(Nanowerk News) For just a dollar, a Penn State-led international collaboration has created a self-contained sensor system capable of monitoring gas molecules in the environment or in human breath. The system combines a nanogenerator with a micro-supercapacitor to harvest and store energy generated by human movement.
The researchers published their approach, which costs just a few dollars for materials and uses widely available equipment, at Nano Letters(“Laser Processing of Tangled Porous Graphene/MXene Nanocomposites for Independent Gas Sensing Systems”). The development was the culmination of years of work led by associated author of Huanyu “Larry” Cheng, James L. Henderson Jr. Memorial Associate Professor of Engineering Science and Mechanics at Penn State.
“This is really a combination of previous studies where we continued the journey of developing wearable gas sensors,” said Cheng. “Most of sensor research and development has centered on the fabrication of device materials. Here, we use a single material to produce multiple components on a single platform that work together as a stand-alone system.”
Cheng and his team previously developed sensors to detect nitrogen dioxide, which can indicate various lung diseases in exhaled breath, and other gases that may signal poor environmental air quality (Microsystems & Nano Engineering, “Moisture resistant, stretchable NOX gas sensors based on laser-induced graphene for environmental monitoring and breath analysis”).
They also found laser-induced graphene foam materials with MXenes, or two-dimensional transition metals that are less reactive than other metals. New materials and new fabrication methods result in elastic sensors that can be bent by human movement. The researchers also applied this approach to produce stretchable micro-supercapacitors that can store the energy generated by such human movements.
In this paper, the group combines these efforts. They first applied the laser to a previously developed 3D porous graphene foam on a flexible substrate. Next, the researchers sprayed MXenes on graphene foam and used another laser to combine the foam and MXenes into a nanocomposite material. They then transferred the nanocomposite material onto the pre-stressed elastomer, which they slowly loosened. This slow discharge destroys nanocomposites, which can be patterned differently for sensors, nanogenerators, and micro-supercapacitors.
“Using this laser is almost like baking a slice of bread: it changes the surface of the bread to something more stable,” says Cheng, explaining that the laser uses carbon dioxide to change the surface of the material. “You end up with a product that is more stable and porous than what you started with. This material allows for more sensitivity in the sensor and more conductivity in other components.”
The laser used is available at most machine shops, according to Cheng.
“These materials are cheap, and more expensive tools are widely available — Penn State alone has hundreds of these lasers,” said correspondent author Cheng Zhang, who is affiliated with Minjang University in China and is a visiting scholar in Cheng’s lab. “Given the low cost and wide availability of materials and tools, this approach can certainly be scaled up for use in clinical settings.”
With the same nanocomposite material that comprises each device, the system components work together seamlessly, according to Cheng. Because the 3D nanocomposite foam is pre-screened, creating a “creased” effect, individual components can also be stretched and bent to adhere to human skin or clothing without losing sensitivity.
“The increased electrical conductivity, mechanical toughness and specific surface area of wrinkled porous graphene/MXenes from simple fabrications provides opportunities for applications in stretchable self-device platforms,” said Zhang.
To demonstrate the proof-of-concept, the research colleagues wore gas sensors under their noses and on their wrists, as well as nanogenerators in their shoes and micro-supercapacitor arrays on their shirts. The person exercises vigorously, with a nano-generator that harvests the energy generated by the movement of their legs. That energy is stored by micro-supercapacitors, which use power to collect and send data from gas sensors to a Bluetooth receiver, where scientists can analyze it. Sensors continuously monitor exhaled breath and the environment for nitrogen dioxide.
“The measurements are consistent with those made by commercial sensors,” said Cheng. He also noted that the system demonstrated a stable level over 50 days in laboratory testing, indicating long-term system stability for real-life applications. “The design and demonstration strategies of this work pave the way for the design, fabrication, and application of next-generation bio-integrated electronics for healthy aging and precision medicine.”