Design and fabrication of flexible wearable sensors

Abstract

This thesis delves into the realm of flexible wearable sensors, aiming to enhance their mechanical and electromechanical properties for optimized health monitoring applications. The journey begins with a meticulous exploration of various sensing mechanisms and materials for wearable sensors, with a significant focus on their role in the context of remote health monitoring, particularly during the COVID-19 pandemic. Emphasising the critical vital signs of respiratory behaviour, body temperature, and blood oxygen levels, the research presents a comprehensive overview of the evolving landscape in wearable sensor technology, highlighting advancements, challenges, and promising opportunities in the field. The investigation further delves into the specific challenges related to piezoelectric nanofiberbased wearable sensors, aiming to amplify their performance for personalized healthcare and human-machine interfaces. This endeavour leads to the proposal of innovative strategies that seek to optimize the piezoelectric properties of nanofiber-based devices through the integration of novel materials and structures, marking a significant step forward in the field of wearable electronics. The thesis presents a new structural design to enhance piezoelectricity of piezoelectric polymers such as polyvinylidene fluoride (PVDF) or poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE). The new design was fabricated by electrospraying zinc oxide (ZnO) or barium titanate (BTO) nanoparticles between the layers of PVDF or PVDF-TrFE nanofibers. The study demonstrates a significant enhancement in the piezoelectric response of the sensors. For example, the voltage output of the sensor showed enhancement of 150% and 1228% in voltage of ZnO+PVDF and BTO+PVDF-TrFE sensors, respectively. The method involves both electrospraying and electrospinning, which enable mass production. This thesis also demonstrates the potential application of these sensors in pressure mapping and movement detection. Furthermore, it presents an innovative design for a stretchable dual-mode sensor capable of measuring both pressure and tensile strain. This sensor consists of a piezoelectric nanofiber mat and electrodes made from conductive polymer nanocomposites, which simultaneously serve as piezoresistive sensors. These piezoelectric nanofiber mats are created by incorporating thermoplastic polyurethane (TPU) into PVDF, endowing them with significant stretchability. The conductive polymer nanocomposite-based electrodes, in conjunction with the piezoelectric nanofiber mats, enable simultaneous measurements of in-plane strain and out-of-plane pressure. The sensor's ability to probe skull deformations during linear acceleration transients has been demonstrated. In conclusion, the thesis not only contributes to the advancement of flexible wearable sensor technology but also addresses critical research questions pertaining to optimal sensing mechanisms, materials integration, and design optimization. By presenting a cohesive narrative that spans literature reviews, experimental investigations, and innovative design approaches, this work sets the stage for future advancements in the field, opening avenues for further exploration and development.