Rahul Bhowmik

Molecular interactions of degradable and non-degradable polymers with hydroxyapatite influence mechanics of polymer-hydroxyapatite nanocomposite biomaterials

Implant materials composed of hard and soft phases (composite materials) have shown much promise for total bone replacement. Interfacial interactions between the components in these composite biomaterials affect the overall mechanical response. Here, the role of interfacial interactions on the load deformation behaviour of soft phase (polymer) have been analysed using constant velocity Steered Molecular Dynamics (v-SMD). From v-SMD simulations, it has been observed that the stiffness of polymers changes significantly when these polymers interact with the hydroxyapatite (HAP) surface. It appears that the reasons for the altered stiffness are different for non-degradable and calcium binding polymers such as polyacrylic acid (PAAc) and degradable and non-calcium binding polymers such as polycapralactone (PCL). These results indicate that mineral proximity affects the mechanical response of both polymers but differently so. The role of different pulling velocities on the load-deformation behaviour of polymers is also analysed. The pulling velocity appears to have a marginal effect on stiffness of the polymers.

Influence of Mineral -Polymer Interactions on Molecular Mechanics of Polymer in Composite Bone Biomaterials

Nanocomposite bone materials of polymers and hydroxyapatite are widely investigated for bone replacement. The mechanical properties of the composites determine the use of these as implant materials. The molecular phenomenon at the interface between mineral and polymer is known to have significant contribution on overall mechanical response of composites. Understanding behavior of interfaces under applied load, and the load transfer mechanisms will lead to development of superior biomaterial composites with desired properties. We have performed Steered Molecular Dynamics (SMD) simulations on the composite system consisting of hydroxyapatite and polyacrylic acid. Our simulations describe the detailed molecular mechanisms responsible at the interface with applied load. Our SMD simulations also indicate that the polymer shows significant changes when it interacts with the mineral. The load-deformation behavior of polymer has shown that the polymer is stiffer when it is interacting with mineral. The binding and unbinding events are also calculated during load transfer in polymer. This work describes specific molecular mechanism responsible for mechanical behavior in composites used as bone biomaterials.

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.

Bioactivity and Mechanical Behavior of Polymer-Hydroxyapatite Composite Biomaterials for Bone Tissue Engineering

Achieving optimal mechanical strength of scaffolds is the key issue in bone tissue engineering. We describe a biomimetic route for design of composites of polymers (polyacrylic acid (PAAc) and polycaprolactone (PCL)) and hydroxyapatite (HAP). The mineral polymer interfaces have a significant role on mechanical behavior as well as bioactivity of the composite systems. We have used a combination of experimental (photoacoustic infrared spectroscopy) as well as modeling (molecular dynamics) techniques to evaluate the nature of interfaces in the composites. Porous composite scaffolds of in situ HAP with PCL are made. Our simulation studies indicate calcium bridging between COO− of PAAc and surface calcium of HAP as well as hydrogen bonding. These results are also supported by infrared spectroscopic studies. PAAc modified surfaces of in situ HAP influence the microstructure and mechanical response of porous composites. Significant differences are present in the mechanical response of in situ and ex situ composite scaffolds. In addition, the growth and mechanism of apatite growth in the in situ and ex situ composites is different. Bioactivity is measured by soaking composite scaffolds in simulated body fluid (SBF). Apatite growth in ex situ composites is primarily by heterogeneous nucleation and that in in situ is primarily by homogeneous nucleation. We also observe that apatite grown on in situ HAP/PCL composites from SBF exhibits higher elastic modulus and hardness. Thus, by influencing the interfacial behavior in bone biomaterials both mechanical response and bioactivity of the composite systems may be modified. The present study gives insight into the interfacial mechanisms responsible for mechanical response as well as bioactivity in biomaterials.Copyright