This paper describes the development of a high-density electronic interface to the central nervous system. Silicon micromachined electrode arrays now permit the long-term monitoring of neural activity in vivo as well as the insertion of electronic signals into neural networks at the cellular level. Efforts to understand and engineer the biology of the implant/tissue interface are also underway. These electrode arrays are facilitating significant advances in our understanding of the nervous system, and merged with on-chip circuitry, signal processing, microfluidics, and wireless interfaces, they are forming the basis for a family of neural prostheses for the possible treatment of disorders such as blindness, deafness, paralysis, severe epilepsy, and Parkinson's disease.

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Wireless implantable microsystems: high-density electronic interfaces to the nervous system

Starting from the basic principles of Tesla's wireless power transfer experiment in the 1890s, this review article addresses the key historical developments of wireless power and its modern applications up to formation of the world's first international wireless power standard “Qi” launched in 2010 for portable electronics. The scientific principles laid down by Nicolas Tesla for wireless power transfer, which still remain valid today, are first explained. Then, modern wireless power applications based on nonradiative (near-field) magnetic coupling for short-range applications are described. Some industrial application examples emerging since the 1960s are highlighted. The article then focuses on the comparison of the horizontal and vertical magnetic flux approaches developed in the early 1990s for low-power planar wireless charging pads. Several critical features such as localized charging, load identification, and freedom of positioning that are essential to wireless charging of portable electronic devices are explained. The core technologies adopted by the Wireless Power Consortium (WPC) for the “Qi” Standard in 2010 are summarized. Finally, the latest research and developments of wireless power transfer for midrange applications based on the domino-resonator concept and their future application potential are described.

Planar Wireless Charging Technology for Portable Electronic Products and Qi

RF wireless interface enables remotely-powered implantable devices. Current studies in wireless power transmission into biological tissue tend to operate below 10 MHz due to tissue absorption loss, which results in large receive antennas. This paper examines the range of frequencies that will optimize the tradeoff between received power and tissue absorption. It first models biological tissue as a dispersive dielectric in a homogeneous medium and performs full-wave analysis to show that the optimal frequency is above 1 GHz for small receive coil and typical transmit-receive separations. Then, it includes the air-tissue interface and models human body as a planarly layered medium. The optimal frequency is shown to remain in the GHz-range. Finally, electromagnetic simulations are performed to include the effect of load impedance and look at the matched power gain. The optimal frequency is in the GHz-range for mm-sized transmit antenna and shifts to the sub-GHz range for cm-sized transmit antenna. The multiple orders of magnitude increase in the operating frequency enables dramatic miniaturization of implantable devices.

/pdf/optimal-frequency-for-wireless-power-transmission-into-3rk2n77jvd.pdf

Optimal Frequency for Wireless Power Transmission Into Dispersive Tissue

Inductive power transfer (IPT) and capacitive power transfer (CPT) are the two most pervasive methods of wireless power transfer (WPT). IPT is the most common and is applicable to many power levels and gap distances. Conversely, CPT is only applicable for power transfer applications with inherently small gap distances due to constraints on the developed voltage. Despite limitations on gap distance, CPT has been shown to be viable in kilowatt power level applications. This paper provides a critical comparison of IPT and CPT for small gap applications, wherein the theoretical and empirical limitations of each approach are established. A survey of empirical WPT data across diverse applications in the last decade using IPT and CPT technology graphically compares the two approaches in power level, gap distance, operational frequency, and efficiency, among other aspects. The coupler volumetric power density constrained to small gap sizes is analytically established through theoretical physical limitations of IPT and CPT. Finally, guidelines for selecting IPT or CPT in small gap systems are presented.

A Survey of Wireless Power Transfer and a Critical Comparison of Inductive and Capacitive Coupling for Small Gap Applications

In this retinal prosthesis project, a rehabilitative device is designed to replace the functionality of defective photoreceptors in patients suffering from retinitis pigmentosa (RP) and age-related macular degeneration (AMD). The device consists of an extraocular and an intraocular unit. The implantable component receives power and a data signal via a telemetric inductive link between the two units. The extraocular unit includes a video camera and video processing board, a telemetry protocol encoder chip, and an RF amplifier and primary coil. The intraocular unit consists of a secondary coil, a rectifier and regulator, a retinal chip with a telemetry protocol decoder, a stimulus signal generator, and an electrode array. This paper focuses on the design, fabrication, and testing of a microchip which serves as the telemetry protocol decoder and stimulus signal generator. It is fabricated by MOSIS with 1.2-mm CMOS technology and was demonstrated to provide the desired biphasic current stimulus pulses for an array of 100 retinal electrodes at video frame rates.

A neuro-stimulus chip with telemetry unit for retinal prosthetic device

Values of mutual inductance between two circular conductors lying in parallel planes have been computed. The results are presented in graphical form and their relevance to the design of radio-frequency coupled implant systems is mentioned.

F. C. Flack

Papers

Mutual inductance of air-cored coils: effect on design of radio-frequency coupled implants.