Reinforcement of experimental composite materials based on amorphous calcium phosphate with inert fillers

Abstract

Objectives

The aim of this study was to examine the influence of the addition of glass fillers with different sizes and degrees of silanization percentages to remineralizing composite materials based on amorphous calcium phosphate (ACP).

Methods

Four different materials were tested in this study. Three ACP based materials: 0-ACP (40 wt% ACP, 60 wt% resin), Ba-ACP (40 wt% ACP, 50 wt% resin, 10 wt% barium-glass) and Sr-ACP (40 wt% ACP, 50 wt% resin, 10 wt% strontium-glass) were compared to the control material, resin modified glass ionomer (Fuji II LC capsule, GC, Japan). The fillers and composites were characterized using scanning electron microscopy. Flexural strength and modulus were determined using a three-point bending test. Calcium and phosphate ion release from ACP based composites was measured using inductively coupled plasma atomic emission spectroscopy.

Results

The addition of barium-glass fillers (35.4 (29.1–42.1) MPa) (median (25–75%)) had improved the flexural strength in comparison to the 0-ACP (24.8 (20.8–36.9) MPa) and glass ionomer control (33.1 (29.7–36.2) MPa). The admixture of strontium-glass (20.3 (19.5–22.2) MPa) did not have any effect on flexural strength, but significantly improved its flexural modulus (6.4 (4.8–6.9) GPa) in comparison to 0-ACP (3.9 (3.4–4.1) GPa) and Ba-ACP (4.6 (4.2–6.9) GPa). Ion release kinetics was not affected by the addition of inert fillers to the ACP composites.

Significance

Incorporation of barium-glass fillers to the composition of ACP composites contributed to the improvement of flexural strength and modulus, with no adverse influence on ion release profiles.

Introduction

Contemporary restorative dental medicine requires not only esthetic materials which restore tooth structures, but also materials which are able to heal carious-affected hard dental tissue . Over the last few decades, efforts have been increased to produce bioactive materials able to reverse the carious process and to remineralize caries-affected tissue. Amorphous calcium phosphate (ACP) based composite resins are intended as remineralizing/anti-demineralizing agents. ACP is well known as a direct precursor of hydroxyapatite (HA) and has a major role in the biomineralization processes of teeth and bones . In an aqueous environment, ACP composite materials release calcium and phosphate ions, providing supersaturating concentrations sufficient to trigger the apatite build-up and remineralize demineralized enamel .

At the same time, those characteristics pose limitations in clinical applications when durability or resistance to crack and deformation upon load are required. Bioactive ACP particles do not have a reinforcing role such as silanized glass or silica fillers, which are used in most of the commercially available dental composite materials. ACP particles are not silanized, as this has an adverse effect on calcium and phosphate ion release without providing any advancement in mechanical properties . Hence, the amount of ACP had to be reduced to the minimal level, which also preserved the remineralizing properties without additionally undermining the mechanical properties. This has lead to the optimization of the composition to 40 wt.% of ACP .

Flexural strength of composite materials is mostly dominated by the degree of conversion of the organic matrix , filler volume and the filler to matrix interfacial relationship . In conventional composites, filler particles increase strength, stiffness and decrease dimensional changes , while their silanization ensures better resin to filler interaction and deflects the fracture line. This contributes to higher flexural strength and an overall improvement of mechanical properties . The filler load is directly correlated to the flexural strength and modulus, as stated by many authors . However, fillers without silanization do not provide sufficiently high flexural strength for composite materials . Composites with silanized fillers show higher flexural strength in comparison to those with unsilanized fillers . A lack of silanized reinforcing fillers in ACP based composite resins have insufficient mechanical properties as a consequence .

This group of authors has recently tested the influence of the addition of silanized nanosilica to the ACP composite resin formulation. The study proved that the principle of the admixture of inert fillers is successful in improving flexural strength and increasing the level of calcium and phosphate ion release in comparison to ACP composites without non-releasing fillers . However, it was emphasized that the agglomeration of silica nanoparticles, due to their large surface area, may contribute to the hydrolytic degradation. This in turn enhances ion release, but it also might negatively affect the long-term mechanical properties. Similar conclusions were drawn in another study which examined the effect of various types of silanized fillers on the degree of conversion of ACP composite resins . Taking into account that the random clustering of ACP particles in resin matrix is also recognized as one of the reasons for diminished strength of ACP composites , any additional agglomeration is an undesired property. Different authors agree that the agglomerated particles could act as strength controlling flaws which initiate crack and consequently lead to fracture .

In contrast, conventional glass microfillers have a lower surface area than nanofillers and do not show the tendency to agglomeration. Thus, it is expected that their addition to ACP composites might provide better interaction to the resin phase and higher strength and modulus. At the same time, it is necessary to examine if the inert glass fillers interfere with the ion release kinetics. The present study was aimed to investigate the effect of glass fillers of various sizes and degrees of silanization on ACP based composite resins. The null-hypothesis was that the addition of fillers does not have an influence on flexural strength, flexural modulus and ion release of ACP based composites.

Materials and methods

Materials

Synthesis of zirconia ACP fillers

The synthesis of Zr-ACP fillers followed the procedure by Skrtic et al. . Zr–ACP precipitated instantaneously in a closed system at 23 °C upon rapidly mixing equal volumes of a 800 mmol/L Ca(NO 3 ) 2 solution, a 536 mmol/L Na 2 HPO 4 solution that contained a molar fraction of 2% Na 4 P 2 O 7 as a stabilizing component for ACP, and an appropriate volume of a 250 mmol/L ZrOCl 2 solution (mole fraction of 10% based on the calcium reactant) . The reaction pH varied between 8.6 and 9.0. The suspension was filtered; the solid phase was washed subsequently with ice-cold ammoniated water and acetone and then lyophilized. ACP fillers were kept in a desiccator to avoid exposure to humidity and premature conversion to apatite until being used in composites.

Formulation of resin

The experimental resin was the same for all ACP based materials containing 62.8 wt% of ethoxylated bisphenol A dimethacrylate (EBPADMA; Esstech, PA, USA), 23.2 wt% of triethylene glycol dimethacrylate (TEGDMA; Esstech), 10.4 wt% of 2-hydroxyethyl methacrylate (HEMA; Esstech), 2.6 wt% of methacryloxyethyl phthalate (MEP; Esstech), 0.2 wt% of the photo oxidant camphorquinone (CQ; Aldrich, WI, USA) and 0.8 wt% of photo reductant ethyl-4- (dimethylamino) benzoate (4E; Aldrich). Using a magnetic stirrer, the monomers and photo activators were mixed (in the absence of blue light) until a uniform consistency was achieved.

Tested materials

The compositions of the three ACP composite materials used in this study, as well as the glass-ionomer control are shown in Table 1 . The ACP fillers, silanized glass fillers ( Table 2 ) and resin were mixed in lightproof containers in a dual asymmetric centrifugal mixing system (Speed Mixer TM DAC 150 FVZ, Hauschild & Co KG, Hamm, Germany) at 2700 rpm for 135 s, followed by pressing of the composite pastes through a three roller mixer (EXAKT 50, EXAKT, Norderstedt, Germany) three times, to ensure paste homogeneity.

Table 1
The composition of tested materials.
Material ACP control (0-ACP) ACP + 10% barium-glass (Ba-ACP) ACP + 10% strontium-glass (Sr-ACP) Fuji II LC capsule a (FII; GC, Tokyo, Japan; LOT 0908197)
Resin or liquid 60 wt.% EBPADMA based hydrophilic resin 50 wt.% EBPADMA based hydrophilic resin 50 wt.% EBPADMA based hydrophilic resin 24 wt.% liquid: PAA, HEMA, proprietary ingredient, 2,2,4-trimethyl hexamethylene dicarbonate, TEGDMA
Fillers 40 wt.% ACP 40 wt.% ACP
10 wt.% Ba fillers
40 wt.% ACP
10 wt.% Sr fillers
76 wt.% (fluoro) alumino silicate glass

a Composition provided by the manufacturer; EBPADMA, ethoxylated bisphenol A dimethacrylate, PAA, polyacrylic acid, HEMA, 2-hydroxyethyl methacrylate, TEGDMA, triethylene glycol dimethacrylate

Table 2
Specifications of inert fillers added to the ACP test materials, as provided by the manufacturer.
Fillers Composition (approximate values; wt%) Size (d50/d99 [μm]) Silanization (wt%) Refractive index Product name/manufacturer
Barium glass (Ba) SiO 2 55.0% 0.77/2.28 6 1.52 GM39923
BaO 25.0% Schott, Germany
B 2 O 3 10.0%
Al 2 O 3 10.0%
F 2.00%
Strontium glass (Sr) SiO 2 60.0% 0.99/2.95 3.2 1.50 G018-163
SrO 15% Schott, Germany
B 2 O 3 15.0%
Al 2 O 3 15.0%
F 2.00%

Methods

Characterization of ACP and glass fillers

The micromorphology of all the fillers used in this study was examined by scanning electron microscopy (SEM; Quanta FEG 400; FEI Company, Netherlands). The fillers were fixed by adhesive foil and were not modified or sputtered for imaging. Images were recorded in low vacuum using a large field detector at a working distance of 10 mm and 4 kV electric potential.

Three-point bending test (3PBT)

Four materials were subjected to the 3PBT: ACP control with no inert fillers (0-ACP), ACP with 10% of barium-glass fillers (Ba-ACP), ACP with 10% of strontium-glass fillers (Sr-ACP) and as control a resin modified glass ionomer FII. Ten samples were made per material. The stainless steel mold with openings 22 mm × 2 mm × 2 mm in size was first coated with silicone spray (Silikonspray, Seidel Medizin GmbH, Buchendorf, Germany). Each side of the mold was then covered with a poly(ethylene terephthalate) film (PET; Frasaco Universal Streifen, Franz Sachs & Co., Tettanang, Germany) and a glass slide. The composite material was dispensed by capsules (KerrHawe Composite Gun; KerrHawe SA, Bioggio, Switzerland). The samples were polymerized with overlapping areas for 200 s from the top and bottom side using a Bluephase C8 LED curing unit (Ivoclar VIvadent, Schaan, Liechtenstein) in a high power mode with a intensity of 1050 mW/cm 2 (mean value from three consecutive light intensity measurements by dental radiometer (CureRite, Dentsply Caulk, Milford, USA)). Glass ionomer samples were immersed in deionized water at room temperature (21 °C) for 15 min after light curing, and then removed from the mold.

All samples were polished with 600 grit silicon carbide abrasive paper (Buehler, Lake Bluff, IL, USA) on each side and stored in deionized water for 24 h at 37 °C. The exact dimensions of each sample were measured before subjecting them to the three-point flexural test at the universal testing machine Zwick Z010 (Zwick Roell AG, Ulm, Germany), operated by the testXpert II software system (version 2.1, Zwick Roell AG, Ulm, Germany) at a 20 mm span with a crosshead speed of 0.75 mm/min. The flexural strength (FS) is calculated according to the formula: σ = 3 FL /2 bh 2 (MPa), where F is the maximum load (force); L is the length of the support span; b is width and h is thickness of the sample. Flexural modulus of elasticity (FM) was determined as: E = FL 3 /4 bh 3 d (GPa), where d represents deflexion of the sample corresponding to the load F .

Micromorphology of ACP composites

Composite samples previously used in the 3PBT were polished with silicon carbide paper (Buehler GmbH, Düsseldorf, Germany) with decreasing roughness – 600, 800 and 1200 grit and afterwards with aluminum oxide powder (Buehler) with deceasing particle size – 1.0, 0.3 and 0.005 μm with distilled water as a medium. The SEM imaging of composite samples was performed in low vacuum using large-field (LFD) and solid-state (SSD) detectors operating at an acceleration voltage of 15 kV and a working distance of approximately 10 mm.

Ion release test (IR) and ion activity product

Calcium and phosphate IR was measured only for the ACP containing materials. A total of 60 samples were made, 20 samples for each material. Teflon rings (IBG Monoforts Vertriebs GmbH; 5 mm inner diameter, 2 mm high) were filled with ACP composite materials dispensed by capsules and covered from both sides with PET films and glass slides. Each sample was polymerized for 80 s (40 s each side) by Bluephase C8 curing unit in high power polymerization mode. The samples were kept at room temperature in a dark container for 24 h before the immersion of each sample in 8 ml of HEPES-buffered saline solution (pH = 7.4) after which all the samples were kept in closed vials in an incubator at 37 °C. Thus, 8.83 mm 2 of sample surface was available for ion release per milliliter of saline solution.

Two approaches were used to attain information on IR. In the first, static approach, samples remained in the same vial for 1, 7, 14, or 28 days (five samples/material/time point). In the second, dynamic approach, the same samples that were eluted for one day in static approach were consecutively transferred into a fresh solution after 7, 14 and 28 days. The concentrations of the eluted calcium and phosphate ions were measured in the solutions after removing the samples using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Spectro Flame EOP, Spectro Analytical Instruments, Kleve, Germany). Standard curves were prepared from serial dilutions of Ca and P standard solutions (TraceCert, Fluka, Buchs, Switzerland).

The ion activity product (IAP) was computed with respect to stoichiometric apatite Ca 10 (OH) 2 (PO 4 ) 6 using Chemist Application (version 1,0,1,0; Micromath Research, St. Louis, MO, USA). The thermodynamic stability of immersion solutions containing Ca 2+ and PO 4 3− ions released from the composite disks was calculated using the Gibbs free energy expression Δ G ° = −2.303(RT/ n )log(IAP/Ksp) (kJ/mole), where R is the ideal gas constant, T is the absolute temperature, n is the number of ions in the IAP ( n = 18 for HAP) and K sp is the thermodynamic solubility product.

Statistical analysis

The results of the 3PBT (ten samples/four materials) and IR test data (five samples/three materials/four time points) for static and dynamic conditions were descriptively expressed as medians with 25–75% quantiles. For statistical analysis, Mann–Whitney U -test and Wilcoxon Rank Sum test were applied for the comparison of the independent and the dependent experimental groups, respectively. The level of significance was set to α = 0.05. For multiple comparisons α was adjusted to α *( k ) = 1−(1 − α ) 1/ k applying the Error Rates Method, where k describes the number of pairwise tests to be considered. All analyses were performed in SPSS 19.0 software (SPSS Inc., Chicago, USA).

Materials and methods

Materials

Synthesis of zirconia ACP fillers

The synthesis of Zr-ACP fillers followed the procedure by Skrtic et al. . Zr–ACP precipitated instantaneously in a closed system at 23 °C upon rapidly mixing equal volumes of a 800 mmol/L Ca(NO 3 ) 2 solution, a 536 mmol/L Na 2 HPO 4 solution that contained a molar fraction of 2% Na 4 P 2 O 7 as a stabilizing component for ACP, and an appropriate volume of a 250 mmol/L ZrOCl 2 solution (mole fraction of 10% based on the calcium reactant) . The reaction pH varied between 8.6 and 9.0. The suspension was filtered; the solid phase was washed subsequently with ice-cold ammoniated water and acetone and then lyophilized. ACP fillers were kept in a desiccator to avoid exposure to humidity and premature conversion to apatite until being used in composites.

Formulation of resin

The experimental resin was the same for all ACP based materials containing 62.8 wt% of ethoxylated bisphenol A dimethacrylate (EBPADMA; Esstech, PA, USA), 23.2 wt% of triethylene glycol dimethacrylate (TEGDMA; Esstech), 10.4 wt% of 2-hydroxyethyl methacrylate (HEMA; Esstech), 2.6 wt% of methacryloxyethyl phthalate (MEP; Esstech), 0.2 wt% of the photo oxidant camphorquinone (CQ; Aldrich, WI, USA) and 0.8 wt% of photo reductant ethyl-4- (dimethylamino) benzoate (4E; Aldrich). Using a magnetic stirrer, the monomers and photo activators were mixed (in the absence of blue light) until a uniform consistency was achieved.

Tested materials

The compositions of the three ACP composite materials used in this study, as well as the glass-ionomer control are shown in Table 1 . The ACP fillers, silanized glass fillers ( Table 2 ) and resin were mixed in lightproof containers in a dual asymmetric centrifugal mixing system (Speed Mixer TM DAC 150 FVZ, Hauschild & Co KG, Hamm, Germany) at 2700 rpm for 135 s, followed by pressing of the composite pastes through a three roller mixer (EXAKT 50, EXAKT, Norderstedt, Germany) three times, to ensure paste homogeneity.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Reinforcement of experimental composite materials based on amorphous calcium phosphate with inert fillers
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