H E A L T H I N N O V A T I O N , O V E R C O M I N G D I S E A S E S A N D P A N D E M I C S

S C I E N T I F I C H I G H L I G H T S

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Revealing the elasto-plastic compressive behaviour of rehydrated mineralised collagen fibres

Biological nanostructured materials such as bone combine unique structural-mechanical properties. X-ray scattering experiments were combined with micromechanical testing and statistical modelling to investigate the nonlinear mechanical properties of bone tissue on the microscopic and molecular level.

Bone diseases such as osteoporosis, osteoarthritis, bone cancer or osteogenesis imperfecta affect millions of people worldwide. Consequently, they have attracted significant attention from the research community to develop better treatment strategies and aid for these patients. While this is of utmost importance, bone itself remains one of the most fascinating biological materials, whose intricate material architecture enables it to combine unique structural-mechanical properties. Made up of mineralised collagen that assembles into mineralised collagen fibres (Figure 6), it reaches outstanding strengths on the micrometre-length scale [1,2,3]. These properties make bone tissue an excellent design template for bio-based nanostructured materials. One of the main constituents of bone is water, and, as in many biological materials, this water plays a significant role in the mechanical interplay of its material architecture. In this work, the influence of water on the mechanical properties of bone tissue at the length-scale of a mineralised collagen fibre was studied and quantified (Figure 6).

To investigate this, in-situ micropillar compression was coupled with simultaneous synchrotron small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) experiments at beamline ID13 (building on an experimental protocol developed previously [2]), and a statistical constitutive model [3] (Figure 7). This approach made it possible to connect the statistical information about the nanostructure that was contained in the scattering data with the micromechanical experiment and the statistical model for evaluation (Figure 7). Only this combination made it possible to access the elasto-plastic micro- and nanomechanical behaviour of rehydrated mineralised collagen fibres.

It was found that rehydration led to a decrease of 65%- 75% in fibre yield stress and compressive strength, and 70% in stiffness, with a 3x higher effect on stresses than strains. While in agreement with results on bone extracellular matrix, the decrease is 1.5-3x higher compared to micro-indentation and macro-compression. Thus, the effect of hydration seems to be strongly mediated by ultrastructural interfaces, which points to an increased ability of collagen fibrils to slide past each other. Hydration influences the strain in the mineral component more than strain in the collagen fibrils, which may be due to increased lubrication compared to the dry tissue state. These results may also provide insights into the mechanical consequences of water-mediated structuring of bone apatite when comparing the observed mineral strain under dry and rehydrated testing conditions. Since a fibril array was excised, removing the reinforcing capacity of the surrounding tissue led to a much more pronounced effect on stiffness and strength compared to dry conditions, which was attributed to fibril swelling. Based on the results, differences leading to higher compressive

Fig. 6: The material architecture of bone tissue, starting with collagen molecules, hydroxyapatite platelets and non-collagenous proteins as its constituents. These are assembled into mineralised collagen fibrils and mineralised collagen fibres, which then form the

extra-cellular matrix (ECM) a highly versatile lamellar ultrastructure. The ECM is then used to form microstructured architectural levels that are combined into organs of different shapes (here sketched as a proximal femur). Image taken and adapted from [4,5].