Adsorption and Acid Erosion Reduction Potential of Polymers on Hydroxyapatite

Objectives: Determination of in vitro surface adsorption and acid erosion reduction potential of anionic polymer functional groups on hydroxyapatite (HAP) employing Quartz-Crystal Microbalance with Dissipation (QCM-D), Atomic Force Microscopy (AFM), and hydroxyapatite dissolution.
Methods: Polymer solutions with equimolar concentrations of carboxylate, sulfonate and phosphate functional groups respectively were tested for adsorption on HAP and acid erosion of the same by QCM-D at 25^oC and flow rate 50μl/min. Flow sequence of 2min water, 8min phosphate buffer solution (PBS), 8min polymer solution and 8min water (repeated twice), 8min citric acid and 8min water (repeated twice) was used. HAP surface topography and roughness was measured by AFM at resonance frequency 320 kHz and spring constant ~42 N/m before and after flow sequence. A fast screening method measured HAP dissolution of polymer-treated HAP discs (n=6) by determining the calcium concentration of a citric acid challenge solution using inductively coupled plasma mass spectrometry (ICP-MS). Effect of pH and molecular weight of phosphate polymer on HAP adsorption and acid erosion was studied.
Results: Type of anionic functional group influenced polymer adsorption on HAP and subsequent acid erosion. HAP-discs treated with phosphate polymer exhibited better polymer adsorption and less acid erosion compared to carboxylate and sulfonate polymers. Lower molecular weight of polymer and lower pH (4.0) of test solution resulted in better adsorption and less acid erosion. Results from QCM-D and AFM were in line with fast screening HAP dissolution method.
Conclusions: QCM-D is a valuable tool for determining hard tissue binding capability and acid erosion reduction potential of materials in oral care application. Polymer coatings with superior binding to HAP surface could be differentiated from those that bind poorly by employment of this method. Effect of toothpaste ingredients on binding capability of above polymers is a useful next step.