Flexible strain sensors with high sensitivity, wide detection range, and low detection limit have continuously attracted great interest due to their tremendous application potential in areas such as health and activity monitoring, smart textiles, human-machine interface, as well as safety and security. High sensitivity and a wide working range are key parameters for a high-quality strain sensor, but it’s difficult to attain both characteristics on the same sensor due to limitations in structure and conductivity.
Now, researchers from Tsinghua University debuted a flexible strain sensor design with a membrane made from stacking parallel and randomly aligned carbon nanofibers (CNF) that achieves both high sensitivity and a wide strain detection range. The stretchable sensor with a lip-language recognition system can directly and quickly translate sentences for people with damaged vocal cords. This can help people with damaged vocal cords navigate daily communication.
For this purpose, the flexible sensor needs to be able to collect information from large movements of facial muscles while also distinguishing more subtle changes. “The only way to meet this requirement is to prepare a flexible strain sensor with both high sensitivity and wide strain detection range,” said first author Peng Bi from Tsinghua University.
Flexible sensors can move and conform to human skin without causing discomfort. These sensors are typically fabricated of elastic polymers with two carbon-nanofiber membranes stacked on top. The design of an integrated membrane contains both parallel aligned (p-CNF) and randomly aligned (r-CNF) carbon nanofibers. The parallel aligned nanofiber membrane exhibits a low strain detection limit and high sensitivity, while the random aligned carbon nanofiber membrane exhibits a large strain detection range. By stacking parallel and randomly aligned carbon nanofiber membranes, the researchers realized a flexible strain sensor exhibits both high sensitivity and a wide working range.
“Remarkably, the obtained p/r-CNF-based strain sensor showed a strain detection limit as small as 0.005% and an ultra-high gauge factor value of up to 1272 for strains under 0.5%,” said Bi. “At the same time, its maximum strain detection limit is 100%, fulfilling the requirements for detecting most human motions.”
As proof of concept, researchers developed an intelligent lip-language recognition system by integrating p/r-CNF strain sensors, an Arduino microcomputer, and a loudspeaker. The system can “read lips,” correctly tracking phonetic symbols by interpreting lip movements, and then execute corresponding instructions, such as output lights or audio signals. According to the researchers, the recognition system has the potential to help people with language disabilities, proving the potential of this strain sensor in health management and medical assistance.
The team demonstrated that the sensor could accurately distinguish large motions, such as joint bending, and also detect minor motions, such as facial expression, eye rotation, pulse, and speaking. At present, the lip-language recognition system can only cope with limited communication scenarios and locations.
“We will build out application scenarios of the lip-language recognition system and improve the comfort and portability of wearing,” said Bi. “We hope that such a wearable device can become a second mouth for people with vocal cord damage and mitigate the effect of this type of injury on someone’s daily life.”
- Peng Bi, Mingchao Zhang, Shuo Li, Haojie Lu, Haomin Wang, Xiaoping Liang, Huarun Liang and Yingying Zhang. Ultra-sensitive and wide applicable strain sensor enabled by carbon nanofibers with dual alignment for human machine interfaces. Nano Research (2022); DOI: 10.1007/s12274-022-5162-0