EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Trees are known to adapt to mechanical requirements by forming material with carefully designed ultrastructure und tuned mechanical properties. They are therefore attractive model systems for bio-inspired materials with optimized properties. Since new wood cells are formed exclusively at the interface between wood and bark (the so called cambium), mechanical stresses at this place at the time of cell formation have an impact on the new cell’s ultrastructure. In particular, the arrangement of the crystalline cellulose microfibrils in the wood cell wall, and most specifically their tilt angle (microfibril angle, MFA), is a powerful means of mechanical optimization. Its variation allows the formation of mechanically vastly different material with stiffness or extensibility variations by a factor of more than 10 [1]. While there are a number of studies on wood ultrastructure and mechanical properties, the direct ultrastructure response of wood to mechanical stresses has not been studied. By synchrotron micro- and nanobeam x-ray diffraction imaging with specifically adapted diffraction geometry [2, 3], we could show the transition between normal wood and compression wood in conifer branches and stems that have been bent in a controlled way. Compression wood is characterized by a very large MFA and usually found on the lower side of the conifer branches, as it is mechanically designed to keep the branch up. We could show that this arrangement is subject to purely mechanical triggers, since we could reverse it by mechanical stimuli to the opposite direction. We also observed an apparently gradual transition to extreme reaction wood in young stems, indicative of a delayed response of the tree to acute stresses.

1. Reiterer, A., et al., Philos. Mag. A, 1999. 79(9): 2173-2184.
2. Lichtenegger, H., et al., J. Appl. Crystallogr., 1999. 32: 1127-1133.
3. Grünewald, T.A., et al., Angew. Chem.-Int. Edit., 2016. 55(40): 12190–12194.