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| Fig. 1: Coronary sinus lead (red arrow) of an implanted cardiac resynchronization device, which coordinates contractions between the left and right ventricles to improve cardiac efficiency. (Source: Wikimedia Commons). |
For millions of patients, implantable medical devices quietly sustain daily life. A pacemaker keeps the heart beating in rhythm, a neural stimulator restores movement or reduces pain, and an insulin pump regulates blood sugar. Despite their life-changing impact, all of these devices share a common constraint: energy. Once placed inside the body, a device must operate for years without direct access to external power. Every joule drawn from a battery shortens its lifetime, and every bit of waste heat must dissipate safely into surrounding tissue.
Today, most implantables rely on lithium batteries that can last anywhere from several years to a decade, depending on the device's energy demands [1]. But each replacement requires invasive surgery, carrying risk and cost. At the same time, more advanced therapies often require more power, intensifying the trade-off between functionality, longevity, and safety. This has prompted interest in alternative approaches, such as harvesting small amounts of energy from body heat, movement, or glucose metabolism. While these methods may not generate enough energy to sustain the device indefinitely, they could extend device lifetimes or reduce the frequency of replacement procedures. This motivates a closer look at the feasibility of harvesting energy from the body itself.
Implantable devices may extend their lifetime by drawing small amounts of energy directly from the body. Thermoelectric generators (TEGs) rely on the Seebeck effect, in which a voltage appears when two sides of a material are at different temperatures. The body's core is a few degrees warmer than the skin, creating a temperature gradient that can be tapped, but the power available is modest, typically only tens of microwatts because heat flows easily through tissue. [2] Another option is mechanical harvesting. Piezoelectric devices generate charge when bent or compressed and could capture energy from the heartbeat, breathing, or muscle movement. Depending on the frequency and intensity of motion, they can deliver anything from a few microwatts to several milliwatts. [3] A third approach is biochemical. Currently available glucose biofuel cells oxidize the body's own glucose to release electrons, producing a steady electrical output. Total power outputs of ~40μW have been reported, but performance typically declines over hours or days as the enzymes degrade [4]
To see how this translates to real lifetimes, consider a pacemaker. Lets say a lithium-iodine battery of ~1 Ah at 2.8 V stores about 10 kJ of energy. A pacemaker hypothetically consuming 25 μW uses only ~0.8 kJ per year, enough to last 10 to 12 years, consistent with clinical replacement schedules. [5] By contrast, an insulin pump at 10 mW would draw ~300 kJ per year, depleting the same battery in weeks. Device demands quickly put these numbers into perspective. Pacemakers consume about 10-50 μW, placing them within reach of thermoelectric or biofuel harvesters. Insulin pumps need tens of milliwatts, and neural stimulators can require pulses of 100 mW or more far beyond what harvesting can supply. As a result, energy harvesting is best viewed as a supplement: viable for low-power devices, but insufficient for high-power applications.
Any move toward alternative power sources must weigh reliability, complexity, and patient safety. Larger batteries extend lifetime but increase device size and complicate implantation. Energy harvesters reduce battery dependence, yet add moving parts or biochemical interfaces that can fail inside the body. Wireless recharging is appealing but requires external alignment and can raise tissue-heating concerns. [6] Hybrid systems combining a conventional battery with micro-harvesters may strike the best balance, providing baseline reliability while capturing small amounts of renewable power. Ultimately, the trade-off lies between maximizing functionality and minimizing the risk, cost, and frequency of replacement surgeries.
The capacities of energy storage sets strict limits on the performance of implantable medical devices. Energy harvesting from the body offers promising supplements, yet current technologies provide only modest power. Future designs must navigate trade-offs between size, safety, and reliability, with hybrid solutions likely to emerge, all towards creating safer and longer-lasting implantable therapies.
© Alice Zhou. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] D. Katz andT. Akiyama, "Pacemaker Longevity: the Worlds Longest-Lasting VVI Pacemaker," Ann. Noninvasive Electrocardiol. 12, 223 (2007).
[2] B. R. M. Kingma et al., "Beyond the Classic Thermoneutral Zone," Temperature 1, 142 (2014).
[3] S. Panda et al., "Piezoelectric Energy Harvesting Systems For Biomedical Applications," Nano Energy 100, 107514 (2022).
[4] A. Zebda et al., "Single Glucose Biofuel Cells Implanted in Rats Power Electronic Devices," Sci. Rep. 3, 1516 (2013).
[5] C. Moerke et al., "New Strategies For Energy Supply of Cardiac Implantable Devices," Herzschrittmacherther. Elektrophysiol. 33, 224 (2022).
[6] L. Lucke and B.Bluvshtein, "Safety Considerations For Wireless Delivery of Continuous Power to IOmplanted Medical Devices," IEEE 6943585, Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. 2014, 286 (2014).
