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Wearable Strain Sensor Using Light Transmittance Helps Measure Physical Signals Better
KAIST researchers have developed a novel 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. This technology, featured in the March 4th issue of ACS Applied Materials & Interfaces as a front cover article, shows great potential for the detection of subtle human motions and the real-time monitoring of body postures for healthcare applications. 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 piezo-resistive and capacitive principles have been developed to meet all these requirements. Conventional piezo-resistive 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, piezo-capacitive strain sensors with better stability, lower hysteresis, and higher stretchability have been suggested. But due to the fact that piezo-capacitive 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 KAIST research team led by Professor Inkyu Park from the Department of Mechanical Engineering suggested that an optical-type stretchable strain sensor can be a good alternative to resolve the limitations of conventional piezo-resistive and piezo-capacitive 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, Professor Park and his researchers chose Ecoflex as an elastomeric substrate with good mechanical durability, flexibility, and attachability on human skin, and the new optical wearable strain sensor developed by the research group actually 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 research team could measure the finger bending motion and used it for robot control. They also developed a three-axes 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. Professor Park said, “In this study, our group developed a new wearable strain sensor platform that overcomes many limitations of previously developed resistive, capacitive, and optical-type stretchable strain sensors. Our sensor could be widely used in a variety of fields including soft robotics, wearable electronics, electronic skin, healthcare, and even entertainment.” This work was supported by the National Research Foundation (NRF) of Korea. Publication: Jimin Gu, Donguk Kwon, Junseong Ahn, and Inkyu Park. (2020) “Wearable Strain sensors Using Light Transmittance Change of Carbon Nanotube-Embedded Elastomers with Microcracks” ACS Applied Materials & Interfaces. Volume 12. Issue 9. Available online at https://doi.org/10.1021/acsami.9b18069 Profile: Inkyu Park Professor email@example.com http://mintlab1.kaist.ac.kr Micro/Nano Transducers Laboratory (MINT Lab) Department of Mechanical Engineering (ME)Korea Advanced Institute of Science and Technology (KAIST) Profile: Jimin Gu Ph.D. Candidate firstname.lastname@example.org http://mintlab1.kaist.ac.kr MINT Lab KAIST ME (END)
Artificial Muscles Bloom, Dance, and Wave
Wearing a flower brooch that blooms before your eyes sounds like magic. KAIST researchers have made it real with robotic muscles. Researchers have developed an ultrathin, artificial muscle for soft robotics. The advancement, recently reported in the journal Science Robotics, was demonstrated with a robotic blooming flower brooch, dancing robotic butterflies and fluttering tree leaves on a kinetic art piece. The robotic equivalent of a muscle that can move is called an actuator. The actuator expands, contracts or rotates like muscle fibers using a stimulus such as electricity. Engineers around the world are striving to develop more dynamic actuators that respond quickly, can bend without breaking, and are very durable. Soft, robotic muscles could have a wide variety of applications, from wearable electronics to advanced prosthetics. The team from KAIST’s Creative Research Initiative Center for Functionally Antagonistic Nano-Engineering developed a very thin, responsive, flexible and durable artificial muscle. The actuator looks like a skinny strip of paper about an inch long. They used a particular type of material called MXene, which is class of compounds that have layers only a few atoms thick. Their chosen MXene material (T3C2Tx) is made of thin layers of titanium and carbon compounds. It was not flexible by itself; sheets of material would flake off the actuator when bent in a loop. That changed when the MXene was “ionically cross-linked” — connected through an ionic bond — to a synthetic polymer. The combination of materials made the actuator flexible, while still maintaining strength and conductivity, which is critical for movements driven by electricity. Their particular combination performed better than others reported. Their actuator responded very quickly to low voltage, and lasted for more than five hours moving continuously. To prove the tiny robotic muscle works, the team incorporated the actuator into wearable art: an origami-inspired brooch mimics how a narcissus flower unfolds its petals when a small amount of electricity is applied. They also designed robotic butterflies that move their wings up and down, and made the leaves of a tree sculpture flutter. “Wearable robotics and kinetic art demonstrate how robotic muscles can have fun and beautiful applications,” said Il-Kwon Oh, lead paper author and professor of mechanical engineering. “It also shows the enormous potential for small, artificial muscles for a variety of uses, such as haptic feedback systems and active biomedical devices.” The team next plans to investigate more practical applications of MXene-based soft actuators and other engineering applications of MXene 2D nanomaterials.
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