Energy harvesting (EH) in humans is a technological approach focused on capturing the small amounts of energy naturally generated or dissipated by the body and converting it into usable electrical power. This process creates perpetual power sources for miniature electronic devices. The goal is to make small, autonomous devices, such as those used for health monitoring and medical treatment, self-sustaining by tapping into the body’s ambient energy. This allows for the development of fully integrated systems that operate without the need for traditional external charging or battery replacement.
The Limitations of Traditional Implanted Power
Traditional implanted medical devices, such as pacemakers and neurostimulators, rely almost exclusively on chemical batteries, primarily lithium-based chemistries. These batteries have a finite lifespan, typically ranging from five to fifteen years, forcing patients to undergo repeat surgeries to replace the power source. This replacement procedure carries inherent risks, is costly, and causes significant patient inconvenience.
The size and volume of the battery also impose strict limitations on the miniaturization and design of the implantable device itself. While modern pacemakers require only about 8 to 30 microwatts (\(\mu\)W) of power, the battery still occupies a significant portion of the device’s total volume. Safety concerns also exist regarding potential material leakage or thermal issues that could arise from the battery chemistry within the body. Energy harvesting offers a solution by supplementing or entirely replacing these conventional power units.
Harnessing Mechanical and Thermal Sources
The human body is a constant source of both mechanical and thermal energy that can be captured by specialized devices. Mechanical energy is derived from both large-scale, deliberate movements and continuous, micro-scale processes. Large-scale activities, such as walking, running, and joint movement, represent significant sources of kinetic energy that can be captured by devices worn externally.
Internally, energy is available from the consistent, rhythmic movements of organs and biological systems. This includes constant pressure changes from the heartbeat, the flow of blood through arteries, the expansion and contraction of the lungs during breathing, and the motion of muscles during activities like gastric peristalsis. These movements provide a continuous, low-frequency mechanical input for implanted harvesters.
Thermal energy is continuously available due to the temperature difference between the body’s internal core and the external environment. The human core temperature is maintained around 37°C, while the skin surface is typically cooler, around 32°C. This consistent temperature gradient between the inner and outer body layers can be converted into electricity.
Core Technologies for Energy Conversion
The conversion of the body’s mechanical and thermal energy into usable electricity relies on specific engineering principles. Piezoelectric generators (PENGs) convert mechanical strain or pressure into an electrical charge. This is achieved using materials that generate a voltage when they are physically deformed, such as by the beating heart or the flexing of a muscle. The PENGs are especially effective at converting the tiny, irregular deformations of internal organs into electrical power.
Thermoelectric generators (TEGs) are solid-state devices designed to convert the thermal gradient into electrical energy through a phenomenon known as the Seebeck effect. TEGs utilize two dissimilar semiconductor materials that are joined together; when one side is exposed to the warmer skin and the other to the cooler ambient air, a voltage is created. This process allows TEGs to capture the energy that the body constantly dissipates as heat.
Triboelectric nanogenerators (TENGs) harvest energy through the combination of contact electrification and electrostatic induction, essentially generating power through friction. When two different materials come into contact and then separate due to movement, an electrical potential is generated. TENGs are highly efficient at scavenging energy from low-frequency, irregular motions like skin contact, muscle vibration, and general body movement.
Current Medical Applications
Energy harvesting technologies are paving the way for a new generation of self-powered medical devices. One of the most promising applications is the perpetually powered cardiac pacemaker, which harvests energy directly from the heart’s movement. This eliminates the need for invasive battery replacement surgeries that patients with traditional pacemakers currently require.
Continuous monitoring sensors are also benefiting from energy harvesting, allowing for uninterrupted health tracking. Devices like glucose sensors, pulse oximeters, and intracranial pressure monitors can be powered autonomously using captured energy. This supports sustained, high-fidelity data collection without the maintenance burden of external power sources.
Furthermore, harvested energy can power advanced neural implants and targeted drug delivery systems. Tiny harvesters can be integrated into devices designed for nerve stimulation or for releasing medication on a precise schedule, ensuring continuous operation. These advancements are transforming implantable and wearable electronics into long-lasting, autonomous systems for precision healthcare.