Fig. 1.

Schematic of the top-down approach to directly transform natural balsa into ultrathin wood films. a Left: schematic of the natural wood with its porous structure. Middle: the microstructure of the cellulose fibers in the wood cell walls. Right: the molecular chains of the cellulose fibers. b Left: schematic of the ultrathin wood with intertwined, compressed wood channels. Middle: the microstructure of the highly oriented cellulose fibers in the ultrathin wood cell walls. Right: hydrogen bond formation between the neighboring cellulose molecular chains. c Meter-long natural balsa wood with a thickness of 300 μm made in the lab