The desire to avoid open-heart surgery just to replace a battery is high on the wish list of medical specialists. Pacemaker batteries last around seven years, on average, and sometimes up to ten years. Elderly people generally have enough health issues to contend with, without having to undergo surgery simply to change a battery in their pacemaker. Any surgical procedure raises the risk of infection, severe bleeding or other complications. Younger patients requiring a pacemaker to correct a heart arrythmia, for example, face the prospect of undergoing frequent operations during their lifetime.
Thus, the autonomously powered pacemaker has long been the target of medical device developers. Converting vibrations from heartbeats into energy to power the pacemaker is the favorite approach, and the subject of many research projects from around the world.
Figure 1: Artist’s impression of a flexible piezoelectric energy generator that will enable the self-powered pacemaker, under development at KAIST. Source: KAIST
The human heartbeat creates vibrations at a low frequency, up to 30 Hz. Energy harvesting at such low frequencies has been difficult due to the brittle properties of the materials from which the harvesters are made. Much of today’s research revolves around new ‘smart’ materials such as flexible polymer nanomaterials. In addition, implantable devices require biocompatibility, years of testing and dedicated approvals.
Meanwhile, in general terms, piezoelectric energy harvesting has significant potential in external, wearable devices. Sports equipment and wellbeing monitoring products are obvious examples. Heart rate and performance monitors for runners, cyclists and gym equipment users are popular. In the medical field, watches, bracelets and patches are increasingly used to monitor patients with heart conditions, diabetes or other ailments. More recently, devices such as Google Glass are showing some potential in the medical and healthcare sector.
This article will review recent advanced research developments destined for piezoelectric-based heart pacemakers, looking specifically at the materials used and the performances claimed. This provides some insight into how the technology will develop for industrial and consumer applications, and in particular, for wearable devices.
Commercially available piezoelectric energy-harvesting devices are becoming smaller, and thus, more applicable to wearable products. Complementary power management circuitry is already available to exploit them. A typical design approach is presented, based on the Midé Technology Volture range of piezoelectric energy harvesters and the Linear Technology DC1459B energy-harvesting demonstration board. The latter is based on the LTC3588-1 piezoelectric energy-harvesting power supply and associated step-up and boost converters.
A research team from the Korea Advanced Institute of Science and Technology (KAIST), working with the Division of Cardiology at Severance Hospital of Yonsei University, has developed a self-powered artificial cardiac pacemaker operated semi-permanently by a flexible piezoelectric nanogenerator.1 In trials, the energy harvested has amounted to 8.2 V and 0.22 mA from very small movements. The research team fabricated high-performance flexible nanogenerators utilizing a bulk single-crystal PMN-PT thin film.
Once successfully developed, such a flexible piezoelectric nanogenerator could be used not only to provide sufficient energy to power a cardiac pacemaker, but also for real-time monitoring. Ultimately, it could be used to power other types of implantable medical devices.
Researchers in the US (University of Illinois, Urbana-Champaign) have been working on a similar approach, with a design for a flexible piezoelectric patch type of device that harnesses energy from the body’s natural movements.2 The patch contains a film made of 500 nm thick ribbons of lead zirconate titanate (PXT) surrounded by gold and platinum electrodes, and encased in polyimide.
Tests have already shown that this biocompatible device can harvest enough energy from heartbeats to power a modern cardiac pacemaker. Modern pacemakers require around 0.3 µW. These patches, when attached to the right ventricle of the heart, were demonstrated to generate up to 0.18 µW/cm². The patches can be multi-layered to produce more than enough power.
In Canada, at the University of Waterloo (UW), researchers are working on a wideband hybrid energy harvester designed to significantly prolong battery life, thereby reducing the number of heart surgeries that need to be performed. The novelty of this design is that it converts ambient vibrations into electricity using a combination of smart materials in order to operate at a wider range of frequencies. If the rate of motion of the vibration source decreases, so does the frequency, and the level of energy produced.
However, even at lower frequencies, this prototype has demonstrated it can continue to exploit the vibration source. Energy harvested in the 8 to 12 Hz range had an output power on the order of 1 mW. Current prototypes are expected to be used to power wireless sensors that can help detect cracks and damage to buildings.
Taking a different approach, the Université Paris-Sud is working on a pacemaker in the form of a leadless capsule that is placed directly in the heart chamber, instead of being attached to the heart wall with leads into the veins. Energy is harvested from the effect of regular blood pressure variations whereby the flexible packaging on the capsule transmits blood forces to an internal 3D electrostatic transducer. The key advantages of this approach, the researchers claim, are greater power density, adaptability to heartbeat frequency changes and the potential for miniaturization.
Experimental testing has determined that a large stroke optimized piezoelectric spiral transducer with complex electrode patterns can produce a power density of 3 µJ/cm³/cycle. The researchers are confident that with further development, the device should provide enough energy to power autonomously the next generation of pacemakers.
Université Paris-Sud has joined forces with the MANpower project, a European consortium led by the Tyndall National Institute at University College Cork.4 The project has significant funding to develop a perpetually powered device for medical implants. Key areas of R&D are low-frequency vibration energy harvesting, high energy density micro energy storage, miniaturization, and biocompatible packaging.
The key issues to address for energy harvesting to be successful for pacemakers, are to ensure that the piezoelectric devices do not interfere with the operation of the heart, thereby exacerbating the problem they are trying to alleviate, and that they will operate for longer than ten years.
Eventually, this advanced research will filter down to industrial and commercial applications. Miniaturization and efficiency are the key parameters for their wider use. Of course, vibration energy harvesting is already widely used to power a wide range of remote sensing devices for machine, vehicle and building monitoring, for example. They are used in some wearable products such as training shoes and other sports equipment, and in health-monitoring devices.
Measurement Specialties has developed a wide range of vibration sensing products, some of which are suitable for implantable devices. For more accessible industrial and commercial applications, the company’s range includes the Minisense 100 series, which can be incorporated into equipment for monitoring vital signs. The 1005939 is a low-cost cantilever style vibration sensor, loaded by a mass. It is specifically designed for operation at low frequencies, from 0 to 40 Hz.
When the beam is mounted horizontally, acceleration in the vertical plane creates bending in the beam, due to the inertia of the mass at the tip of the beam. Strain in the beam creates a piezoelectric response, which may be detected as a charge or voltage output across the electrodes of the sensor. It measures just 18 x 7 mm overall.
The Midé Technology Volture range of piezoelectric vibration energy harvesters has been designed to meet growing demand for delivering remote power to wireless sensor networks. The devices can replace batteries, or complement an energy storage source to extend maintenance periods. The lowest frequency product in the range, the V22BL, has a sensing range of 26 to 110 Hz.
Midé overcomes the disadvantage of normally brittle piezoelectric materials, the characteristic that the new research is aiming to avoid by exploring smart materials, by pre-attaching the electrical leads and then packaging the device in a protective skin. The Volture harvesters are robust and hermetically sealed for use in harsh environments.
Another useful feature of the Volture range is its compatibility with the LTC3588 piezoelectric power supply circuit from Linear Technology. Indeed, Midé’s EHE004 Energy Harvesting Conditioning Circuit exploits the LTC3588 to convert the AC output from its Volture harvester to a regulated DC output. It comprises a full-wave rectifier with integrated charge management and DC to DC conversion. The DC output can be configured to 1.8, 2.5, 3.3 and 3.6 V, and the board includes a 200 µF storage capacitor.
The LTC3588-1, described as a nanopower energy-harvesting power supply, also features 450 nA quiescent current undervoltage lockout mode with a wide hysteresis window, so that charge can accumulate on an input capacitor until the buck converter can efficiently transfer some of the stored charge to the output. See a typical application circuit in Figure 2.
Figure 2: The LTC3588-1 piezoelectric energy-harvesting power supply in a sample application circuit.
A number of evaluation boards are available, including the DC1459B-A demonstration board, featuring the LTC3588-1. A Quick-Start Guide can be downloaded, which provides a detailed explanation of how it works, complete with circuit diagrams.
Research into new materials for piezoelectric energy harvesting, specifically for autonomously powered pacemakers and other implantable devices, is likely to yield smaller, more flexible and more efficient devices for use in the industrial and commercial field, as well as for wearable products for health monitoring and sports equipment.