Transparent wood-based materials: A new step toward sustainability and circularity

Wood is a natural composite material mainly consisting of three components, i.e., cellulose, hemicellulose, and lignin. It exhibits a complex hierarchical structure characterized by open channels, aligned in the growth direction, having specific porosity at micro-, meso-, and macro-scale, and an extended opacity, due to adsorption phenomena because of the presence of lignin and scattering, as different refractive indexes characterize its components. Even if during the historical ages some of its applications have been replaced by other materials, wood still covers a large part of common uses, which range from biomass for energy recovery to material for the building sector, or from artifacts to household/furniture manufacturing. Despite its real invention dating to 1992, only about ten years ago two independent research groups, one from the University of Maryland (USA) and the other from the Royal Institute of Technology (Sweden) rediscovered and started to thoroughly investigate the so-called transparent wood (TW). TW can be derived from almost any wood biomass through specific chemical treatments focused on lignin. These aim to completely remove this component from wood or to eliminate the chromophore groups present in the pristine material, hence obtaining, after direct densification or after infiltration with a suitable polymer resin, a new material with very high transparency, toughness, and lightness. These characteristics can further be combined with other specific features (such as environmental protection, flame retardancy, photoluminescence, and energy storage ability, among others), which open the way toward the development of new, up-to-date, advanced, and sustainable materials for both structural and functional purposes, fulfilling the current concepts of circular economy and sustainability. The present review is aimed at providing the reader with an overview of the characteristics of transparent wood, describing the latest applications and, finally, discussing some challenging issues and perspectives for possible developments in the forthcoming years.