Humidity responsive self-actuated cellulose-based materials

Toward the production of humidity-responsive self-actuators, the humidity actuation in some plants (e.g. pinecones) has been mimicked. For this purpose, a cellulose-based material (cellulose acetate), renowned for its hygroscopic behavior, was used as a lamina for bi-layered self-actuators. The hygroscopic properties of the membrane material were evaluated via gravimetric measurement and thermomechanical analysis, at constant temperature (T = 25 °C), in a wide range of relative humidity (RH = 21–76%). The "variable surface concentration" model was used for the description of sigmoidal moisture diffusion resulted from the gravimetric measurements. The interpolation of this model to the experimental data allowed to have a polynomial function of relative humidity for the concentration at saturation (Csat), with constant values of: the diffusion coefficient D = 3.35×10-6 mm2⁄s, and the relaxation factor  = 0.026 s-1. Thermomechanical Analysis provided the measurement of the induced hygroscopic expansion (εhygro) resulting in a polynomial function of relative humidity. The coefficient of hygroscopic expansion (α) has then been obtained by the ratio of εhygro and Csat as function of relative humidity. These properties were used to build a COMSOL Multiphysics finite element model to predict the sigmoidal moisture diffusion, moisture concentration, induced hygroscopic expansion, and the relevant induced bending deformation of self-actuated bi-layers for different levels of relative humidity. The results of the finite element model reproduce the real-time hygro-mechanical response of bi-layered composite consisting of a cellulose acetate membrane coupled to a non-hygroscopic substrate.