Leachability of ACP Composites: Effect of Extraction Conditions

Objective:  To assess, by ¹H NMR spectroscopy, the effect of different extraction conditions on the leachability of monomers and photo-initiators from an experimental amorphous calcium phosphate (ACP) composite.

Method:  Light-cured resins and their ACP composites were extracted in either acetone (7 days, 23 ^oC, magnetic stirring) or saline/ethanol (1:1 mass ratio) mixture (14 days, 23 ^oC, 30 s mixing 2 times/day).  The resins consisted of urethane dimethacrylate (UDMA), 2-hydroxyethyl methacrylate (HEMA), methacryloyloxyethyl phthalate (MEP) and poly(ethylene glycol) extended UDMA (PEG-U), with camphorquinone (CQ) and ethyl-4-N,N-dimethylamino benzoate (4EDMAB) as initiators and butylated hydroxytoluene (0.01 mass-%) as an inhibitor.  ¹H NMR spectroscopy was employed to qualify and quantify leachables. Data were statistically analyzed by ANOVA and multiple pair-wise comparisons (t-test).

Result:  All monomers and 4EDMAB were identified in both extraction experiments, while CQ was not detected.  Up to13.9 mass-% and 6.2 mass-% of the copolymers and composites, respectively, were extracted in acetone.  Monomers varied between 0.30 % to 14.29 % (copolymers) and 0.12 % to 10.39 % (composites) of initial content. 4EDMAB showed the highest leachability (33.06 % and 24.66 % in copolymer and composite extracts, respectively). When composite data were normalized with respect to initial amounts of resin, the differences between copolymer and composite became marginal. In ethanol/saline experiments, the extractable portions were 2.5 mass-% and 7.3 mass-% of copolymers and composites, respectively. Monomers varied between 0.20 % to 37.32 % (copolymers) of initial content.  4EDMAB showed a leachability of 22.70 %.  Composite samples are currently being analyzed. 

Conclusion:  Results exemplify a strong effect of the extraction conditions on leachability. The acetone extraction data suggest that in highly cross-linked experimental resins and composites, leachability is generally limited by the mobility of polymer chains which restricts pathways for monomer to leach out of the system.

Support: NIDCR Grant R01DE13169, ADAF and NIST.