Environmental Sensor Based on Stretchable Textile
Authors: Lee Hyung-Kun , Kim Do Yeob , Lee Sung Q , Youm Wooseup 
Published: February 2020

Flexible and stretchable electronic devices have been widely studied because of various applications such as wearable devices and body-attachable devices based on facile human-machine interfaces. Research on this field has focused on the development of flexible, high-performance electronics devices ensures stability for physical/chemical stress and facile modification with various shapes. Chemical sensors, environmental sensors and biosensors along with physical sensors based on the flexible and stretchable devices have been reported through extensive investigations on various elastic substrates designs such as serpentine (P. Cutruf et al. Appl. Phys. Lett. 2014, 104, 021908), wavy (H. Fei et al. J. Vac. Sci. Technol. A
2009, 27, L9) and mogul (H.-B. Lee et al. Adv. Mater. 2016, DOI:10.1002/adma.201505218) patterns and sensing materials such as carbon nanotubes (S. Park et al. Nanoscale
2013, 5, 1727), reduced graphene oxides (N. O. Weiss et al. Adv. Mater. 2012, 24, 5782) and silver nanowires (P. Lee et al. Adv. Mater. 2012, 24, 3326). These sensors have been applied as tactile sensors, strain sensor, gas sensors, etc. The authors reported a method of graphene coating on electrospun fibers and commercially available yarns to convert them into conductive yarns (Y. J. Yun et al. Adv. Mater. 2013, 25, 5701) or chemical sensors (Y. J. Yun et al. Nanoscale
2014, 6, 6511; Y. J. Yun et al. Sci. Rep. 2015, 5, 10904) by utilizing molecular glues such as bovine serum albumin (BSA) and polydopamine, and found that the method was effective on increasing durability to mechanical and chemical stresses. The gas sensors coated with reduced graphene oxides on electrospun nylon fiber and commercial cotton yarn show 3 times higher responses to nitrogen dioxide (NO2) gas and durability to bending stress and washing treatment. We report a stretchable gas sensor by coating reduced graphene oxides on a stretchable yarn that can be elongated up to 400% compared to its original length without changing an electrical resistance. Optimal coating concentration of graphene oxide and its effective reducing condition were investigated to prepared stretchable and durable gas sensor using BSA as a molecular glue. We found that stretchable yarn showed no change of resistance upon 5000 times stretching treatments with 100% strain. Furthermore, the stretchable yarn showed 45% response to 1 ppm of NO2 gas under 45% relative humidity, which is 3 times higher response to what we obtained using cotton yarn coated with reduced graphene oxide (15% reponse@NO2 1.25 ppm). The stretchable yarn under 200% strain shows a similar response to the response of the yarn under no strain, which is one of the requirements for the stretchable electronic devices in order to find practical applications without limitation. The explanation of the sensitive response to NO2 gas and durability to mechanical stress will be covered through SEM (scanning electron microscope), Raman and TEM (transmission electron microscope) analyses. One of the advantages of reduced graphene oxides as a sensing material is the response selectivity to NO2 gas compared with other gases such as VOCs (volatile organic compounds). The stretchable yarn coated with reduced graphene oxides shows a selective response to NO2 gas rather than acetone, ethanol, carbon monoxide, carbon dioxide, ethylene, etc. NO2 is known as one of the most common exhausted gases from automobiles. The selective response to NO2 of the stretchable yarn can be considered as an environmental monitoring sensor. However, the sensor composed of reduced graphene oxide has a notorious slow recovery behavior that hinder an application of the sensor. Lastly, we found an effective recovery method of the sensor through UV irradiation for the practical application of the stretchable yarn as NO2gas sensor. (This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted fi nancial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20142020500160))