Researchers at North Carolina State University have developed a stretchable strain sensor with 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 demonstrate the utility of the sensor by creating new health monitoring and human-machine interface devices.
Strain is a measure of how much a material deforms from its original length. For example, if a rubber band is stretched to twice its original length, its strain will be 100%.
“Measuring strain is useful in many applications, such as devices that measure blood pressure and technologies that track body motion,” said corresponding author Yong Zhu, the Andrew A. Adams Distinguished Professor of Mechanical and Aerospace Engineering at NC State .
“But so far, there has been a trade-off. Sensitive strain sensors — capable of detecting small deformations — cannot be stretched very far. On the other hand, sensors that can be stretched to greater lengths are generally not very sensitive. The new sensor we developed Both sensitive and capable of withstanding significant deformation,” Zhu said. “Another feature is that the sensor is very robust even when over-strained, meaning it is less likely to break when applied strain accidentally exceeds the sensing range.”
The new sensor consists of a network of silver nanowires embedded in an elastic polymer. Polymers have a pattern of parallel cuts of uniform depth, alternating from either side of the material: one cut from the left, then one from the right, then one from the left, and so on.
“This feature — the patterned cut — enables a wider range of deformations without sacrificing sensitivity,” said Shuang Wu, a recent Ph.D. and lead author on the paper. Graduated from North Carolina State University.
The sensor measures strain by measuring a change in electrical resistance. As the material stretches, the resistance increases. The cuts on the sensor surface are perpendicular to its stretching direction. This does two things. First, the cutout deforms the sensor significantly. Because the cuts in the surface pull apart, creating a zigzag pattern, the material can withstand large deformations without reaching a breaking point. Second, when the cut is pulled apart, this forces the electrical signal to travel farther, zigzagging up and down.
“To demonstrate the sensitivity of the new sensors, we used them to create a new wearable blood pressure device,” Zhu said. “To demonstrate how far the sensor can deform, we created a wearable device that monitors the movement of a person’s back, which could be used in physical therapy.”
“We also demonstrated a human-machine interface,” Wu said. “Specifically, we used the sensors to create a three-dimensional touch controller that can be used to control video games.”
“The sensor can be easily integrated into existing wearable materials, such as fabrics and sports tapes, for practical applications,” Zhu said. “And all of this is just scratching the surface. We think there will be a range of additional applications as we continue to use this technology.”
This work was supported by the National Science Foundation, grant number 2122841; the National Institutes of Health, grant number R01HD108473; and the U.S. Department of Defense, grant number W81XWH-21-1-0185.
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