Thermally drawn polymer fibers for advanced sensing fabrics

• Main Author: Wang, Zhe
• Other Authors: Wei Lei
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Functional materials
• Online Access: https://hdl.handle.net/10356/145523
• Institution: Nanyang Technological University

Fabrics are one of the earliest forms of human expression. However, they have not evolved much in the long history of being an essential product in our daily life. Enlightened by the rapid development in functional electronic devices over the past few decades, escalating research interests have been focused on achieving functional fabrics for various applications, for example, detecting vital signs, monitoring environmental change, and transmitting data. As the basic building blocks of fabrics are fibers, the functionalization and scalable fabrication of fibers are highly demanded. Fibers with diverse functions such as sensing, actuating, imaging, energy harvesting, and light emitting are crucial for achieving various functional fabrics and supporting their potential applications. Among all the fiber fabrication methods, a preform-to-fiber thermal drawing technique has been broadly employed for the mass production of longitudinally uniform fibers with an extended length up to tens of kilometers. This technique is applicable to a wide selection of materials, including both structural materials, such as polymers and elastomers, and functional materials such as metals and semiconductors, and even materials in liquid phase, indicating extensive sensing applications. Further, complex inner and outer architectures can be constructed in fibers by the design of the preform or modification of the thermal drawing process. The tremendous possible combinations of materials and architectures in fibers also make them privileged in fabricating wearable triboelectric nanogenerator (TENG), which is frequently used as self-powered touch sensors. In this thesis, thermal and touch sensing fibers and fabrics are developed based on the thermal drawing technique. First, a thermoelectric fiber is fabricated by thermal co-drawing a macroscopic preform containing a semiconducting glass core and a polymer cladding to deliver thermal sensing function at fiber-optic length scales with excellent flexibility and uniformity. The resulting thermoelectric fiber sensor operates in a wide temperature range with high sensitivity and accuracy while offering superior flexibility with the bending curvature radius below 2.5 mm. Additionally, a single thermoelectric fiber can either sense the spot temperature variation or locate the heat/cold spot on the fiber. As a proof of concept, a two-dimensional 3 × 3 fiber array is woven into a fabric to simultaneously detect the temperature distribution and the position of heat/cold source with the spatial resolution of millimeters. The results demonstrate the feasibility of the fabrication of large-area, flexible, and wearable temperature sensing fabrics for wearable electronics and advanced artificial intelligence applications. Second, a two-step soluble-core thermal drawing process is developed. It consists of the traditional thermal drawing and simple post-draw processing to further extend the potential of soft fiber electronics. An ultra-stretchable conductive fiber is achieved, which maintains excellent conductivity even under 1900% strain or freefalling from 1-m height with 1.5 kg load. Moreover, the resulted fiber also acts as a stretchable TENG for self-powered self-adapting multi-dimensional sensing based on the coupling effect of contact electrification and electrostatic induction. A touch sensing net fabric is built from the stretchable fiber and attached to sports gear to monitor sports performance while bearing strong and sudden impacts. This scalable fabrication approach combines with TENG design enables various in-fiber structures, leading to the further promotion of self-powered sensors and large-scale device integration. Last, to further improve the performance of fiber-based TENG devices and extend the applications of thermally drawn fibers, high-resolution designer micro/nanostructures are achieved on fiber surface by introducing direct imprinting in thermal drawing (DITD) technique. A wide variety of regular and irregular micro/nano-scale surface patterns are successfully created on the entire surfaces of hundreds-meter long fibers with different inner structures and materials. Such a thermal imprinting process is simulated and confirmed by experimental measurements to illustrate the feasibility of the DITD technique. Pattern resolution, repeatability, and pattern depth control are further examined, showing a featured size as small as tens of nanometers. To explore the application prospect of the DITD technique, nanopatterns are fabricated on fibers to form plasmonic metasurfaces. Moreover, double-sided patterned fibers are produced to construct wearable TENGs with enhanced performance compared to fibers with flat surfaces. Further, a self-powered wearable touch sensing fabric with 12 sensing nodes is fabricated using the double-sided patterned fibers. The self-powered wearable touch-sensing fabric can precisely locate single and multiple touchpoints on curved surfaces, revealing a bright future of the DITD technique in wearable multifunctional fiber-based electronics and smart fabrics.