A stretchable system is that which will harvest energy from human breathing & motion to be used in wearable health monitoring devices could also be possible, consistent with a world team of researchers, led by Huanyu Larry Cheng, Dorothy Quiggle Career Development Professor in Penn State’s Department of Engineering Science & Mechanics.
According to Cheng, current versions of batteries & supercapacitors powering wearable & stretchable health monitoring & diagnostic devices have many shortcomings including low energy density & limited stretchability.
“This is something quite different than what we’ve worked on before, but it’s an important part of the equation,” Cheng said, noting that his research group & collaborators tend to focus on developing the sensors in wearable devices. “While performing on gas sensors & other wearable devices, we always got to combine these devices with a battery for powering. Using micro-supercapacitors gives us the ability to self-power the sensor without the need for a battery.”
An alternative to batteries, micro-supercapacitors are energy storage devices which will complement or replace lithium-ion batteries in wearable devices. Micro-supercapacitors have a little footprint, high power density & the ability to charge & discharge quickly. However, according to Cheng, when fabricated for wearable devices, conventional micro-supercapacitors have a “sandwich-like” stacked geometry that displays poor-flexibility, long ion diffusion distances & a complex integration process when combined with wearable electronics.
This led Cheng & his team to explore alternative device architectures & integration processes to advance the use of micro-supercapacitors in wearable devices. They found that arranging micro-supercapacitor cells in a serpentine, island-bridge layout allows the configuration to stretch & bend at the bridges, while reducing deformation of micro-supercapacitors, the islands. When combined, the structure becomes what the researchers called as “micro-supercapacitors arrays.”
“By using an island-bridge design when connecting cells, the micro-supercapacitor arrays displayed increased stretchability & allowed for adjustable voltage outputs,” Cheng said. “This allows the system to be reversibly stretched up to 100%.”
By using non-layered, ultrathin zinc-phosphorus nanosheets & 3D laser induced graphene foam a highly porous, self-heating nanomaterial to construct the island-bridge design of the cells, Cheng & his team saw drastic improvements in electric conductivity & the number of absorbed charged ions. This proved that these micro-supercapacitor arrays can charge & discharge efficiently & store the energy needed to power a wearable device.
The researchers also integrated the system with triboelectric nanogenerator, an emerging technology that converts mechanical movement to electrical energy. This combination created self-powered system.
“When we’ve this wireless charging module is based on the triboelectric nanogenerator, we will harvest energy based on motion, like bending your elbow or breathing & speaking,” Cheng said. “We are able to use these everyday human motions to charge the micro-supercapacitors.”
By combining this integrated system with a graphene-based strain sensor, the energy storing micro-supercapacitor arrays charged by the triboelectric nanogenerators are ready to power the sensor, Cheng said, showing the potential of this technique to power wearable, stretchable devices.