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TECHNOLOGY 35
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packed along fibre direction. flexural performance, with the longitudinal flexural strength
The anisotropic structure further exacerbates the directional approaching 400 MPa.
differences in the mechanical properties of wood. Compression This represents a significant advantage over compressed
densification significantly improves the longitudinal tensile wood, where the longitudinal flexural strength varies considerably

strength of wood (compressed wood >> natural wood), depending on whether the load axis is aligned with the
and also enhances the transverse tensile strength when compression or non-compression direction (96 MPa vs. ~400
perpendicular to the compression direction (compressed wood MPa).
>> natural wood). The resulting sustainable, super-strong wood emerges as a
However, when the load axis is parallel to the compression strong competitor for replacing traditional structural materials
direction, the transverse tensile strength of compressed such as metals and alloys.
densified wood is considerably lower than that of natural
wood, primarily due to microdefects, stress concentrations, Chemicals And Raw Materials
viscoelastic behaviour, and shape memory effects. The basswood used in this study is scientifically classified as

Additionally, compression densified wood faces issues such Tilia amurensis. It primarily grows in Northeast China.
as expansion and rebound along the compression direction. The material was purchased from Oriental Aolong Wooden
Although hydrogen bonding or chemical treatments can Crafts. Chemicals used for removing lignin from wood were
partially improve dimensional stability, the issue of low transverse sodium hydroxide (NaOH), and sodium sulphite (Na2SO3),
tensile strength in the compression direction persists, limiting both of which were purchased from Sinopharm Chemical
its application in complex mechanical environments. Reagent. The anhydrous lithium chloride (LiCl) was purchased
Therefore, the development of a new isotropic densification from Xilong Scientific. The solvents used included N,N-

technology that improves the mechanical properties and dimethylacetamide (purchased from Sinopharm Chemical
dimensional stability of wood in all directions is key to Reagent) and deionised water.
addressing this problem and will open new avenues for the
broader application of wood.
In this paper, we report a self-densification strategy to
transform porous natural wood directly into a dense and
super-strong material without hot-pressing.
Our approach involves setting free cellulose fibrils from
the lignin confinement through partial delignification, then

the cellulose fibrils moving inward and filling the cell lumen
via lithium chloride/dimethylacetamide (LiCl/DMAc) swelling
process, followed by air-drying.
Unlike unidirectionally compressed densified wood, our
self-densified wood exhibits uniform shrinkage in the transverse
area, resulting in greatly improved mechanical properties in Fig. 1. Schematic of self-densification strategy to transform (a) natural wood
all directions. into (b) self-densified super-strong wood without hot-pressing, compared
with (c) traditional compressed densified wood. (d) Photographs of the
Multi-scale aligned wood fibres, as functional units
compressed densified wood featuring one-direction reinforcement and
assembled with enhanced hydrogen bonds, exhibit excellent self-densified wood featuring multi-direction reinforcement.