Researchers at North Carolina State University have developed an extendable strain sensor that has an unprecedented combination of sensitivity and range, allowing it to detect even small changes in strain with a greater range of motion than previous technologies. Researchers demonstrated the sensor’s utility by creating new human-machine interface and health monitoring devices.
Strain is a measure of how much a material deforms from its original length. For example, if you stretched a rubber band to twice its original length, its deformation would be 100%.
“And strain measurement is useful in many applications, such as devices that measure blood pressure and technologies that track physical motion,” says Yong Zhu, corresponding author of a paper on the work and Andrew A. Adams Distinguished Professor of Mechanical and Aerospace Engineering to NC status.
“But until now, there has been a trade-off. Sensitive strain sensors, which can detect small strains, can’t be stretched very far. On the other hand, sensors that can be stretched to longer lengths are typically not very sensitive. The new sensor we developed is both sensitive and able to withstand significant deformation,” says Zhu. “An additional feature is that the sensor is extremely robust even when subjected to excessive stresses, meaning that it is unlikely to break when the applied strain accidentally exceeds the sensing range.”
The new sensor consists of a network of silver nanowires embedded in an elastic polymer. The polymer has a pattern of parallel cuts of uniform depth, alternating from both sides of the material: one cut from the left, followed by one from the right, followed by one from the left, and so on.
“This feature — the patterned cuts — is what allows for a wider range of deformation without sacrificing sensitivity,” says Shuang Wu, who is the paper’s first author and a recent Ph.D. NC State graduate.
The sensor measures strain by measuring changes in electrical resistance. As the material stretches, the strength increases. The cuts in the sensor surface are perpendicular to the direction in which it is stretched. This does two things. First, the cuts allow the sensor to deform significantly. Because the cuts in the surface open up, creating a zigzag pattern, the material can withstand substantial deformation without reaching its breaking point. Secondly, as the cuts open, this forces the electrical signal to travel further, traveling up and down the zigzag.
“To demonstrate the sensitivity of the new sensors, we used them to create new blood pressure wearables,” says Zhu. “And to demonstrate the extent to which sensors can be distorted, we created a wearable device to track movement in a person’s back, which has utility for physical therapy.”
“We also demonstrated a human-machine interface,” Wu says. “In particular, we used the sensor to create a three-dimensional touch controller that can be used to control a video game.”
“The sensor can be easily incorporated into existing wearable materials such as sports fabrics and tapes, which are convenient for practical applications,” Zhu says. “And all of this is just scratching the surface. We think there will be a number of additional applications as we continue to work with this technology.”
The work was done with support from the National Science Foundation, grant number 2122841; the National Institutes of Health, under grant number R01HD108473; and the United States Department of Defense, under license number W81XWH-21-1-0185.
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Materials provided by North Carolina State University. Original written by Matt Shipman. Note: Content can be edited for style and length.