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

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 at this length scale 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 measurements in both dehydrated and rehydrated conditions at the microscale. Single lamella bone microbeams of four human donors, aged 65-94 yrs, were machined via FIB and tested both in air and fully submerged in Hank's Balanced Salt Solution (HBSS) to investigate the effect of (de)hydration and to a certain extent, of age, on bone mechanics. Bending moduli 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 in samples from older donors could be observed in the rehydrated state. These results confirm directly the importance of water for the mechanical properties of bone tissue at the microscale. Moreover, the trend of lowered capability of energy dissipation in older donors may contribute to a decrease of fracture toughness and thus an increase in bone fragility with age.

Automated summary

Bone tissue has unique mechanical properties due to its complex hierarchical structure. Testing of bone structures at the microscale using techniques like FIB and AFM has shown the importance of water for bone mechanics and the potential increase in bone fragility with age, linked to decreased energy dissipation.