A major challenge in self-powered wearable sensors for health care monitoring is distinguishing different signals when they occur at the same time. Researchers from Penn State and China’s Hebei University of Technology addressed this issue by uncovering a new property of a sensor material, enabling the team to develop a new type of flexible sensor that can accurately measure both temperature and physical strain simultaneously but separately to more precisely pinpoint various signals.
“This unique sensor material we’ve developed has potentially important applications in health care monitoring,” said Huanyu “Larry” Cheng, James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics (ESM) at Penn State and co-corresponding author of the study published in Nature Communications. “By accurately measuring both temperature changes and physical deformation or strain created by a healing wound and measure that by separating the two signals, it could revolutionize the tracking of wound healing. Doctors could get a much clearer picture of the healing process, identifying issues like inflammation early on.”
The researchers aimed to accurately measure temperature and strain signals without cross talk by using laser-induced graphene, a two-dimensional (2D) material. Like all 2D materials including regular graphene, laser-induced graphene is one to a few atoms thick with unique properties, but with a twist. Laser-induced graphene (LIG) forms when a laser heats certain carbon-rich materials — like plastic or wood — in a way that converts their surface into a graphene structure. The laser essentially “writes” the graphene directly onto the material, making it a simple and scalable way to produce graphene patterns for electronics, sensors and energy devices.
LIG has been used before in various applications. Previously, Cheng and his team have used LIG for gas sensors, electrochemical detectors for sweat analysis, supercapacitors, and more. However, the researchers said they believe they discovered a new property of LIG for the first time that makes it ideal for a multi-purpose and accurate sensor.
“In this particular study, we kind of stumbled upon the fact that this material also has thermoelectric properties,” Cheng said. “We believe this is the first time anyone has reported laser-induced graphene having thermoelectric capabilities. And that’s really important for what we’re trying to do here, which is to separately measure both temperature changes and physical strain or deformation.”
Thermoelectric properties in a material refer to the ability to convert temperature differences into electrical voltage and vice versa, enabling such materials to be used for applications like energy harvesting and temperature sensing. According to Cheng, this newly identified thermoelectric property of LIG makes it easy to separate the two sensor measurements and ideal for health care applications such as a sensor embedded in a bandage.
“When you have materials that are sensitive to both temperature and strain, it can be tricky to tell which signal is causing changes in the material,” Cheng said. “But by using this thermoelectric effect in the laser-induced graphene, we can essentially decouple those two measurements. We can look at the electrical resistance to get information about the strain, while also measuring the thermal voltage to determine the temperature. This is why doctors could use it to track both temperature fluctuations and physical changes in the wound site and give a much clearer picture of how the healing is progressing.”
He also noted that the sensor is highly sensitive, detecting temperature changes as small as 0.5 degrees Celsius. The material’s design takes advantage of the way porous graphene and thermoelectric components work together, making it nearly four times better at converting heat into electricity. The sensor can also stretch up to 45%, as well as conform to different shapes and surfaces, without losing function.
“The porous structure of this material creates a lot of tiny spaces and channels that allow it to interact with its surroundings in a very sensitive way,” Cheng said. “This makes it well-suited for interfacing with human soft tissues, in contrast to more rigid thermoelectric materials, such as ceramic-based ones.”
Since the thermoelectric aspect of LIG also means it can generate electrical power when there is a temperature difference, LIG sensors are self-powered. According to Cheng, this could be particularly useful for continuous monitoring in clinical settings and for other applications, such as helping detecting fires in remote locations.
In addition to refining the sensor, the team is developing a wireless system that will allow people to monitor the data from the sensor remotely. This will make it possible to track important information, such as temperature or strain, in real time using smartphones or other devices.
“For example, a doctor could monitor a patient’s condition from a distance, or emergency responders could receive alerts about dangerous temperature changes,” Cheng said. “These advancements aim to make the technology more accessible and effective, helping to improve health monitoring and safety in everyday situations.”
Along with Cheng, other paper authors include Ankan Dutta, graduate student in engineering science and mechanics at Penn State; and Li Yang, Xue Chen, Hui Zhang, Zihan Wang, Mingyang Zin, Shuaijie Du and Guizhi Xu, all from Hebei University of Technology.
The National Institutes of Health and the U.S. National Science Foundation supported this work.
