Nanocomposite Single-Electrode Capacitive Sensors for Intelligent and Wearable Cardiovascular Health Monitoring

Abstract

Cardiovascular disease remains one of the leading causes of death worldwide. Progressive conditions such as heart failure and hypertension require continuous, rather than intermittent, monitoring to ensure timely diagnosis and effective management. Wearable sensors have emerged as a promising solution, offering noninvasive, real-time physiological tracking. However, current technologies, which rely on biopotential or pulsation sensing, are often limited by rigid form factors, susceptibility to motion artifacts, and discomfort during prolonged use. Most wearable sensors also target superficial signals at peripheral sites such as the wrist or fingertip, providing limited insight into systemic cardiovascular health. In contrast, deep-tissue signals from central organs offer a comprehensive perspective: blood volume changes in the heart and central aorta reflect whole-body perfusion, while subsurface vessel signals reveal only local circulation. Capturing these deep-tissue signals requires high sensitivity and sufficient penetration depth (λL), highlighting the need for advanced sensing platforms. This dissertation advances flexible, wearable nanocomposite capacitive sensors for deep-tissue cardiovascular monitoring. By enhancing λL and sensitivity, the proposed sensors aim to overcome key limitations of existing systems and improve usability in real-world settings. Chapter 1 reviews wearable sensor technologies for cardiovascular monitoring, including electrocardiography, optical, impedance, capacitive, and pressure sensors, while highlighting persistent challenges in λL and sensitivity. It introduces nanocomposite capacitive sensors as a promising alternative, offering multimodal sensing capability and improved sensitivity for continuous, unobtrusive cardiovascular monitoring. Chapter 2 focuses on enhancing λL through the design and characterization of a novel single-electrode capacitive sensor based on carbon nanotube (CNT) nanocomposites. The roles of surface area and high-aspect-ratio structure are examined, demonstrating a λL of 124 mm in a surrogate tissue model. The sensor demonstrates the ability to detect deep-tissue signals, particularly blood volume, across surrogate, human, and animal models. In an internal hemorrhage protocol, the sensor enables real-time, noninvasive monitoring of cardiac blood volume changes during active bleeding. Chapter 3 focuses on improving pressure sensitivity by incorporating an auxetic structural design into a nanocomposite sensor for detecting cardiac-induced vibrations. The synergistic effect of auxetic deformation and material percolation yields a pressure sensitivity of 2.0 kPa-1 and a detection limit of 0.4 Pa. The sensor detects subtle cardiac motion and leg arterial pulses without a cuff while maintaining high mechanical compliance. Combined with the enhanced λL and machine learning models, the system enables unobtrusive, continuous blood pressure monitoring, validated through human subject testing. Chapter 4 advances quantitative cardiac volume monitoring through a novel electrode geometry designed to improve lateral resolution. A kirigami-patterned central aperture is introduced to confine the electric fields, enhancing sensitivity and lateral resolution for tracking cardiac boundaries. Configured as a wearable sensor array, the system maps cardiac contour and estimates cardiac volume with a mean absolute error of 7.9 mL relative to echocardiography. Collectively, this work demonstrates how nanocomposite materials and structural design strategies can simultaneously improve λL and sensitivity while preserving flexibility. This establishes nanocomposite capacitive sensors as a viable platform for intelligent, continuous cardiovascular health assessment.