Analysis and Optimization of Four-Coil Planar Magnetically Coupled Printed Spiral Resonators

Fully implanted battery-free high power platform for chronic spinal and muscular functional electrical stimulation

Electrical stimulation of the neuromuscular system holds promise for both scientific and therapeutic biomedical applications. Supplying and maintaining the power necessary to drive stimulation chronically is a fundamental challenge in these applications, especially when high voltages or currents are required. Wireless systems, in which energy is supplied through near field power transfer, could eliminate complications caused by battery packs or external connections, but currently do not provide the harvested power and voltages required for applications such as muscle stimulation. Here, we introduce a passive resonator optimized power transfer design that overcomes these limitations, enabling voltage compliances of ± 20 V and power over 300 mW at device volumes of 0.2 cm2, thereby improving power transfer 500% over previous systems. We show that this improved performance enables multichannel, biphasic, current-controlled operation at clinically relevant voltage and current ranges with digital control and telemetry in freely behaving animals. Preliminary chronic results indicate that implanted devices remain operational over 6 weeks in both intact and spinal cord injured rats and are capable of producing fine control of spinal and muscle stimulation.

Remote terahertz wireless power transfer with self-resonating spoof plasmonic structures

In this paper, we use a pair of self-resonating subwavelength spoof plasmonic structures to achieve remote non-radiative terahertz wireless power transfer, while nearly without affecting the electromagnetic environment of free space around the structure. The resonating frequency and quality factor of the magnetic dipole mode supported by the spoof plasmonic structures can be freely tuned by tailoring the geometric structure. By putting the weak source and detector into the self-resonating structures, we can find that the effective non-radiative terahertz power transferring distance can reach several hundred times the radius of the structures. Finally, we also demonstrate the efficient wireless power transfer capability for the multi-target receiving system. These results may provide a novel approach to the design of non-radiative terahertz wireless power transfer and communications.

A Comprehensive Review of In-Body Biomedical Antennas: Design, Challenges and Applications

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.

Simplified Mutual Inductance Calculation of Planar Spiral Coil for Wireless Power Applications

In this paper, a simplified method for the calculation of a mutual inductance of the planar spiral coil, motivated from the Archimedean spiral, is presented. This method is derived by solving Neumann's integral formula in a cylindrical coordinate system, and a numerical tool is used to determine the value of mutual inductance. This approach can calculate the mutual inductances accurately at various coaxial and non-coaxial distances for different coil geometries. The calculation result is compared with the 3D finite element analyses to verify its accuracy, which shows good consistency. Furthermore, to confirm it experimentally, Litz wire is used to fabricate the sample spiral coils. Finally, the comparison of a simplified method is also studied relative to the coupling coefficient. The accuracy of the calculation results with the simulation and the measurement results makes it a good candidate to apply it in wireless power applications.

Wireless Power Transfer Techniques for Implantable Medical Devices: A Review

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.

Towards a Miniaturized 3D Receiver WPT System for Capsule Endoscopy

The optimization, manufacturing, and performance characterization of a miniaturized 3D receiver (RX)-based wireless power transfer (WPT) system fed by a multi-transmitter (multi-TX) array is presented in this study for applications in capsule endoscopy (CE). The 200 mm outer diameter, 35 μm thick printed spiral TX coils of 2.8 g weight, is manufactured on a flexible substrate to enable bendability and portability of the transmitters by the patients. The 8.9 mm diameter—4.8 mm long, miniaturized 3D RX—includes a 4 mm diameter ferrite road to increase power transfer efficiency (PTE) and is dimensionally compatible for insertion into current endoscopic capsules. The multi-TX is activated using a custom-made high-efficiency dual class-E power amplifier operated in subnominal condition. A resulting link and system PTE of 1% and 0.7%, respectively, inside a phantom tissue is demonstrated for the proposed 3D WPT system. The specific absorption rate (SAR) is simulated using the HFSS^TM software (15.0) at 0.66 W/kg at 1 MHz operation frequency, which is below the IEEE guidelines for tissue safety. The maximum variation in temperature was also measured as 1.9 °C for the typical duration of the capsule’s travel in the gastrointestinal tract to demonstrate the patients’ tissues safety.

Wireless power transfer analysis of circular and spherical coils under misalignment conditions for biomedical implants

In this paper, a 3-coil inductive WPT system having circular shaped external coil and miniaturized spherical shaped implantable coil is designed and optimized at 13.56 MHz to overcome reduced PTE under misalignment conditions. The external coil is placed at a distance of 10 mm from the implantable coil. The proposed design is referred to as C-S WPT system. To evaluate the performance of the C-S WPT system, a 3-coil inductive WPT system having circular shaped external coil and circular shaped implantable coil is designed and is referred to as C-C WPT system. The impacts of perfect alignment, lateral and/or angular misalignments on the overall PTE of the systems are analyzed and compared. At perfect alignment, the PTE of C-C and C-S WPT systems are 59.8% and 52.6%, respectively. Though the PTE of C-C WPT system is slightly higher than C-S WPT system, there is a sharp decrease in PTE of C-C system for angles greater than 40°. At 90° of angular misalignment, the PTE's of C-C and C-S WPT systems are 0.4% and 5.6% respectively. When the coils are laterally displaced by a distance of 10 mm, the C-C and C-S WPT systems produce PTE of 33.4% and 27.1% respectively. The C-C and C-S WPT systems attain PTE of 1.6% and 5.5% respectively at 10 mm displacement and 50° rotation. The simulated systems are fabricated to analyze the result in real-environment. The measurement results show that the C-S WPT system provides better PTE when compared to C-C WPT system under angular and a combination of angular and lateral misalignments.

Evaluation of Specific Absorption Rate in Three-Layered Tissue Model at 13.56 MHz and 40.68 MHz for Inductively Powered Biomedical Implants

This paper presents an optimized 3-coil inductive wireless power transfer (WPT) system at 13.56 MHz and 40.68 MHz to show and compare the specific absorption rate (SAR) effects on human tissue. This work also substantiates the effects of perfect alignment, lateral and/or angular misalignments on the power transfer efficiency (PTE) of the proposed WPT system. Additionally, the impacts of different tissue composition, input power and coil shape on the SAR are analyzed. The distance between the external and implantable coils is 10 mm. The results have been verified through simulations and measurements. The simulated results show that the SAR of the system at 40.68 MHz had crossed the limit designated by the Federal Communications Commission and hence, it is unsafe and causes tissue damage. Measurement results of the system in air medium show that the optimized printed circuit board coils at 13.56 MHz achieved a PTE of 41.7% whereas PTE waned to 18.2% and 15.4% at 10 mm of lateral misalignment and 60° of angular misalignment respectively. The PTE of a combination of 10 mm lateral misalignment and 60° angular misalignment is 21%. To analyze in a real-environment, a boneless pork sample with 10 mm of thickness is placed as a medium between the external and implantable coils. At perfect alignment, the PTE through pork sample is 30.8%. A RF power generator operating at 13.56 MHz provides 1 W input power to the external coil and the power delivered to load through the air and tissue mediums are 347 mW and 266 mW respectively.