In this paper, an efficient free-positioning wireless power transfer (WPT) system for charging multiple devices is proposed. For the system, a planar transmitting (Tx) coil that consists of a spiral coil and concentric multiple parallel loops and switchable impedance matching networks (IMNs) at a transmitter are developed. Using the Tx coil, mutual inductance between the Tx coil and the receiving (Rx) spiral coils is made uniform in the effective charging area. The parallel loops are placed in the interior of the Tx coil, and the magnetic field is enhanced and controlled by adjusting the space in each parallel loop. For optimal design of the coil, an analysis method using an equivalent circuit is explained in detail. It is found that the current of one loop of a parallel loop is more than the input current and negative current flows on the other loop, while the total current of each parallel loop is constant. An array of parallel capacitors with a switch as the switchable IMN is used for the maximum power transfer efficiency corresponding to the number of devices. The reflection coefficients of the fabricated WPT system are under -10 dB at 6.78 MHz, regardless of the number of devices and positions. The WPT system has the maximum power transfer efficiencies of 93% for two devices and simultaneously transfers uniform power to up to four devices with high efficiency.

A planar coil of multiple loops connected in series and in parallel is proposed for wireless power transfer (WPT). The current direction in each loop is mixed in forward and in reverse. The proposed coil gives uniform mutual inductance and enables simple impedance matching for free‐positioning WPT.

Wireless power transfer (WPT) is currently sparking more attention towards the application of wireless charging for mobile devices and electric vehicles

Wireless biodegradable electronics offer a transformative approach to transient biomedical applications by combining fully implantable, resorbable architectures with untethered communication and power delivery. These systems address key limitations of conventional implants, including infection risk, foreign body response, and the need for surgical retrieval. As biodegradable implants are designed to disappear after fulfilling their function, wireless operation is essential to avoid permanent components such as transcutaneous wires. Advances in bioresorbable materials have enabled electronic components capable of functioning over clinically relevant timescales before safely degrading in vivo. Wireless communication techniques, including radio frequency telemetry, LC resonators, and ultrasound‐mediated links, enable real‐time data transmission with minimal energy requirements. Complementary power delivery strategies, such as inductive and capacitive coupling, acoustic energy transfer, photovoltaic harvesting, and transient batteries, support autonomous function across diverse anatomical sites. These integrated platforms have demonstrated utility in neural recording and stimulation, pressure monitoring, cardiac rhythm regulation, gastrointestinal leak detection, immune response tracking, and spatiotemporally controlled drug delivery. This review outlines recent advances in wireless biodegradable electronics, spanning materials, system design, and clinical applications, and provides a foundation for future development of transient implants tailored to short‐term therapeutic and diagnostic needs.

An effective calculation method for AC resistance and the inductance of planar rectangular spiral coils is proposed. For the calculation, a volume filament model and its equivalent circuit are provided. By using the method, AC resistance and inductance are calculated accurately in megahertz frequency range by including both skin and proximity effects. The method is verified with experiments and used to optimise rectangular spiral coils for the maximum quality factor ( Q ‐factor) at 6.78 MHz. A small planar rectangular coil is shown to have the maximum Q ‐factor when the width of the innermost coil loop is about a half of that of the outermost coil loop.

An efficient wireless power transfer (WPT) system is proposed using two self-resonant coils with a high-quality factor (Q-factor) over medium distance via an adaptive impedance matching network using ferrite core transformers. An equivalent circuit of the proposed WPT system is presented, and the system is analyzed based on circuit theory. The design and characterization methods for the transformer are also provided. Using the equivalent circuit, the appropriate relation between turn ratio and optimal impedance matching conditions for maximum power transfer efficiency is derived. The optimal impedance matching conditions for maximum power transfer efficiency according to distance are satisfied simply by changing the turn ratio of the transformers. The proposed WPT system maintains effective power transfer efficiency with little Q-factor degradation because of the ferrite core transformer. The proposed system is verified through experiments at 257 kHz. Two WPT systems with coupling efficiencies higher than 50% at 1 m are made. One uses transformers at both Tx and Rx; the other uses a transformer at Tx only while a low-loss coupling coil is applied at Rx. Using the system with transformers at both Tx and Rx, a wireless power transfer of 100 watts (100-watt light bulb) is achieved.