Nanocomposites Containing CaF2 Nanoparticles: Thermal Cycling, Wear and Long-Term Water-Aging

Objectives:  Fluoride (F) releasing dental restoratives are promising to promote remineralization and combat caries. The objectives of this study were to develop nanocomposites containing calcium fluoride nanoparticles (nCaF₂), and investigate the long-term mechanical durability including wear, thermal-cycling and long-term water-aging behavior.

Methods: Two types of fillers were used: nCaF₂ with a diameter of 53 nm, and glass particles of 1.4 µm. Four composites were fabricated with fillers of: (1) 0% nCaF₂ + 65% glass; (2) 10% nCaF₂ + 55% glass; (3) 20% nCaF₂ + 45% glass; (4) 30% nCaF₂ + 35% glass. Three commercial materials were also tested. Specimens were subjected to thermal-cycling between 5°C and 60°C for 10⁵ cycles, three-body wear for 4x10⁵ cycles, and water-aging for 2 years.

Results:   After thermal-cycling, the nCaF₂ nanocomposites had flexural strengths in the range of 100-150 MPa, 5 times higher than the 20-30 MPa for resin-modified glass ionomer (RMGI). The wear scar depth showed an increasing trend with increasing nCaF₂ filler. Wear of nCaF₂ nanocomposites was within the range of wear for commercial controls. Water-aging decreased the strength of all materials. At 2 years, flexural strength was 94 MPa for nanocomposites with 10% nCaF₂, 60 MPa with 20% nCaF₂, and 48 MPa with 30% nCaF₂. They are 3-6 fold higher than the 15 MPa for RMGI (p<0.05). SEM revealed air bubbles and cracks in RMGI, while composite control and nCaF₂ nanocomposites appeared dense and solid.

Conclusions:  Combining nCaF₂ with glass particles yielded nanocomposites with long-term mechanical properties that were comparable to those of a commercial composite with little F release, and much better than those of RMGI controls. These strong long-term properties, together with their F-release being comparable to RMGI as previously reported, indicate that the nCaF₂ nanocomposites are promising for load-bearing and caries-inhibiting restorations.  Supported by NIH DE14190 and DE17974 (HX).