Flexible, stretchable, and wearable strain sensors are becoming a suitable candidate for next-generation electronics due to their enormous potential in various applications, such as healthcare monitoring systems, electronic skins, and soft robotics. Particularly, textile-based strain sensors, which can be used for electronic textiles (e-textiles), have become a highly promising possibility for healthcare and wearable electronics owing to the inherent softness, wearability, lightweight and warmth of textiles. Textile-based strain sensors can be easily integrated into garments to promote the development of next-generation e-textiles. Currently, various types of textile-based strain sensor have been widely explored by using stretchable polymeric textiles (fiber/yarn and fabric) as supporting substrate materials and diverse conductive nanomaterials as strain-sensing materials. This thesis reports the design, fabrication, characterization, mechanism, and application of ultraflexible one-dimensional (1D) polyurethane (PU) yarn-based strain sensors. As the substrate material, PU yarn shows superior stretchability which can be stretched to around 500% strain. For the strain-sensing materials, advanced carbon-type nanomaterials and noble metal nanomaterials are proved to be the excellent strain-sensing materials which can endow strain sensors with high electro-mechanical performance. In this study, one advanced carbon-type nanomaterial-graphene nanosheet (GNS), two noble metal nanomaterials in different forms-gold film (AuF) and silver nanowire (AgNW) were selected. The main contributions of this thesis are summarized as follows:

First, a core-sheath structured strain sensor which was composed of the superelastic core PU yarn, a multilayer strain-sensing sheath GNSs/AuF/GNSs, and a thin wrapping layer polydimethylsiloxane (PDMS) film was prepared. In detail, the multilayer sheath-GNSs/AuF/GNSs was coated on the PU yarn by utilizing layer-by-layer (LbL) assembly method combining with sputtering, and the PDMS layer was wrapped by using a scalable dip-coating approach. The coupling effect among core material, sheath materials, and wrapping material was deeply investigated. The special combination of the PU yarn, the multilayer GNSs/AuF/GNSs, and PDMS wrapping layer enabled the strain sensor to achieve superior electro-mechanical performance, excellent sensitivity, broad strain-sensing range, and good waterproof property simultaneously. Specifically, the yarn strain sensor exhibited high gauge factor (GF: 661.59), wide working range to 75% strain, outstanding stability for around 10000 stretch/release cycles, and superior water resistance.

Second, based on the outstanding electro-mechanical performances, the applications of the GNSs/AuF/GNSs/PU yarn strain sensor were further explored. It could be readily integrated into textiles including medical textile bandage and textile glove to monitor various human motions (phonation, pulse, finger bending, and walking) and effectively control a hand robot, respectively. Thereby showing enormous potential in textile electronics, wearable electronics, and biomedical electronics which used for healthcare-related applications such as human health condition monitoring, preventive healthcare, rehabilitation care, and robotics control-related applications such as controlling a hand robot to hold or catch some objects.

Third, a novel strain sensor which was inspired by the structure of crepe roll was fabricated. The strain sensor was constructed by the superelastic core PU yarn, a crepe roll-structured (CRS) multilayer sheath, and a thin polydopamine (PDA) wrapping layer. In between, the unique CRS multilayer sheath was assembled by a novel integration of different dimensional nanomaterials-1D AgNWs and 2D GNSs. The resultant CRS strain sensor showed significant broadened strain-sensing range from micro-scale to large-scale deformations (0.01%~125% applied strain), superior and stable sensitivity, and excellent waterproofness, providing huge possibilities in applying in multiscale deformative motions detection.

Fourth, owing to the superior performance from micro-scale to large-scale deformations, the 1D CRS strain sensor in practical applications was further investigated. It demonstrated multifunctionality in practical applications, such as motion detection, tactile sensing, and proprioception of its surface-mounted body. Specifically, when it was mounted on a Sprague-Dawleyw (SD) rat skin, the strain sensor could monitor and distinguish the rat’s respiratory motions under anesthesia and post-anesthesia states. Moreover, it could be integrated into tactile sensing wearable fabric for digital display and smart glove for proprioception of multi-joint finger bending, showing considerable potential for the development of next-generation e-textiles, wearable electronics, and biomedical electronics. Finally, it could be coupled with endoscopic robot and then used for proprioception of the antagonistic flexion/extension motions of its flexible continuum body, providing possibility in assisting medical endoscopy such as inspecting inside of patient’s lumen.

In summary, the structure design, fabrication approaches, fundamental electro-mechanical properties, mechanisms, and potential applications of PU yarn-based strain sensors with strain-sensing nanomaterials in different forms were systematically investigated. The fabricated strain sensors demonstrated superb sensitivities, wide and tunable strain-sensing ranges, outstanding stabilities, and diverse applications. This study could shed light on the development of next-generation e-textiles with superior performance and multifunctionality for potential application in healthcare systems.