Experimental
designfor
uniaxial
tensileTensile properties of bone extracellular matrix at the microscalemeasurements
at, Rolfthemicroscale
Daniele Casari, Laszlo Pethö , Patrik Schürch
Brönnimann, Laetitia Philippe ,aaaaa

Johann Michlera, Philippe Zyssetb, Jakob Schwiedrzika

aEmpa, Swiss Federal Laboratories for Materials Science and Technology, Thun, Switzerlandb Institute for Surgical Technology and Biomechanics, University of Bern, Bern, Switzerland

Abstract: A micromechanical tensile setup was designed to probe focused ion beam (FIB) fabricated samples, while reducing measurement errors due to possible misalignment
between the gripper and the sample. Ovine osteonal bone was tested in both axial and transverse orientations on the length scale of a single lamellae inside a scanning electron
microscope in order to identify the elastic modulus, strength and strain at failure. Fracture surfaces were analysed in order to understand the dominant failure mechanisms.

1. Introduction

2. Experimental

Hierarchical structure of cortical bone tissue.
(adapted from Reisinger et al., Biomech Model Mechanobiol. 2010)

The mechanical properties of bone aredefined by the individual elements thatcompose its hierarchical structure andthe interaction between these acrossdifferent length scales. To understandhow bone can combine antagonistic
properties like toughness and strength, it
is important to characterize themechanical properties of bone atdifferent length scales.

a) Sample handlingb) Sample fabricationc) Misalignment

Sample fabrication

70 µm

Multiscale biomechanics is an emerging field aiming at
increasing the precision of mechanical models on the organ
level by considering bone’s hierarchical architecture. The studyof bone micromechanics might allow to enrich existing finiteelement (FE) models for improved fracture risk prediction.
Nevertheless there are major experimental challenges
encountered when testing miniaturized samples that need tobe solved to obtain reliable data:

Sample design

Microtensile setup

Gripper design (self-alignment)
In plane misalignment study:

FE calculations for fracture analysis.
(Zysset et al., Bone. 2015)

Different testing methodology
from macroscale is required!
1 µm

3. Microtensile properties

20 um

4. Fracture surfaces
AxialCanaliculi

FIB RedepositionTransverse

strain rates ≈ 3·10−4 s−1

Property

Axial
Tension

Axial
Compression [1]Transverse
TensionTransverse
Compression [1]Strength, σmax (GPa)Strain, ε(σmax) (-)
Elastic modulus, E (GPa)0.47 ± 0.03
0.023 ± 0.002
33.6 ± 0.7

0.75 ± 0.06
0.054 ± 0.017
31.1 ± 6.5

0.12 ± 0.03
0.013 ± 0.004
13.9 ± 1.2

0.59 ± 0.04
0.121 ± 0.025
16.5 ± 1.5

Collagenfibril[1] J. Schwiedrzik et al. Nature Materials, 13, 740–747 (2014)

Similarly to micropillar compression [1], microtensile tests on
the length scale of a single lamella showed a size effect and aclear anisotropy of the mechanical properties of lamellar bone.
While elastic properties were comparable between compression
and tension for both loading directions, failure strength andstrains were different. Bone micromechanical properties showed
an increase in value by a factor of 2.8 from macroscopic
mechanical properties, highlighting the influence of thehierarchical structure. Generally, tensile properties exhibited
lower values than compressive properties and axial samples
were stronger than transverse samples. The anisotropy is more
pronounced in tension compare to compression. This is in linewith previous observations, that showed that in bovine bone
the mineralized collagen fibrils (MCF) are mainly aligned withthe longitudinal axis of the osteons.

Collagenfibrils

Canaliculi

Osteon axis
Scanning transmission microscopy
on axial lamellar bone

Two distinct failure modes were identified from the in situ observation and post-failure
high resolution scanning electron micrographs. In the case of axial specimens, canaliculi
were always found on the fracture surface, presumably acting as a stress concentration
point and causing the failure. While observing brittle failure in dry conditions, the roughsurfaces suggest MCF pull-out as a possible toughening mechanism. We expect to see
this mechanism to be more pronounced in hydrated conditions since the MCF will beable to undergo larger deformations before failure [2]. Efforts are being undertaken toextend the testing methodology to probe lamellar bone in hydrated conditions. In
transverse samples the MCF are oriented perpendicular to the applied load andextrafibrillar matrix (EFM)-fibrils debonding was observed as the principal failure
behavior.
[2] J. Schwiedrzik et al. Acta Biomaterialia, 60, 302-314 (2017)

5. Conclusions
Ø Uniaxial microtensile properties of bone were characterized on the length scale of a single lamella using a new experimental setup.
Ø Tensile tests revealed size effects and anisotropy of the micromechanical properties of bone.
Ø Axial samples had consistent strength of 0.47 ± 0.03 GPa and their failure was associated to the presence of canaliculi.
Ø Transverse samples exhibited lower strength of 0.12 ± 0.03 GPa and showed interface failure between the MCF and the EFM.

Grants: # 165510 # 174192

Daniele Casari
Swiss Federal Laboratories forMaterials Science and Technology
Feuerwerkerstrasse 39
3602 Thun
Switzerlanddaniele.casari@empa.ch+41 58 765 62 31