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TECHNOLOGY 39
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flexural strength is relatively lower (~96 MPa); however, when
the bending load axis is parallel to the non-compression
direction, its strength significantly increases to ~328 MPa.
The remarkable improvement of mechanical properties

is attributed to the unique self-densification strengthening at
multiple-scales. Self-densified wood maintains a highly densely
ordered alignment structure.
During the densification process and tensile fracture surface,
highly aligned nanocellulose fibres were densely packed and
arranged longitudinally, significantly increasing the interfacial
area among nanofibers.
At the molecular scale, during the swelling and dehydration
processes, the abundant hydroxyl groups in cellulose molecular

chains facilitate numerous cycles of hydrogen bond formation,
breaking, and reformation.
The densification process promotes the formation of numerous Fig. 3. Superb longitudinal mechanical properties of self-densified wood.
(a) Tensile stress-strain curves of natural and self-densified wood. (b)
hydrogen bonds between the highly aligned cellulose chains.
Specific tensile strength of the resulting self-densified wood compared
Simultaneously, the microstructure of wood cells transitions to natural wood and various mechanical-compressed wood. Polymer-
infiltrated compressed wood (PICW); thermo-mechanical compressed
from a tubular to a dense columnar configuration, significantly
wood (TMCW); delignified compressed wood (DCW); delignified and
reducing both defect density and cell size. partially fiber surface dissolved compressed wood (D&PSDCW); delignified
The enhanced hydrogen bond density, combined with the resin-infiltrated compressed wood (DRICW). (d) Impact toughness and (e)
Shore D hardness of natural wood, mechanically compressed wood, and
reduction in structural defects and dimensions, synergistically self-densified wood. All data were results from longitudinal testing of the
contributes to the ultrahigh strength of self-densified wood. wood. (f) Compressive stress-strain curves and (g) compressive strength of
natural wood, mechanical-compressed wood, and self-densified wood. The
Furthermore, to demonstrate the universality, balsa wood
testing direction, as shown in the embedded diagram (f), is the longitudinal
was also processed by using the same method, resulting in compression direction of the wood. (h) Longitudinal flexural strength for
a 14-fold increase in tensile strength. natural wood, mechanical-compressed wood, and self-densified wood.
Longitudinal flexural stress-strain curves for (i) natural wood, (j) mechanical-
All these results collectively confirm that the super-strong compressed wood, and (k) self-densified wood. When the bending load axis
wood can be obtained through a microstructure regulation is parallel to the compression direction, the longitudinal flexural strength
is low (~96 MPa), but when parallel to the non-compression direction,
strategy without hot-pressing.
it increases significantly (~328 MPa). The direction of the force axis in
both wood and self-dense wood has no significant effect on longitudinal
flexural strength, showing uniform consistency. PICW, polymer-infiltrated
Transverse Section Reinforcement
compressed wood; TMCW, thermo-mechanical compressed wood; DCW,
We further investigated the tensile behaviour in the transverse delignified compressed wood; D&PSDCW, delignified and partially fibre
section to better understand the comprehensive mechanical surface dissolved compressed wood; DRICW, delignified resin-infiltrated
compressed wood; NW, natural wood; SW, self-densified wood.
properties of the self-densified wood.
Transverse mechanical compression (perpendicular to the compression direction.
the longitudinal direction) is usually essential to fabricate the This anisotropic microstructure further intensifies the

densified wood. Such unidirectional compression enables the anisotropy of the wood's mechanical properties. The transverse
wood cell walls to collapse and become densely packed along tensile strength perpendicular to the compression direction is