Nanocomposites with Ca and PO4 release: effects of reinforcement, dicalcium phosphate particle size and silanization

Abstract

Objectives: Nanoparticles of dicalcium phosphate anhydrous (DCPA) were synthesized in our laboratory for the first time and incorporated into a dental resin. Our goal was to develop a mechanically strong dental composite that has Ca and PO(4) ion release to combat tooth caries, and to investigate the effects of whisker reinforcement, DCPA particle size and silanization.

Methods: DCPA nanoparticles and two larger DCPA particles were used with nano-silica-fused whiskers as fillers in a resin matrix. Composite mechanical properties were measured via three-point flexure, and the release of Ca and PO(4) ions were measured versus time.

Results: Using DCPA nanoparticles with a diameter of 112nm, the composite at a DCPA:whisker mass ratio of 1:1 had a flexural strength (mean+/-S.D.; n=5) of (112+/-17)MPa, not significantly different from (112+/-14)MPa of a commercial non-releasing composite; both were higher than (29+/-7)MPa for the composite at DCPA:whisker of 1:0 (p<0.05). The composite with DCPA particle size of 112nm released Ca to a concentration of 0.85mmol/L and PO(4) of 3.48mmol/L, higher than Ca of 0.67mmol/L and PO(4) of 1.11mmol/L using DCPA with 12microm particle size (p<0.05). Silanization of DCPA increased the composite strength at DCPA:whisker of 1:0 compared to that without silanization, but decreased the Ca and PO(4) release (p<0.05). Increasing the DCPA particle surface area increased the Ca and PO(4) release.

Significance: Decreasing the DCPA particle size increased the Ca and PO(4) release; whisker reinforcement increased the composite strength by two- to three-fold. The nano DCPA-whisker composites, with high strength and Ca and PO(4) release, may provide the needed, unique combination of stress-bearing and caries-inhibiting capabilities.

Figure 1

Microscopy of the three DCPA powders. (A) TEM microscopy of nano-particles of DCPA with arrows indicating particles of about 50 nm in size. (B) SEM of a commercial DCPA powder ground for 24 h, yielding a median particle diameter of 0.88 μm. (C) SEM of the as-received commercial DCPA powder with a median diameter of 12.0 μm.

Figure 2

Mechanical properties of the composites. Each value is the mean of five measurements with the error bar showing one standard deviation (mean ± sd; n = 5). “Whisker” refers to the nano-silica-fused whiskers that were silanized. DCPA was not silanized. The DCPA:whisker number is a mass ratio. The hybrid control is a commercial composite (TPH) with no release of Ca or PO₄. Decreasing the DCPA particle size slightly decreased the composite properties; whisker reinforcement substantially improved the composite mechanical properties.

Figure 3

Effect of DCPA silanization on composite strength. “Whisker” refers to the nano-silica-fused whiskers that were silanized. DCPA was either silanized or not silanized. Each value is mean ± sd; n = 5. At DCPA:whisker of 1:0, DCPA silanization increased the composite strength. At DCPA:whisker of 1:1, DCPA silanization had no significant effect. These trends were true for both 0.112-μm DCPA (A) and 0.88-μm DCPA (B). The horizontal bar indicates values that are not significantly different (p > 0.1).

Figure 4

Ca and PO₄ release: effect of DCPA particle size. “Whisker” refers to the silanized nano-silica-fused whiskers. DCPA was not silanized. The numbers (e.g., 12 μm, refer to the DCPA particle size). Each value is mean ± sd; n = 3. The nano-composite with DCPA particle size of 0.112 μm had significantly higher Ca and PO₄ release than the other two composites with DCPA sizes of 0.88 μm and 12 μm. The hybrid control composite had no detectable Ca or PO₄ release.

Figure 5

Ca and PO₄ release: effect of DCPA silanization. “Whisker” refers to the nano-silica-fused whiskers that were silanized. The composite contained DCPA powder (particle size of 0.112 μm) that was not silanized. Each value is mean ± sd; n = 3.

Figure 6

Effect of DCPA particle surface area on Ca and PO₄ release for composite containing unsilanized nano DCPA at DCPA:whisker of 1:0. The particle specific surface area was calculated by A = 6/(d ρ), where d is diameter and ρ is density (2.89 g/cm³ for DCPA). (A–B) Initial Ca and PO₄ release from Fig. 4A and 4C (7-day data). (C–D) 56-d release data from Fig. 4A and 4C. Each Ca and PO₄ concentration value is mean ± sd; n = 3. Smaller particles with a larger surface area had more ion release.