Researchers Streeeetch Wearable Sensor Capabilities
Researchers at North Carolina State University have developed a stretchable strain sensor they claim has an unprecedented combination of sensitivity and range, allowing it to detect even minor changes in strain with greater range of motion than previous technologies. They demonstrated the sensor’s utility by creating new health monitoring and human-machine interface devices.
Strain is a measurement of how much a material deforms from its original length. For example, if you stretched a rubber band to twice its original length, its strain would be 100%.
“And measuring strain is useful in many applications, such as devices that measure blood pressure and technologies that track physical movement,” Yong Zhu, corresponding author of a paper on the work and the Andrew A. Adams distinguished professor of Mechanical and Aerospace Engineering at North Carolina State University, says in a university press release.
“But to date there’s been a trade-off. Strain sensors that are sensitive—capable of detecting small deformations—cannot be stretched very far. On the other hand, sensors that can be stretched to greater lengths are typically not very sensitive. The new sensor we’ve developed is both sensitive and capable of withstanding significant deformation,” Zhu added in the release. “An additional feature is that the sensor is highly robust even when over-strained, meaning it is unlikely to break when the applied strain accidentally exceeds the sensing range.”
The new sensor consists of a silver nanowire network embedded in an elastic polymer. The polymer features a pattern of parallel cuts of a uniform depth, alternating from either side of the material: one cut from the left followed by one from the right followed by one from the left, and so on. The patterned cuts enable a greater range of deformation without sacrificing sensitivity.
The sensor calculates strain by measuring changes in electrical resistance. As the material stretches, resistance increases. The cuts in the surface of the sensor are perpendicular to the direction that it is stretched. This does two things. First, the cuts allow the sensor to deform significantly. Because the cuts in the surface pull open, creating a zigzag pattern, the material can withstand substantial deformation without reaching the breaking point. Second, when the cuts pull open, this forces the electrical signal to travel further, moving up and down the zigzag.
To demonstrate how far the sensors can be deformed, the researchers created a wearable device for monitoring motion in a person’s back, which has utility for physical therapy. They also demonstrated a human-machine interface by using the sensor to create a three-dimensional touch controller that can be used to control a video game. The sensor can be incorporated into existing wearable materials such as fabrics and athletic tapes, convenient for practical applications.
The paper, “Highly Sensitive, Stretchable, and Robust Strain Sensor Based on Crack Propagation and Opening,” is published in the journal ACS Applied Materials & Interfaces. Shuang Wu, a recent Ph.D. graduate at North Carolina State, is first author on the paper, which was co-authored by Katherine Moody, a Ph.D. student at North Carolina State, and by Abhiroop Kollipara, a former undergraduate at North Carolina State.
The work was done with support from the National Science Foundation and the U.S. Department of Defense.
