Nanocellulose-clay composites as substrates for printed humidity sensors

Flexible and biodegradable nanocomposite films were developed as substrates of printed humidity sensors, using cellulose nanofibers (CNFs) that were reinforced with naturally abundant nanoclays, microfibrous sepiolite and lamellar cloisite. The addition of nanoclays to the CNF matrix led to notable changes in film morphology, surface roughness, wettability, and optical properties, as confirmed by structural, spectroscopic, and microscopic characterization. These modifications also improved the mechanical properties of the films, increasing stiffness and tensile strength. Importantly, bending tests revealed that the substrates preserved their mechanical and electrical functionality under repeated deformation, confirming their suitability for flexible electronic systems. The humidity sensing performance was evaluated by measuring impedance magnitude and phase responses across a range of relative humidity levels. The type and concentration of clay strongly influenced the sensing behavior; sepiolite enhanced water affinity and extended the detection range, while cloisite promoted a more capacitive response with greater phase sensitivity. Both nanocomposites demonstrated reliable signal stability and response linearity with sensitivities of –0.043 Ω/%RH and –0.071 Ω/%RH for sepiolite and cloisite nanocomposites, respectively, across wide relative humidity ranges. Additionally, the biodegradability of the sensors was proved in fungal growth assays. Overall, this study demonstrates that CNF-based nanocomposites containing natural clays offer a sustainable and tunable platform for the fabrication of flexible, low-impact humidity sensors and contribute to the advancement of transient and eco-friendly electronic technologies.