4774 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013 A Novel Mat-Based System for Position-Varying Wireless Power Transfer to Biomedical Implants Qi Xu , Hao Wang , Zhaolong Gao , Zhi-Hong Mao , Jiping He , and Mingui Sun Key Laboratory of Image Processing and Intelligent Control of Education Ministry, Department of Control Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Department of Neurosurgery, University of Pittsburgh, Pittsburgh, PA 15213 USA Department of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, PA 15261 USA Department of Bioengineering, Arizona State University, Tempe, AZ 85287 USA Wireless power transfer via magnetically resonant coupling is a new technology to deliver power over a relatively long distance. Here, we present a mat-based design to wirelessly power moving targets based on this technology. Our design is specifically applied to tran- scutaneously power medical implants within free-moving laboratory animals. Our system comprises a driver coil array, a hexagonally packed transmitter mat, a receiver coil, and a load coil, and generates a nearly flat magnetic distribution over a defined area to produce an approximately constant power output independent of the location of the receiver coil. This paper also describes a novel power receiver coil design of the same shape as the exterior of the implant, allowing for maximum magnetic coupling, eliminating the space restrictions due to the coil within the implant, and matching the resonant frequencies of the implant and the transmitter coil. Our new transmitter and receiver designs significantly reduce the size of a biomedical implant and may provide a lifetime power supply to implanted circuits without the need for an internal battery. Our designs are also useful in various other applications involving moving targets, such as part of a robot or a vehicle. Index Terms—Biomedical implant, driver-coil array, magnetically resonant coupling, mat-based design, wireless power transfer (WPT). I. INTRODUCTION I MPLANTABLE devices have seen increasingly wide- spread use in health and medical applications. These devices have been widely used to locally stimulate internal organs and/or monitor vital health indicators. Continuing tech- nological advances have resulted in increasingly miniaturized and functional implantable electronic circuits [1]. However, development of a reliable electrical power supply to implants located subcutaneously within a biological body (e.g., a human or an animal) has not advanced at a similar pace and has re- mained an unresolved limitation in such devices. Improvements in battery energy density have not negated the need for periodic surgeries to replace a depleted battery nor the high costs and health risks associated with such intrusions. In the case of rechargeable batteries, successful applications are hindered by various issues related to battery size, weight, longevity, toxicity, and safety [2]. Wireless power transfer (WPT) is a promising technology capable of addressing limitations in implantable devices. This technology not only negates the risk of infection due to cables passing through the skin, but also minimizes the size of the device by excising bulky components such as batteries [3]. Decades of efforts by the research community and associ- ated industry have resulted in inductively coupled systems consisting of a pair of coils separated by the skin, such as the system that powers an artificial heart [3]. In these systems, WPT Manuscript received April 21, 2012; revised June 29, 2012; accepted De- cember 26, 2012. Date of publication February 20, 2013; date of current version July 23, 2013. Corresponding author: Q. Xu (e-mail: xuqi@hust.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2013.2245335 Fig. 1. Components of the magnetically coupled resonant WPT consisting of four coils (driver, primary, secondary, and load coils). efficiency depends on coil size, structure, physical spacing, relative location, and surrounding tissue properties. Due to a low -factor and limited coupling between coils, however, the two-coil inductive system often suffers from a rapid decrease in power transfer efficiency as the power delivery distance increases. Significant challenges arise when a relatively large amount of power is transmitted over a relatively long distance. If the power transfer efficiency is low, the lost energy may pro- duce excessive heat in biological tissue, compromising human safety. Efficient power transfer mechanisms are required by high-power implants to be effective. Wireless electricity represents a new WPT technique based on strongly coupled resonance via evanescent fields in the midrange of coil separation [5], [6]. The WPT system based on wireless electricity typically contains four coils—namely, driver, primary, secondary, and load coils as shown in Fig. 1. Coupled-mode theory (CMT) has been used to analyze the mechanisms of power transfer in such systems [6]. Detailed the- oretical and quantitative analyses have shown that an efficient midrange wireless energy link can be established between a pair of resonators of the same resonant frequency [5], [6]. This type of WPT has the ability to operate in a strong coupling mode due to the magnetic resonance between the power transmitter and receiver. When the WPT system is properly designed, energy 0018-9464/$31.00 © 2013 IEEE