Characterization of Two-Turns External Loop Antenna With Magnetic Core for Efficient Wireless Powering of Cortical Implants

We present a two-turns loop antenna with a magnetic core to transfer power wirelessly to a miniature 2 \times 2 \times 2\hbox{-mm}^{3} cubic loop antenna in a cortical implant. Compared to a same-sized single-turn loop antenna, it achieves reduction in the specific absorption rate (SAR) and transmits 20 mW (11%) more power to the implant by maintaining the same link power efficiency. We also investigate the optimum distance of the external antenna from the body in terms of the link power efficiency and SAR. Moreover, we present a new method of estimating the worst-case voltage amplitude incident at the front end of the implanted microsystem under different impedance matching conditions. We have fabricated the simple and two-turns loop antennas and attested the link power efficiency of both in a liquid phantom mimicking the dielectric properties of the human skin at 100 MHz.