Wearable technology relies on energy to function efficiently, just like any other technological device. Fortunately, a promising array of energy sources exists for wearables, from sunlight and radio waves to the body’s own heat and motion. As technology continues to mature, the harnessing of these energy sources is becoming increasingly feasible, potentially liberating wearables from the need for batteries. This development has garnered significant attention from various companies and researchers.

According to Alper Bozkurt, co-director of the Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) at North Carolina State University, we often take energy for granted by plugging devices into outlets. However, energy must still be generated. The best-known wearable energy-harvesting technology is solar power, which captures electrons from sunlight or ambient light. But solar is just the tip of the iceberg.

Researchers have explored a diverse range of energy-harvesting methods to replace wearable batteries effectively. This includes piezoelectric and triboelectric generators, which use mechanical strain and electrostatic properties of materials to produce electricity. Electromagnetic induction, a well-established phenomenon, generates small but useful amounts of current through motion.

While wearables typically have low power requirements, they must also be comfortable to wear. Large, cumbersome energy-harvesting solutions like backpack-sized solar panels are not practical for most wearables. This has led to a surge in research into energy harvesting, including hybrid approaches that combine multiple methods.

For instance, Wei Gao at the California Institute of Technology developed “electronic skin” or e-skin. E-skin is a sensor-embedded device applied directly to the skin, enabling the monitoring and transmission of vital health indicators such as heart rate, body temperature, blood sugar, and metabolic byproducts. Gao’s first e-skin used built-in fuel cells powered by a patient’s sweat to generate electricity, effectively charging the device’s sensors and data transmission. The e-skin sustained a capacitor for about 60 hours, with voltage translating to stored electrons.

In subsequent developments, Gao’s team created a second-generation e-skin that harnessed kinetic energy from movement, generating triboelectricity. This innovation used thin layers of Teflon, copper, and polyimide that slide against each other during motion, yielding up to 0.94 milliwatts of power.

To further enhance energy harvesting for wearable devices, some researchers are turning to 3D printing. They’ve used this technology to create multimodal health-tracking systems called “e3-skin,” incorporating various sensors, hydrogel-coated electrodes, and a microsize supercapacitor powered by a solar cell. These customized components enable early detection and diagnosis of various health conditions.

Energy harvesting also extends beyond human use cases. Researchers are exploring this technology to track animals, where battery-powered and solar-based solutions often fall short. For instance, scientists at the University of Copenhagen, the Technical University of Denmark, and Germany’s Max Planck Institute of Animal Behavior developed the “Kinefox,” a self-recharging GPS tracker for wildlife. Inspired by self-winding watches, the device transforms an animal’s movement into energy, enabling continuous tracking without frequent battery replacement.

The Kinefox is open source, with files published on GitHub, and costs significantly less than traditional wildlife trackers. In the future, researchers are in talks to develop microgenerators specifically designed for animals. A broader outlook emphasizes the combination of unique materials and sustainable energy harvesting systems. One such development combines piezoelectric composites with carbon-fiber-reinforced polymer to create durable, efficient energy harvesters for various applications, including wearables and infrastructure systems for structural health monitoring.

Ultimately, the sweet spot in energy harvesting will be in data analysis, aligning energy-harvesting capabilities with the data needed for specific use cases. It’s crucial to ensure that energy is harvested and utilized efficiently, delivering meaningful data that addresses real-world needs.

By Impact Lab