Strong nanocomposites with Ca, PO(4), and F release for caries inhibition

Abstract

This article reviews recent studies on: (1) the synthesis of novel calcium phosphate and calcium fluoride nanoparticles and their incorporation into dental resins to develop nanocomposites; (2) the effects of key microstructural parameters on Ca, PO(4), and F ion release from nanocomposites, including the effects of nanofiller volume fraction, particle size, and silanization; and (3) mechanical properties of nanocomposites, including water-aging effects, flexural strength, fracture toughness, and three-body wear. This article demonstrates that a major advantage of using the new nanoparticles is that high levels of Ca, PO(4), and F release can be achieved at low filler levels in the resin, because of the high surface areas of the nanoparticles. This leaves room in the resin for substantial reinforcement fillers. The combination of releasing nanofillers with stable and strong reinforcing fillers is promising to yield a nanocomposite with both stress-bearing and caries-inhibiting capabilities, a combination not yet available in current materials.

Figure 1.

CaF₂ nanocomposite (A) TEM micrograph of CaF₂ nanoparticles. BET measurement yielded a specific surface area, A = 35.5 m²/g. With the density of CaF₂, ρ = 3.18 g/cm³, the CaF₂ nanoparticle diameter, d = 6/(Aρ)= 53 nm. (B) Fluoride release from CaF₂ nanocomposite. Fluoride ion (F) release was calculated as the release rate per hour per composite specimen surface area vs. immersion time. (Adapted from Xu et al., 2008a, with permission.)

Figure 2.

Effect of releasing filler level. (A) PO₄ release from the nanocomposite vs. filler mass fraction. Ca release had a similar trend (not shown). Increasing the filler level increased the ion release. (B) Effect of DCPA volume fraction in the resin, V_DCPA. The released PO₄ concentration at 56 days was related to V_DCPA by: PO₄ = 67V_DCPA^2.6, with a correlation coefficient r = 0.99. The Ca concentration was related to V_DCPA: Ca = 4.46V_DCPA^1.6 (not included). (Adapted from Xu et al., 2007a, with permission.)

Figure 3.

Effect of Ca-PO₄ particle size. (A) PO₄ release from composite vs. DCPA particle size. (B) PO₄ concentration at 56-day vs. DCPA particle surface area. Increasing the particle surface area increased the PO₄ release from the composite. Ca ion release (not shown) also increased with decreasing DCPA particle size and increasing particle surface area. (Adapted from Xu et al., 2007b, with permission.)

Figure 4.

Effect of Ca-PO₄ filler silanization on ion release. (A) PO₄ and (B) Ca ion release from nanocomposite containing 65% of DCPA nanopowder. The composite containing unsilanized DCPA released more ions than did the composite containing silanized DCPA. (Adapted from Xu et al., 2007b, with permission.)

Figure 5.

Flexural strength of composites. Specimens were tested either without immersion, or with immersion in water at 37°C for 1 day or 56 days. The ion-releasing nanocomposite matched the strengths of a commercial non-releasing hybrid composite. Both composites had strengths about two-fold that of a resin-modified glass ionomer. The box at the left axis indicates the reported strengths of previous Ca-PO₄ composites before immersion. The filled box at the right axis indicates reported strengths of previous Ca-PO₄ composites after immersion. The unfilled box at the right axis indicates strengths for non-releasing, stress-bearing composites (Xu et al., 2008b).

Figure 6.

Fracture toughness (K_IC) via a single-edge-notched-beam approach. Horizontal line indicates similar values (p > 0.1). The two control composites are: indirect inlay/onlay composite (Concept, Ivoclar, Amherst, NY, USA) and prosthetic composite (Artglass, Heraeus Kulzer, Germany). The unfilled box at the right axis indicates reported K_IC for non-releasing, stress-bearing composites; the filled box indicates reported K_IC of glass ionomer and resin-modified glass ionomer (Xu et al., 2008b).

Figure 7.

Three-body wear. (A) Wear scar depth and (B) diameter. Horizontal line indicates values not significantly different (p > 0.1). Nanocomposite with Ca-PO₄ release matched the wear of a commercial indirect composite without ion release. Box at right axis indicates wear of amalgam measured in the same way (Xu et al., 2008b).