attributed to suppression of the thermal mo-
tion of the polymer chains in the presence of
more inorganic nanowires ( 27 ); this is sche-
matically depicted in Fig. 3C. When the inor-
ganic phase content is small, the distance (d)
between nanowires is larger than the critical
distance,dc= 2.88 nm (table S4), and the poly-
mer chains become flexible, thus decreasing
stiffness, hardness, and strength. With an in-
crease in the inorganic phase, the mobility of
the polymer is partially confined by the suitable
distance (d=dc), providing sufficient support
and a strong interface connection which leads
to a simultaneous increase in stiffness, hard-
ness, and strength. As the concentration of
nanowires increases, the distance between
them becomes smaller than the critical dis-
tance (d<dc), which creates a strong confine-
ment for the polymer chains (the thermal
mobility of the polymer chains almost dis-
appears) and provides strong support for
the composite ( 28 ). As a result, the stiffness
and hardness of ATE-5 are the highest, and
higher than those of tooth enamel. However,
this partially sacrifices the polymer’s mobility
and its ability to adapt to changing interfaces
between the organic and inorganic phases,
leading to a slight decrease in strength and
toughness of ATE-5, which can also be observed
in cellulose composites ( 29 ). Regardless, the
comprehensive mechanical performance of
ATE-5 is still outstanding for overall stiffness,
hardness, strength, viscoelasticity, and tough-
ness. FTIR analysis (fig. S29) of A-ZrO 2 /PVA
interfaces imply that there also exists chem-
ical bonding between Zr4+and–OH of PVA,
and these strong chemical bonds strengthen
the interface connection. Furthermore, con-
sidering that both the HA nanowires and the
PVA matrix are closely connected to the AIP
through chemical bonding as illustrated in
Fig. 3C, right, the AIP provides a buffer layer
which can not only facilitate the stress transfer
but also enhance the inorganic-organic inter-
face connection, which efficiently contributes
to the outstanding mechanical performance
of ATE.
The improved mechanical properties ob-
served during bending tests can be attri-
buted to fracture-resistant deformation and
crack deflection (Fig. 4, A to D). Specifically,
when an external load was applied to the
sample, the nanowires initially slid, dissipat-
ing a considerable amount of energy as a
result of tight binding between the organic
phase and the inorganic amorphous layer.
Similar to other biomaterials and composites,
the confinement of the organic phase in the
gap between the nanowires maximized the
contribution of interfaces but also restricted
their motion, thus improving crack deflection
(Fig. 4E). Pull-out of the nanowires (Fig. 4, E 1
and E 2 ) and fracturing of the sample with
large-range crack deflection (Fig. 4C) dissipated
a large amount of energy. Moreover, the pull
out nanowires can connect to each other to
restrict the further failure of the sample. Crack
splitting, bridging, and bunching (Fig. 4E)
also occurred during crack propagation, which
can further dissipate energy and result in ATE’s
excellent flexural toughness without sacrificing
strength ( 30 ). In comparison, ATE-NAIP ex-
hibited a relatively small crack deflection
owing to the lack of amorphous restriction.
Additionally, ATE-NOM exhibited almost brittle
fracture (fig. S30).
To investigate the role of the enamel-like
hierarchical architecture on increased stiffness,
hardness, and viscoelasticity, we observed the
permanent deformation zone obtained by a
maximum load of 200 mN from the top view.
We attributed their outstanding mechanical
performance to sliding nanowires and crystal-
amorphous phase-facilitated energy dissipation.
Upon closer observation of the nanoindentation
zone undergoing permanent deformation, we
found jagged nanoscale cracks growing along
the indenter (Fig. 4F, yellow arrows) and in-
terface delamination (Fig. 4F, orange arrows)
generated by the sliding, bending, and fracture
of the nanowires. This can dissipate energy
by transferring it from one nanowire to the
organic layer and the adjacent nanowire,
thus avoiding collapse of the structure and
enhancing the stiffness, hardness, and vis-
coelasticity of ATEs simultaneously ( 31 ). Sim-
ilar mechanical behavior is also detected in
enamel ( 18 ) (Fig. 4G), which means that the
complex structure of ATE with the three iden-
tified structural elements engenders enamel’s
mechanical performance.
In summary, we have engineered a multi-
scale assembly pathway to macroscale analogs
of tooth enamel, revealing atomic, nanoscale,
and microscale organization of inorganic nano-
structures similar to the original biomaterial.
The designed biomimetic composite retaining
the structural complexity of the biological pro-
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ACKNOWLEDGMENTS
We thank M. Liu for help with analyzing and discussing
these results.Funding:This work was supported by the
National Key R&D Program of China (2020YFA0710403 to L.G.,
2020YFA0710401 to Y.W.), the National Natural Science
Foundation of China (51532001 and 51772011 to L.G.; 51802010
and 52073008 to H.Z.; 51922017 and 51972009 to Y.Y.),
AFOSR FA9550-20-1-0265 and Graph Theory Description of
Network Material and NSF 1463474 to N.K.Author
contributions:L.G., H.Z., L.J., and N.K. conceived
this project. H.Z., S.L., M.G., Y.L., and X.Z. prepared the
artificial tooth enamel. L.G., H.Z., and S.L. performed the
morphology and spectroscopy characterization. H.Z. and S.L.
carried out the mechanical tests, including nanoindentation,
bending test and in situ tensile test. H.Z., Y.Y., and S.L.
performed the FIB cutting slice and TEM observation. Y.W.
and X.D. provided the natural tooth enamel and performed
the atom probe tomography. L.G., N.K., L.J., H.Z., S.L.,
Y.W., and X.D. analyzed the data and wrote this paper. All
authors participated in the discussions of the research.
H.Z., S.L., Y.W., and Y.Y. contributed equally to this work.
Competing interests:The authors declare no competing
interests.Data and materials availability:All data are available
in the main text or the supplementary materials.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abj3343
Materials and Methods
Figs. S1 to S30
Tables S1 to S4
References ( 32 – 92 )6 May 2021; accepted 7 December 2021
10.1126/science.abj3343556 4 FEBRUARY 2022•VOL 375 ISSUE 6580 science.orgSCIENCE
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