Dimitroula Matsouka

Spacer fabrics are a versatile textile construction that can be modified and adapted for a wide range of uses. While the behaviour of spacer fabrics in compression has been extensively studied, their time dependant properties such as creep and stress relaxation have not been investigated equally well. In this research project warp-knit spacer fabrics underwent consecutive compression cycles and the resulting stress degradation over time curves were analysed in order to develop a mathematical and mechanical model of the material behaviour. Based on this analysis it was found that the complexity of spacer structure results in the creep behaviour being connected to the creep bending of interconnecting yarns.

The drive for integration of electronic components into textile substrates has been a major aspect of research into production of smart textiles. Electronic components as such, are rigid, inflexible, do not easily follow body movements and can cause hypersensitivity of the skin upon prolonged contact due either to the presence of irritants or the rigid nature of the materials. Another important disadvantage of electronic components, in wearable applications, is that while textiles and textile like materials can withstand the usual cleaning / care treatments, those same treatments can have detrimental effects on the transmission lines and on any exposed electronic parts. This paper investigates a method for the protection of transmission lines made of conductive yarn seams on specimens containing both electronic and textile elements integrated on a textile substrate as well as the protection of the electronic parts. The reliability and durability of the transmission line on the combined specimen is determined by the measurements of the value of an electrical resistance located on a rigid PCB, after repeated wash cycles.

Piezoelectricity was first described by the Curie brothers in the late 1800s. The first materials investigated were natural materials such as bone and wood and single crystals such as quartz. Then in 1946 it was discovered that BaTiO3 ceramic can be made piezoelectric through a poling process. This was followed by the discovery of lead zirconate titanate solid solutions (PZT) in 1954 of very strong lead effects which is still widely used in piezoelectric applications. In 1969, Kawai discovered large piezoelectricity in elongated and poled films of polyvinylidene fluoride (PVDF) opening the way for research into piezoelectric polymers. Piezoelectric polymers exhibit low density and excellent sensitivity and are mechanically tough and respond better to fatigue situations. Since 2010, research has focused on the production of melt-spun piezoelectric textile fibers, with the aim of integrating sensing/energy-harvesting capabilities into smart textile structures. In this chapter, a technological overview of the state-of-the-art research into piezoelectric, melt-spun, textile fibers will be presented. The methods used for the characterization of the fibers will also be discussed with special concentration on the electric response of the fibers after mechanical stimulation.

Wearable electronics have evolved from electronic components secured on the human body with straps and belts to partial electronic components integration onto textile structures. Cables and standard components defeat the purpose of the wearable approach by being bulky, rigid and especially not being able to withstand standard textile cleaning/care methods (washing, dry-cleaning, etc.). New 3D textile structures can provide a promising solution. In this research project, we examined the capacitive behaviour of specially prepared 3D weft knitted textile fabrics. The samples knitted specially for this project incorporated conductive outer layers and an insulating inner layer. The outer layers form the plates of the capacitor and the insulating layer plays a role of the dielectric material between the two plates. The structure of these 3D knits allows for inherent capacitive behaviour of the material. These 3D weft-knitted fabrics can be produced on usual existing knitting machines, without any need of dedicated, specialized or expensive equipment. The expected values of the capacitance, based on theoretical calculations, satisfactorily approach the values derived from the measuring process. The ability to customize the structure and hence the capacitance of the 3D fabrics-based capacitors is a positive point towards the design of the textile-based electronics systems in the future. Therefore, the development of textile capacitors based on the 3D fabrics is expected to be an essential contribution to the integration of the wearable system concept.

Polymers such as polyvinylidene difluoride, polypropylene and polyamide-11 show great promise for providing light-weight, flexible and fibrous piezoelectric materials that can be integrated into technical textile fabric structures for energy harvesting applications. Durability is an important parameter for the textiles and especially for functional and smart materials. This research work provides an insight on the piezoelectric behaviour of polypropylene, polyamide-11 and polyvinylidene difluoride in terms of peak-to-peak voltage generation capabilities after washing at 40°C with the addition of detergent as described in test method BS EN ISO 105-C06:2010. It was observed that the peak-to-peak voltage generated by polypropylene monofilaments retained similar values with only slight differences, while the monofilaments of polyvinylidene difluoride and polyamide-11 showed higher peak-to-peak voltage generation after washing. These changes have been explained using the changes in the crystallinity and phase, as determined by Fourier transform infrared spectroscopy analysis.