Design and development of electrically conducting textile sensors for smart textiles and apparel

Abstract

The thesis presents a systematic study of sensing behaviour of the polypyrrole (PPy)-coated textile materials for large strain sensing purposes. This research is aimed at creating a conducting textile with superior sensing performance including low electrical resistance, high strain sensitivity and good environmental stability. It is also aimed to provide better understanding of the strain sensing mechanism and interaction between the PPy coating and textile substrate. The material system, fibre architecture and fabrication techniques have been investigated for producing flexible fabric strain sensors for particular applications. The study covers five related aspects, namely (1) fabrication techniques of the PPy-coated fabrics, (2) sensing performance of the PPy-coated fabrics with various stabilisation treatments, (3) characterisation of the PPy-coated fabrics fabricated under extremely low polymerisation temperature, (4) sensing mechanism of the PPy-coated fabrics and (5) product development using the newly developed flexible fabric strain sensors. Fabrication techniques including (1) screen printing followed by chemical vapour deposition and (2) padding followed by chemical vapour deposition have been further developed in this project. The two fabrication processes and the conventional solution polymerisation technique are thoroughly described and compared. The chemical vapour deposition method can produce thinner, denser and uniformly distributed PPy coating layer on the textile substrate thereby enhancing the sensing performance. Thermal annealing by vacuum drying and vacuum heat treatment plus large sized dopant application are the stabilisation treatments investigated for enhancing the environmental stability of the PPy-coated fabrics. Various characterisation techniques have been utilised to investigate the properties of PPy coating layer treated with different stabilisation methods. The study of the PPy-coated fabrics and pure PPy powders with different stabilisation treatments reveals that all the treatment methods can improve thermal stability, PPy chain ordering, surface smoothness and fabric hydrophobicity, resulting in enhancing the environmental stability as well as strain sensitivity. The most promising fabrication process with the above mentioned stabilisation treatments has been identified for producing high sensing capacity. Electrically conductive fabrics prepared by the chemical vapour deposition process under extremely low polymerisation temperature (<-20C) exhibit excellent strain sensitivity factor of over 400 at a strain of 50%. Particle size analyser, dynamic force mode and surface potential modes of scanning probe microscopy have been employed to characterise the properties of PPy coating on textile substrates. It is found that low temperature polymerisation produces smaller particle size of PPy with thinner, denser and smoother PPy coating on textile substrate which is beneficial to the sensing performance. The present research discovers that the excellent strain sensing behaviour of the PPy-coated Tactel/Lycra knitted fabrics are mainly attributed to the high performance of the PPy-coated PU yarn as well as the excellent property of the knitted fabric structure. In-situ scanning electron microscopy observation demonstrates the crack-opening and crack-closing mechanism on the fibre surface. Electrical resistance of PPy-coated PU fibres with different pre-extension levels has been investigated in order to simulate the actual situation of PU fibres inside the knitted fabric substrate. The deformation mechanism of weft knitted fabric structure has also been studied. The newly developed fabric strain sensors with excellent sensing performance are potentially applicable to many practical fields such as health and well being, wearable bio-medical monitoring, rehabilitation, toy and entertainment etc. This thesis demonstrates a prototype of instrumental dancing garment using the fabric sensor as a soft switch to control the instrumental music during dancing.