there was only minimal contraction. Although
there is a correlation between local strains and
amount of intrafibrillar mineral, it cannot be
perfect because strains induced by mineral-
ization obviously extend through elastic inter-
actions over larger areas. This is illustrated
schematically in Fig. 3E. Finally, the macro-
scopic contraction does not increase linearly
with time. It is faster in the first hour and
then changes approximately linearly (fig. S9B).
The origin of this chemomechanical effect is
not obvious, but it is most likely due to the
replacement of water by mineral during the
mineralization process. This is probably sim-
ilar to bone mineralization (i.e., when the
mineral is hydroxyapatite), where it has been
proposed that water is pushed from intra-
fibrillar to extrafibrillar compartments and
replaced by precursors under the effect of the
Gibbs-Donnan equilibrium ( 25 ) or capillary
transport ( 7 ). Dehydration experiments have
shown that the removal of water shortens the
collagen molecules by changing their con-
formation ( 16 ). This suggests that the contrac-
tion due to dehydration and mineralization
has the same origin. Indeed, mineralization
causes dehydration and water replacement,
thereby changing the osmotic equilibrium
that leads to a shrinkage of the triple-helical
pitch in certain regions of the collagen mol-
ecules ( 16 ).
To find out whether the contraction of col-
lagen also leads to a compression of the SrCO 3
mineral particles embedded in the fibrils, we
used synchrotron wide-angle x-ray scatter-
ing to extract 2D WAXS patterns (Fig. 4A). The
crystallography of the orthorhombic unit cell
is presented in fig. S13. To determine a po-
tential compression or dilatation of the lattice,
we compared the position of (110), (002), and
(200) rings with the peak positions from a
reference sample (table S1), which was obtained
by a heat treatment of a SrCO 3 -mineralized
tendon to induce a relaxation of any strains
in the crystal lattice by thermal degradation of
the organic matrix. The morphology and the
crystal orientation in respect to the orienta-
tion of the collagen fibrils were also analyzed
at the end of the in-operando measurement
using TEM. A typical example of a mineralized
collagen fibril is shown in Fig. 4C. The TEM
images revealed that the mineral matrix con-
sisted of small SrCO 3 nanocrystals, which
were well coaligned, forming a nearly single-
crystalline matrix. This was demonstrated
especially by selected area electron diffraction
(SAED). As the dominant crystallographic ori-
entation of nanocrystals parallel to the colla-
gen fibrils, a <100> direction was determined
(Fig. 4C, inset).
In mineralized tendons, WAXS measure-
ments showed a pronounced compression of
crystals along the <200> direction, but an
elongation in the perpendicular <002> direc-
tion (Fig. 4B). A compressive strain in the
<200> lattice direction was measured to be
−0.033% and−0.063% along vertical and hori-
zontal directions, respectively. In the per-
pendicular <002> direction, an expansion was
measured to be 0.049 and 0.056% along ver-
tical and horizontal directions, respectively.
The orientation of the collagen molecules was
along the vertical direction. Taking 62 GPa as
a bulk modulus of SrCO 3 ( 26 ), the prestress on
nanocrystals in collagen fibrils was estimated
to be between 20 and 40 MPa. This amounts
tothesamemagnitudeascompressiveloadon
hydroxyapatite minerals in bone ( 27 ).
We also studied the mineralization with
Ca 10 (PO 4 ) 6 (OH) 2 , CaF 2 , CaCO 3 , SrWO 4 , and
SrSO 4. In all these cases, mineralization oc-
curred (fig. S14A and S15), and a correspond-
ing contractile stress was measured in the
collagen fibers (Fig. 4D and fig. S14B). The
stress values at the end of the mineraliza-
tionprocess,asshowninfig.S14B,havebeen
plotted as“final stress”in Fig. 4D. In some
cases (SrSO 4 , SrWO 4 ), the final stress corre-
sponds to a plateau value; in other cases, it
continued to increase up to 120 hours, when
the experiments were terminated. The kinetics
of stress generation varied for the different
minerals (fig. S14). For the SrCO 3 system, the
final stress was determined by the final min-
eral content, independently of the pH of the
solution, which regulated the stress rate in
the initial stage. There seems to be a linear
relation between final stress and mineral vol-
ume fraction in strontium-based inorganic
species (Fig. 4D). Although Ca 10 (PO 4 ) 6 (OH) 2
is close to this hypothetical line, CaF 2 and
CaCO 3 are far away from it. This indicates
that the precipitation of different minerals
leads to contraction of different degree. Fi-
nally, it is interesting to note that the apatite
content in our artificially mineralized tendon
is about 72 wt % (fig. S4), which is of the same
order of magnitude as compact bone, which
has ~65% mineral and 25% collagen by weight,
with the rest being water ( 28 ). With other min-
erals, the inorganic content can even be some-
what higher, up to 88 wt % (fig. S4). However,
we cannot exclude some residual mineral on
the surface of all specimens in the TGA mea-
surements, so that the mineral content may
generally be overestimated.
This work demonstrates that chemome-
chanical coupling between precipitation and
collagen contraction that was previously ob-
served for hydroxyapatite in bone occurs for a
wide range of minerals. Furthermore, our in-
vestigations also reveal that the stress of the
collagen fibrils is transferred to the embedded
mineral. As a result, its crystal lattice is strongly
compressed parallel to the fibrils in the range
of 20 to 40 MPa. This phenomenon not only
reveals an intriguing property of collagen; it
also provides an exciting concept for enhanc-ing the mechanical properties of hybrid ma-
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ACKNOWLEDGMENTS
We thank S. Amini for help with the heat treatment of samples,
P. Leibner for support with mechanical testing, D. Werner for
assistance with micro-CT measurements, C. Schmitt for help
during Raman mapping, and W. Fang for help with intrafibrillar
mineralization of various inorganic materials.Funding:This
work was financially supported by the National key Research and
Development Program of China (2021YFA0715700), by the Max
Planck Society, and by the National Natural Science Foundation of
China (51832003, 51902236). P.F. also acknowledges support
by the German Research Foundation within SFB1444 and through
the Cluster of Excellence“Matters of Activity. Image Space Material”
EXC 2025.Author contributions:H.P., W.W., and P.F. conceived
the main idea of the project; Z.F. codirected the project. H.P.,
Z.F., and W.W. designed and performed the experiments; N.H.
participated in the construction of the mechanical testing setup; and
E.S. and C.L. participated in the collection and analysis of data. H.P.,
W.W., and P.F. analyzed the experimental results, prepared the
figures, and wrote the manuscript. All authors discussed the results
and commented on the manuscript.Competing interests:The
authors declare that they have no competing interests.Data and
materials availability:All data are available in the main text or the
supplementary materials, as well as at Edmond–Open Research
Data Repository of the Max Planck Society ( 29 ).SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abm2664
Materials and Methods
Supplementary Text
Figs. S1 to S15
Table S1
References ( 30 – 32 )
Movies S1 and S25 September 2021; accepted 17 February 2022
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