Resonant hair humidity sensors for disposable applications: Revisit the hair hygrometer

Hairs have long been used for humidity sensing with the aid of their unique property, elongation upon the increase of ambient humidity. However, relatively long strands of hairs are required to induce measurable length change. While such long strands of hairs amplify the readout signal, they, in turn, compromise the spatiotemporal resolution. Here, we revisit the hair hygrometer, firstly demonstrated by a Swiss physicist and geologist, Horace Bénédict de Saussure in 1783, by making suspended string resonators with a short segment of human hairs. A short segment of human hair is placed onto a substrate with a through-hole and then firmly fixed under tension to make a taut suspended string. As a final step, a thin gold layer is locally deposited onto the suspended hair through a shadow mask. While the prepared hair resonator is driven by a piezo actuator, its resonance frequency is optically measured. Measurements confirm that the resonance frequency of the hair resonator decreases as the relative humidity increases. The water absorption decreases both the tension and the stiffness of the hair resonator and increases the inertial mass of the hair resonator, all of which contribute to the reduction of the resonance frequency.