Analysis of hygro-elastic properties of biocomposites to develop moisture-induced self-shaping actuators

Hygroscopically active materials have been developed with self-shaping ability, converting the water sensitivity of natural fibres into the driving force of the actuation (as observed in natural actuators such as pine cone scales). The actuation mechanism relies on the anisotropic swelling behaviour of natural fiber organized in a typical biocomposite bilayer structure. The aim of this study is to provide a better insight into the actuation mechanism and reliability of hygromorph biocomposites (HBCs) by evaluating the bending response as a function of fiber biochemistry and microstructure. The results obtained clearly show that cellulose content and microfibrillar angle in fibers influences the water uptake and swelling behaviour in biocomposites and therefore influences the curvature and the actuation speed of corresponding HBCs. Due to the effect of cycling (wetting/drying) the porosity is increased, not only accelerating the actuation but also greatly weakening the return to the initial position. Modifying the fiber nature and thus its biochemistry and microstructure leads to an improvement in the durability of HBCs.