Microbeam bending of hydrated human cortical bone lamellae from the central region of the body of femur shows viscoelastic behaviour

Abstract Bone is a biological tissue with unique mechanical properties, owing to a complex hierarchical structure ranging from the nanoscale up to the macroscale. To better understand bone mechanics, investigation of mechanical properties of all structural elements at every hierarchical level and how they interact is necessary. Testing of bone structures at the lower microscale, e.g. bone lamellae has been least performed and remains a challenge. Focused ion beam (FIB) milling is an attractive technique for machining microscopic samples from bone material and performing mechanical testing at the microscale using atomic force microscopy (AFM) and nanoindentation setups. So far, reported studies have been performed on bone samples of animal origin, mostly in a dehydrated state, except for one study. Here we present an AFM-based microbeam bending method for performing bending measurement in both dehydrated and rehydrated conditions at the microscale. Single lamella bone microbeams of four human donors, aged 65–94 y, were machined via FIB and tested both in air and fully submerged in Hank's Balanced Salt Solution (HBSS) to investigate the effect of dehydration and to a certain extent, age on bone mechanics. Bending moduli obtained were found to reduce up to 5 times after 2 h of rehydration and no trend of change in bending moduli with respect to age could be observed. Mechanical behavior changed from almost purely elastic to viscoelastic upon rehydration and a trend of lower dissipated energy of older samples could be observed, but only in the rehydrated state. These results confirm directly the importance of water for the mechanical properties of bone tissue. Moreover, the possible trend of lower capability of energy dissipation in older donors could have implications in decrease of fracture toughness and thus increase in bone fragility with age.