Extracting wood elastic constants at the level of the cell wall layers by nano-indentation with atomic force microscopy

Linking the ultrastructural organization of wood cell walls to their mechanical behavior remains a major challenge due to their hierarchical structure and the strong anisotropy induced by the nearly uniform orientation of cellulose microfibrils. In particular, determining the elastic constants of individual cell wall layers at the nanoscale remains experimentally challenging. Here, we introduce an experimental strategy that combines AFM nanoindentation with a microfibril-angle (MFA)-resolved mechanical analysis to identify transverse isotropic elastic properties of the main wood cell wall layers, as well as trends in their viscoplastic properties. By exploiting the continuous variation of the effective MFA, generated by cutting samples at controlled orientations, the approach provides a large and robust dataset that enables the identification of longitudinal, transverse and shear elastic moduli through reverse analysis. The method is applied to poplar wood to characterize the mechanical behavior of the S 2 layer, in normal and opposite wood, and to the gelatinous G layer, in tension wood. The results reveal marked differences in anisotropy and shear response between lignified S 2 layers and the non-lignified cellulose-rich G layer. Comparison with values reported using other experimental methods highlights the relevance and robustness of the proposed approach for probing the elastic properties of wood cell wall layers. Analysis of viscoplastic parameters as a function of the MFA reveals marked and unexpected differences between the S 2 and G layers, as well as with the evolution of the macroscopic viscous dissipation with MFA reported in the literature.