Wireless inductive power transmission systems can potentially supply wearable devices. Power cables or batteries can be eliminated by implementing a wireless power transfer system, making the wearable devices less obtrusive to users. However, rigid coils can cause discomfort to users in wearable applications. The novel screen-printed flexible coils on textiles reported here are intended to be a low-cost and comfortable solution when integrated into clothing. A constant-width circular-spiral

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... coil has been designed to minimize the detrimental effect of the low conductivity of the screen-printed flexible conductors on the efficiency of the wireless power transfer system. The coils are printed on 65/35 polyester/cotton textile with a screen-printed Fabink-UV-IF1 interface layer coating. The interface layer provides a relatively flat and smooth surface to prevent the permeation of the conductive paste into the textile and allows the printing of finer-profile coils. A 5 V 1.2 W DC output has been achieved by a wireless power transfer system using the printed flexible coils with Qi standard circuitry; a DC-DC efficiency of 37% has been measured.