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.