# Knitted Microwave Transmission Line for Wearable Electronics
## 1 Introduction  
Wearable systems are made of electronic devices designed to be worn on the body, often integrated into clothing or accessories. These systems leverage advancements in technology to provide a wide range of functionalities, enhancing various aspects of daily life [1]. Wearable systems can be used for health monitoring, tracking vital signs, detecting anomalies, and providing early warnings for potential health issues [2]. They can also be used for fitness and activity tracking, measuring steps, heart rate, sleep patterns, and calorie consumption to support fitness goals [3]. Additionally, wearable systems can be employed for environmental sensing, monitoring air quality, noise levels, and other environmental factors to assess exposure and inform decision-making [4]. Furthermore, wearable systems can enhance personal safety by detecting falls, emergencies, or threats, and providing immediate assistance or alerting designated contacts [5]. They can also offer augmented reality experiences, enhancing the physical world with digital information and providing immersive interfaces [6]. Wearable systems can be used for data transmission, collecting and transmitting data for analysis, research, or communication purposes [7; 8; 9; 10; 11]. Finally, they can provide entertainment and gaming experiences, offering interactive content and social connectivity [12]. The versatility of wearable systems enables them to cater to diverse needs and preferences, making them a growing and influential segment of the technology landscape. Wearable electronics benefits from technology development, especially the miniaturization of integrated circuits and improvement of power density available from rechargeable batteries. Since high-frequency transceivers have become a one-chip solution, high-frequency wireless transmission has become available for wearable systems. This, however, requires textile antennas and textile transmission lines to connect them with electronic devices. The development of textile-based microwave transmission lines is essential for the realization of fully integrated wearable electronic systems, as presented in Figure 1. Microwave transmission lines serve as crucial conduits for delivering radio frequency (RF) signals from transceivers to antennas and need to have a fixed impedance (usually 50 \(\Omega\)) and low signal attenuation in high-frequency ranges. In the context of wearable devices, where antennas are often fabricated from textile materials [13; 14; 15; 16], the use of textile transmission lines offers significant advantages. By employing a fully textile-based structure, the entire system becomes inherently flexible and lightweight, ensuring comfort and ease of wearability [17; 15; 16]. This integration of textile components not only enhances the aesthetic appeal of wearable devices, but also facilitates seamless integration with clothing and other textiles, providing a more natural and unobtrusive user experience. Textile transmission lines can be fabricated with different technologies. The embroidery of conductive yarn into dielectric textile substrate was used in [18]; however, the limited precision of the embroidery process affected the performance of the structure. Another paper presents textile lines that were made by sewing electro-conductive strips onto a non-conductive textile substrate for the composition of textile materials [19]. This approach, however, requires a precise positioning of the upper and lower layers relative to each other and their stabilization during the stitching process. Also, screen-printing technology has been applied for textile transmission line fabrication [20]. Using this technology, the minimal feature size depends not only on the paste, the stencil, and the number of printing passes, but also on the textile substrate; it has been shown that wide geometrical tolerances are not suitable for microwave structures.