Flexible sensors with tannin-modified vertical graphene arrays for the highly sensitive detection of humidity and strain

It is still a challenge for multi-functional flexible sensors to simultaneously improve the sensitivity to different external stimuli. Here, we construct vertical graphene arrays on the surface of a natural latex film with good stretchability. Then, the graphene was modified by tannic acid to endow it hydrophilicity. The modification of tannic acid not only improved the humidity sensing ability of the sensor, but also greatly improved its sensitivity to strain response. The strain response sensitivity was increased from 24.0 to 858.7 at 50 % strain, and the lower detection limit was also increased to 0.1 %. The sensor also showed good response stability in the cyclic strain test in 2000. The excellent strain response behavior enables the sensor to detect human joint motion and emotional fluctuations. The modification of tannic acid improved the hydrophilicity of graphene and made the sensor have good humidity response ability. The modified sensor has humidity sensing capability in the range of 0–100 % humidity. The sensor can detect 0.5 % humidity change in dry environment and 2 % humidity change in high humidity environment. The high sensitivity to humidity also allows the sensor to detect the movement of wet objects in a non-contact manner. The research results show that the sensor has potential applications in wearable devices, electronic skin, intelligent artificial limbs and other fields.