(765i) Synthesis, Characterization, and Cytocompatibility Studies of Nanocrystalline Magnesium and Strontium Substituted β-Tricalcium Phosphate

Calcium phosphates (CaPs) are widely used as synthetic bone graft substitutes for the treatment of orthopedic, craniofacial, and dental defects. Current state of the art bone tissue regenerative strategies involve the use of inorganic components, such as CaPs, coupled with growth factors, of the likes of recombinant human platelet derived growth factor (rhPDGF) and recombinant human bone morphogenic protein 2 (rhBMP-2). The use of growth factors has significantly improved the overall efficacy of synthetic bone graft substitutes; however, their active involvement in several in vivo processes other than bone regeneration has led to increased safety concerns, particularly with respect to tumorigenesis. In addition, the inclusion of growth factors significantly increases the cost and complexity of the device, further increasing healthcare expenditures.

An alternative approach which has been hypothesized to optimize scaffold performance is the utilization of non-traditional synthesis and processing techniques which result in materials that more closely mimic not only the composition, but also the structure of native mineralized tissues. In order to test this hypothesis, in the current work a low temperature chemical synthesis approach that results in the formation of nanocrystalline β-tricalcium phosphate (β-TCP), similar to that of native mineralized bone tissues has been utilized. Furthermore, divalent biocompatible ions such Mg and Sr, which can be found in enamel, dentin, and bone but are rarely used in commercially available CaP scaffolds, have also been included during the synthesis step in order to evaluate the effect ionic substitution on the manipulation of the physicochemical properties and explore the in vitro biocompatibility.

X-ray diffraction (XRD) indicated that increasing Mg and Sr amounts during synthesis led to the formation of nanocrystalline β-TCP. Furthermore, thermal analysis, Rietveld refinement, and elemental analysis confirmed the increasing presence of a secondary amorphous phase as Mg and Sr content was increased. Interestingly, the presence of this amorphous phase appeared to promote live cell attachment, determined using the MC3T3-E1 mouse osteoblast cell line and live/dead staining along with scanning electron microscopy studies. Cell viability, studied using the MTT assay, indicated that for all conditions viability was comparable to that of undoped β-TCP and significantly improved in comparison to tissue culture plastic. An elemental analysis was performed on cell culture media which confirmed the increased concentrations of Mg and Sr in the media. The osteogenic differentiation of MC3T3-E1 and human mesenchymal stem cells was also studied on these materials and in both cases osteogenic gene expression was significantly upregulated on scaffolds with increased dopant levels.

In conclusion, biphasic mixtures of nanocrystalline β-TCP, prepared with increased dopant concentrations, and a secondary amorphous phase supported increased levels of osteogenic differentiation in comparison to undoped β-TCP. These results indicate that the combination of low temperature processing with ionic substitutions may be a viable method to produce novel CaP scaffolds which require lower concentrations of exogenous growth factors while achieving a similar overall efficacy. Results of these studies will be presented.