Nanomechanics and ultrastructural studies of cortical bone : fundamental insights regarding structure-function, mineral-organic force mechanics interactions, and heterogeneity

Abstract

Although the mechanics of bone has been studied extensively at the micro- and macro-scale, the nano-scopic level is perhaps the most illuminating as this is the length scale at which the individual constituents interact. Bone is made primarily up of type I collagen, hydroxyapatite mineral, a variety of non-collagenous proteins, and water. A multitechnique experimental and modeling approach methodology was used break down several of the deformation mechanisms that exist in bone at the nanoscale including the effect of mineral content, cohesive-frictional plasticity, increased ductility through mechanical heterogeneity, and intermolecular forces. To show that mineral content had a significant effect on both nanomechanical properties and ultrastructural deformation mechanisms of bone, partial and complete demineralization was carried out to produce samples ranging from -0-58 wt.% mineral content. Nanoindentation experiments perpendicular to the osteonal axis found a 4-6x increase in stiffness for the -58 wt.% sample compared to the completely demineralized -~0 wt.% samples. These results are discussed in the context of in situ and post-mortem AFM imaging studies which shed light on nanoscale mechanisms of deformation including collagen fibril deformation and pressure induced structural transitions of the mineral component.

(cont.) A finite element elastic-plastic continuum model was able to predict the nanomechanical properties of the different samples on loading and unloading. In addition, the ultrastructural origins of the strength of bone, which is critical for proper physiological function, were also investigated. A combination of dual nanoindentation, 3-D elastic-plastic FEA using a Mohr-Coulomb cohesive-frictional strength criterion, and angle of repose measurements was employed, suggesting that nanogranular friction is responsible for increased resistance to plasticity in compression, and that cohesion originates from within the organic matrix itself, rather than organic-mineral bonding. Nanomechanical heterogeneity is also expected to influence elasticity, damage, fracture, and remodeling of bone. The spatial distribution of nanomechanical properties of bone was quantified for the first time at the length scale of individual collagen fibrils. The results show elaborate patterns of stiffness ranging from 2 to 35 GPa which do not correlate directly with topographical features and hence, are attributed to underlying local structural variations. A new energy dissipation mechanism is proposed arising from nanomechanical heterogeneity, offering a graceful means for ductility enhancement, damage evolution, and toughening.

(cont.) This hypothesis is supported by finite element simulations which incorporate the nanoscale experimental data and predict markedly different biomechanical properties compared to a uniform material, through nonuniform inelastic deformation over larger areas and increased energy dissipation. The fundamental concepts discovered here are applicable to a broad class of biological materials and may serve as a design consideration for biologically-inspired materials technologies. Stem cell-based gene therapy and tissue engineering have been shown to be an efficient method for the regeneration of critical-size bone defects. Despite being an area of active research over the last decade, no previous knowledge of the intrinsic structural and nanomechanical properties of such bone tissue exists. The nanomechanical properties of engineered bone tissue derived from genetically engineered mesenchymal stem cells (MSCs) overexpressing the rhBMP2 gene, grown in vivo in an ectopic as well as a radial defect site of immunocompetent mice is compared to native bone adjacent to the transplantation sites. Supplementary experiments showed that the two types of bone had similar mineral contents, overall microstructures showing lacunae and canaliculi, chemical compositions, and nanoscale topographical morphologies.

(cont.) Nanoindentation experiments revealed that the small length scale mechanical properties were statistically different for the engineered bone derived from the ectopically implanted site. For data derived from the radial defect site experiments, results were both statistically similar and different, depending on the particular animal. The intermolecular forces (i.e. electrostatics, van der Waals, hydrogen bonding, etc.) that exist in bone are also expected to play a significant role in determining its morphology, structural integrity, interaction with bone fluid and its constituent biomolecular species as well as synthetic bone implant materials. Understanding of the macromolecular mechanisms of biomineralization in bone is an area of significant interest that is widely applicable to a wide range of materials applications, both biological in origin and synthetic. High resolution force spectroscopy (HRFS) allows for direct measurement of forces between a nanosized probe tip functionalized with SAMs which are molecules of uniform structure, charge, and chemistry as a function of separation distance from the sample in fluid. This data on approach was compared to electrostatic double layer theory, and the surface charge density (a) was estimated numerically. The normal electrostatic double layer forces were measured for the first time between a SAM functionalized probe tip and bone of varying mineral content.