Stretch Goals in Energy Harvesting

Nothing will get a wearable electronic device tossed in the drawer of forgotten toys faster than frequent battery replacements or recharges. The electronics in these devices may have advanced to the point that we can have very powerful and inexpensive computers and sensors strapped to our bodies, but innovation in battery technologies has simply not been able to keep pace. To make these wearables more practical — and acceptable to end users — technological innovations in power delivery are sorely needed.

A quantum leap in battery design may be a way down the road, so exploring alternatives is likely to be our best option at this point in time. Energy harvesting has emerged as a potential solution to our present woes, however, these systems also have their own share of problems. Piezoelectric energy harvesters, for example, are very efficient and can produce enough energy to power many wearable devices, yet they are hard and brittle. Outside the confines of a research lab, they are likely to fail under normal operating conditions.

But thanks to the work of a team led by researchers at the Daegu Gyeongbuk Institute of Science and Technology, piezoelectric energy harvesters may soon be much more practical for real-world use in wearable devices. Using their novel techniques, they have been able to produce energy harvesters that are flexible and stretchable, while still maintaining high levels of efficiency in converting movement to electricity.

The new energy harvester designed by the team combines high piezoelectric efficiency with enhanced stretchability by leveraging lead zirconate titanate (PZT), a material known for its excellent piezoelectric properties but traditionally limited by its hardness and brittleness. To overcome these challenges, the team engineered PZT into a three-dimensional structure that is deformation-insensitive, allowing it to maintain high energy efficiency while being stretchable.

Additionally, the researchers introduced a novel curvature-specific coupling electrode design, which segments the electrodes into distinct regions based on curvature. This configuration prevents the cancellation of electrical energy caused by opposing local strains in the material, enabling more efficient energy harvesting. The result is a device with an energy efficiency 280 times higher than conventional stretchable piezoelectric harvesters, achieving unprecedented power density and practical utility in applications ranging from wearable devices to in vivo energy harvesting.

To validate the technology experimentally, a single strap of the energy-harvesting material was tested under a variety of conditions. Experimental data was found to closely match theoretical predictions, demonstrating the reliability of the curvature-specific coupling mechanism. The device achieved a peak power density of 8.34 mW per cubic centimeter under an optimal external resistance of 50 megaohms. Additional tests, including wrapping the device around curved body parts like knees and fingers, showed consistent voltage generation during stretching and releasing motions, further demonstrating its practical applicability for wearable energy harvesting.

The team’s future research will focus on experimental validation of their technology with other materials and geometries. They hope this work will pave the way for more robust and adaptable wearable energy harvesting systems.