Micromechanical Modeling of Hydroxyapatite Whisker Reinforced Polymer Composites and Cortical Bone Tissue

A new micromechanical model was developed to predict the elastic moduli of hydroxyapatite (HA) whisker reinforced polymer biocomposites based upon the elastic properties of each phase and the reinforcement volume fraction, morphology and preferred orientation. The effects of the HA whisker volume fraction, morphology and orientation distribution were investigated by comparing two and three-dimensional model predictions with experimentally measured elastic moduli or stiffness coefficients, respectively, for HA whisker reinforced high density polyethylene (HDPE) composites. Predictions using experimental measurements of the HA whisker aspect ratio distribution and orientation distribution were also compared to common idealized assumptions. The best model predictions were obtained using the experimentally measured HA whisker aspect ratio distribution and orientation distribution. The same micromechanical model developed and validated for HA whisker reinforced composites was also applied to the elastic properties of human cortical bone tissue. The orientation distribution of apatite crystals in bone was measured and statistically correlated to the elastic anisotropy of bone tissue. The relationship was further investigated using the micromechanical model. The orientation distribution of apatite crystals was shown to be a significant factor contributing to the elastic anisotropy of cortical bone tissue.