Wireless Power Transfer for Small Implantable Devices by using Smartphones with Relay Coils

We present a wireless power transfer (WPT) technique for small implantable devices by using smartphones' transmitting coils at 13.56 MHz of near-field communication (NFC). Since the implantable devices and their receiving coils are smaller than typical NFC coils, the smartphones cannot transmit their available power due to the transmitters' output impedance mismatch, and the received power is limited. We propose placing a relay coil in front of the smartphone to adjust the input impedance of the transmitting coil and solve its mismatch. For a $7 \times 7\ \text{mm}$ sized receiving coil at 1 cm distance, a relay coil with a 40 mm diameter and 1 cm gap is chosen optimally by analysis. The measured result shows over 10 times increase of the received power from 8.8 mW to 92.5 mW by using the relay coil. The calculated power transfer efficiencies (PTE) are compared with the estimated values from the measurement and their close agreements verify the analysis.