Viscoelastic properties of bone: Mechanical spectroscopy studies on a chicken model

The mechanical behaviour of bone exhibits a time-dependent response, as a consequence of its viscoelastic character. New materials for biomedical applications should have similar time-dependent mechanical behaviour as bone (viscoelastic biocompatibility). Hence, it is important to evaluate the properties of bone and to understand the underlying mechanism responsible for its damping features. In this work the solid-state rheological properties were studied in chicken bone. Fresh ulna specimens were tested using dynamic mechanical analysis in the temperature range −50 to 80 °C, at a constant frequency of 1 Hz. A broad tan δ peak was found between −50 and 30 °C, that was attributed to melting of water and molecular mobility involving collagen molecules. A minimum in the loss factor was detected at physiological temperature, which is consistent with isothermal results previously reported. It was hypothesised that this behaviour would have a physiological significance and could be associated to some damage process occurring within the organic phase caused by energy absorption during cyclic loading. This minimum is not detected in the demineralised bone or in collagen, indicating that the specific interactions between the collagen molecules and the apatite crystals, as well as the complex hierarchical organisation of the bone structure, are designed to reduce damping at functional time scales. A broad loss process was observed at low temperature in collagen. Local molecular motions at low temperatures occurring in collagen molecules could enhance bone toughness. Creep tests also revealed bone viscoelasticity features, and are useful to monitor irrecoverable strain upon static loads, which could be mainly a consequence of structural damage, rather than pure-viscous flow.