Moisture-induced self-shaping flax-reinforced polypropylene biocomposite actuator

Pine cone scales exhibit moisture-induced bending actuation which is driven by their bilayered architecture made up of tissues with different hygroscopic properties. Inspired by this mechanism, we describe the behaviour of moisture-induced self-shaping biocomposites displaying a bilayer structure. Even though plant fibres are assumed to display weaknesses as a reinforcing agent in the field of structural biocomposites, i.e. due to their high water absorption and swelling, this behaviour can be used as a driving force when considering an additional biocomposite function such as actuation. A reversible bending is obtained when the studied biocomposite is immersed and then taken out of water. In fact, moisture displacement during wetting / drying steps appears to exert a linear control on the amplitude and rate of movement. Although motion is relatively slow compared to natural actuators such as pine cones, we can nevertheless envisage further material optimizations.