Wood can be the starting point for a large variety of nanostructured polymer composites. The wood cell wall is a sophisticated nanocomposite of several biopolymers with carefully controlled structure at molecular and nanoscale. The biosynthesis has similarities with features of industrial composites processing, but the scale is micrometers to nanometers/molecular level. At cell wall scale, the reinforcing cellulose nanoscale fibrils are combined with hemicellulose polysaccharides and then small lignin molecules are polymerized.

In chemical kraft pulping, most of the lignin is removed and the resulting wood fiber cells primarily contain cellulose fibrils and hemicelluloses. We can disintegrate the fibrils from the fiber and use them in bottom-up preparation of new composites. The organization of those cellulose nanofibrils (CNF) into oriented fibrils with controlled interfibril distance then becomes the same challenge as for all other nanofibers available. One remaining advantage with CNFs is that they can readily form stable hydrocolloids, with the use of controlled surface charge.

We may also use the nanofibril structure in porous wood pulp fibers for the purpose of chemical manipulation. Examples include controlled polymerization strategies inside the pulp fiber cell wall or controlled cell wall swelling followed by prepolymer impregnation. Such fibers are “nanostructured” and can provide favorable reinforcement mechanisms to polymer matrices.

Wood veneer can also be used as a reinforcement substrate by itself. By careful control, wood can be delignified and yet preserve its cellular structure. Such substrates have been impregnated by monomers, luminescent dyes, quantum dots, inorganic nanoparticles and clay platelets as well as metal ions. One may also manipulate the cell wall nanostructure by the precipitation of plasmonic nanoparticles and magnetic nanoparticles from metal salts. This makes it possible to synthesize complex composites where the wood substrate is combined with inorganic nanoparticles and a continuous polymer matrix.

A particular challenge is composites which combine mechanical performance with high optical transmittance. The polymer matrix needs to match the refractive index of the substrate but the presence of optical defects at the microscale also needs to be minimized by careful processing. In the ideal structure for low extent of light scattering, the cellulose reinforcement needs to be distributed homogeneously at small scale. Since there are not so many transparent materials, such cellulosic composites offer potential in solar cells, lasing, translucent windows, lighting, building panels with structural color, thermochromic and photochromic materials, electrochromic mirrors and more. The challenge of sustainable development means that green chemistry, biobased constituents and low embodied energy must be integrated in wood-based composites developments and form criteria for new material concepts.