A Tunable Resonance Cantilever for Cardiac Energy Harvesting THOMAS W. SECORD and MILAD C. AUDI University of St. Thomas, St. Paul, MN 55105, USA (Received 20 June 2018; accepted 22 January 2019) Associate Editors Ajit P. Yoganathan and Scott C. Corbett oversaw the review of this article. Abstract Purpose—Energy harvesting from cardiac motion is an attractive means to avoid the use of batteries in implantable sensors and pacemakers. A single implantable de- vice would ideally integrate both sensing and self-powering functionality. Methods—This work describes a novel elec- tromagnetic system that achieves high sensitivity detection of the heart rate while simultaneously providing adaptive energy harvesting capability using a tunable resonance cantilever mechanism. Results—Our prototype design exhibits tunability of reso- nant frequency across the range of physiologic heart rates at a combination of lengths and angular orientations. Our initial prototype also produces between 3.0 lW and 20.6 lW of power at heart rates of 79–243 bpm, respectively. Conclusions—The prototype device can harvest sufficient energy to sustain implantable cardiac devices such as a leadless pacemaker. The system in this paper has the potential to eliminate batteries in certain implantable cardiac devices and thereby improve overall patient monitoring and treatment. Keywords—Energy harvesting, Resonance, Cardiology, Elec- tromagnetic induction, Implantable medical devices, Pace- makers. INTRODUCTION Implantable devices that operate inside or near the heart have direct access to an immense collection of physiologic signals and therapy delivery methods. Classical cardiac devices such as pacemakers and implantable cardioverter defibrillators (ICDs) measure fundamental diagnostic signals including electrocar- diogram (ECG), heart rate, respiration rate, and tho- racic impedance. However, these devices require additional power budget to provide their wide func- tionality, and their continual energy expenditure for therapy delivery necessitates regular device explant and replacement. Therefore, efficient energy harvesting from the mechanical energy in cardiac motion is a highly desirable design feature that could eliminate the need for batteries in both sensing and therapy delivery devices. Both the anatomical placement and design archi- tecture fundamentally dictate the success of a device intended to capture energy from heart motion. Fig- ure 1 shows an example of endocardial placement of our device near the right ventricular apex that would provide access to useful cardiac signals while also being ideally placed for direct integration into a right ven- tricular leadless pacemaker. In addition to proper placement, an energy harvesting device should be de- signed to match the frequency content of heart motion, which is particularly challenging given an excess of 200% variation in heart rate. The design discussed in this paper utilizes tunable resonance to achieve this goal. Several researchers have explored power generation for biomedical devices and a summary is provided in Romero et al. 21 Most vibrational energy harvesters for biomedical applications obtain energy from motion of the extremities. In addition to being distant for cardiac applications, limb motion is completely absent during many hours of the day and during sleep. Therefore, cardiac motion, blood flow, arterial deformation, and arterial pressure fluctuations hold the best promise of continuous energy harvesting over a well-defined and predictable frequency range. More generally, several devices have been explored for energy harvesting at all regions of the frequency spectrum and from a variety of energy input sources. A useful summary is provided by Khalig et al. 11 Within this broad range of research, electromagnetic systems (such as the device we propose) have been explored. For example, von Bu¨ren and Tro¨ster 27 describe a de- vice that uses a magnetic generator, yet it uses a flex- Address correspondence to Thomas W. Secord, University of St. Thomas, St. Paul, MN 55105, USA. Electronic mail: thomas.secord@stthomas.edu, audi0003@stthomas.edu Cardiovascular Engineering and Technology (Ó 2019) https://doi.org/10.1007/s13239-019-00402-9 BIOMEDICAL ENGINEERING SOCIETY Ó 2019 Biomedical Engineering Society