Penn State wearable sensor tracks temperature and strain

12 Feb 2025

Laser-induced graphene allows multimodal sensing for better health monitoring.

Wearable sensors with their own integrated power generation capability are an attractive alternative to relying on external batteries, and a number of thermoelectric, photovoltaic and other technologies have been built into sensors to do the job.

Distinguishing between the different input signals in self-powered wearables has remained a challenge, however, and low-cost, self-powered, dual-parameter high-performance sensors have proven hard to fabricate.

A project at Penn State University and China's Hebei University of Technology has now developed a wearable sensor able to detect both temperature and strain values, decoupling the two parameters for independent measurement.

Published in Nature Communications, the breakthrough points to multimodal wearables able to detect several parameters, with medical applications an immediate target.

"This unique sensor material we've developed has potentially important applications in health care monitoring," said Huanyu Cheng from Penn State.

"By accurately measuring both temperature changes and physical deformation or strain created by a healing wound, and 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 sensor is built around laser-induced graphene (LIG), an approach to graphene synthesis in which laser irradiation converts carbon-rich precursor molecules into 3D porous graphene in predetermined shapes and formations. This allows a laser to essentially write a graphene pattern onto another material.

LIG has been used for a number of industrial sensing applications, but its ability to convert temperature differences into electrical voltage and vice versa for the kind of multimodal detection needed by developers of wearable sensors was not clear until now, according to Penn State.

Medical treatments and fire alarms

"We stumbled upon the fact that this material also has thermoelectric properties," commented Cheng. "We believe this is the first time anyone has reported laser-induced graphene having thermoelectric capabilities."

LIG's thermoelectric effect means that its electrical resistance can indicate the strain being experienced by the material, while the voltage being generated relates to the temperature. For clinicians monitoring wound recovery, both temperature fluctuations and physical changes are important to the healing process.

The project manufactured a stretchable porous graphene foam 300-microns thick on a siloxane substrate, controlling the laser-induced structure so that a large number of randomly stacked 2D graphene flakes were present. Trials on mice confirmed that this sensor monitored the lowering of temperature as inflammation from a wound decreased, along with the reduction in strain as the wound healed over.

The sensor proved to be highly sensitive, detecting temperature changes as small as 0.5 degrees Celsius, conforming to different shapes and surfaces without losing function. This combination might also allow the same material to be used as a fire detector, registering any abnormal temperature increase in the vicinity and triggering an alarm.

"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 ceramic-based thermoelectric materials."