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The efficiency of an on-body wireless power transfer system for implant powering is defined by how the electromagnetic energy interacts with the lossy, heterogeneous, and dispersive body tissues. The objective of this study is to discuss the methodology and evaluate the theoretical bounds for the frequency-dependent electromagnetic energy transfer efficiency. We propose a simplified model that uses a finite tissueequivalent phantom enclosing an implantable receiver surrounded by a medium that represents a transmitter matched to the wave impedance of the body. This model is used to study different cases and evaluate the wireless power transfer efficiency as a function of the operating frequency and implantation depth. The obtained results can be used as a guideline to choose the design parameters and constraints of the on-body power source and gauge its performance against the predicted maximum achievable efficiency.

The Magnetic Resonant Wireless Power Transfer (RWPT) is one of the best suitable techniques for mid-range applications. Therefore, there are different approaches to enhance RWPT efficiency, many of them based on metamaterial devices. In this work, the GRadient-INdex (GRIN) technique is applied to increase the coupling between transmitter and receiver coils and collimate the magnetic field, reducing its dispersion. This proposed lens topology operates at 28 MHz and is formed by a 5x5 planar and periodic arrangement of Split Ring Resonators (SRR) unit cells. In each one, a lumped capacitor is added to reduce the lens's size and radially create a refractive index gradient. Then, a four-coil RWPT system is designed at the same operating frequency. Finally, the coils and lenses are prototyped, and the RWPT efficiency is measured in three cases: without any lenses, with a conventional (uniform) metasurface, and with the proposed GRIN lens. The experimental results demonstrate that the RWPT efficiency, which is about 8% without any lenses, becomes almost four times higher (33%) after the inclusion of metasurface GRIN lenses. When the same measurement is carried out with a uniform lens, typical topology in literature, the obtained efficiency is 17.59%, nearly half of the obtained with the proposed metasurface.

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