Researchers Develop Self-Powered, Stretchable Micro-Supercapacitors For Health and Diagnostic Wearable Devices

Researchers from Penn State University along with collaborating researchers from Minjiang University and Nanjing University in China have developed a self-powered, stretchable system that could be used in wearable health-monitoring and diagnostic devices. The research was led by Huanyu “Larry” Cheng, Dorothy Quiggle Career Development Professor in Penn State's Department of Engineering Science and Mechanics. Findings from the research were published in the journal Nano Energy.

 

Researchers develop technology that could potentially power wearable and stretchable health devices. Image used courtesy of Penn State University.

 

What are Micro-Supercapacitors?

Micro-supercapacitors are miniature versions of supercapacitors and can range from microns to centimeters. They are energy storage devices that can either be used alongside lithium-ion batteries (LIBs) or can replace them in consumer wearables. 

Micro-supercapacitors have some notable qualities such as a high power density, small footprint, and can charge and discharge quickly. Despite this, the “sandwich-like” layered geometry of conventional micro-supercapacitors has been observed to show poor flexibility when designed for wearable devices. Additionally, they display long ion diffusion distances, and when integrating with wearable electronics can be complex.

Batteries and supercapacitors currently available for powering wearable and stretchable health-monitoring devices throw up a few hiccups when it comes to performance, including limited stretchability and low energy density. 

Cheng’s research team and collaborators have previously focused on developing the sensors in wearable devices. “While working on gas sensors and other wearable devices, we always need 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,” said Cheng in a recent news release.

 

The Research

To tackle the challenges facing conventional battery, supercapacitor, and micro-supercapacitor technology, Cheng and his colleagues decided to manipulate device architecture in a unique way. The researchers arranged micro-supercapacitor cells in a snaking, serpentine fashion, and island-bridge layout. This architecture allowed the bending and stretching of the configuration at the bridges, all the while reducing the deformation of the micro-supercapacitors. The researchers refer to the combined structure as “micro-supercapacitor arrays”.

In the same news release, Cheng commented: “By using an island-bridge design when connecting cells, the micro-supercapacitor arrays displayed increased stretchability and allowed for adjustable voltage outputs.” Cheng added: “This allows the system to be reversibly stretched up to 100%.”

 

Professor Huanyu Cheng. Image used courtesy of Penn State University 

 

To create the island bridge structure of the cells, Cheng and colleagues used a highly-porous, self-heating nanomaterial known as 3D laser-induced graphene foam, and non-layered, ultrathin zinc-phosphorus nanosheets. From using these materials, the researchers saw substantial improvements in electric conductivity and the number of absorbed charged ions. This provided evidence that micro-supercapacitor arrays can discharge and charge efficiently and provide energy storage to power wearable devices. 

To make the system self-powering, the researchers made further alterations. They added a triboelectric nanogenerator. This technology can be used for converting mechanical movement to electrical energy. The researchers also added a graphene-based strain sensor.

“When we have this wireless charging module that’s based on the triboelectric nanogenerator, we can harvest energy based on motion, such as bending your elbow or breathing and speaking,” Cheng said. “We are able to use these everyday human motions to charge the micro-supercapacitors.”

Additionally, the energy-storing micro-supercapacitor arrays provide power to the graphene-based strain sensor which shows the potential for the system to power stretchable, wearable devices.