# Textile-Based Flexible Coils for Wireless Inductive Power Transmission
## 1 Introduction  
Wireless power transmission (WPT) using inductive coupling has been employed in numerous applications [1], in particular where cable-free devices are desired. E-textile technology covers the integration of electronics into clothes to achieve a wearable flexible implementation that is easy to use. Furthermore, the watt-level power supplied by WPT makes it possible to improve the functionality of wearable devices and reduce their size and weight by eliminating power cables or batteries [2] and make the wearable devices less intrusive for users. Most current research involves the use of coils fabricated either by winding copper wire or by using a track fabricated onto a PCB [3; 4]. These rigid inflexible coils are uncomfortable and so less suited to e-textile applications due to their impact on the user. In this paper, screen-printed flexible coils have been designed and fabricated for a WPT system. These flexible coils provide a convenient and comfortable option to integrate a WPT system into clothing which is less invasive and more comfortable for users, by having improved flexibility and breathability over the conventional wire-wound or PCB-based coil fabrication approaches. Screen printing is a well-established, low-cost, and textile-compatible technology for fabrication which can be readily applied to volume production [5; 6]. In this paper, the textile used is 65/35 polyester/cotton which has widespread use in everyday clothing. Conventionally flexible printed electronics for e-textiles are fabricated by printing the conductive paste to form the electronic circuitry onto a substrate material, such as a polyimide film, which is then attached to the host textile by a mechanical fixing method such as sewing, as such the printed electronics are not directly integrated with the actual textile. Printing the conductive paste directly on the textile causes several problems in manufacture andapplication. Firstly, a textured or porous substrate causes a higher effective resistance as conductive paste sinks into the gaps between the textile fibers [7]. Secondly, conductive paste printed directly on textile without a supporting structural material is brittle, and when the textile is deformed the conductor can be fractured, increasing the resistance of conductive tracks, or even causing an open circuit [8]. Finally, the profile of a printed conductive paste is poorly defined when printed onto a rough substrate. These factors lead to unpredictability and variation of the electrical properties which can cause the coils of the WPT system to diverge from their tuned operational frequency or cause the driver circuit to fail. An interface layer is therefore required on the textile to create a smooth surface for the printing of subsequent conductive layers with uniform thickness. The flexible coils used in this work are printed onto interface-coated textile achieved by screen printing to produce novel printed coils that are directly integrated with the textile substrate.  
This paper is organized as follows: in Section 2, the theory of inductive power transmission is discussed to introduce the essential parameters used in determining the behavior of the flexible coils. Section 3 describes the specific design for the flexible coils in this work, based on the essential parameters identified in Section 2. The practical fabrication of the coils is described in detail in Section 4. Section 5 describes the experimental performance evaluation of flexible coils and the WPT system. The essential parameters of coils are detailed in Section 6 and the performance of the WPT system using the printed flexible coils is given in Section 7. Considerations for the safe operation of the WPT system are identified in Section 8. Final conclusions are given in Section 9.