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In this work, a dual-polarized wide-angle energy harvester operating at 3.75 GHz is proposed for ambient RF energy harvesting applications. A $4\times 4$ array harvester can efficiently harvest incident RF waves from all polarizations, resulting in high RF-AC efficiency of 94% and final RF-DC efficiency of 34% at 0.02 W/m2 input power density, or −15 dBm input power. This performance is preserved under a wide incident angle up to 65 degrees in TE mode and 75 degrees in TM mode. Owing to its simple power combining configuration, the array can be easily expanded to incorporate more unit cells, increasing the amount of collected power. Not only having the RF-DC conversion function, it is also equipped with a power management circuit to create a usable output voltage level. With this advantage, the harvester is suitable to be employed in real-life applications in which the polarization and angle of incidence are unknown and the incident power is very low.

Wireless power transfer (WPT) presents a safe and robust method for powering biomedical implants. The range and misalignment tolerance restrictions of the WPT system are the main factors, conducive to the transfer efficiency degradation. Especially, the implanted device is invisible and difficult to arrange in alignment with the transmitting coil. This paper proposes a butterfly-shaped transmitting (BS-Tx) coil of a magnetic resonance WPT system for biomedical implants. The BS-Tx coil prototypes with the changing driven current directions, parallel and anti-parallel current directions, are verified and analyzed. Besides, an equivalent circuit model of a coupling two-coil system with an optimal pair of capacitors method for the matching network is mathematically derived to obtain the maximized power transfer efficiency. The BS-Tx coil enhances the efficiency of the system in the distance, angle, and axial misalignment tolerances. A practical WPT system is fabricated and measured to verify the transfer efficiency through the real biological tissue and exhibits a significant efficiency improvement. Finally, a hypothetical context of retinal prosthesis application is demonstrated as a potential application to retinal implants. The proposed system shows a low specific absorption rate, that is satisfied the human safety regulations.