Shape recovery and anomalous swelling of steam-compressed wood by swimming ring-like expansion of cell lumina

To investigate the mechanism of shape fixation of steam-compressed wood, Japanese cedar wood specimens were compressed radially by 50% using saturated water vapor at 160 °C and observed during shape recovery in different liquids, including water and dimethyl sulfoxide (DMSO). Shape recovery was evaluated based on the dimensional changes in wet conditions (SRw) and those in dry conditions (SRd). SRw in DMSO was significantly greater than that in water, but this recovery observed in DMSO was not recognized in SRd. The contradiction arose from significant reversible cellular deformation: the compressed cells were reversibly expanded with the swelling of the cell wall, and this swimming ring-like cellular deformation increased SRw in DMSO. At room temperature, the SRd values in various liquids were closely connected to the swelling of the wood cell walls, rather than to the degree of softening. This suggested that the plastic expansion of cells, as well as the softening of the wood cell walls, caused the shape recovery of compressed wood. This mechanism was qualitatively expressed by an elasto-plastic model equipped with an inflator representing the swimming ring-like expansion of the cell lumina.