A liquid power-ultrasound based green fabrication process for flexible strain sensors at room temperature and normal pressure

Flexible strain sensors with large stretchability, high sensitivity and good stability are highly desirable because of their potential applications in electronic skins and health monitoring systems. In this work, a liquid power-ultrasound based fabrication process for flexible strain sensors is reported. This method uses the acoustic cavitation and acoustic streaming in multi-walled carbon nanotube (MWCNT) water solution sonicated by power ultrasound of 19.9 kHz, to deposit MWCNT nanomaterials onto a 200 μm-thick polydimethylsiloxane (PDMS) substrate. The prepared strain sensor has a wide sensing range (≥420 % strain), high sensitivity (gauge factor, GF=8.4～68.3) and excellent stability (>10000 cycles at 50 % strain). The fabrication process is implemented at room temperature (25℃) and normal pressure (1 atm), and does not cause the atomization of nano solution. Also, it uses a minimum quantity of organic solvent (N,N-Dimethylformamide, DMF) as the disperser, and the remained MWCNTs in the fabrication process can be recycled. Apart from these green features, the fabrication process is not selective to the nano materials in the solution. Therefore, the proposed technique can be used in the fabrication of flexible strain sensors, electronic skin, and other flexible nano devices in an environment friendly way.