Mechanical properties of visible light-cured resins reinforced with hydroxyapatite for dental restoration
Introduction
Dentine can be regarded as a natural composite, consisting of an organic matrix of mainly collagen plus a small amount of citrate and an inorganic filler constituted of nanoscopic (10–100 nm) hydroxyapatite (HAP) crystals. A dental resin reinforced with dispersed HAP crystals seems, in principle, a favorable restorative material for human tooth tissues [1], [2], as it is for bone tissues [3]. The use of HAP in restorative dentistry offers several promising advantages, including intrinsic radio-opaque response, enhanced polishability and improved wear performance, since synthetic HAP has a hardness similar to that of natural teeth. Finally, this material is less expensive than most of the fillers commonly used (e.g. barium or zinc glasses, quartz, zirconia, silica or alumina [2], [4]). The main disadvantage of HAP is its high refractive index when compared to those of light-activated polymers normally used in dental restoration [5]. Because of its bicompatibility, the use of HAP in both skeletal and dental restorations is a fundamental area of research [6], [7].
In the present work, either microscopic or nanoscopic-HAP was used as filler material. Microscopic particles are easily mixed with dental resins to a high proportion and are widely used in dentistry as the reinforcing material. On the other hand, nanoscopic particles have more similarities to natural tooth mineral phase as far as crystal size is concerned. Additionally, the high surface area of the nanoscopic particles would offer a good mechanical interlocking with the polymer matrix.
Most marketed dental composites contain the visible light-cured Bis-GMA monomer (Table 1) [8], [9]. In order to achieve a final consistency suitable for the incorporation of particulate fillers as well as to increase the degree of conversion, a low molecular weight methacrylate diluent is added to the highly viscose Bis-GMA monomer [10], [11]. In this work, TEGDMA (Table 1) was chosen because it is the most commonly used diluent. HEMA (Table 1) is widely used as adhesive, since it facilitates self-adhesion to the mineral phase of dental tissues [11]. HEMA was also selected as a diluent, since its tendency to interact with HAP was expected to improve the matrix/filler coherence.
To induce matrix/filler interaction, the filler is usually impregnated on the surface with a bifunctional coupling agent [12], [13], [14]. In most resin composites, the filler particles are impregnated with a silane coupling agent. Several authors have already studied the interaction of silane with HAP [5], [15]. In this study, citrate (Table 1) was primarily chosen as a coupling agent because it is one of the components of natural tooth tissues, is more biocompatible than silane and interacts easily with calcium ions [16]. Polyalkenoic acids, such as polymaleic or polyacrylic acids, are used in commercial restorative formulations owing to their tendency to interact with dental tissues [17]. A strong interaction between the monomers of these acids and the HAP crystals is also predictable. Thus, malate, acrylate and methacrylate (Table 1) were also chosen, together with citrate, as coupling agents. The four organic compounds hold carboxylic functional groups that can interact with HAP surface [16], [17], [18].
The main objective of this work was to gain more insight into the use of HAP as reinforcing filler in dental restorative materials, by analyzing the mechanical properties of the formulated composite resins. Conventional flexural and hardness mechanical tests were used. In comparison with the scanning electron micrographs, a more accurate description of these properties is given.
Section snippets
Composites preparation
Table 2 lists the composition of the 14 experimental restorative materials. The organic matrix consisted of 60 wt% Bis-GMA (Freeman Chemical Corporation, UK) and 40 wt% diluent comonomer. Either TEGDMA (series 1) in a mole fraction of 0.54 or HEMA (series 2) in a mole fraction of 0.72 was used as a comonomer. Camphorquinone and dimethyl aminoethyl methacrylate were added as the initiator and coinitiator, respectively. Diluents, initiator and coinitiator were procured from Fluka, Switzerland. As a
HAP particles
Fig. 1 shows the habit of the precipitated HAP particles. The needle-like nanoparticles of 50–100 nm length (Fig. 1(a)) had a high specific surface area of ∽120 m2/g, while the microscopic 1–5 μm particles (Fig. 1(b)) had only ∽5 m2/g.
HAP-coupling agent
Fig. 2 shows the FTIR spectra of the precipitated HAP microparticles (a) and HAP nanoparticles (b); and the HAP microparticles after sorption of the used coupling agents (c–f). The characteristics HAP bands of PO43− and OH− appeared at 1090–1030 cm−1(band A)/600–560 cm−1
Discussion
Most dental restorative materials contain mixtures of Bis-GMA and TEGDMA in the percentage 60:40 wt%, since it was found to be an appropriate ratio for a high degree of polymerization [9]. Although monomers similar to HEMA, such as 3-hydroxypropil methacrylate, HPMA [21], have been used as diluents for Bis-GMA, HEMA has not been examined to date, the main reason being probably the high degree of hydrophilicity of this monomer. Several studies underlined the strong affinity of HEMA monomer with
Conclusions
The elasticity modulus and surface hardness of the experimental composites were increased by the incorporation of both uncoated and coated HAP, but the flexural strength decreased in most of the cases. HEMA can be used at the same time as a viscosity-reducer monomer, as a HAP coupling agent and as a HAP adhesive material. In general, mechanical properties were improved by the addition of a coupling agent, with the exception of materials containing malic acid. Filler/matrix coherence was
Acknowledgements
This investigation was supported by Research Projects CICYT: MAT98-0976-CO2-01 and CICYT: MAT98-0937-C02.
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