Electrical properties of stretchable and skin-mountable PDMS/MWCNT hybrid composite films for flexible strain sensors

Flexible strain sensors based on carbon nanofillers have great potential in the application of skin-adhesive sensors, wearable sensors, and tactile sensors, due to their superior electrical properties. Herein, the electrical properties of highly sensitive PDMS/MWCNT strain sensors made by vacuum filtration method were investigated. In order to obtain the electrical percolation curve of the flexible conductive films, first different samples were made with the same surface area but with different wt. % of CNTs. Then, depending on CNT content, the obtained conductive films exhibited initial electrical resistance in the range of 12.5 K omega to 22.8 M omega. The piezoresistive films with the CNT concentration of 1.4 to 2.9 mg/mm2 had shown superior resistance drop, so this interval was determined as the percolation threshold region. According to the SEM images, the nanocomposite layer thickness of the flexible strain sensors in this region was 790 nm to 1210 nm. Afterward, the percolation curve was obtained using curve fitting to the experimental data and the exact value of the percolation threshold was defined as phi c=1.992mg/mm2. Finally, in order to determine the minimum gauge factor (GFmin) of the sensors in percolation region, a flexible strain sensor in the upper limit of this region was selected and the piezoresistive properties of the selected sample were investigated.