A Green Strategy Based on In Situ Lignin Fusing and Reorganization: Synergistic Enhancement of Dimensional Stability and Permeability in Bamboo

Bamboo’s utility is limited by inherent anisotropy, which leads to dimensional instability, low permeability that hinders treatment, and modification methods lacking eco-friendliness and performance. Therefore, this study proposes a green strategy, employing combined deep eutectic solvent and heat treatment at 140 °C, based on in situ lignin dissolution, regeneration, migration, and reassembly to synergistically improve the dimensional stability and permeability of bamboo. Results showed that in modified bamboo, lignin from the compound middle lamella and cell wall corners migrated to the secondary wall; hemicellulose and amorphous cellulose underwent degradation or reconstruction; and cellulose, hemicellulose, and lignin were reorganized. These changes significantly enhanced the dimensional stability of bamboo, with dry shrinkage rate and wet swelling rate reduced by approximately 65 and 53%, respectively. Simultaneously, the redistribution of lignin facilitated stress transfer, leading to an increase in bamboo’s flexural strength and elastic modulus by approximately 11.5 and 5.7%, respectively. The depolymerization of lignin introduced abundant macropores within the bamboo, reducing material density, markedly improving permeability, and leading to a 56.9% increase in water absorption. Furthermore, the modified bamboo exhibited excellent humidity regulation, photothermal conversion, flame retardancy, and antifungal properties. This strategy provides a green and sustainable solution for the high-value and multifunctional utilization of bamboo.