Electromagnetic field focusing for short-range wireless power transmission

Wireless power transmission enables remotely-powered implantable devices to reduce the risk of wire snapping, and replacement and corrosion of embedded batteries. However, current autonomous implants remain large in scale due to the operation at very low frequency and the use of unwieldy size of antennas. This paper will first review that the optimal frequency is about 2 orders of magnitude higher than the conventional wisdom; and thereby the power receiving coils can be reduced by more than 100 fold without sacrificing either power efficiency or range. To further maximize the received power, using an array of magnetic current sources increases the power transfer efficiency by an additional 10 dB. The optimal current source distribution is analytically solved and the theoretical upper-bound on the efficiency is investigated. The optimal solution reveals that a finite dimensional source is sufficient to approach the theoretical upper-bound. The efficiency at the low GHz-range is 16-dB higher than that at the low MHz-range for typical depth of the implant.