QUANTIFICATION OF CALCIUM PHOSPHATE NUCLEATION KINETICS TOWARDS DEVELOPMENT OF BIOMIMEMETIC DENTAL REFERENCE MATERIALS
William C. Miles, Sheng Lin-Gibson
National Institute of Standards and Technology
Polymers Division, Gaithersburg, MD 20899
Polymeric dental restoratives are used extensively in clinical settings but have considerable drawbacks in that more than half of current restorative procedures are to replace previous restoratives.1 Failure mechanisms are not well understood due to patient variability and a lack of criteria for material property characterization.2 Thus, there is a need for clinically relevant standard reference materials to understand these mechanisms. Biomimetic approaches to synthesize reference materials are of particular interest, but biological mineralization processes are exceedingly complicated. This research seeks to understand the factors that fundamentally influence nucleation kinetics of calcium phosphate and how these factors affect crystal structure and morphology. By understanding these factors, we hope to develop methods for controlled mineralization that mimic biological processes.
Dynamic light scattering was utilized to examine nucleation of calcium phosphate from a solution of CaCl2 and K2HPO4. Polyelectrolytes containing different functional groups were suspended in the solution. The nucleation rate of calcium phosphate in the presence of these materials was quantified using light scattering, and the crystal structure and morphology was examined using X-ray diffraction and TEM. The effects of temperature and solution ionic strength were also examined.
Previous studies have indicated that the carboxylic acid in poly(acrylic acid) (PAA) complexes with calcium ions and slows nucleation of calcium phosphate.3 Our experiments agreed with this analysis. However, our experiments also showed a marked increase in nucleation rate with the addition of polyethyleneimine (PEI), a positively charged polyelectrolyte. The current hypothesis is that the local basicity of the amine groups in the PEI is responsible for the increase in nucleation rate. This will be tested by addition of polyelectrolytes with positive charge but with decreasing basicity. Understanding the mechanism by which the nucleation rate is affected may then provide a means to control calcium phosphate nucleation and growth through patterned substrates with carefully chosen functionalities.
1. Sarrett, D. C., Clinical challenges and the relevance of materials testing for posterior composite restorations. Dental Materials 2005, 21, (1), 9-20.
2. Burke, F. J. T.; Wilson, N. H. F.; Cheung, S. W.; Mjor, I. A., Influence of patient factors on age of restorations at failure and reasons for their placement and replacement. Journal of Dentistry 2001, 29, (5), 317-324.
3. Girija, E. K.; Yokogawa, Y.; Nagata, F., Apatite formation on collagen fibrils in the presence of polyacrylic acid. Journal of Materials Science-Materials in Medicine 2004, 15, (5), 593-599.