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Bone features a hierarchical architecture combining antagonistic properties like toughness and strength. In order to better understand the mechanisms leading to this advantageous combination, its postyield and failure behaviour was analyzed on the length scale of a single lamella. Micropillars were compressed to large strains under hydrated conditions to measure their anisotropic yield and post-yield behaviour. An increase in strength compared to the macroscale by a factor of 1.55 and a strong influence of hydration with a decrease by 60% in yield stress compared to vacuum conditions were observed. Post-compression transmission electron microscopic analysis revealed anisotropic deformation mechanisms. In axial pillars, where fibrils were oriented along the loading axis, kink bands were observed and shear cracks emerged at the interface of ordered and disordered regions. Micromechanical analysis of fibril kinking allowed an estimate of the extrafibrillar matrix shear strength to be made: 120±40MPa. When two opposing shear planes met a wedge was formed, splitting the micropillar axially in a mode 1 crack. Making use of an analytical solution, the mode 1 fracture toughness of bone extracellular matrix for splitting along the fibril direction was estimated to be 0.07MPam. This is 1-2 orders of magnitude smaller than on the macroscale, which may be explained by the absence of extrinsic toughening mechanisms. In transverse pillars, where fibrils were oriented perpendicular to the loading axis, cracks formed in regions where adverse fibril orientation reduced the local fracture resistance. This study underlines the importance of bone's hierarchical microstructure for its macroscopic strength and fracture resistance and the need to study structure-property relationships as well as failure mechanisms under hydrated conditions on all length scales., Statement of Significance: Bone's hierarchical architecture combines toughness and strength. To understand the governing deformation mechanisms, its postyield behaviour was analyzed at the microscale. Micropillars were compressed in physiological solution; an increased strength compared to macroscale and an influence of hydration was found. Transmission electron microscopy revealed cracks forming in regions with adverse fibril orientation in transverse pillars. In axial pillars kink bands were observed and shear cracks emerged at the interface of ordered and disordered regions. It was estimated that bone's fracture toughness for splitting between fibrils is significantly smaller than on the macroscale. This study underlines the importance of bone's hierarchical microstructure and the need to study structure-property relationships on all length scales., (Copyright © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.)

Osteogenesis imperfecta (OI) is a genetic, collagen‐related bone disease that increases the incidence of bone fractures. Still, the origin of this brittle mechanical behavior remains unclear. The extracellular matrix (ECM) of OI bone exhibits a higher degree of bone mineralization (DBM), whereas compressive mechanical properties at the ECM level do not appear to be inferior to healthy bone. However, it is unknown if collagen defects alter ECM tensile properties. This study aims to quantify the tensile properties of healthy and OI bone ECM. In three transiliac biopsies (healthy n = 1, OI type I n = 1, OI type III n = 1), 23 microtensile specimens (gauge dimensions 10 × 5 × 2 μm3) were manufactured and loaded quasi‐statically under tension in vacuum condition. The resulting loading modulus and ultimate strength were extracted. Interestingly, tensile properties in OI bone ECM were not inferior compared to controls. All specimens revealed a brittle failure behavior. Fracture surfaces were graded according to their mineralized collagen fibers (MCF) orientation into axial, mixed, and transversal fracture surface types (FST). Furthermore, tissue mineral density (TMD) of the biopsy cortices was extracted from micro–computed tomogra[hy (μCT) images. Both FST and TMD are significant factors to predict loading modulus and ultimate strength with an adjusted R2 of 0.556 (p = 2.65e−05) and 0.46 (p = 2.2e−04), respectively. The influence of MCF orientation and DBM on the mechanical properties of the neighboring ECM was further verified with quantitative polarized Raman spectroscopy (qPRS) and site‐matched nanoindentation. MCF orientation and DBM were extracted from the qPRS spectrum, and a second mechanical model was developed to predict the indentation modulus with MCF orientation and DBM (R2 = 67.4%, p = 7.73e−07). The tensile mechanical properties of the cortical bone ECM of two OI iliac crest biopsies are not lower than the one from a healthy and are primarily dependent on MCF orientation and DBM. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. [ABSTRACT FROM AUTHOR]

Tissue fixation is a prevalent method for bone conservation. Bone biopsies are typically fixed in formalin, dehydrated in ethanol, and infiltrated with polymethyl methacrylate (PMMA) Since some experiments can only be performed on fixed bone samples, it is essential to understand how fixation affects the measured material properties. The aim of this study was to quantify the influence of tissue fixation on the mechanical properties of cortical ovine bone at the extracellular matrix (ECM) level with state-of-the-art micromechanical techniques. A small section from the middle of the diaphysis of two ovine tibias (3.5 and 5.5 years old) was cut in the middle and polished on each side, resulting in a pair of mirrored surfaces. For each pair, one specimen underwent a fixation protocol involving immersion in formalin, dehydration with ethanol, and infiltration with PMMA. The other specimen (mirrored) was air-dried. Six osteons were selected in both pairs, which could be identified in both specimens. The influence of fixation on the mechanical properties was first analyzed using micropillar compression tests and nanoindentation in dry condition. Additionally, changes in the degree of mineralization were evaluated with Raman spectroscopy in both fixed and native bone ECM. Finally, micro tensile experiments were conducted in the 3.5-year fixed ovine bone ECM and compared to reported properties of unfixed dry ovine bone ECM. Interestingly, we found that tissue fixation does not alter the mechanical properties of ovine cortical bone ECM compared to experiments in dry state. However, animal age increases the degree of mineralization (p = 0.0159) and compressive yield stress (p = 0.041). Tissue fixation appears therefore as a valid conservation technique for investigating the mechanical properties of dehydrated bone ECM. [Display omitted] • Influence of fixation on the mechanical and mineralization properties was examined in dry ovine bone extracellular matrix. • Novel micro mechanical tests were applied to compare the mechanical properties in fixed and native dry ovine bone biopsies. • Mechanical properties of fixed bone were not statistically different compared to native dry bone extracellular matrix. [ABSTRACT FROM AUTHOR]