Microfibril Angle-Moisture Synergy Enables Fatigue Resistance in Wood Cell Walls.

Wood cell walls have long been recognized as natural fatigue-tolerant composites, yet the nanoscale mechanisms underlying their cyclic recovery remain difficult to resolve experimentally. Using large-scale molecular dynamics simulations, we reveal from an atomistic perspective how wood cell walls recover and resist fatigue, with the microfibril angle (MFA) and moisture synergistically regulating cyclic loading capacity. Higher MFAs can channel strain into interfacial slip and nanoscale delamination, dissipating energy before damage accumulates. Cyclic loading induces reversible reconfiguration of hydrogen bonds (HBs) in cellulose nanofibrils, redistributing stress and causing progressive hardening associated with an incomplete recovery of cellulose nanocrystalline packing. Moisture and higher MFAs act synergistically to balance energy dissipation and structural stability by promoting matrix plastic deformation, HB recovery, and multicycle ductility. Our work provides a mechanistic framework for understanding how wood cell walls can recover, harden, dissipate energy, and delay failure under cyclic deformation, offering bioinspired design principles for high-performance, sustainable materials.