A
team of international researchers, led by Huanyu "Larry" Cheng,
Dorothy Quiggle Career Development Professor in Penn State's Department of
Engineering Science and Mechanics, has developed a self-powered, stretchable
system that will be used in wearable health-monitoring and diagnostic devices.
Courtesy: Penn State College of Engineering.
A
stretchable system that can harvest energy from human breathing and motion for
use in wearable health-monitoring devices may be possible, according to an
international team of researchers, led by Huanyu "Larry" Cheng,
Dorothy Quiggle Career Development Professor in Penn State's Department of
Engineering Science and Mechanics.
The
research team, with members from Penn State and Minjiang University and Nanjing
University, both in China, recently published its results in Nano Energy.
According
to Cheng, 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.
"This
is something quite different than what we have worked on before, but it is a
vital part of the equation," Cheng said, noting that his research group
and collaborators tend to focus 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."
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, 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 and a complex integration process when combined with
wearable electronics.
This led
Cheng and his team to explore alternative device architectures and 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 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-supercapacitors arrays."
"By
using an island-bridge design when connecting cells, the micro-supercapacitor
arrays displayed increased stretchability and 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 and 3-D laser-induced
graphene foam—a highly porous, self-heating nanomaterial—to construct the
island-bridge design of the cells, Cheng and his 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.
"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."
By
combining this integrated system with a graphene-based strain sensor, the
energy-storing micro-supercapacitor arrays—charged by the triboelectric
nanogenerators—are able to power the sensor, Cheng said, showing the potential
for this system to power wearable, stretchable devices.