Modeling the Role of Interfaces on Mechanical Response in Composite Bone Biomaterials

Polymer-hydroxyapatite (HAP) composites have potential use as bone replacement materials and are also the subject of several recent research studies. The molecular interactions between the mineral and polymer are known to have significant role on mechanical response of the composite system. We have used molecular dynamics to model the interaction between the polymers and HAP. Molecular dynamics studies require force field parameters for both molecules. Some force fields are described in literature representing the structure of hydroxyapatite reasonably well. Yet, the applicability of these force fields for studying the interaction between dissimilar materials (such as mineral and polymer) is limited, as there is no accurate representation of polymer in these force fields. We have derived the parameters of CVFF (consistent valence force field) for monoclinic hydroxyapatite. These parameters are validated by comparing the computationally obtained unit cell parameters, vibrational spectra and atomic distances with XRD and FTIR experiments. Using the previously obtained parameters of HAP and available parameters of polymer (polyacrylic acid), interaction study was performed using MD simulations between these dissimilar molecules. The MD simulations indicate that several hydrogen bonds and chelation bonds may form between HAP and polyacrylic acid depending upon the exposed surface of HAP. Also, the favourable planes of HAP where polyacrylic acid is most likely to attach are obtained. We have also simulated the mineralization of HAP using a “synthetic biomineralization”. These modeling studies are supported by photoacoustic spectroscopy experiments on both porous and non porous composite samples for potential joint replacement and bone tissue engineering applications.