Smaller, Smarter, Ultra-Low-Power Sensors Raise Potential for Energy Harvesting in Medical Implants



Energy harvesting has long been regarded as having considerable potential in the field of medical implants with the body itself providing the energy source. Some progress had been made, but with today’s ultra-low-power circuits and sensors, efforts to exploit energy harvesting are accelerating. At last, the objective of autonomous operation and smaller, battery-free designs can be realized.

This article will touch on some of the advanced research projects underway to illustrate the potential of energy harvesting, not only in the medical implant sector, but also in other fields. We will review some of the components already available with the potential to exploit body energy sources to power medical devices.

These components include piezoelectric film transducers, such as those available from Measurement Specialties; tiny thin-film, solid-state batteries from Cymbet, and RF transceivers from Texas Instruments.

Battery-free

The principle criticism of energy harvesting for many applications in the past has been the low levels of energy generated through all but solar cells. However, significant advances in extremely-low-power, ultra-miniature electronics, including RF circuitry and wireless sensors, are generating strong interest in energy harvesting for medical implants. When batteries can be eliminated, or their life extended significantly, then implants will become smaller, more convenient, more reliable, longer lasting, and more effective.

As developers continue to push the boundaries of miniaturization and ultra-low-power operation, some inroads have already been made to develop implants that benefit from energy-harvesting techniques, both from internal and external sources.

Cardiac pacemakers, powered by piezoelectric energy harvested from the heartbeat itself, are now viable. A European consortium of researchers, led by CEA-Leti in France, is developing a low-power cardiac pacemaker, operating at 5 µW instead of 25 µW, powered by energy generated by the patient’s heartbeats. Eliminating the battery avoids having to replace it every five to ten years. It also means the device can be made smaller (see Figure 1).

Figure 1: In a traditional pacemaker design, the battery takes up a significant area of the implantable device.

The team is aiming, eventually, to reduce the size of the cardiac stimulator eight-fold to less than 1 cm³. This would enable it to be implanted directly onto the epicardium, obviating the need to attach a lead into the heart through a vein. The consortium is investigating both piezoelectric and electrostatic (electrets) techniques for the mechanical to electrical conversion process, harvesting power in the 20 Hz region. Initially, the techniques are expected to provide an output power of around 10 µW, via a power-conversion circuit.

In the US, researchers at the Department of Aerospace Engineering at the University of Michigan are testing a piezoelectric energy-harvesting device with the potential of using the beating heart to generate enough electricity to power a pacemaker. The device is about half the size of the batteries currently used in pacemakers (see Figure 2 below). Trials have shown that it can generate around 10 µW of power, around eight to ten times more than is required by modern pacemakers.

Figure 2: Piezoelectric energy harvester that could operate from a beating heart, developed by researchers at the University of Michigan.

Biological battery

Cochlear implants have been available for a few years, and are reported to be more effective than traditional hearing aids. However, they typically comprise an internal and external section, with the microphone, sound processor, and battery in the external unit. The battery remains a limiting factor, particularly as processors become more powerful and computations made more complex. Some devices use rechargeable batteries and some require regular replacement.

Research into more highly-integrated devices is underway. Work at the University of Utah has demonstrated a proof of concept of implanting the microphone in the middle ear. This is claimed to partly remove the need for an outer hearing aid. However, the battery still has to be recharged at night by wearing a charger behind the ear.

Figure 3: A device developed by MIT to harness energy from the inner ear’s biological battery to power cochlear implants.

Elsewhere, researchers are investigating converting chemical energy in the inner ear to power cochlear implants. A team of researchers led by a group from Massachusetts Institute of Technology (MIT) is working on harnessing the inner ear’s biological battery, located in the cochlear (See Figure 3).

The level of electrical voltage is too low however, even to power today’s ultra-low-power circuitry. A tiny capacitor will store the charge, and power-conversion circuitry is required for power management. Once operational, the device would be self-sustaining, the researchers claim. Devices that harness this biological battery could also be designed to monitor biological activity in the ears of patients, or to deliver drugs or therapies.

The sweet spot

Engineers at MIT are also developing glucose fuel cells to power neural implants. The fuel cell operates by stripping electrons from glucose molecules to create a small electric current. Glucose is present in the blood and other body fluids. The fuel cell can be integrated, together with ultra-low-power circuitry, onto a silicon chip, to enable entirely self-powered devices such as brain implants. Such implants are being developed to help people with spinal cord injuries or who have suffered strokes.

Advances in neuromodulation have resulted in implants that influence the nervous system to control pain, and can help eliminate the tremor in patients suffering from Parkinson’s disease. The smaller and more reliable these devices can be made, with the help of energy harvesting, the more viable they become.

Ingestible electronic devices are becoming more widely used, and through energy harvesting and/or the use of tiny solid-state batteries, can perform a number of tasks. The ‘Pillcam’, the size of a large vitamin capsule, is used like an endoscope to visualize the digestive system, detecting abnormalities as it passes through, thereby avoiding subjecting the patient to lengthy and uncomfortable procedures.

Figure 4: The ingestible Pillcam developed by Given Imaging.

Some ingestible devices can be programmed to trigger and receive data from biomarkers, monitor gastric problems, or stimulate the repair of damaged tissue. They can be used to detect when drugs enter the bloodstream and monitor the effects. MEMS-based ingestible event markers are activated and powered using stomach electrolytes.

Targeted drug delivery for certain types of cancerous tumors is another important application for ingestible implantable devices. The ability to direct an active device to a precise location, to minimize the amount of drug administered and to avoid damaging adjacent cells, is proving particularly efficacious.

Body pump

Exploiting body heat to power electronic devices is an obvious candidate for energy harvesting. Miniature thermoelectric generators (TEGs), that produce energy from the temperature differential between the skin and the outside air, are already being used to run e-health devices such as blood sugar monitors, and to power sports and fitness equipment. For medical implants, the challenge is to generate energy internally. Chip-based TEGs are now envisioned that can be inserted under the skin, or in the skull, exploiting the small temperature differences between the brain and skin tissue. In theory, enough energy can be produced to power neurotechnological implants.

RF technology has not only revolutionized implantable and ingestible medical devices by wirelessly sending data to monitoring systems, but it can also be used to power implants. Radio and electromagnetic signals sent to a small coil in an implantable device can produce enough current for it to operate. Ongoing research anticipates minute devices that can be injected directly into the bloodstream, and wirelessly propelled via an external magnetic field to the correct location, and powered up to perform specific tasks.

Of course, electronic medical devices must meet a gamut of stringent health standards. Issues such as tissue heating, biocompatibility, hygiene, safety, and reliability are critical. Compatibility, or at least non-interference with medical equipment such as MRI scanners and radiotherapy systems, also has to be considered.

Markets and Commercialization

Electronic component manufacturers and medical equipment companies are succeeding in qualifying energy harvesting and related devices for use in implants. Many, including Analog Devices, Freescale, Maxim, Microsemi, and Texas Instruments have set up separate medical business divisions. The global microelectronics medical implant market is growing rapidly and is projected to reach $24.8 billion by 2016, according to BCC Research.¹ The fastest growing segment is neurostimulators at 10.5% cagr, with energy harvesting cited as an enabler.

Another report from MarketsandMarkets² published last year, forecasts the Medical Bionic Implant market to reach $17.82 billion by 2017. Predominantly covering artificial organs and organ-related implants, the report states that new and improved technologies are slated to propel the growth in this market. However, barriers to adoption include the high cost of devices and development, limited surgical expertise, and insurance/reimbursement issues.

Piezoelectric power

Measurement Specialties is a well-established supplier of piezoelectric film sensors to the OEM medical marketplace for a range of applications. Custom components are manufactured in ISO13485 certified facilities in order to meet medical-specific quality requirements. Pressure, force, temperature, humidity, and position sensors are incorporated into a range of patient monitoring and treatment equipment.

The firm’s piezoelectric film technology is used in a wide range of standard industrial sensor, transducer, and accelerator devices. Designers interested in the detailed process behind this technology and its potential for energy scavenging should see the company’s Application Note.³

Figure 5: Multi-purpose piezoelectric film sensor, LDT1-028K, from Measurement Specialties.

Meanwhile, the LDT-028K piezoelectric film transducer is a multi-purpose device for vibration sensing. The piezo film element is laminated to a sheet of Mylar polyester, and produces a usable electrical signal output when forces are applied to the sensing area. The dual-wire lead attached to the sensor allows a circuit or monitoring device to process the signal. Output voltage is 10 mV to 100 V, depending on the force and circuit impedance. Minimum impedance is 1 Mohm, while 10 Mohms or higher is recommended.

An evaluation kit is available that demonstrates the use of these devices, as well as other Measurement Specialties sensors for experimentation and development of a range of medical and non-medical applications, including remote monitoring systems.

Biocompatible batteries

In some applications, the next best thing to battery-free operation is to use energy harvesting techniques with a rechargeable form of energy storage. The EnerChip™ thin-film, solid-state batteries from Cymbet Corporation are radically different from typical battery devices, being fabricated on a silicon wafer using semiconductor process techniques. This means that the bare die can be integrated and packaged with conventional circuitry to save both space and cost. In the bare die form, these batteries are claimed to be one hundred times smaller than a non-rechargeable coin cell and last three times longer.

Packaged parts, ten times smaller than a coin cell, are also available with or without charge control and power management functionality. The CBC050-M8C, is rated for 50 µAh at 3.8 V. It is an ideal onboard power source for very-low-power circuitry and smart sensors. It can be recharged thousands of times and can be coupled to energy-harvesting devices.

Designed from the outset as eco-friendly devices, the EnerChip parts have just been demonstrated as biocompatible for implantable devices. They have wider applications too, in wireless sensor nodes and other systems. To experiment with these innovative batteries, the CBC-EVAL-05B EnerChip evaluation kit is available, supplied with a selection of batteries that can be connected in various ways. A universal energy-harvesting evaluation kit, the CBC-EVAL-09, accepts inputs from piezoelectric, TEG, or electromagnetic power sources, and features the EnerChip batteries.

Figure 6: Cymbet’s CBC-EVAL-09 universal energy-harvesting evaluation kit.

Wireless communications

Ultra-low-power RF transceivers are critical for energy harvesting-based sensors for implantables as they need to communicate vital information. The Texas Instruments CC1101 low power, sub-1 GHz RF transceiver is one example. It is primarily intended for the ISM (Industrial, Scientific, and Medical) and SRD (Short Range Device) frequency bands, but it can easily be programmed to specific frequencies such as the 400 to 406 MHz range, normally allocated for communication between implantable devices and external equipment.

The RF transceiver is integrated with a configurable baseband modem. In a typical system, the CC1101 is used in conjunction with an ultra-low-power microcontroller, such as TI’s CC430. For ultra-low-power, battery, and energy-harvesting applications, it can be used with the TPS62730 step-down converter with bypass mode.

Summary

The RF, processing, and sensor technology used in medical implants is evolving fast, becoming smaller, smarter, and much lower power. As a direct result, energy-harvesting techniques are back in vogue, enabling longer lasting and more versatile, battery-free implants. Energy sources derived from the body itself include piezoelectric (vibration), chemical, and heat differential. For some short-term implantable devices, external energy sources such as RF power can be used.

Demand is increasing for ultra-low-power sensors, controllers, power management circuitry, and miniature rechargeable energy-storage devices, as well as energy-harvesting devices themselves, for use in medical implants. Some may need to be customized, but OEMs will be looking at commercially available devices that meet their power and size constraints, which may have been developed for other application areas where small size and autonomous, battery-free operation are desirable.

References:
  1. BCC Research
  2. MarketsandMarkets
  3. Measurement Specialties Application Note on piezoelectric technology for energy harvesting
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