Facile green strategy for high-strength, wet-resistant, and transparent wood film via alkali cell wall swelling

The development of sustainable, high-performance alternatives to petroleum-derived plastics remains a critical challenge in materials science, especially under the global carbon neutrality goals. Herein, we reported a facile polymer-free strategy to fabricate high-strength, flexible, and biodegradable all-wood films (WFs) by inducing cell wall swelling within a delignified wood skeleton via simple alkaline treatments under varied conditions. Notably, by employing cold-alkali swelling (7 wt% NaOH at −6 °C for 24 h) followed with hot-pressing, the resultant WF-C24 with densified multilayered structure exhibited an ultra-high tensile strength (530.1 ± 28.3 MPa), superior toughness (26.9 MJ m −3 ), and a folding endurance of 7636 times, surpassing most of the reported wood films. They also demonstrated high transparency (71% at 600 nm), extremely low thermal conductivity (0.079 W m −1  K −1 ), good thermal stability, and remarkable wet tensile strength (~55 MPa), greatly exceeding typical cellulose-based films. The cold-alkali treatment provided extensive cell wall swelling combined with partial dissolution and regeneration which facilitate to form strong hydrogen bonding networks at cell wall interphases, leading to a “self-adhesive” wood film without any chemical adhesives. Besides, the WFs were fully recyclable and could be reproduced into cellulose papers with increased mechanical strength after repeated cycles, highlighting its sustainable recycling merits. Complete biodegradation within three months in natural soil condition further affirmed its environmental compatibility. These findings established a promising scalable and sustainability pathway for the cold-alkali-treatment wood films as bioplastic in advanced applications, such as heat insulating material, green packaging, and flexible electronics.