One of the key challenges in self-powered wearable sensors for healthcare monitoring is the ability to distinguish between different signals that occur simultaneously. Researchers have now addressed this issue by discovering a new property of sensor materials, leading to the development of a flexible sensor that can accurately measure both temperature and physical strain independently, enabling more precise identification of various signals.

A team of researchers from Penn State (University Park, PA, USA) and Hebei University of Technology (Tianjin, China) aimed to measure temperature and strain signals without interference using laser-induced graphene, a two-dimensional (2D) material. Like other 2D materials such as graphene, laser-induced graphene is just one to a few atoms thick, offering unique properties but with a distinct feature. Laser-induced graphene (LIG) forms when a laser is used to heat certain carbon-rich materials like plastic or wood, converting their surface into a graphene structure. Essentially, the laser "writes" the graphene directly onto the material, making it a simple and scalable method for creating graphene patterns for use in electronics, sensors, and energy devices. While LIG has previously been used in a variety of applications, including gas sensors, electrochemical detectors for sweat analysis, and supercapacitors, the researchers believe they have uncovered a new property of LIG that makes it particularly suitable for an accurate and multi-functional sensor.

Thermoelectric properties refer to the ability of a material to convert temperature differences into electrical voltage, and vice versa, which is useful for applications such as energy harvesting and temperature sensing. The researchers identified this thermoelectric property in LIG, which allows the sensor to separate temperature and strain measurements effectively. This makes the material ideal for healthcare applications, such as a sensor embedded in a bandage. Additionally, the sensor is highly sensitive, detecting temperature variations as small as 0.5 degrees Celsius. The design of the material takes advantage of how porous graphene and thermoelectric components work together, making it nearly four times more efficient at converting heat into electricity. The sensor is also capable of stretching up to 45% and can conform to different shapes and surfaces without losing its functionality.

Because the thermoelectric properties of LIG allow it to generate electrical power from temperature differences, the sensor is self-powered. According to the researchers, this feature makes it especially useful for continuous monitoring in clinical environments and other applications, such as detecting fires in remote areas. In addition to refining the sensor itself, the research team is developing a wireless system that will enable users to remotely monitor sensor data. This would allow the real-time tracking of crucial information, such as temperature or strain, through smartphones or other devices.

"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.” 

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