materials, comparing them with each other and
with natural tooth enamel (Fig. 2A and fig. S20).
The averageEandHof ATE, calculated from
the quasistatic nanoindentation, reach 105.6 ±
12.1 GPa and 5.9 ± 0.6 GPa, respectively. These
values are higher than those of natural enamel
(Fig. 2A), whereas the inorganic content of ATE
(78.06 wt %, fig. S21) is far less than that of
enamel (>96 wt %) ( 18 ). The presence of high-
modulus ZrO 2 is one reason that the modulus
of ATE surpasses that of enamel. The twofold
increase in stiffness and hardness compared
with ATE-NAIP and ATE-NOM indicates the
necessity of the nanoscale crystal-amorphous
interface and parallel organization of columns
at the microscale to reach the high values of
macroscale stiffness and hardness seen in ATE.
This conclusion can be confirmed by compar-
ing the stiffness and hardness of ATE to those
of stiff natural biomaterials (e.g., teeth, nacre,
bones of many animals), previously reported
HA-based composites, and ceramic-polymer
composites ( 19 ) (Fig. 2B and table S1).
ATE exhibits high viscoelasticity without
sacrificing its stiffness and hardness ( 20 ), (Fig.
2C and table S2). The average storage mod-
ulus of ATE can be up to 78.6 ± 9.8 GPa with a
frequency of 10 Hz, whereas the average damping
coefficient (tand) of ATE reaches ~0.07 (Fig. 2A
and fig. S20, C and D), which exceeds the limits of
traditional engineering materials with similar
storage modulus such as ceramics and metals
( 20 ) (usually 0.001 to 0.01). The viscoelastic
figure of merit (VFOM, defined as the product
of E′and tand)isashighas5.5GPa,ninetimes
as high as the limitation of traditional engi-
neering materials (~0.6 GPa) and six times as
high as in the ZnO-based enamel-like compo-
sites ( 2 ). The data for ATE-NAIP and ATE-NOM
composites (Fig. 2A) point to the importance
of the AIP and hierarchical organization with
microscale alignment. Considering that clin-
ically relevant loadings for natural tooth enamel
occur at a typical frequency of 1 Hz ( 21 ), we have
also tested the viscoelastic performance of ATE
at a frequency of 1 Hz. The E′andtandof ATEsare 73.5 ± 10.1 GPa and ~0.075, respectively
(fig. S22), comparable to the values for 10 Hz.
ATE achieved a flexural strength of ~142.9 MPa
and fracture strain of 0.018, which are superior
to those of enamel (Fig. 2A and fig. S20E). The
flexural strength and fracture strain of ATE are,
respectively, ~2 and ~10 times as high as those
of HA ceramic ( 22 ) (fig. S23). The nearly two-
fold reduction of flexural strength in ATE-NAIP
and ATE-NOM (Fig. 2A) compared with ATE
further supports the necessity of multiscale
engineering of organic-inorganic composites.
Single-edge notched beam (SENB) tests were
carried out ( 23 ) and their mechanical behavior
were observed through in situ bending with an
ESEM. The typical stress-strain curves from
SENB tests were plotted in fig. S24. The initial
fracture toughness (Kic) of ATE (2.0 ± 0.5 MPa m1/2)
is higher than both ATE-NAIP (1.0 ± 0.1 MPa m1/2)
and ATE-NOM (1.2 ± 0.1 MPa m1/2) (fig. S25),
meaning that ATE shows more resistance to
the initial crack during deformation. Increased
toughness during crack propagation can beSCIENCEscience.org 4 FEBRUARY 2022¥VOL 375 ISSUE 6580 553
Fig. 2. Mechanical properties of ATE.(A) Left, schematic illustration of
ATE, ATE-NAIP, ATE-NOM, and enamel; right, mechanical performance
of different enamel-like composites with Young’s modulus (E), hardness
(H), storage modulus (E′), damping coefficient (tand), flexural strength (s),
and toughness (Kjc), for ATE, ATE-NAIP, ATE-NOM, and enamel plotted as a
radar map. (B) Young’s modulus and hardness of ATE and referenced
engineering materials including biomaterials and HA/other ceramic-based
composites. The performance of ATE is outstanding. (C) Storage
modulus and damping coefficient of ATE compared with biomaterials,
HA-based composites, ceramics, and ceramic-based composites.
(D) Flexural strength and toughness of ATE compared with biomaterials
and HA-based composites.RESEARCH | REPORTS