absorption ( 17 ), which may contribute to im-
proved mechanical properties.
We evaluated the mechanical performance
of ATE by both nanoindentation (for stiffness,
hardness, and viscoelasticity, with load direc-
tion parallel to the nanowires) and the three-
point bending test (for strength and toughness,
with load direction perpendicular to the nano-
wires). To explore the role of each structural
element, we also produced two benchmark
HA-based composites, namely ATE from nano-
wires with no AIP (ATE-NAIP) (fig. S18) anda composite with an HA@A-ZrO 2 nanowire
loading similar to ATE but with no ordered
microstructure (ATE-NOM) (fig. S19). We eval-
uated Young’s modulus (E), hardness (H),
storage modulus (E′), damping coefficient
(tand), strength (s), and toughness (Kjc) of these552 4 FEBRUARY 2022•VOL 375 ISSUE 6580 science.orgSCIENCE
Fig. 1. Tooth enamel and synthesis of artificial tooth enamel (ATE).
(A) Optical photographs of tooth enamel, including a closeup view and a
sectional slice perpendicular to the midcoronal cervical plane. (B) Scanning
electron microscope (SEM) image of tooth enamel. (C) Transmission electron
microscopy (TEM) image of tooth enamel. (D) Enlarged TEM image taken from
the red zone in (C). (E) Molecular structure of HA with amorphous intergranular
phase (AIP). (F) Schematic illustration of construction of the mimicked tooth
enamel process; the corresponding SEM and TEM images of HA and HA@A-ZrO 2
are displayed on the left and right side, respectively. The exposedÐOH on HA
and the balance between nucleation and growth of ZrO 2 precursor is the key to
formation of the amorphous thin layer. (G) The tensile mechanical performance
of HA with and without AIP, showing that the AIP indeed makes the nanowire
stronger and tougher. (H) Schematic illustration of micro- and macro-assembly
of the HA@A-ZrO 2 nanowires coupled with PVA. (I) Optical photographs of
ATE, including a closeup view and a sectional slice perpendicular to the midcoronal
cervical plane. (J) SEM image of ATE. (K) TEM image of ATE. (L) Enlarged
TEM image taken from the blue zone in (K). (M) Molecular structure of HA and
amorphous ZrO 2 (A-ZrO 2 ).RESEARCH | REPORTS