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Wireless power transmission (WPT) is a critical technology that provides an alternative for wireless power and communication with implantable medical devices (IMDs). This article provides a study concentrating on popular WPT techniques for IMDs including inductive coupling, microwave, ultrasound, and hybrid wireless power transmission (HWPT) systems. Moreover, an overview of the major works is analyzed with a comparison of the symmetric and asymmetric design elements, operating frequency, distance, efficiency, and harvested power. In general, with respect to the operating frequency, it is concluded that the ultrasound-based and inductive-based WPTs have a low operating frequency of less than 50 MHz, whereas the microwave-based WPT works at a higher frequency. Moreover, it can be seen that most of the implanted receiver’s dimension is less than 30 mm for all the WPT-based methods. Furthermore, the HWPT system has a larger receiver size compared to the other methods used. In terms of efficiency, the maximum power transfer efficiency is conducted via inductive-based WPT at 95%, compared to the achievable frequencies of 78%, 50%, and 17% for microwave-based, ultrasound-based, and hybrid WPT, respectively. In general, the inductive coupling tactic is mostly employed for transmission of energy to neuro-stimulators, and the ultrasonic method is used for deep-seated implants.

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Wireless Power Transfer Approaches for Medical Implants: A Review

We present a compact, magnetically dispersive engineered surface for resonant inductive wireless power transfer (WPT) that significantly enhances the performance of an inductive link, increasing the efficiency and/or working distance. The proposed metasurface measures 6 cm $\times 6$ cm $\times0.1$ cm, achieving a greatly reduced thickness over a 3-D metamaterial, especially relative to the long wavelength at the chosen operating frequency of 5.77 MHz, which is in the range of typical wireless biomedical implants. Such feature is obtained by virtue of an extremely compact connected double-spiral unit cell, which provides high inductance values despite the small size. We prove through numerical simulations that the proposed metasurface considerably lowers the electric field level with respect to traditional two- or three-coil WPT systems, while maintaining the same magnetic field level at the receiver. In addition, we conducted efficiency measurements on a fabricated prototype, confirming the performance of our design. The excellent efficiency enhancement, combined with the resulting low electric field, makes the proposed metasurface an attractive solution for a number of WPT applications, especially where the exposure in terms of specific absorption rate (SAR) is one of the major concerns, such as in biomedical implants and rechargeable devices.

A Compact Magnetically Dispersive Surface for Low-Frequency Wireless Power Transfer Applications

With the recent advancement and progress in the field of wireless power transfer (WPT), there is an ever increasing demand for high power transfer efficiency (PTE) of the WPT systems and improved transfer distance for the end-users. However, some existing WPT systems have limited PTE and transfer distance as they take the inductive coupling approach, where the PTE dramatically decreases as the distance between (Tx) and receiver (Rx) coils increases. Alternatively, magnetic resonance coupling (MRC) is used as a mid-range WPT approach, for which the insertion of metamaterials (MTMs) between Tx and Rx coils is exploited to improve efficiency. MTMs are artificially engineered materials that show uncommon electromagnetic properties, such as evanescent wave amplification and negative refractive characteristics, which could be utilized for the enhancement of PTE. In this article, a comprehensive review on recent progresses in the MTM-based WPT systems is reported, where previously reported MTM-based WPT systems are compared in terms of various parameters such as configurations, operating frequencies, dimensions and PTE. Also, the PTEs of these systems were plotted as a function of the normalized transfer distance. This review is expected to provide an insight for understanding the trends of the MTM-based WPT systems and serve as a reference for researchers who work on WPT systems and their applications.

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Wireless Power Transfer Systems Using Metamaterials: A Review

The state-of-the-art and emerging design approaches of double-tuned RF coils for X-nuclei, brain MR imaging and spectroscopy: A review.

Spiral resonators (SRs) are one of the most common typologies of resonant magnetic unit cell for the realization of metamaterials. The precise knowledge of their lumped electric properties ( RLC parameters) is of crucial importance in the metamaterial design. Thus, an accurate and unambiguous procedure for estimating the value of the RLC lumped parameters of compact SRs is introduced. The proposed procedure relies on a rigorous approach allowing a complete characterization of SRs also in terms of $Q$ -factor. The method is general and valid for other shapes of resonators. The estimations have been finally verified by performing measurements on fabricated SRs through a magnetic probe.

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Accurate Extraction of Equivalent Circuit Parameters of Spiral Resonators for the Design of Metamaterials

In this letter, we develop an accurate and effective analytical framework to arbitrarily manipulate the frequency response of magnetic metasurfaces. In particular, we demonstrate the possibility to homogenize the response of the unit cells constituting a finite slab, avoiding undesired truncation effects and, consequently, performance degradation. In addition, we also show that the control over the current flowing in each array element can be arbitrary, thus allowing exotic properties accomplishment. For instance, we prove that the magnetic field distribution reconfigurability can be easily achieved. We finally demonstrate the reliability of the procedure based on the theoretical circuit model through full-wave simulations performed over meaningful test cases. The possibility to accurately control the response of magnetic metasurfaces can be extremely important for enhancing performance in numerous applications, as for instance wireless power transfer and magnetic resonance imaging.

An Analytical Approach for the Arbitrary Control of Magnetic Metasurfaces Frequency Response

In this article, we introduce a general analytical procedure to unambiguously characterize a metasurface through its lumped circuital equivalent in resonant inductive Wireless Power Transfer (WPT) applications. The proposed model incorporates the finite extent of the slab, as well as the WPT near field operative regime and the presence of the particular driving/receiving coils arrangement, providing quantitative and easy-to-handle parameters which can be manipulated to achieve WPT performance enhancement. We first develop the theoretical background aimed at the lumped parameters extraction, which reveals, for WPT applications, more accurate and robust with respect to the conventional sub-wavelength homogenization theories based on infinite slab extent and impinging plane wave hypotheses. We provide some general guidelines for the design of metasurfaces for WPT performance enhancement based on the derived circuit model; afterwards, we numerically design a test-case consisting of two resonant coils (driver and receiver, respectively) with an interposed passive metasurface to verify the developed theory. Finally, we show some measurements performed on a fabricated prototype, that present an overall excellent agreement with both the lumped model and the numerical simulations.

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An Accurate Equivalent Circuit Model of Metasurface-Based Wireless Power Transfer Systems

In this article, we introduce a novel array-based system for inductive wireless power transfer (WPT) applications and a procedure for its design, which can be interpreted as a generalization of the traditional three-coil systems where the intermediate passive resonant coil (i.e., the transmitter) is replaced with an array of nonresonant concentric loops. We demonstrate that the new WPT array system provides flexibility with respect to the characteristic parameters of the inductive link (i.e., efficiency and gain), thus allowing for their optimal tradeoff depending on the specific WPT application: this is achieved by controlling the current amplitude flowing in each element of the array through the addition of an appropriately selected reactive load. Furthermore, we demonstrate that it is possible to obtain, with the WPT array, focused magnetic field distributions, leading to a potential reduction in the receiver dimensions. Such configuration can be made effectively equivalent to a traditional three-coil system in terms of physical dimensions and performance, through the addition of an appropriate reactance to each loop of the array. To demonstrate the validity and usefulness of the proposed system and design procedure, we first performed extensive numerical simulations of a traditional three-coil system and compared its performance with different array configurations. We then fabricated and experimentally characterized several prototypes, observing an excellent agreement between the measurements and simulations. The innovative features of the proposed design can be extremely useful in many WPT applications requiring a fine optimization in efficiency, gain, and field distribution.

Danilo Brizi

Papers

A Compact Magnetically Dispersive Surface for Low-Frequency Wireless Power Transfer Applications

Accurate Extraction of Equivalent Circuit Parameters of Spiral Resonators for the Design of Metamaterials

An Analytical Approach for the Arbitrary Control of Magnetic Metasurfaces Frequency Response

An Accurate Equivalent Circuit Model of Metasurface-Based Wireless Power Transfer Systems

On the Design of Planar Arrays of Nonresonant Coils for Tunable Wireless Power Transfer Applications