Wireless charging is a technology of transmitting power through an air gap to electrical devices for the purpose of energy replenishment. The recent progress in wireless charging techniques and development of commercial products have provided a promising alternative way to address the energy bottleneck of conventionally portable battery-powered devices. However, the incorporation of wireless charging into the existing wireless communication systems also brings along a series of challenging issues with regard to implementation, scheduling, and power management. In this paper, we present a comprehensive overview of wireless charging techniques, the developments in technical standards, and their recent advances in network applications. In particular, with regard to network applications, we review the static charger scheduling strategies, mobile charger dispatch strategies and wireless charger deployment strategies. Additionally, we discuss open issues and challenges in implementing wireless charging technologies. Finally, we envision some practical future network applications of wireless charging.

Wireless Charging Technologies: Fundamentals, Standards, and Network Applications

Neural implants have emerged over the last decade as highly effective solutions for the treatment of dysfunctions and disorders of the nervous system. These implants establish a direct, often bidirectional, interface to the nervous system, both sensing neural signals and providing therapeutic treatments. As a result of the technological progress and successful clinical demonstrations, completely implantable solutions have become a reality and are now commercially available for the treatment of various functional disorders. Central to this development is the wireless power transfer (WPT) that has enabled implantable medical devices (IMDs) to function for extended durations in mobile subjects. In this review, we present the theory, link design, and challenges, along with their probable solutions for the traditional near-field resonant inductively coupled WPT, capacitively coupled short-ranged WPT, and more recently developed ultrasonic, mid-field, and far-field coupled WPT technologies for implantable applications. A comparison of various power transfer methods based on their power budgets and WPT range follows. Power requirements of specific implants like cochlear, retinal, cortical, and peripheral are also considered and currently available IMD solutions are discussed. Patient's safety concerns with respect to electrical, biological, physical, electromagnetic interference, and cyber security from an implanted neurotech device are also explored in this review. Finally, we discuss and anticipate future developments that will enhance the capabilities of current-day wirelessly powered implants and make them more efficient and integrable with other electronic components in IMDs.

Wireless Power Transfer Strategies for Implantable Bioelectronics

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Security and privacy issues in implantable medical devices

Traditional power supply cords have become less important because they prevent large-scale utilization and mobility. In addition, the use of batteries as a substitute for power cords is not an optimal solution because batteries have a short lifetime, thereby increasing the cost, weight, and ecological footprint of the hardware implementation. Their recharging or replacement is impractical and incurs operational costs. Recent progress has allowed electromagnetic wave energy to be transferred from power sources (i.e., transmitters) to destinations (i.e., receivers) wirelessly, the so-called wireless power transfer (WPT) technique. New developments in WPT technique motivate new avenues of research in different applications. Recently, WPT has been used in mobile phones, electric vehicles, medical implants, wireless sensor network, unmanned aerial vehicles, and so on. This review highlights up-to-date studies that are specific to near-field WPT, which include the classification, comparison, and potential applications of these techniques in the real world. In addition, limitations and challenges of these techniques are highlighted at the end of the article.

Opportunities and challenges for near-field wireless power transfer: A review

Wireless charging is a technology of transmitting power through an air gap to electrical devices for the purpose of energy replenishment. The recent progress in wireless charging techniques and development of commercial products have provided a promising alternative way to address the energy bottleneck of conventionally portable battery-powered devices. However, the incorporation of wireless charging into the existing wireless communication systems also brings along a series of challenging issues with regard to implementation, scheduling, and power management. In this article, we present a comprehensive overview of wireless charging techniques, the developments in technical standards, and their recent advances in network applications. In particular, with regard to network applications, we review the mobile charger dispatch strategies, static charger scheduling strategies and wireless charger deployment strategies. Additionally, we discuss open issues and challenges in implementing wireless charging technologies. Finally, we envision some practical future network applications of wireless charging.

Implantable biomedical sensors and actuators are highly desired in modern medicine. In many cases, the implant's electrical power source profoundly determines its overall size and performance. The inductively coupled coil pair operating at the radio-frequency (RF) has been the primary method for wirelessly delivering electrical power to implants for the last three decades. Recent designs significantly improve the power delivery efficiency by optimizing the operating frequency, coil size and coil distance. However, RF radiation hazard and tissue absorption are the concerns in the RF wireless power transfer technology (RF-WPTT) [4], [5]. Also, it requires an accurate impedance matching network that is sensitive to operating environments between the receiving coil and the load for efficient power delivery. In this paper, a novel low-frequency wireless power transfer technology (LF-WPTT) using rotating rare-earth permanent magnets is demonstrated. The LF-WPTT is able to deliver 2.967 W power at ~ 180 Hz to an 117.1 Ω resistor over 1 cm distance with 50% overall efficiency. Because of the low operating frequency, RF radiation hazard and tissue absorption are largely avoided, and the power delivery efficiency from the receiving coil to the load is independent of the operating environment. Also, there is little power loss observed in the LF-WPTT when the receiving coil is enclosed by non-magnetic implant-grade stainless steel.

A Low-Frequency Versatile Wireless Power Transfer Technology for Biomedical Implants

Providing electrical power to an implantable microelectronic device (IMD) via wireless power transfer technology is critical to implant's efficacy. The inductive coupling based wireless power transfer technology has been the primary approach to remotely power an IMD [1]. Reducing the coil size and improving efficiency are the two primary design goals for the power harvesting component in an IMD. A pulse-width-modulated (PWM) boost converter has been demonstrated to efficiently convert the low-voltage (much less than the regular diode's turn-on voltage) AC power generated by a miniaturized receiving coil to a desired high-voltage DC power [2]. In [2], the switching frequency of the PWM is much higher than the frequency of the input AC power. The high switching frequency not only dissipates more power during the switching, which deteriorates the converter's overall efficiency, but also prevents the circuit from handling the high-frequency input AC power. In this paper, a low switching frequency AC to DC boost converter is demonstrated. By aligning the PWM signal with the input AC, the boost converter is able to convert the AC power, whose amplitude is 500 mV at the open-circuit condition, to a >5 V DC output. The converter demonstrated in this paper has the potential to significantly improve the conversion efficiency by reducing the dissipated power associated with the PWM switch, especially when the input AC is at higher frequencies.

A low switching frequency AC-DC boost converter for wireless powered miniaturized implants

High-Q inductive coils are essential components in biomedical implants for efficient wireless charging and effective wireless sensing. The planar spiral coil (PSC) that can be easily optimized and reliably fabricated by lithographic tools is a preferred candidate. To support the inductive coupling at MHz range, the size of PSCs used in implants is much larger than those used in wireless communication circuits. Therefore, to achieve high Q, it is imperative to reduce the metal trace's unit-length-resistance. In this paper, multiple parallel-connected metal traces, instead of a conventional single trace, have been employed to reduce the unit-length-resistance by mitigating the skin effect. Although the approach was used to make stranded wires for mega-watts transmission systems, it has been used to design PSCs for the first time. The parallel-trace PSC exhibits 38%&#x2223C;53% improvements in Q when it resonates with a capacitor at &#x2223C;10 MHz. Measurement results also indicate that there is &#x2223C;10% inductance reduction in the parallel-trace PSC compared to the single-trace PSC of the same design. Measurement results also indicate that, in a parallel-trace PSC, the length difference between the parallel-connected, side-by-side traces when they are winded into a coil, and the dielectric environment difference when they are placed in different layers, can be neglected when the operating frequency is less than the PSCs self-resonating frequency. Utilizing widely-available planar fabrication technologies, the parallel-trace PSC can be widely adopted in biomedical implants.

A parallel-trace high-Q planar spiral coil for biomedical implants

The application of biomedical implants is far reaching, encompassing biological monitoring, as well and diagnosis, treatment, and therapeutic treatment applications. Delivering electrical power to biomedical implants wirelessly has many advantages, and is extensively researched at this time. The basis for the current wireless power transfer technology (WPTT) is inductive coupling. Thus, enhancing the inductive coupling efficiency plays an important role to improve the efficacy of WPTT. Planar Spiral Coils (PSCs) are widely used in biomedical implants for inductive coupling because they can be easily optimized and reliably batch-fabricated. In this paper, an innovative and easy-to-implement method is demonstrated to dramatically improve the coupling efficiency of PSCs. By incorporating a ferrite layer under the PSC, its induced open-circuit voltage is improved by up to 48% in a low frequency rotating-magnet based WPTT system. At radio frequencies, the mutual inductance between two PSCs using a layer of ferrite material is improved by 50%. Because adding a ferritic layer in the PSC is still a planar fabrication process, the described method could be widely used to enhance the inductive coupling efficiency of PSCs that are used in biomedical implants.

Coupling enhancement of planar spiral coils using planar ferrite for biomedical implants

A miniature batteryless implantable wireless pressure sensor that can be used deep inside the body is desired by the medical community. MEMS technology makes it possible to achieve high responsivity that directly determines the operating distance between a miniature implanted sensor and the external RF probe, while providing the read-out. In this paper, for the first time, an analytical expression of the system responsivity versus the sensor design is derived using an equivalent circuit model. Also, the integration of micro-coil inductors and pressure sensitive capacitors on a single silicon chip using MEMS fabrication techniques is demonstrated. Further, the derived analytical design theory is validated by the measured responsivity of these sensors.

Di Lan

Papers

A Low-Frequency Versatile Wireless Power Transfer Technology for Biomedical Implants

A low switching frequency AC-DC boost converter for wireless powered miniaturized implants

A parallel-trace high-Q planar spiral coil for biomedical implants

Coupling enhancement of planar spiral coils using planar ferrite for biomedical implants

The responsivity of a miniaturized passive implantable wireless pressure sensor