Heart diseases are a major and growing public health concern, affecting millions of people and costing tens of billions of dollars each year. Among them, arrhythmias are one of the most common heart disorders but can be quite easily treated by using pacemakers.
The first pacemaker implantation dates back to 1958, and obviously, many improvements have been performed since then, making these devices safer and more comfortable. Yet, researchers and medical groups want to go further, and imagine today the pacemakers of the future, relying on Micro-Electro-Mechanical Systems (MEMS) technologies. Smaller and smarter, these devices will reduce costs and further improve patients’ comfort and safeness.
1. Pacemakers, market and needs for improvements
Pacemakers have been used for years, delivering electrical impulses to regulate the beating of the heart. Today, more than 3 million people worldwide have pacemakers and about 1’000’000 new devices are implanted each year . Hence, the market of pacemakers is forecasted to reach $5.1 billion in 2018 with an annual growth rate of 11 percent by then .
As a consequence, the competition is tough with many players involved (Medtronic, St-Jude, Boston Sc, Sorin Group, Biotronik), and innovation is an excellent lever to gain market shares. The research in this area is today focused on small devices using MEMS technologies, paving the way to technological breakthrough cardiac implants.
Actually, today’s pacing systems have two main drawbacks, offering opportunities of improvements and differentiations from the competition .
The first major way of improvement concerns the lead. Indeed, present pacemakers are made of two elements (figure 1a):
(i) a pulse generator (the “pacemaker”) , placed under the skin in the chest, that gathers an energy source (battery) and the impulse control system. (ii) a lead inserted directly in the heart through a vein, delivering the impulses.
As a matter of fact, leads are not always easy to position, may be subject to displacement and erosion, which may lead to conduction or insulation failures.
The second major way of improvement is the pacemaker’s battery lifetime. Over the years, various power sources have been used for pacemakers (batteries, plutonium). Unfortunately, these sources are limited and must be replaced every five to ten years, leading to new surgeries to change the pulse generator, and inherent (but preventable) costs.
In fact, size reduction thanks to MEMS devices makes it possible to imagine tiny pacemakers fixed directly on the heart wall (epicardium), limiting the quantity of energy given to the heart at each cycle, paving the way to leadless pacemakers powered by energy harvesting (figure 1b) and therefore solving the problems of leads and autonomy at the same time.
The pacemakers of the future will be small, leadless and everlasting thanks to energy harvesting (EH).
(b) Figure 1: (a) current pacemaker and (b) future leadless pacemaker powered by energy harvesting
Since the pacemaker is inserted into the heart, only energy coming from the human body is available. As presented in , four ambient sources can theoretically be exploited: radiant sources, thermal sources, mechanical sources and biochemical sources. However, only vibrations and biochemical products are available in the heart, as the body is opaque and thermo-regulated.
Extracting energy thanks to biochemistry is generally complicated and especially in the harsh environment of the body, leading to packaging challenges, that are hard to solve (semi-permeable membranes’ lifetimes, clogging).
On the other hand, heartbeat’s vibration energy harvesting offers a great opportunity to develop hermetic systems, not in contact with any human body fluids; it was then chosen by HBS consortium (HBS for Heart Beat Scavenger) to power Sorin’s future pacemakers.
HBS is a consortium of 4 European companies (Sorin Group, Tronics, Cedrat, Easii IC) and 2 research centers (CEA-LETI, TIMA) (figure 2) that decided to combine their expertise in 2010 to:
(i) Develop an EH-powered pacemaker by harvesting the mechanical energy produced by the movements of the heart and eliminating the need for batteries that must be replaced every five to ten years. (ii) Reduce the size of a cardiac pacemaker by a factor of eight, from 8 cm3 to 1 cm3. This reduction will make it possible to attach the pacemaker directly to the endocardium, eliminating the need for intravenous introduction of cardiac leads.
Figure 2: HBS consortium to develop the pacemaker of the future
Heartbeats Energy harvesters
HBS Heartbeats energy harvesters belong to the family of Vibration Energy Harvesters that have been increasingly studied since the 2000s. Yet, due to the strong constraints in terms of size (<1cm³) and frequency (a few Hz), developing such devices is quite challenging.
1. Harvesting vibrations from heartbeats
Harvesting energy from vibrations consists in developing mass-spring resonant devices that turn vibrations into a relative movement between two elements (mechanical-to-mechanical converter) that is then turned into electricity thanks to a mechanical-to-electrical converter (piezoelectric, electrostatic, electromagnetic devices)  (figure 3).
(b) Figure 3: (a) Two-steps conversion to turn vibrations into electricity and (b) general model of a vibration energy harvester
The output power of these devices is low, generally in the order of 10µW per gram of mobile mass, but this is enough to power basic actions such as detecting vibration peaks or delivering energy pulses to the heart.
But, the heart is a tough environment for the mechanical-to-mechanical converter. The global size of the device must be small (1cm³) and the resonant frequency of the mass-spring system low (in theory, equal to heartbeats frequency, that is to say 1-3Hz). Such constraints lead to large mass displacements and fragile devices that are not compatible with pacemakers’ requirements (size, lifetime, reliability).
Fortunately, while beating at 1-3Hz, in-vivo acceleration measurements (figure 4a) showed that significant power can be harvested on the 20Hz range (figure 4b) turning HBS into a challenging but viable project.
Figure 4: (a) Typical vibration spectrum in the right atrium and (b) theoretical available power per gram of mobile mass in the ventricle as a function of the resonant frequency of the device
As for the mechanical-to-electrical converter, two transduction technologies are currently under investigation: piezoelectric devices are studied at TIMA while CEA-LETI focuses on electrostatic devices using electrets (electrically charged dielectrics, equivalent to magnets in electrostatic). It is also noteworthy that electromagnetic devices cannot be used due to MRI incompatibilities.
How it works 2. Piezoelectric devices
Piezoelectric converters use piezoelectric materials that generate charges under stress or strain. A cantilever-based piezoelectric energy harvester has been chosen to harvest energy from heartbeats (figure 5). The mass added at the free end enables to increase the output power and to decrease the mass-spring resonant frequency down to 20Hz while keeping small dimensions.
Figure 5: HBS piezoelectric device developed by TIMA
3. Electrostatic devices
Electrostatic converters are capacitive structures made of two plates separated by air, vacuum or any dielectric materials. In these devices, a relative movement between the two plates generates a capacitance variation and then electric charges .
As explained in , electrostatic devices are well-suited for size reduction and enable to decouple the mechanical system (mass-spring) from the mechanical-to-electrical converter. An electret-based converter has been chosen to enable a direct mechanical-to-electrical conversion. A schema of the device is introduced in figure 6a; patterned electrets are presented in figure 6b. Once again, a mass is added in order to increase the output power.
Figure 6: (a) Electret-based energy harvester developed by CEA-LETI and (b) patterned electrets
A first MEMS electrostatic prototype has been manufactured using cleanroom processes and is presented in figure 7a. The energy harvester has a total volume of 1cm³ and an output power of 10µW will be available as soon as the device is implanted in the heart. A schema of the future complete autonomous pacemaker, with its vibration energy harvester, is represented in figure 7b.
(b) Figure 7: (a) MEMS electret-based vibration energy harvester (b) schematic of the future device
Piezoelectric or electrostatic devices deliver an AC output voltage that cannot be used as is to supply electronic devices: a power management circuit is required.
————– 1. For more information on electrostatic vibration energy harvesters: Electrostatic conversion for vibration energy harvesting, S. Boisseau, G. Despesse, B. Ahmed Seddik, Small-scale Energy Harvesting, Intech, 2012
Power management ICs
HBS’s whole conversion chain is presented in figure 8. A power converter and a buffer are inserted between the energy harvester and the device to supply. In order to respect the size constraints, the buffer and the power converter are integrated circuits and designed by Easii IC.
Figure 8: Power conversion chain for HBS energy harvesters
In both cases, the power converter (figure 9) is a buck converter (step-down) which enables to get the best of the energy harvester by optimizing power extraction. A supercapacitor (buffer) stores the energy and supplies the electronic circuit, aimed at measuring heartbeats and delivering the adequate amount of energy at the right moment, with a constant voltage of 3V.
Figure 9: Buck converter developed by Easii IC
Thanks to this conversion chain and power consumption optimizations of the electronic circuits, heartbeats energy harvesting will be enough to generate pulses up to 3-4 Hz.
5. Conclusions and perspectives
HBS project has begun in 2010 and fully functional prototypes should be manufactured by the end of the year. The industrialization is expected within five to ten years, after validation tests and agreements from health administrations.
HBS project is a further step towards autonomous medical implants and could be applied to many devices such as cochlear implants, insulin pumps, glucose sensors…
This project is supported by Sorin and labeled by Minalogic. The authors would like to thank their partners and coworkers for their contribution to this article, and especially, S. Basrour and M. Colin (TIMA), A. Makdissi (Sorin), B. Challiol and J.P. Goglio (Easii IC), J.S. Danel, A.B. Duret, G. Despesse and T. Hilt (CEA-LETI).
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.