Effect of HEMA on bonding efficiency of 10-MDP to hydroxyapatite

The functional monomer 10-methacryloxydecyl dihydrogen phosphate (10-MDP) ionically bonds to hydroxyapatite (HAp) (Yoshida et al., JDR 2004). This bond is very stable, as was confirmed by the low dissolution rate of 10-MDP's calcium salt in water. Objectives: To study potential interference of HEMA on the chemical bonding efficiency of 10-MDP to HAp. Methods: We examined the chemical interaction of 5 experimental adhesive solutions, containing 0.005 mol 10-MDP and 0.228 mol H2O with EtOH and HEMA respectively varying following the mol ratio's 0.10:0 (exp-1), 0.75:0.25 (exp-2), 0.05:0.05 (exp-3), 0.25:0.75 (exp-4) and 0:0.10 (exp-5), with HAp by means of XRD (X-ray Diffraction) and solid-state NMR (Nuclear Magnetic Resonance: 31P MAS NMR, 31P CP-MAS NMR and 1H MAS NMR). In addition, QCM (Quartz Crystal Microbalance) equipped with a HAp sensor was used to study the potential adsorption of HEMA (5wt% in H2O) onto HAp. Results: XRD revealed nano-layering of CaMHP2 for exp-1 and also, though less intensively, for exp-2, but not for exp-3/4/5. NMR disclosed a strong peak at -1.96 ppm for exp-1 that can be assigned to CaMHP2. All other solutions showed a significantly less intense peak that should be assigned to 10-MDP molecules adsorbed onto HAp, and this irrespective of the mol ratio of EtOH to HEMA. Finally, QCM showed that HEMA molecules are adsorbed to HAp as well. Conclusion: This experiment showed that HEMA restrained nano-layering or the formation of CaMHP2, since both 10-MDP and HEMA molecules compete to interact with the HAp surface.