Ecoflex-Passivated Graphene-Yarn Composite for a Highly Conductive and Stretchable Strain Sensor.

We present a flexible strain sensor based on a graphene-yarn composite obtained by spray coating of graphene nanoplates. To improve the stretchability, graphene nanoplates were spray-coated instead of dip-coated on pre-stretched yarn. The spray-coating method yielded not only 3.68 times higher conductivity but also 2.1 times higher stretchability compared to the dip-coating method. The sensor spray-coated 400 times showed a high stretchability of 310%. Here, the relative resistance change (ΔR/R0) was 2.27 when a tensile strain of 50% was applied to the strain sensor. In addition, the fabricated sensor was coated with a protective layer of Ecoflex to minimize environmental effects. The passivated graphene-yarn composite sensor had a higher resistance than the unpassivated sensor because the Ecoflex film penetrated the conductive graphene nanoplates; however, the response to strains of up to 200% did not degrade after passivation. Furthermore, we demonstrated that our sensor can be used in wearable applications for monitoring individual finger movements and the wrist pulse.