Researchers have developed a wearable strain sensor based on the modulation of optical transmittance of a carbon nanotube (CNT)-embedded elastomer. The sensor is capable of sensitive, stable, and continuous measurement of physical signals.
A wearable strain sensor must have high sensitivity, flexibility, and stretchability as well as low cost. Those used especially for health monitoring should also be tied to long-term solid performance and be environmentally stable. Various stretchable strain sensors based on piezoresistive and capacitive principles have been developed to meet all these requirements.
Conventional piezoresistive strain sensors using functional nanomaterials, including CNTs as the most common example, have shown high sensitivity and great sensing performance; however, they suffer from poor long-term stability and linearity as well as considerable signal hysteresis. As an alternative, piezocapacitive strain sensors with better stability, lower hysteresis, and higher stretchability have been suggested. But due to the fact that piezocapacitive strain sensors exhibit limited sensitivity and strong electromagnetic interference caused by the conductive objects in the surrounding environment, these conventional stretchable strain sensors are still facing limitations that are yet to be resolved.
A research team suggested that an optical-type stretchable strain sensor can be a good alternative to resolve the limitations of conventional piezoresistive and piezocapacitive strain sensors because they have high stability and are less affected by environmental disturbances. The team then introduced an optical wearable strain sensor based on the light transmittance changes of a CNT-embedded elastomer, which further addresses the low-sensitivity problem of conventional optical stretchable strain sensors.
In order to achieve a large dynamic range for the sensor, researchers chose Ecoflex as an elastomeric substrate with good mechanical durability, flexibility, and attachability on human skin. The new optical wearable strain sensor shows a wide dynamic range of 0 to 400%. In addition, the researchers propagated the microcracks under tensile strain within the film of multi-walled CNTs embedded in the Ecoflex substrate, changing the optical transmittance of the film. By doing so, it was possible for them to develop a wearable strain sensor having a sensitivity 10 times higher than conventional optical stretchable strain sensors.
The proposed sensor has also passed the durability test with excellent results. The sensor’s response after 13,000 sets of cyclic loading was stable without any noticeable drift. This suggests that the sensor response can be used without degradation, even if the sensor is repeatedly used for a long time and in various environmental conditions.
Using the developed sensor, the team could measure the finger-bending motion and use it for robot control. They also developed a three-axis sensor array for body posture monitoring. The sensor was able to monitor human motions with small strains such as a pulse near the carotid artery and muscle movement around the mouth during pronunciation. The sensor could be widely used in a variety of fields including soft robotics, wearable electronics, electric skin, and healthcare.