Telemetry for Implantable Medical Devices
Part 1 – Media Properties and Standards
By Rudolf Ritter, Jonas Handwerker, Tianyi Liu, and Maurits Ortmanns
NOTE: This is an overview of the entire article, which appeared in the Spring 2014 issue of the IEEE Solid-State Circuits Magazine.
Click here to read the entire article.
This article is the first part of a three-part series on telemetry for implantable medical devices (IMDs) Parts 2 and 3 will be published in upcoming issues of the IEEE Solid-State Circuits Magazine. Over the last 50 years, IMDs have become an important tool for medical doctors, life sciences researchers, and mankind in general. They have been used to restore lost function, treat disorders, or monitor biological parameters with significant impact on either life quality, health, or understanding the human body.
Part one of this article discusses the challenges such devices face. A common challenge is their location within the body and determining how best to access the system. In order for the IMDs to operate successfully, commands must be sent and sensor signals received. Choosing the power mode for IMDs presents a significant challenge, not only in how much power is needed but, even more, in how much space is available, what distance is required, and the device’s reliability and duration. For example, a cardiac pacemaker must have a reliable energy source, while an inner ocular IMD might not provide enough room for a battery thus requiring remote, wireless power delivery.
On the data side, the requirements are even more numerous: from very low to very high data rates, small and large distance, uni-or bidirectional with half or full duplex, and variable constraints on data integrity and bit-error rate.
One thing all IMDs have in common is the need for wireless operation and remote control of the device. The main reasons for this are patient convenience and health, risk of inflammation, and the reliability of penetrating wires. In the context of IMDs, telemetry is commonly used for remote, wireless bidirectional communication as well as the wireless powering of the device.
In this first part of the three-part article, the researchers discuss three main topics: electromagnetic (EM) properties, optical properties, and regulations and standards. The dielectric properties of tissue are characterized by several dispersion mechanisms. The resulting effects, which are the main interest for the IMD engineer, are how deep the EM signal penetrates the tissue, what losses occur, and how much heat is caused to the tissue. There are two main influences for high-absorption and low-penetration depth: high water tissue content and high frequency. When models were exposed to a simulated emission, they yielded an estimated field strength distribution which allows the specification of the telemetry system and, more importantly, the estimation of absorption and tissue heating. The most common measure is the specific absorption ratio (SAR), measured by determining the temperature increase on a short EM exposure and by multiplying the result with the specific heat capacity.
The article discusses considerations related to exposure to optical radiation, explaining that photons traveling through tissue experience various interactions which can be described with four parameters: absorption, scattering, the anisotropy factor, and the refractive index. The figure below illustrates, for instance, the maximum eye exposure as a function of duration.
The maximum permissible exposure for the eye based on IEC 60825.
IMD regulations and standards were developed in response to more than 100,000 incidents reported in the late 1980s and 1990s involving cardiac-type medical devices. Several thousand of them were most likely caused by EM interference. The first regulations were implemented by the Medical Implant Communication Service (MICS) band in the 1990s and later the Wireless Medical Telemetry Service (WMTS) and the MedRadio band for medical device communication. Such regulations are needed for two reasons: first, to limit the interference of what the transmitter could emit to other devices in the vicinity, and second, to keep safe limits of exposure to the human body to avoid adverse effects such as tissue heating by absorption or electrostimulation.
Read the full article for more detailed information.
ABOUT THE AUTHORS
Rudolf Ritter received the Dipl.Ing. (FH) degree in communications engineering from the University of Applied Sciences, Ulm, Germany, in 2009 and the M.S. degree in electrical engineering with honors from the University of Ulm, Germany, in 2012. Since 2012, he has been pursuing the Ph.D. degree with a focus on high-speed, wide-bandwidth, and low-power continuous-time sigma-delta modulators for wireless receiver applications together with the Institute of Microelectronics at the University of Ulm, Germany.
Jonas Handwerker received the B.Sc. and M.Sc. degrees in microsystems engineering from IMTEK, University of Freiburg, Germany, in 2008 and 2011, respectively. From 2009 to 2010, he interned in the integrated circuits group at the Robert Bosch Research and Technology Center in Palo Alto, California. Since 2012, he has been working toward his Ph.D. degree at the Institute of Microelectronics at the University of Ulm, Germany, with his main research focus on IC and MEMS design for μNMR imaging and spectroscopy as well as MRI gradient field correction.
Tianyi Liu received the B.S. degree in applied physics from Southeast University, Nanjing, China, in 2006. Later, he furthered his studies in the Key Laboratory of MEMS of Ministry of Education in School of Electronic Science and Engineering and completed the M.S. degree in January, 2009. Since December 2009, he has been working as a research assistant/Ph.D. candidate in the Institute of Microelectronics, Faculty of Engineering and Computer Science, University of Ulm, Germany. His current research interests are transcutaneous optical telemetric link design, tissue optics, and brain machine interface.
Maurits Ortmanns received Dr.-Ing. from IMTEK, University of Freiburg, Germany, in 2004. From 2004 to 2005 he was with sciworx GmbH, Hannover, Germany, as a project leader in mixed-signal electronics and from 2006 to 2007 as an assistant professor at the University of Freiburg. Since 2008 he has been a full professor at the University of Ulm, Germany. His research interests include mixed-signal integrated circuit design, self-correcting data converters, and implantable neural interface circuits. He served as a program committee member of ESSCirC, DATE, ECCTD, ICECS; as associate editor of IEEE Transactions on Circuits and Systems I, as guest editor for IEEE Journal of Solid-State Circuits, and is currently a program and executive committee member of ISSCC.