for measuring oxygen saturation ref. . Oxygen saturation calculation relies on the distinct absorption properties of oxygenated and deoxygenated hemoglobin at different wavelengths, with combined light sources used to exploit these profiles. Multi-light-source PPG sensors enable non-invasive, two-dimensional oxygen saturation mapping, promising advancements in real-time and postoperative monitoring of tissues, wounds, and organs ref. . Khan et al. developed a flexible printed sensor array using organic light-emitting diodes (OLEDs) and photodiodes, capable of detecting reflected light and accurately measuring oxygen saturation, with an average error of 1.1%. This system also generated two-dimensional oxygenation maps of the forearm under ischemic conditions ref. . Traditional multi-source sensors, however, suffer from high power consumption, limiting their use in continuous monitoring. Lee and colleagues addressed this by creating a reflective, ultra-low-power patch-type sensor using red and green OLEDs and organic photodiodes, operating at just 24 milliwatts ref. . Although multi-light-source sensors mitigate noise issues associated with single substrates, signal precision remains affected. To overcome this, Lee's team developed a fiber-optic quantum dot PPG system, enhancing sensitivity and minimizing substrate-related noise ref. . Multi-light-source sensors, by generating richer datasets, create a better foundation for algorithmic processing, especially machine learning. Franklin et al. integrated multiple synchronized PPG sensors into a wireless dermal system, using data to build a support vector machine model for categorizing hemodynamic states influencing blood pressure, cardiac output, and vascular resistance (Fig. 2e) ref. .

PPG sensors are among the most energy-intensive components in wearable devices due to their reliance on LED light sources. Self-powered sensors address this challenge by eliminating the need for frequent recharging, thereby allowing continuous monitoring and enhancing the practical utility of wearable technologies. A promising solution for self-powering PPG sensors involves the integration of solar cells. Jinno et al. developed an ultraflexible, self-powered organic optical system for PPG sensors, combining air-stable polymer light-emitting diodes, organic solar cells, and organic photodetectors (Fig. 2f) ref. . Similarly, Sun et al. introduced a solution-process approach for fabricating wearable self-powered PPG sensors with a three-layer device architecture for organic photovoltaics, photodetectors, and light-emitting diodes. Their device demonstrated comparable performance and enhanced stability compared to a sophisticated reference device with evaporated electrodes. This integration facilitates the development of a self-powered, long-term health monitoring system ref. .

In summary, despite the extensive use of PPG sensors for monitoring physiological signals, such as pulse wave, their signals remain susceptible to various factors, including skin temperature, skin pigmentation, physical activity, and lighting conditions ref. . Furthermore, miniaturization and flexibility of the devices are needed to enable continuous, long-term, comfortable monitoring. Moreover, in specific patient demographics, including individuals with hypotension or compromised blood circulation, PPG sensors are unable to capture physiologically relevant cardiovascular signals effectively. Addressing these challenges is of paramount importance for the advancement of PPG sensors in the future.

Pressure sensors detect pulse waves by measuring the pressure transmitted to the skin surface from the cyclic expansion and contraction of blood vessel walls with each heartbeat. These sensors convert pressure variations into electrical signals, providing extensive cardiovascular information. Unlike PPG sensors that capture signals from capillaries, pressure sensors typically measure pulse waves from larger arteries, offering richer physiological insights. Traditional Chinese Medicine (TCM) practitioners similarly palpate the wrist to assess pulse characteristics, reflecting their analogous function ref. . Although not yet widely adopted in clinical practice, their ability to sustain continuous monitoring under challenging conditions (movement and underwater) endows them with potential for large-scale deployment in the future.

Pressure sensors can be classified based on operational principles, including resistive, capacitive, magnetoelectric, piezoelectric, and triboelectric sensors ref. . Resistive pressure sensors are widely used, relying on changes in the conductive pathway structure within the material due to external pressure. This shift alters the sensor's resistance, enabling pressure inference ref. . To enhance sensitivity, resistive sensors often employ microstructural patterns, such as pyramids and microspheres ref. , improving responsiveness to subtle pressure changes. Lee et al. developed a high-performance electronic skin with interlocked microspheres, achieving excellent conformal adaptability for precise pulse measurements (Fig. 2g) ref. . Variations in the arterial pulse location can complicate sensor placement; to streamline this process, Baek et al. created a method for spatiotemporal measurement of arterial pulse waves using wearable active matrix pressure sensors, fabricated via inkjet printing of thin-film transistor arrays, enhancing diagnostics in cardiovascular diseases (Fig. 2h) ref. .

Capacitive pressure sensors measure pressure through capacitance changes between conductors, offering faster response times than resistive sensors and reduced sensitivity to temperature variations ref. . While both capacitive and piezoresistive sensors share similarities in design, capacitive sensors may introduce non-linearity, necessitating complex data processing for accurate output. Lv et al. developed a self-wrinkling dielectric layer using Multi-Walled Carbon Nanotubes and Polydimethylsiloxane, achieving enhanced sensitivity with a linear range extending up to 21 kPa and a detection limit of 0.2 Pa which covers the 0.5-5 kPa window for extracting skin-surface pulse signals (Fig. 2i) ref. . Yang et al. introduced an innovative fabrication method for a flexible pressure sensor that demonstrates remarkable sensitivity and precision in pulse detection ref. . Ruth et al. improved manufacturing processes with a pyramid microstructured layer, yielding tunable and reproducible sensors for applications like extracorporeal pulse sensing (Fig. 2j) ref. .

Magnetic-effect-based sensors like Hall sensors leverage changes in magnetic fields due to mechanical deformation, distinguishing themselves with high durability and reliability for physiological signal measurement. However, challenges remain in their application for wearable devices, including the mechanical properties of traditional materials ref. . Zhao's group shows that the magnetoelastic effect can be harnessed in flexible fibers to yield a textile magnetoelastic generator that converts arterial pulses into electrical signals. The detection limit reaches as low as 0.05 kPa, well below the 0.5 kPa required for epidermal pulse‑wave sensing, enabling more precise measurement of human pulse signals. The sensor has been demonstrated to operate unencapsulated on sweat-saturated skin and even underwater, a capability that paves the way for all‑weather, long‑term pulse monitoring (Fig. 2k) ref. .

Self-powered pressure sensors, utilizing piezoelectric or triboelectric principles, alleviate energy constraints for continuous monitoring ref. . Chu et al. created a piezoelectric pulse sensing system capable of detecting subtle vibrations in the human radial artery, showcasing precision and stability, akin to TCM diagnostic methods ref. . Su et al. explored piezoelectric sensors based on muscle fiber analogs to enhance robustness and adhesion, suitable for pulse wave measurement and motion monitoring (Fig. 2l) ref. . Despite their promise, piezoelectric sensors face challenges, including thermal sensitivity and limited static pressure measurement capabilities ref. . Triboelectric sensors operate on the contact electrification effect, offering flexibility and affordability for diverse applications in biomedicine and sports ref. . Motion artifacts is a common challenge in pulse testing. While clinically, patients typically remain stationary in bed, making this issue negligible, motion by subjects cannot be entirely avoided during wearable, long-term pulse signal monitoring. Therefore, acquisition devices must minimize or suppress motion artifacts. Meng et al. designed a highly sensitive and conformal pressure sensor inspired by kirigami structures to measure pulse waves during movement, exhibiting excellent sensitivity and stability. Moreover, this sensor is integrated into a wireless cardiovascular monitoring system capable of real-time transmission of pulse signals to a smartphone, and it operates normally during motion ref. . Fang et al. developed a lightweight textile triboelectric sensor for continuous pulse monitoring in dynamic environments, achieving high fidelity and accuracy (Fig. 2m) ref. . While triboelectric sensors represent a promising direction for environmentally friendly pressure sensing, they still require advances in stability and linear response to broaden their practical applications ref. .