With the rapid advances that have been seen in areas like processing and sensing, and the simultaneous miniaturization of these technologies in recent years, it can be hard to understand why wearable electronic devices have not really taken off in the way that many expected. After all, wearables offer a friction-free way to monitor our health, improve communication, simplify human-machine interactions, provide new forms of entertainment, and much more. But all of these applications are being held back, in large part, by one limitation of today’s devices — energy storage capacity.
Wearables need to be small and lightweight if they are going to stay on the body and out of the junk drawer. But to keep these devices powered up for extended periods of time, they need large batteries. If developers skimp on battery size in favor of comfort, then they will need to be recharged too frequently. Neither option is acceptable to most users, so researchers have started experimenting with energy harvesting techniques that could extend the life of an otherwise undersized battery.
The device converts motion into electricity (📷: A. Pratap et al.)
These techniques involve reclaiming energy from sources like motion or heat that would otherwise be lost, and converting it into electricity. Triboelectric nanogenerators (TENGs) are a popular type of energy harvester that convert motion into electricity. However, they often require complex fabrication processes to produce. Toxic chemicals are also frequently necessary for their construction, which is not exactly something anyone wants in contact with their body all day long.
But now, researchers at Boise State University have developed a TENG that is made with biocompatible and eco-friendly materials. Furthermore, their energy harvesters can be fabricated with just a 3D printer. These factors could make energy harvesting technologies not only more practical, but also easier and less costly for developers to experiment with.
The team has created a fully printed TENG using a novel composite ink composed of poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (PVBVA) and Ti₃C₂Tₓ MXene nanosheets. This formulation enables additive manufacturing of high-performance, skin-compatible devices that can be directly integrated into wearables.
LEDs and a stopwatch were powered by the energy harvester (📷: A. Pratap et al.)
Traditional TENGs rely on fluorinated polymers and toxic solvents that pose health and environmental risks. In contrast, the researchers used ethanol in this work, which is a safer, biocompatible solvent, to create their inks. PVBVA, while more environmentally friendly, does normally lag behind fluorinated materials in charge generation, however. The team overcame this performance issue by incorporating 5.5 mg/mL of MXene, which is known for its exceptional electrical conductivity and high surface area.
Initial testing showed that a TENG created with their methods could achieve an impressive 252 volts of open-circuit voltage, 2.8 microamperes of short-circuit current, and a peak power density of 760 milliwatts per square meter. Furthermore, the device maintained mechanical stability even after 10,000 bending cycles, demonstrating its suitability for long-term wearable use.
The team’s TENG can not only harvest energy from human motion, such as walking, running, knee bending, and jumping, but can also capture ambient energy from sources like rainwater. To demonstrate its versatility, the researchers powered LEDs and a stopwatch using the printed device. This opens up new possibilities in the future for fully self-powered sensors and other devices that are both portable and environmentally sustainable.