evaluated by the so called R-curve effect ( 23 ),
enabling us to calculateKjc.Thistypeof
toughness for ATE wasKjc=7.4±0.4MPam1/2,
a 3.7-fold increase compared withKic= 2.0 ±
0.5 MPa m1/2(Fig. 2A and fig. S20F) and also
higher than the fracture toughness of ATE-
NAIP (1.6-fold increase), ATE-NOM (3.8-fold
increase), enamel (1.6-fold increase), and refer-
enced HA ceramics (12.3-fold increase) ( 22 )
(Fig. 2A). When compared with biomaterials
and HA-based composites ( 24 ) (Fig. 2D and
table S3), ATE shows an excellent combina-
tion of high strength and high toughness. High
toughness of ATE can also be observed with
bending tests for angles unusually high for
ceramic materials (fig. S26). Additionally, the
strength and toughness of ATE tested in the
direction parallel to the nanowires are also
analyzed (fig. S27). Because of the anisotropic
structure of ATE, the strength and toughness
of ATE parallel to the nanowires are lower
than those perpendicular to the nanowires
but are still higher than those of enamel with
the same direction (fig. S27).
The excellent combination of mechanical
properties can be attributed to ATE’s hierarchi-
cal enamel-mimetic structure and the design of
inorganic and organic constituents. To under-
stand the functional mechanism between the
inorganic and organic constituents, we synthe-
sized five types of ATEs with varying content of
inorganic nanofillers (HA@A-ZrO 2 and PVA
ratios range from 1:1 to 5:1 and are defined as
ATE-1 to ATE-5; the corresponding inorganic
nanofillers content increased from 41.28 to
78.06%, fig. S21), and tested their mechanical
properties (fig. S28). We found that stiffness
(E), hardness (H), and viscoelasticity (VFOM)increased with increasing percentage of in-
organic nanofillers reaching the highest value
for ATE-5; strength and toughness increased
with the content of inorganic phase reaching
a plateau for ATE-3 (fig. S28). A greater con-
centration of inorganic nanofillers equates to
a denser composite and smaller distance be-
tween inorganic nanowires, which can be dem-
onstrated by the shift of peaks to higher degrees
(Fig. 3A), as seen in the small-angle x-ray dif-
fraction ( 25 ) spectrum. The distances between
inorganic nanofillers affect the mobility of the
polymeric chains, which can be evaluated by dif-
ferential scanning calorimetry ( 26 ). The glass
transition temperature (Tg)ofPVAstrongly
shifted toward the higher values with the con-
centration of inorganic components increasing,
and even disappeared for ATE when there are
more inorganic components (Fig. 3B), which is554 4 FEBRUARY 2022•VOL 375 ISSUE 6580 science.orgSCIENCE
Fig. 3. Polymer confinement in ATE.(A) Small-angle x-ray diffraction (SAXD) of ATEs with different HA@A-ZrO 2 nanowires contents. (B) Differential scanning
calorimetry (DSC) analysis of ATEs with different inorganic content and PVA. (C) Schematic illustration of polymer confinement and chemical bonding at the
A-ZrO 2 /PVA interfaces in ATE.
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