3D Printing Wood Tissue
Abstract: Biomass from forests provides society with energy, materials and chemicals, thus contributing to the circular bioeconomy. The majority of biomass is found in the wood tissue of trees. Its composition and hierarchical structure originates from the synthesis and bottom-up assembly of biopolymers which involve numerous genes, hormones and exogenous factors. A technology for bottom-up fabrication of materials is 3D printing. In 3D printing, material is assembled layer-by-layer and thereby offers the potential to build up hierarchical complex structures with control of design and material properties. 3D printing wood is not as straight forward as for plastics since wood can’t be processed by melting. Also, printing wood involves the assembly of multiple polymers since wood is a composite material. Inspired by the composition, crosslinking mechanism, anisotropy and structural design of natural wood tissue, this work has established a platform for 3D printing wood biopolymers into hierarchical wood-like structures. The platform consists of extrusion-based 3D printers, designed printing pathways, and wood based solutions and dispersions which are called inks. We found that inks of both cellulose dissolved in ionic liquid and dispersions of cellulose nanofibrils (CNF) were printable due to their shear thinning properties. Good printing fidelity of cellulose solutions required a continuous gel formation. Printing on a coagulating gel allowed non-solvent to diffuse through the print and instantly regenerate cellulose. Diffusion through multiple layers was however challenging making it difficult to 3D print large constructs. CNF (1-4 wt%) exhibits a yield stress, and stops flowing when leaving the nozzle which facilitated the printing of multilayered structures, i.e. an ear. This also contributed to the printing resolution (≈ 300 μm). However, without crosslinking, the printed CNF could not withstand mechanical force. Hence, CNF was mixed with crosslinkable biopolymers. The mixed inks remained printable for CNF concentrations above 2 wt%. The crosslinking time was below 10 minutes and gel strength increased with the concentration of crosslinkable biopolymers. Inks containing alginate were ionically crosslinked and formed reversible hydrogels. Enzymatic crosslinking, similar to the polymerization of monolignols in the wood cell wall was obtained by substituting carboxylic groups (COOH) of hemicelluloses with tyramine. Hydrogels with tunable mechanical properties were obtained by varying the degree of substitution by using xylan, or TEMPO oxidized galactoglucomannan with degrees of oxidation from 10 to 60%. A computational fluid dynamics simulation tool was studied as a complement to 3D printing tests of new inks to evaluate printability. By simulation, it was easy to isolate parameters such as printing speed and printing height to study their influence on printing fidelity. Finally, natural bottom up assembly of wood tissue was substituted with 3D printing. G-code substituted genome and the cellulose was extruded by a printer head instead of the rosette. Structures that resemble morphological features found in wood were prepared by computer aided design and printed with all wood based inks. Control of printing paths provided anisotropic features resembling the micro fibril angle of the cell wall. The breakthrough of this work is the 3D shaping of wood by a bottom up process. Consequently, products assembled by wood biopolymers can transform from 2D (paper, board, films, textiles) to 3D. The concepts developed in this work can be employed in future applications of 3D printing with wood based materials, such as garments, electronics, wound dressings and packaging.
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