Current versions of batteries and supercapacitors powering wearable and stretchable health-monitoring and diagnostic devices have many shortcomings including low energy density and limited stretchability.
An alternative to batteries, micro-supercapacitors are energy storage devices that can complement or replace lithium-ion batteries in wearable devices. Micro-supercapacitors have a small footprint, high power density, and the ability to charge and discharge quickly; however, when fabricated for wearable devices, conventional micro-supercapacitors have a “sandwich-like” stacked geometry that displays poor flexibility, long ion diffusion distances, and a complex integration process when combined with wearable electronics.
Researchers developed alternative device architectures and integration processes to advance the use of micro-super-capacitors in wearable devices. They found that arranging micro-supercapacitor cells in a serpentine, island-bridge layout allows the configuration to stretch and bend at the bridges, while reducing deformation of the micro-supercapacitors — the islands. When combined, the structure becomes what the researchers refer to as micro-supercapacitor arrays. By using an island-bridge design when connecting cells, the micro-supercapacitor arrays displayed increased stretchability and allowed for adjustable voltage outputs, allowing the system to be reversibly stretched up to 100%.
By using non-layered, ultrathin, zinc-phosphorus nanosheets and 3D laser-induced graphene foam — a highly porous, self-heating nano-material — to construct the island-bridge design of the cells, the team saw drastic improvements in electric conductivity and the number of absorbed charged ions. This proved that these micro-supercapacitor arrays can charge and discharge efficiently and store the energy needed to power a wearable device.
The researchers also integrated the system with a triboelectric nanogenerator — an emerging technology that converts mechanical movement to electrical energy. This combination created a self-powered system. With the wireless charging module that’s based on the triboelectric nanogenerator, energy can be harvested based on motion such as bending an elbow, breathing, and speaking.
By combining this integrated system with a graphene-based strain sensor, the energy-storing micro-supercapacitor arrays — charged by the triboelectric nano-generators — are able to power the sensor.