The microwave dielectric properties of cotton fabrics in the fabric thickness direction are investigated in relation to the fabric construction, thread count and solid volume fraction (SVF) under the five different relative humidity (RH) conditions. The dielectric constants of the fabric samples exhibits increasing trends with RH, and this is primarily due to more abundant free water in the samples. For most of woven and knitted samples, an increase in thread count results in a higher dielectric constant and this is associated with the increase in SVF. By comparing the woven and knitted samples of the same SVFs, it is found that the woven fabrics tend to have higher dielectric constants than the knitted samples at near 2.45 GHz. This observation of the dielectric properties of cotton fabrics clearly indicates that although the SVF is primarily responsible for the resulting dielectric properties of fabrics, other structural parameters must also be considered in dielectric analysis. Based on in-situ analysis of yarn orientations in the given samples, we claim that the yarn orientation plays an important role in the microwave dielectric properties of cotton fabrics.

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A fabrication and characterization procedure is detailed for a flexible planar antenna integrated into textiles by interfacing thin metal-coated fabric sheets on a polyester fabric substrate. From the full-wave electromagnetic simulations and measurements, it is observed that the low dielectric dissipation in the porous woven polyester enables the fabric antenna to achieve a high gain of 8.4 dBi. It is comparable to other antennas fabricated with engineered substrates of low-loss polymer composites. Using this antenna, the impact of cylindrical concave bending deformation is observed in terms of the impedance matching and radiation performance. The simulated and measured results agree reasonably well. A 1.2% frequency shift is observed when the antenna is bent concavely along its length, while bending along its width showed only a marginal impact. On the other hand, the gain is reduced by as much as 1.0 and 0.5 dB when the antenna is bent along its length and width, respectively. The impact of padding layers was also investigated when placed above the radiating patch and below the ground plane. Because the textile padding layers have complex permittivity closer to air due to their highly porous structure, it is expected to observe only small influence on the radiation performance. However, the simulations and measurements show that padding the radiating patch lowers both the operating frequency and the realized gain by up to 1.6% and by up to 0.9 dB, respectively, due to dielectric loading and dissipation.

Breast hyperthermia is a non-invasive cancer treatment, where breast temperature is mildly elevated by a localized electromagnetic (EM) irradiation to deactivate and damage cancer cells. The emerging needs associated with this medical modality include the development of a highly wearable microwave applicator with a low power requirement to enable a more patient-friendly and continuous hyperthermia therapy. As a potential solution, we propose a textile antenna that consists of a copper-plated woven polyester fabric as a radiating patch and a ground plane and a woven polyester fabric as a dielectric substrate and a padding layer. The porous nature of these textile materials enables construction of a lightweight and flexible antenna with a low dielectric loss for a more comfortable hyperthermia treatment. By incorporating a synthetic breast tissue for a model study, the temperature rises were measured to be 3.3 °C and 1.9 °C at 5 mm and 15 mm depths, respectively, after 15 min of heating (input power of 1 W). This suggests that the textile-based approach could be an effective solution for comfortable and long-term applications of breast hyperthermia therapy.

Deciphering how the dielectric properties of textile materials are orchestrated by their internal components has far-reaching implications. For the development of textile-based electronics, which have gained ever-increasing attention for their uniquely combined features of electronics and traditional fabrics, both performance and form factor are critically dependent on the dielectric properties. The knowledge of the dielectric properties of textile materials is thus crucial in successful design and operation of textile-based electronics. While the dielectric properties of textile materials could be estimated to some extent from the compositional profiles, recent studies have identified various additional factors that have also substantial influence. From the viewpoint of materials characterization, such dependence of the dielectric properties of textile materials have given rise to a new possibility—information on various internal components could be, upon successful correlation, extracted by measuring the dielectric properties. In view of these considerable implications, this invited review paper summarizes various fundamental theories and principles related to the dielectric properties of textile materials. In order to provide an imperative basis for uncovering various factors that intricately influence the dielectric properties of textile materials, the foundations of the dielectrics and polarization mechanisms are first recapitulated, followed by an overview on the concept of homogenization and the dielectric mixture theory. The principal advantages, challenges and opportunities in the analytical approximations of the dielectric properties of textile materials are then discussed based on the findings from the recent literature, and finally a variety of characterization methods suitable for measuring the dielectric properties of textile materials are described. It is among the objectives of this paper to build a practical signpost for scientists and engineers in this rapidly evolving, cross-disciplinary field.