Wireless power transfer (WPT) systems have become increasingly suitable solutions for the electrical powering of advanced multifunctional micro-electronic devices such as those found in current biomedical implants. The design and implementation of high power transfer efficiency WPT systems are, however, challenging. The size of the WPT system, the separation distance between the outside environment and location of the implanted medical device inside the body, the operating frequency and tissue safety due to power dissipation are key parameters to consider in the design of WPT systems. This article provides a systematic review of the wide range of WPT systems that have been investigated over the last two decades to improve overall system performance. The various strategies implemented to transfer wireless power in implantable medical devices (IMDs) were reviewed, which includes capacitive coupling, inductive coupling, magnetic resonance coupling and, more recently, acoustic and optical powering methods. The strengths and limitations of all these techniques are benchmarked against each other and particular emphasis is placed on comparing the implanted receiver size, the WPT distance, power transfer efficiency and tissue safety presented by the resulting systems. Necessary improvements and trends of each WPT techniques are also indicated per specific IMD.

This paper presents closed form expressions for self-inductance and mutual inductance of circular wire-wound coils (WWCs) used in near-field wireless power transfer systems. The calculation of the radius of the coils, inspired from the Archimedean spiral found in many biological organisms, is used to model the self-inductance of single- and multilayer spiral coils. The value of the mutual inductance is determined by expressing the Taylor expansion of Neumann’s integral for constant current-carrying wires. Formulas for the mutual inductance are also derived for misaligned magnetically coupled coils enabling the rapid but accurate calculation of power transfer efficiency in real-life applications. Self-inductance and mutual inductance values are computed using the 3-D electromagnetic software package Maxwell 3D, and these values demonstrate excellent agreement compared with the proposed models. WWCs of different geometrical configurations have been manufactured to validate experimentally the accuracy of the proposed models. Comparison of analytical and experimental results indicates that the proposed models are capable to accurately predict the self-inductance and mutual coupling rapidly. The proposed modeling paves the way for the time efficient optimization of near-field wireless power transfer links.

This paper describes the design, fabrication, packaging, and performance characterization of a conformal helix antenna created on the outside of a $\text{10\,mm\,}\times \,\text{30\,mm}$ capsule endoscope designed to operate at a carrier frequency of 433 MHz within human tissue. Wireless data transfer was established between the integrated capsule system and an external receiver. The telemetry system was tested within a tissue phantom and in vivo porcine models. Two different types of transmission modes were tested. The first mode, replicating normal operating conditions, used data packets at a steady power level of 0 dBm, while the capsule was being withdrawn at a steady rate from the small intestine. The second mode, replicating the worst-case clinical scenario of capsule retention within the small bowel, sent data with stepwise increasing power levels of –10, 0, 6, and 10 dBm, with the capsule fixed in position. The temperature of the tissue surrounding the external antenna was monitored at all times using thermistors embedded within the capsule shell to observe potential safety issues. The recorded data showed, for both modes of operation, a low error transmission of 10−3 packet error rate and 10−5 bit error rate and no temperature increase of the tissue according to IEEE standards.

In-body biomedical devices (IBBDs) are receiving significant attention in the discovery of solutions to complex medical conditions. Biomedical devices, which can be ingested, injected or implanted in the human body, have made it viable to screen the physiological signs of a patient wirelessly, without regular hospital appointments and routine check-ups, where the antenna is a mandatory element for transferring bio-data from the IBBDs to the external world. However, the design of an in-body antenna is challenging due to the dispersion of the dielectric constant of the tissues and unpredictability of the organ structures of the human body, which can absorb most of the antenna radiation. Therefore, various factors must be considered for an in-body antenna, such as miniaturization, link budget, patient safety, biocompatibility, low power consumption and the ability to work effectively within acceptable medical frequency bands. This paper presents a comprehensive overview of the major facets associated with the design and challenges of in-body antennas. The review comprises surveying the design specifications and implementation methodology, simulation software and testing of in-body biomedical antennas. This work aims to summarize the recent in-body antenna innovations for biomedical applications and indicates the key research challenges.

High-efficiency power transfer at a long distance can be efficiently established using resonance-based wireless techniques. In contrast to the conventional two-coil-based inductive links, this paper presents a magnetically coupled fully planar four-coil printed spiral resonator-based wireless power-transfer system that compensates the adverse effect of low coupling and improves efficiency by using high quality-factor coils. A conformal architecture is adopted to reduce the transmitter and receiver sizes. Both square architecture and circular architectures are analyzed and optimized to provide maximum efficiency at a certain operating distance. Furthermore, their performance is compared on the basis of the power-transfer efficiency and power delivered to the load. Square resonators can produce higher measured power-transfer efficiency (79.8%) than circular resonators (78.43%) when the distance between the transmitter and receiver coils is 10 mm of air medium at a resonant frequency of 13.56 MHz. On the other hand, circular coils can deliver higher power (443.5 mW) to the load than the square coils (396 mW) under the same medium properties. The performance of the proposed structures is investigated by simulation using a three-layer human-tissue medium and by experimentation.

This paper presents the modeling, implementation, and testing of a cross-type receiver (RX) used for wireless power transfer (WPT) into an endoscopic capsule. A 3-D configuration for the receiver is proposed to increase the wireless link power transfer efficiency (WL-PTE) and enhance its robustness to angular variations of the capsule. The diameter and thickness of the flexible transmitter coils are 200 mm and 185 μm, respectively. The diameter of the miniaturized RX is 8 mm, which is appropriate for integration within current commercially available capsules. Experimental results demonstrate a WL-PTE of 1.3% and complete system efficiency of 0.9% inside a suitable tissue phantom of the relative permittivity of 300 at 5 MHz. A maximum variation of only 8.4% of the WL-PTE across 90° orientation of the RX was also measured. The minimum WL-PTE of 1% is the recorder for 7-cm translational displacement inside the phantom. A simulated specific absorption rate of 0.86 W/Kg was calculated, which is well below the limit allowed for medical applications. A maximum temperature variation of 2.2 °C was also measured for an operating duration of 10 h, which is typical of a capsule journey inside the gastrointestinal tract. The proposed system represents a viable proposition for WPT in capsule endoscopy.