Enhanced strain gradient in structural wood for high flexoelectricity

Unlike piezoelectricity, flexoelectricity is not limited by crystalline symmetry and is therefore a more widespread electromechanical property of solids, inspiring applications in flexoelectronics, sensing, actuating and energy harvesting. It has been widely investigated in synthetic materials such as crystals, ceramics and metals, but remains unexplored in natural biomaterials like wood. Here, we report the observation of high flexoelectricity in wood, achieved through structural modification via delignification combined with compression. The structural wood exhibits a high flexoelectric coefficient (36.72 nC·m^-1), comparable to dielectric ceramics, and several times greater than semiconductors and polymers. The observed high flexoelectricity arises from the molecular-level charge asymmetry in cellulose's monoclinic crystalline structure, amplified by the anisotropic and hierarchical architecture of wood. Furthermore, we illustrate the practical potential by developing a flexible wood-based sensor capable of detecting subtle changes in human motion. This work provides a route to exploiting flexible wood for electromechanical applications.