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Lecture

WEB Mineralised collagen fibre elasto-plasticity: In situ experiments and modelling of bone’s fundamental building block

Wednesday (23.09.2020)
09:00 - 09:15 C: Characterization 1
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Bone is made of mineral nanocrystals and mineralised collagen fibrils, abundantly available ingredients, and grown at mild temperatures. It combines high stiffness, toughness and strength at low specific weight through a unique hierarchical setup. To mimic it in engineering materials a key gap in understanding its fundamental building block, the mineralised collagen fibre, needs to be closed. We present micro- and nanomechanical data as well as nanoscale imaging which we use in a statistical model that explains mineralised collagen fibre elasto-plasticity.

We used ultra-short pulsed laser ablation and focused ion beam milling to machine 14 micropillars (6x12 μm) from individual mineralised collagen fibres. We conducted combined micropillar compression and in situ small angle X-ray scattering/X-ray diffraction at beamline ID13 of the European Synchrotron Radiation Facility to determine fibre, fibril, and mineral strains [1]. Micropillars were compressed until 12% apparent fibre strain and exposed to X-rays every 5 s. We performed phase-contrast CT with 20 nm voxel size at beamline ID16A. From this data, we analysed fibril orientation using an auto-correlation approach [2], tissue density [3] and fibril diameter. We integrated the experimental data in a novel statistical mechanical model that describes the micro- and nanomechanical behaviour of mineralised collagen fibril arrays. To calculate load transfer between components, we embedded two classical shear lag models [4].

We found small strain ratios of 22:5:2 between fibre-fibril-mineral levels with the maximum for fibrils towards apparent strength and for mineral nanocrystals towards apparent yielding outlining the load transfer between organic and inorganic components [1]. This spurious ratio agrees with literature on macroscopic samples [5]. The model allowed us to identify a heterogeneous deformation of fibrils under compression which explained the small experimental strain ratios in our samples as well as in the literature. We saw that a variability of 10-15% in micro- and nanomechanical properties is present in the micropillars and that fibril diameter influences hardening behaviour and strength when we generalise the model towards other fibril-reinforced composites [6].

Findings from our combined in situ testing and statistical modelling aim to inform the design of future bio-inspired materials to tackle the socio-economic burden of bone-related diseases that affect millions of people worldwide.

[1] Groetsch et al., Acta Biomaterialia, 89:313-329, 2019.
[2] Varga et al., Acta Biomaterialia, 9:8118-8127, 2013.
[3] Langer et al., PLOS One, 7:e35691, 2012.
[4] Gao, International Journal of Fracture, 138:101-137, 2006.
[5] Gupta et al., PNAS, 103:17741-17746, 2006.
[6] Schwiedrzik et al., Nature Materials, 13:740-747, 2014.

Speaker:
Dr. Alexander Groetsch
Heriot-Watt University & Swiss Federal Laboratories for Materials Science and Technology (Empa)
Additional Authors:
  • Prof. Aurélien Gourrier
    Université Grenoble Alpes, CNRS, LiPhy
  • Dr. Peter Varga
    AO Research Institute Davos
  • Dr. Alexandra Pacureanu
    European Synchrotron Radiation Facility
  • Prof. Françoise Peyrin
    Université de Lyon, CNRS UMR 5220, Inserm U1206, INSA Lyon, UCBL Lyon 1, Creatis
  • Dr. Johann Michler
    Swiss Federal Laboratories for Materials Science and Technology (Empa), Laboratory for Mechanics of Materials and Nanostructures
  • Dr. Jakob Schwiedrzik
    Swiss Federal Laboratories for Materials Science and Technology (Empa), Laboratory for Mechanics of Materials and Nanostructures
  • Prof. Philippe K. Zysset
    University of Bern, ARTORG Centre for Biomedical Engineering Research
  • Prof. Uwe Wolfram
    Heriot-Watt University, School of Engineering and Physical Sciences