Effect of mineral content on the nanoindentation properties and nanoscale deformation mechanisms of bovine tibial cortical bone

Abstract

In this paper, a multitechnique experimental and numerical modeling methodology was used to show that mineral content had a significant effect on both nanomechanical properties and ultrastructural deformation mechanisms of samples derived from adult bovine tibial bone. Partial and complete demineralization was carried out using phosphoric and ethylenediamine tetraacetic acid treatments to produce samples with mineral contents that varied between 37 and 0 weight percent (wt%). The undemineralized samples were found to have a mineral content of ~58 wt%. Nanoindentation experiments (maximum loads ~1000 μN and indentation depths ~500 nm) perpendicular to the osteonal axis for the ~58 wt% samples were found to have an estimated elastic modulus of ~7–12 GPa, which was 4–6× greater than that obtained for the ~0 wt% samples. The yield strength of the ~58 wt% samples was found to be ~0.24 GPa; 3.4× greater than that of the ~0 wt% sample. These results are discussed in the context of in situ and post-mortem atomic force microscopy imaging studies which show clear residual deformation after indentation for all samples studied. The partially demineralized samples underwent collagen fibril deformation and kinking without loss of the characteristic banding structure at low maximum loads (~300 μN). At higher maximum loads (~700 μN) mechanical denaturation of collagen fibrils was observed within the indent region, as well as disruption of interfibril interfaces and slicing through the thickness of individual fibrils leading to microcracks along the tip apex lines and outside the indent regions. A finite element elastic-plastic continuum mechanical model was able to predict the nanomechanical behavior of all samples on loading and unloading.