Optimal transmit dimension for wireless powering of miniature implants

RF wireless interface enables remotely powered implantable devices. To maximize the received power under the safety constraint, we analytically solved the optimal current source distribution and investigated the theoretical upper-bound of the efficiency obtainable with planary vertical magnetic current sources. The optimal solution reveals that a finite dimensional source is sufficient to approach the theoretical upper-bound. At the low MHz-range, a coil of approximately 2 cm in diameter is adequate. At the low GHz-range, multiple coils each with different phases and magnitudes are required. However, the efficiency at the low GHz-range is 16-dB higher than that at the low MHz-range for typical depth of the implant.