A Compact Magnetically Dispersive Surface for Low-Frequency Wireless Power Transfer Applications

We present a compact, magnetically dispersive engineered surface for resonant inductive wireless power transfer (WPT) that significantly enhances the performance of an inductive link, increasing the efficiency and/or working distance. The proposed metasurface measures 6 cm × 6 cm × 0.1 cm, achieving a greatly reduced thickness over a 3-D metamaterial, especially relative to the long wavelength at the chosen operating frequency of 5.77 MHz, which is in the range of typical wireless biomedical implants. Such feature is obtained by virtue of an extremely compact connected double-spiral unit cell, which provides high inductance values despite the small size. We prove through numerical simulations that the proposed metasurface considerably lowers the electric field level with respect to traditional two- or three-coil WPT systems, while maintaining the same magnetic field level at the receiver. In addition, we conducted efficiency measurements on a fabricated prototype, confirming the performance of our design. The excellent efficiency enhancement, combined with the resulting low electric field, makes the proposed metasurface an attractive solution for a number of WPT applications, especially where the exposure in terms of specific absorption rate (SAR) is one of the major concerns, such as in biomedical implants and rechargeable devices.