Transparent wood for a bio-based future: A critical review

Transparent wood (TW) is an emerging class of bio-based composites that combines high optical transmittance, mechanical strength, and tunable light scattering. This review provides a critical and integrated overview of recent advances in the design and processing of TW, with particular emphasis on how modifications of the wood template, through chemical, structural, and cell-wall engineering, govern optical and mechanical performance. Tailoring haze is highlighted as a key design parameter, enabling precise control of light diffusion and optical response for applications ranging from clear glazing to diffusive solar harvesting and smart window systems. Beyond functional low- and high-haze TW, the review also addresses the emerging class of aesthetic TW, in which visual appearance, diffuse light transmission, and architectural integration are prioritized over maximum transparency. Dynamic and functionalized TW systems, including thermo- and photo-responsive structures, luminescent films, and composites incorporating micro- and nano-scale fillers, are discussed as routes toward adaptive and multifunctional materials. Finally, the review critically examines the challenges associated with scalability, reproducibility, standardization, and sustainability, including processing bottlenecks, measurement comparability, and life-cycle considerations. By linking material design, optical scattering mechanisms, and manufacturing constraints, this work outlines a coherent pathway from laboratory-scale innovation to the development of industrially viable, biobased, and circular TW materials.