Bio-inspired functional wood-based materials – hybrids and replicates

Wood is a CO2 storing material from renewable resoures with excellent mechanical properties and a sophisticated hierarchical structure from the nanoscale of cell wall polymers up to the macroscale of tree size. In recent years, one can observe an increasing research interest in modifying and functionalising wood cell and cell wall assemblies as well as cell wall components. In this context, three fundamental approaches aiming at developing novel and advanced bio-inspired and bio-based functional materials can be identified. At the level of bulk wood, this review will cover two research directions of wood functionalisation which have in common the utilisation of the hierarchical structure at different length scales, but differ in terms of the preservation of the organic scaffold of the wood cell wall. In those protocols that modify and functionalise wood cell walls with the emphasis on retaining their structural integrity, hybrid materials leading to polymer-or mineral-related wood products are developed. In the second wood functionalisation approach, the hierarchical structure of wood is used as a template to engineer wood-derived inorganic non-metallic materials. The third approach is at the level of cell wall components, where bio-based materials from wood cellulose nanofibres (CNF) are discussed. The use of CNF allows for designing structures with different porosities and new combinations of anisotropic properties beyond the predetermined hierarchical structure of wood. The review will cover fundamental aspects and various protocols for the three to date surprisingly separately treated approaches with a focus on synthesis procedures and characterisation of the modified materials with respect to the targeted functionalisation as well as potential fields of application.