Cylindrical relative humidity sensor based on poly-vinyl alcohol (PVA) for wearable computing devices with enhanced sensitivity

In this paper, a wearable resistive-type humidity sensor is fabricated on a cylindrical organic substrate for the first time. This sensor contained polyvinyl-alcohol (PVA) as the sensing material, which was applied by the dip-coating method at the middle, and copper (Cu) thin film was applied by RF magnetron sputtering for the side electrodes. The resistance was measured for 60–90%RH. Thus, the value was decreased by hydrogen bonds between the hydroxyl groups and water molecules in the highly humid condition. In addition, as the dip-coating speed and number of coats varied, corresponding factors α and β, which were related to the activation energy, thickness, and/or conductance, were suggested based on Ohm's law and conductivity equations. Moreover, to verify its durability as a wearable computing device, repetitive bending tests were performed with different bending times and radii.