Wood-water interaction: fundamental understanding, advanced characterization and functional frontiers

Water plays a fundamental role in wood science and technology, not only governing the multi-scale structure and properties of wood, but also affecting wood processing and application as well as the design of wood-based functional materials. Following a structure–property–function logic, this review summarizes current understanding of wood-water interactions. We outline the hierarchical anatomical structure and chemical composition of wood; distinguish states of water in wood; and clarify the fiber saturation point as the hygroscopic limit versus the cell wall saturation capacity. We discuss sorption isotherms, hysteresis, and diffusion with a coupled poromechanical perspective, highlighting the molecular mechanisms of hygroscopic response of the cell wall components. We summarize the corresponding in-situ characterizations and connect them to modeling across length scales. Facing the functional frontier, we introduce hygromorphic actuation and one-time self-shaping, directional liquid transport and separation, and 4D-printed programmable wood composites. This review establishes an up-to-date understanding of wood-water relations, integrates in-situ techniques with modeling across length scales, and provides inspiring insights for the design and engineering use of wood-based functional materials.