Abstract
Wireless energy transfer is a broad research area that has recently become applicable to implantable medical devices. Wireless powering of and communication with implanted devices is possible through wireless transcutaneous energy transfer. However, designing wireless transcutaneous systems is complicated due to the variability of the environment. The focus of this review is on strategies to sense and adapt to environmental variations in wireless transcutaneous systems. Adaptive systems provide the ability to maintain performance in the face of both unpredictability (variation from expected parameters) and variability (changes over time). Current strategies in adaptive (or tunable) systems include sensing relevant metrics to evaluate the function of the system in its environment and adjusting control parameters according to sensed values through the use of tunable components. Some challenges of applying adaptive designs to implantable devices are challenges common to all implantable devices, including size and power reduction on the implant, efficiency of power transfer and safety related to energy absorption in tissue. Challenges specifically associated with adaptation include choosing relevant and accessible parameters to sense and adjust, minimizing the tuning time and complexity of control, utilizing feedback from the implanted device and coordinating adaptation at the transmitter and receiver.
Highlights
Implantable medical devices have become a huge market, with over 20 million individuals estimated to have an implanted medical device and over $300 billion in associated costs in the U.S.in 2000
While a transcutaneous system can be optimized for a particular configuration, wireless transcutaneous energy transfer in practice is complicated by unpredictability and variability of the physiological environment
The paper is organized as follows: Section 2 covers background topics relevant to the review of the literature on adaptive transcutaneous systems; Section 3 provides an overview of the design process for an adaptive system and the strategies that have been implemented or proposed for each stage of the process; Section 4 details implementations in the literature for implantable devices and transcutaneous power transfer; Section 5 provides a summary of the literature and a discussion of remaining challenges in the field
Summary
Implantable medical devices have become a huge market, with over 20 million individuals estimated to have an implanted medical device and over $300 billion in associated costs in the U.S. Implantable medical devices have already effected vast improvements in patient monitoring and treatment by eliminating percutaneous cables that are prone to infection and limit patient mobility These include continuous therapy devices, such as the implantable cardioverter/defibrillator (ICD), electronic pacemaker, implantable neurostimulators and fully-implantable drug delivery pumps [2,3,4,5,6]. Implantable sensing devices are important in situations where the biological signals to be accessed are inside the body and cannot be reliably sensed non-invasively [10] This includes wireless implantable biosensors, such as those used with insulin pumps, commercial sensors are tethered to facilitate removal [11]. An implanted biosensor may only need intermittent power to perform a sensor reading These device considerations determine which powering methods are feasible for an implanted device
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