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TECHNOLOGY 37
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The charpy impact properties of the wood were tested
using pendulum impact testing machine and the general size
of samples were 60 mm by 10 mm by 3 mm.
For all the tests, at least five specimens of each group

were tested, and the average values were reported with
their standard deviations. The shear tests and pullout tests
of wood nails were performed using a universal mechanical
testing machine.
For shear tests, the nails were hammered into a 3-layer
basswood block (thickness of each layer: 1 cm).
For pullout tests, the nails were hammered into a basswood
blocks with the depth of 1 cm.

Fabrication And Microstructure Analysis
We synthesised the super-strong, self-densified wood via the
microstructural regulation strategy. After regulation, natural
wood underwent uniform shrinkage in the transverse area Fig. 2. Microstructure and mechanism underlying the wood self-densification
while maintaining its longitudinal dimension, resulting in a 79.0 process. Photographs of (a) natural wood and (b) self-densified wood,
indicating significant volume shrinkage. (c) Densities of the natural and
percent reduction in volume and a nearly threefold increase in
self-densified wood. Scanning electron microscopy (SEM) images of (d)
density (1.25 g/cm³), which is similar to that of mechanically partially delignified wood cell, (e) swelling wood cell, and (f) self-densified

compressed wood. wood cell. The cellulose fibrils first moved inward and filled the cell
lumen, then self-densified into a dense and solid material. (g) Schematic
This transverse area shrinkage with unchanged longitudinal illustration of self-densification process (S1, S2 and S3 are the outer
length was especially noticeable in irregularly shaped wood, layer, middle layer, and inner layer of the secondary wall of wood cell
walls, respectively.). (h) Schematic illustration showing hydrogen bonding
where spheres became ellipsoids and cones sharpened.
formation between adjacent cellulose fibrils during air-drying. The SEM
In order to study the microstructural evolution of the self- images of (i) partially delignified wood cell wall featuring fibrils density of
288 fibres per μm2, and (j) intermediate swelling wood cell wall featuring
densified wood, we used the SEM to observe the swelling
fibrils density of 102 fibres per μm2.
and densification processes.
Natural wood possesses a hierarchical cellular structure Fourier-transform infrared spectroscopy (FT-IR) indicated the
with aligned cellulose fibrils confined in the cell wall matrix. The effective removal of lignin and hemicellulose as the characteristic

lignin and hemicellulose deposited in the cell wall are tightly peaks assigned to lignin and hemicellulose (C = O stretching
bonded with cellulose fibrils through covalent and hydrogen vibration) significantly reduce.
bonds, which fix the position of the cellulose fibrils. Then, the partially delignified wood was swollen by the LiCl/
First, a common NaOH/Na2SO3 delignification process DMAc treatment. Due to the restriction of the middle lamella
was used to partially remove lignin and hemicellulose from and residual lignin (residual lignin:12.9%), the cellulose fibrils
the wood cell walls. The lignin content decreased from 26.6 swelled and moved evenly inward and filled the cell lumen.
percent to 12.9 percent due to delignification. The secondary wall and S1 layer, to some extent, guide

This process freed the cellulose fibrils from the constriction the swelling direction, leading to significant radial expansion
of lignin, giving cellulose fibrils the ability to move more easily. of plant fibres.