Rhubarb petioles inspire biodegradable cellulose fibre-reinforced PLA composites with increased impact strength

The petioles of red rhubarb are load-bearing structures which need to combine sufficient stiffness with high toughness. Although mostly consisting out of water, the petioles showed a remarkable Charpy impact strength, partly based on their beam-like structure: outermost bark fibre bundles provide stiffness and strength, and surround a core reinforced with ductile vascular bundles, which dissipate impact energy at two hierarchical levels: helical fibres in the vascular bundles are straightened, and both helical fibres and bundles are pulled-out of the parenchyma. Using technical composites made from stiff and ductile cellulose fibres as a model system, we investigated the functional significance of the fibre arrangement further. The impact resistance of bio-inspired composites exceeded that of composites with identical fibre-fraction but random fibre-distribution by more than a factor of two, while their tensile and flexural properties did not differ significantly, suggesting promising new routes for the design of tough, bio-compatible composites.