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Midfield wireless power transfer has proved its advantages in power delivery to miniature implantable medical devices located deeply in the human body; however, the delivered power level is still low. Besides, design proposals for a compact source structure concentrating the transmitter field on a miniature implant remain challenges. This letter presents a novel compact transmitting structure (TX) at the midfield band and shows the capability of focusing the magnetic field inside the human tissue. Source performance has been verified by implantable planar inverted-F antenna (PIFA) with a total size of 9 × 13 mm ${^2}$ (RX) that positioned in the heart tissue layer at a 55 mm separated distance of TX and RX. In addition, the power transfer efficiency has been demonstrated by a measurement setup using pork muscle, which obtained a good match of the transmission coefficient between the simulation and experiment results. Consequently, for 1 W of output power from the midfield transmitter, the received energy could be achieved more than 5.6 mW via the implantable antenna at a subwavelength distance.

This letter presents a novel multiband rectenna for the ambient wireless energy harvesting of an autonomous internet of thing (IoT) sensor. A bow-tie antenna with slits is proposed to obtain four frequency bands at 840 MHz, 1.86, 2.1, and 2.45 GHz, respectively. Furthermore, a multiband rectifying circuit combined of four single ring-loop rectifiers is designed with high conversion efficiency. Finally, a power management circuit with storing and converting the harvested voltage is proposed for smoothing dc output voltage. The energy harvester can be used in practical wireless sensor applications.

The free-positioning wireless power transfer (WPT) system has drawn attention in recent years. Traditionally, a WPT system can transfer energy in one or two directions on the same plane, but it leads the restrictions of angle and axis misalignment between a transmitter and a receiver coil. In this paper, we propose a free-positioning WPT system using a three-dimensional cubic-shaped transmitting coil for portable device charging. A small receiving coil is placed inside the transmitter to achieve the transferred energy through the magnetic resonant coupling. In addition, the equivalent circuit and the mutual inductance between the Tx and Rx coils are analyzed. Finally, a practical experiment is implemented to verify the transfer performance, which can reach up to about 50% power transfer efficiency. The proposed system can charge in spatial freedom.

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.

Wireless power transfer presents a robust method to provide a safe and convenient solution for powering biomedical implants. The implanted device is necessarily minimized conducive to the transfer efficiency degradation. This paper presents a wireless power transfer for retinal prosthesis using an artificial intelligent algorithm to optimize the power transfer efficiency with the minimizing implanted device dimension. A novel transmitter is designed based on the conformal strongly coupled magnetic resonator, which is proposed to attach to the glasses lens. An implanted receiver is optimized to obtain the highest efficiency with the minimal dimension of 9.7 x 9.7 x 1.9 mm3. The WPT system achieves approximately 17.5 % at the operating ISM frequency band of 433 MHz and a distance between transmitter and receiver of 20 mm.

Midfield wireless power transfer has been proven as a solution for biomedical micro-implant that can operate at nearly any location in the body. Some previous experiments using an external patterned structure as optimal source demonstrated magnetic and current density focusing characteristic on multilayers tissue, therefore, significantly enhancing efficiency for wireless powering to miniature receivers than conventional inductively coupled coils. This paper proposed a novel compact and simple source transmitter while conserving convergence effect on human tissue.

In this paper, a new metamaterial prototype is designed using a Genetic algorithm (GA) method to improve the gain of the microstrip patch antenna. This metamaterial is Zero-Index Metamaterials (ZIMs), which consists of a 5 × 5 matrix with a dimension of 40 mm × 40 mm × 1.2 mm. The metamaterial is positioned above a prove-fed patch antenna with a resonating frequency of 5.8 GHz. Simulation results show that the gain enhancement of microstrip patch antenna is from 7.6 dBi to 11.4 dBi. The GA method presents a significant performance improvement of the antenna design and optimization.

This paper presents a 5.8 GHz rectenna array for wireless energy harvesting applications. A novel different-mode rectenna array has rectifier circuits integrated with circularly polarized antennas is proposed. A single antenna element has four ports with 90° phased deviation, which produce circular polarization and bias the rectifying diodes for operating as a full-wave rectifier. In simulation results, the maximum RF-to-DC conversion efficiency was achieved approximately 58% at the RF power density of 0.233 W/m2 when the optimal load resistance is 501 Ω.

In recent years, wireless power technology has been promoted for recharging systems for portable devices that are used in everyday life. The conventional technology used for this purpose, which is based on magnetically coupled resonators, has provided promising results but is limited in range and direction at the receiving terminal. In this paper, we propose an omnidirectional wireless power transfer (WPT) system with a novel cubic transmitter to achieve relatively high efficiency. Specifically, a single power source is utilized to drive the current of the transmitter without phase and current control methodology. Energy delivery is transmitted to the receiver through magnetic resonant coupling in the medium-range WPT mode. In addition, an equivalent circuit model of a coupling two-coil system is derived and mathematically analyzed. The efficiency of the proposed omnidirectional WPT system depending on the various distances between the transmitter and the receiver, as well as the transmitter structure, is evaluated via analysis and implementation. Finally, practical experimental results from the resonant coupling system confirm the theoretical analysis of the cubic transmitter and the omnidirectional power transfer capability, which demonstrate approximately 60% power transfer efficiency.

A new approach to designing a broadband and highly efficient class-E power amplifier based on nonlinear shunt capacitance analysis is proposed. The nonlinear shunt capacitance method accurately extracts optimum class-E power amplifier parameters, including an external shunt capacitance and an output impedance, at different frequencies. The dependence of the former parameter on the frequency is considered to select an optimal value of external shunt capacitor. Then, upon determining the latter parameter, an output matching network is optimized to obtain the highest efficiency across the bandwidth of interest. An analytical approach is presented to design the broadband class-E power amplifier of a MOSFET transistor. The proposed method is experimentally verified by a 140–170 MHz class-E power amplifier design with maximum added power efficiency of 82% and output power of 34 dBm.

In this paper, a dual-band high frequency (HF) and ultra-high frequency (UHF) radio-frequency identification (RFID) tag antenna is presented that operates in the 13.56 MHz band as well as in the 920 MHz band. A spiral coil along the edges of the antenna substrate is designed to handle the HF band, and a novel meander open complementary split ring resonator (MOCSRR) dipole antenna is utilized to generate the UHF band. The dual-band antenna is supported by a single-feeding port for mono-chip RFID applications. The antenna is fabricated using an FR4 substrate to verify theoretical and simulation designs, and it has compact dimensions of 80 mm × 40 mm × 0.8 mm. The proposed antenna also has an omnidirectional characteristic with a gain of approximately 1 dBi.

Free-positioning wireless power transfer (WPT) system has been drawn attention in recent years. Traditionally, WPT system can transfer energy in one or two directions on the same plane leading to the restrictions of angle and axis misalignment between a transmitter and receiver coil. In this paper, we propose a wireless power transfer system with a half-rectangular prism (HRP) transmitting coil for free-positioning. A small receiving coil is placed inside the transmitter to achieve the transferred energy through the magnetic resonant coupling. It is shown that the wireless power transfer system can charge in spatial freedom.

This paper proposes a multiple-receiver wireless power transfer (WPT) system using a cubic transmitter. The theoretical analysis of the system is implemented to achieve a general formulation of the power transfer function. The system presents a pair of capacitors consisted of a series and a shunt capacitor to match the coil impedance to the source and load impedance. The desired values of lumped elements are also given to compute the scattering parameter as well as the power transfer efficiency (PTE).