# Textile-Based Flexible Coils for Wireless Inductive Power Transmission
## 4 Fabrication
### Conductive Track Layer  
The conductive paste was printed onto the interface layer and cured to form the planar spiral coil; a low sheet resistivity is required to minimize resistive losses with the coil. Thermally cured silver polymer paste Fabink-TC-AG4002 from Smart Fabric Inks Ltd. (Smart Fabric Inks Ltd., Southampton, UK) [23] was used to print the conductive layer; it has 20 m\(\Omega\)/\(\square\) sheet resistance and high flexibility, the maximum permissible bend radius for the printed conductor is dependent on the number of layers printed and resulting geometry, the ink has been found to be stable once cured and to perform well with repeated bending cycles though is not machine washable at present due to the increased bending during the process. From the datasheet, the recommended curing temperature of Fabink-TC-AG4002 is 110 to 130 \({}^{\circ}\)C, and the recommended curing time is 2 to 10 min, this curing parameters are for the recommended deposit thickness of 25 \(\upmu\)m and a printed area of up to 100 mm\({}^{2}\). After fully curing, the 25 \(\upmu\)m printed thickness of the silver layer shrinks to 16-18 \(\upmu\)m. These recommended curing parameters from the manufacturer are intended for printed conductive layer in applications such as printed resistors. Since the textiles maximum curing temperature is 130 \({}^{\circ}\)C, curing at a higher temperature is not an option. It is investigated in following section that the designed coil, which has a printed area over 100 cm\({}^{2}\), required an increased curing duration to provide a homogeneous cured layer. In the uncured liquid state, the conductive paste contains a solvent which has higher resistivity than the conductive silver particles so consequently the DC resistance of a conductive track decreases as the solvent evaporates throughout the curing process. Therefore, the DC resistance of a track is an indicator of the state of the curing process. The appropriate curing time was identified by identifying the relationship between curing state and DC resistance. The DC resistance of a conductor is used as a standard test to establish when the curing process has been completed and volatile components of the conductive paste have been removed, at which point the conductor's properties should have reached their optimum. For this test a conductive sub-layer was printed with two deposits onto the interface layer or the previously cured conductive layers and then cured at 130 \({}^{\circ}\)C for 5 min. This was then followed by measurement of the DC resistance. The sample was then cured for a further 5 min and the DC resistance measured again. This curing and measuring cycle was repeated until the DC resistance of the coil stabilized indicating that it is fully cured. This process was completed for samples of conductive tracks composed of one, two, four and eight conductive sub-layers on an interface-coated textile for curing durations from 15 to 60 min. All the numbers of sub-layers tested show consistent decreases in the normalized DC resistance for curing times from 10 to 40 min. For curing durations over 40 min they show varying levels of increase in DC resistance. This occurs with all numbers of sub-layers, but the effect is reduced as the total thickness increases and the number of sub-layers increases. The cause of this effect is due to interactions between the interface layer and the conductive layer which occur once the conductive paste is over-cured. Figure 5: SEM image of the interface and conductive layer on textile. The interface layer provides a smooth surface for conductive layer over the textile. Comparative tests for the same conductive layer printed directly onto Kapton without an interface layer show no increase in DC resistance for curing durations over 40 min. The final selected curing process for a 140 mm \(\times\) 140 mm printed coil conductive layer is 40 min at 130 \({}^{\circ}\)C, which minimizes the DC resistance and avoids increases due to over curing.