Advanced Cardiovascular Wearables Through Dispersion-Enhanced Negative Triboelectrification in Lignin-Skin-Integrated Triboelectric Sensors

Cardiovascular disease is the leading cause of death globally, with an annual mortality rate exceeding 1.04 million in the U.S. alone. Continuous and real-time cardiovascular monitoring is critical for early detection and intervention. However, existing electrocardiography (ECG) and photoplethysmography (PPG) sensors often struggle with real-world challenges such as motion artifacts, signal interference, and skin-condition leading to performance degradation of up to 60%. These issues hinder the effective measurement of critical cardiovascular parameters such as heart rate variability (HRV), especially under several skin conditions. Skin-integrated triboelectric sensors (SITS) offer a promising alternative by converting mechanical energy from subtle skin deformations into electrical signals, allowing for continuous and non-invasive health monitoring. The sensor mechanism integrates skin, utilizing its natural tribopositive properties to enhance contact electrification. However, SITS development faces challenges, including finding sustainable materials with high triboelectric performance and maintaining durability under varied physiological conditions. To address these limitations, we explore lignin, a naturally abundant biopolymer, due to its high tribonegativity. Despite its promising characteristics, lignin has faced challenges related to poor dispersibility, which affect its triboelectric performance. Traditional solvents and processing techniques result in molecular aggregation, limiting its effectiveness as a triboelectric material. Overcoming these challenges required advances in dispersion engineering to unlock lignin full electronegative potential for skin-integrated sensors.
Here, we developed a tribonegativity-engineered wireless Lignin-Skin-Integrated Triboelectric Sensor (w-LiSITS) that monitors cardiovascular physiological parameters with high precision under several skin conditions. We achieved an aqueous lignin dispersibility of 100 times higher than previously reported using Hansen solubility parameters. This enables the scalable production of ultrathin (down to 1 µm) lignin films. During processing, the tribonegativity is preserved through guaiacyl and syringyl structures of lignin, resulting in a surface potential of 5.5 eV and increasing triboelectric performance by a factor of 10 compared to other biopolymers. Source-specific molecular selection allowed us to demonstrate the highest electronegative performance for a biopolymer in skin-integrated sensors. In addition, we also increased the hydrophobicity, resulting in a contact angle of 89°, maintaining over 90% performance in sweat conditions. Moreover, we proved that several types of nanomanufacturing techniques, including thin-film printing and spraying, enabled the production of uniform lignin films for the w-LiSITS. The sensor was evaluated for cognitive-cardiovascular assessment and demonstrated over 95% accuracy in classifying mental workload using principal component analysis of heart rate, HRV, augmented index, and differential volumetric pulse. The device also exhibited reliable heart rate recovery tracking, with high accuracy during monitoring of recovery to baseline within 10 seconds after physical exertion, and continuous heart rate monitoring under various skin conditions with less than 10% variation in signal amplitude for the augmented index, and differential volumetric pulse.
This work demonstrates that w-LiSITS provide a sustainable, high-performance solution for continuous, real-time cardiovascular monitoring, overcoming limitations of ECG and PPG sensors, particularly under challenging conditions like sweat and physical exertion.