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Over the past decades, electronics engineers developed increasingly small, flexible and sophisticated sensors that can pick up a wide range of signals, ranging from human motions to heartrate and other biological signals. These sensors have in turn enabled the development of new electronics, including smartwatches, biomedical devices that can help monitor the health of users over time and other wearable or implantable systems.

Strain sensors, which are designed to convert mechanical force into electrical signals, are among the most widely used sensing devices within the electronics industry, as they can be valuable for tracking both human movements and health-related biological signals. While these sensors are already embedded in many electronic devices, most existing solutions are only able to track movements in one direction.

Sensors that can accurately pick up movements and forces in multiple directions could be highly advantageous, as they could be applied to a wider range of scenarios. In addition, these sensors could be embedded in existing electronic devices to broaden their functions or enhance their capabilities.

Researchers at Peking University recently developed a new promising strain sensor that can detect deformations in multiple directions. This sensor, presented in a paper in ACS Sensors, was fabricated using tiny carbon-based cylindrical structures known as carbon nanotubes.

"Flexible and stretchable sensors have garnered significant attention in the fields of human–computer interaction, motion capture, and health monitoring," Yongsheng Yang, Qinqi Ren and their colleagues wrote in their paper. "Presently, most sensors are limited to capturing motion in a single direction and lack the capability to analyze multidirectional deformations in the real world. A single device capable of detecting multidirectional deformations has always been a high expectation and a daunting challenge."

To develop a sensor that can detect multidirectional deformations, Yang, Ren and their colleagues grew vertically aligned carbon nanotubes onto a thin silicon wafer. They then moved this wafer on a highly flexible material, which could be easily integrated into wearable devices.

"We realize the idea of using a single sensor for multidirectional sensing by adopting a 'one-step' rolling process to transfer vertically aligned carbon nanotubes grown on a silicon wafer onto a flexible Ecoflex substrate," wrote the researchers. "The entire preparation process is simple and efficient. Distinct conductive paths form along different directions controlled by the rolling process and the pattern design of carbon nanotubes, thus resulting in a sensitive directional dependence."

The rolling process employed by the team enables the formation of different conductive paths (i.e., routes through which electricity can flow). When they evaluated their sensor's performance, the researchers found that it could pick up deformations in multiple directions with high precision.

"The sensor exhibits remarkable performance, including a wide operating range (0–120%), high sensitivity (GF = 126.6), short response time (64 ms), and good stability (over 4,000 cycles under strain 40%)," wrote the researchers. "The sensors are demonstrated for detecting motion signals and monitoring human health, ranging from subtle motion signals to large deformation.

"These sensor characteristics fulfill the requirements of various practical scenarios and have an immense potential for applications in human–computer interaction interfaces, intelligent robots, and in situ health monitoring."

In the future, the new strain sensor developed by this team of researchers could be improved further and integrated in a wide range of electronics. Most notably, it could be used to develop more advanced biomedical devices, smartwatches, fitness trackers, prosthetic limbs and robotic systems.

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