# Textile-Based Flexible Coils for Wireless Inductive Power Transmission
## 9 Conclusions  
A constant-width single-layer circular-spiral coil has been designed and fabricated using screen printing on an interface-coated flexible textile. This provides a novel means of producing a flexible coil directly integrated with a host textile as opposed to methods such as the addition of a flexible PCB-based coil to a separate textile by a method such as sewing, the integration of the coil directly with the textile can befit the end-user's comfort. The functionality of this flexible coil for use in a WPT system is examined in this paper. The experimental results from the tests of the flexibility and electrical characteristics of the screen-printed coil have been described and show that the flexible screen-printed coils on 65/35 polyester/cotton textile achieve an acceptable flexibility. This has been achieved by using an interface layer (Fabink-UV-IF1) to facilitate the successful printing of the conductive paste with the desired geometry and resolution and to prevent the spreading and permeation of the conductive paste into the textile. The electrical characteristics of screen-printed coils have been measured and the results have been compared with theoretical calculations of the SERs, capacitance and inductance of the coils and found to agree within a reasonable range of 17%. A 1.51 W DC output has been achieved by a WPT system using Qi wireless power standard compliant driver and receiver circuitry in conjunction with the screen-printed flexible coils.  
The ESRs and parasitic capacitance of the printed coils lead to increased losses in the coils when compared to wound copper coils resulting in the loss of energy in the inductive coupling process and therefore reducing the overall efficiency of the WPT system. A comparison of the DC to DC transfer efficiency of the WPT system with different types of coils has shown the DC to DC efficiency of the printed coils to be limited to 37% compared to a 52% efficiency recorded for the wound copper coils. The separation distance of two coils was found to be the major influencing factor on the DC to DC efficiency compared with the effect of deformation of the flexible coils from a planar form.  
[PERSON], [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON], [PERSON] and [PERSON] analyzed the data; [PERSON] contributed materials and fabrication expertise; [PERSON], [PERSON] and [PERSON] wrote the paper.  
The authors wish to acknowledge the support of the UK Engineering and Physical Sciences Research Council (EPSRC). Part of this work was performed under the SPHERE IRC funded by the UK Engineering and Physical Sciences Research Council (EPSRC), Grant EP/K031910/1. The authors also acknowledge funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no. 260034 for the project titled: 'TIBUCON: Self Powered Wireless Sensor Network for HVAC System Energy Improvement Towards Integral Building Connectivity', which is part of the ICT for Sustainable Growth Research Program. [PERSON] also gratefully acknowledges EPSRC support through his Fellowship 'Energy Harvesting Materials for Smart Fabrics and Interactive Textiles' (EP/I005323/1). The data for this paper can be found at doi.org/10.5258/SOTON/D0401.  
The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.