Disparate micro-mechanical behaviors of adjacent bone lamellae through in situ SEM micropillar compression

The mechanical properties and anisotropy of a single osteon are attributed to the micro-mechanical behaviors of adjacent bone lamellae. The microstructure-property relationships could reveal the basis of the micro failure mechanisms of lamellar bone. In order to quantitatively evaluate the micro-mechanical behaviors of adjacent bone lamellae inside a single osteon, six micropillars with similar sizes (diameters of 1075 nm, 1091 nm, 1215 nm, 1086 nm, 1113 nm and 1092 nm, aspect ratio of 2:1) inside three adjacent lamellae were fabricated to avoid a size effect. The in situ scanning electron microscopy compressive experiments directly revealed the collagen fibril orientation-dependent similar micro-mechanical behaviors of the spaced micropillars and the disparate behaviors of adjacent micropillars. A couple of spaced micropillars exhibited similar performances, including in relation to strength, ductility, stress fluctuation amplitude, anisotropic deformation behavior, elastic recovery, and partial brittle failure mode. The central micropillars exhibited the lowest strength, ductility, and stress fluctuation amplitude, accompanied by isotropic, slight recovery behaviors and a partial failure mode. A deformation theory was proposed to explain the effect of collagen fibril orientation on the micro-mechanical behaviors of micropillars. A small orientation angle of collagen fibrils was verified to enhance the strength, ductility, and stress fluctuation amplitude of micropillars through the characterization of bent collagen fibrils and oblique hydroxyapatite crystals.