To evaluate the strength and ion release of experimental composites containing TEGDMA-functionalized calcium phosphate particles.
Seven composites containing equal parts (in mols) of BisGMA and TEGDMA and 60 vol% of fillers were manipulated. Filler phase was constituted by silanized barium glass and 0% (control), 10% or 20% (volume) of dicalcium phosphate dihydrate (DPCD) particles, either non-functionalized or functionalized with two different TEDGMA contents. DCPD particles were synthesized and characterized by X-ray diffraction (XRD), elemental analysis, surface area and dynamic light scattering. Composites were tested for degree of conversion (DC) by near-FTIR. Biaxial flexural strength (BFS) was determined after 24 h and 28 days in water. Calcium and phosphate release after 7 days was assessed using inductively coupled plasma optical emission spectrometry (ICP-OES). Data were analyzed by ANOVA/Tukey test (alpha:5%).
XRD confirmed the crystalline structure corresponding to DCPD. Elemental analysis revealed particles with zero, 14% or 22% TEGDMA, with similar D 50 (around 19 μm) and surface areas from 3.5 to 11.4 m 2 /g. The presence of DCPD did not reduce DC. After 24 h, functionalization (both 14% and 22% TEGDMA) improved composite strength in comparison to non-functionalized DCPD, both at 10% and 20% levels. After 28 days, BFS of materials containing 10% functionalized DCPD were statistically similar to the control containing only barium glass. Among composites containing 10% DCPD, particle functionalization with 14% TEGDMA did not jeopardize ion release.
At 10 vol%, the use of TEGDMA-functionalized CaP particles improved composite strength in relation to non-functionalized particles, while maintaining similar ion release levels.
Over the years, the incorporation of calcium orthophosphates and bioactive glass particles in resin-based materials has been studied as a possible path to promote tooth remineralization and reduce the risk of secondary caries at the tooth/restoration interface . These particles behave as ion sources, releasing calcium and phosphate to the surrounding medium and creating a supersaturated environment that favors mineral deposition on the tooth.
Unfortunately, the addition of CaP particles to unfilled resins significantly lowers their strength . This impaired mechanical behavior is ascribed to the lack of a strong chemical bond between the bioactive particles and the resin phase, as well as the low cohesive strength of the CaP agglomerate itself . Several approaches have been tested to increase the mechanical properties of bioactive composites. For example, an experimental composite containing hydroxyapatite particles functionalized with acrylic acid showed a significantly higher flexural strength in comparison with a control material containing unmodified CaP particles . Silanization of CaP particles was also tested and, though composite strength increased in comparison to the use of non-silanized particles, ion release was negatively affected .
The association between reinforcing fillers and bioactive particles seems to offset some of the negative effect of the latter on composite mechanical properties. For example, when 40 wt% of tetracalcium phosphate (TTCP) was added to an unfilled resin, 20 wt% of silanated glass had to be added in order to recover the flexural strength of the unfilled resin . In another study, the addition of 10 wt% reinforcing fillers to an experimental material containing 40 wt% amorphous calcium phosphate (ACP) also contributed to improve composite strength, with no adverse effect on ion release .
Recently, DCPD nanoparticles functionalized with triethylene glycol dimethacrylate (TEGDMA) were synthesized . Among the different calcium orthophosphate phases, dicalcium phosphate dihydrate (DCPD, CaHPO 4 .2H 2 O) presents a relatively high solubility , which makes it more suitable as ion source than less soluble phases, such as hydroxyapatite (HA). Also, its refractive index (1.54–1.55) is similar to barium glass (1.53) . HA, for example, has a refractive index of 1.63–1.67, which compromises light transmission during photoactivation, as well as esthetics . By adding the monomer to one of the reacting solutions ( i.e ., prior to DCPD crystallization), it would attach to the Ca 2+ at the nanoparticle surface at early stages of its growth, when reactivity is at its maximum. Also, the TEGDMA in the functionalizing layer would co-polymerize with monomers of the resin matrix, increasing the compatibility between the nanostructured agglomerates and the resin. Preliminary studies showed that the incorporation of TEGDMA-functionalized DCPD in a BisGMA:TEGDMA resin matrix increased biaxial flexural strength by 32% in comparison with a material containing non-functionalized particles.
This investigation is a follow-up from a previous study where the substitution of silanized barium glass with proprietary, non-functionalized DCPD nano-structured agglomerates in experimental composites resulted in up to 35% reduction in fracture strength . In the present investigation, TEGDMA-functionalized DCPD particles were tested partially replacing the reinforcing fillers. The ultimate goal was to determine if the use of functionalized DCPD particles would improve composite strength (in comparison to the material containing non-functionalized particles) without jeopardizing ion release. Additionally, the effect of the amount of TEGDMA in the functionalizing layer was also verified. The null hypotheses were: (1) replacing 10 vol% or 20 vol% of silanized glass fillers by DCPD particles does not affect composite degree of conversion and strength (prior and after aging in water for 28 days), regardless of the DCPD content or the presence of a TEGDMA functionalizing layer, (2) ion release is not affected by DCPD content or by functionalization of the DCPD particles.
Synthesis and characterization of dicalcium phosphate dihydrate (DCPD) nanoparticles
DCPD (CaHPO 4 .2H 2 O) nanoparticles were synthesized through the reaction between ammonium phosphate (NH 4 ) 2 HPO 4, and calcium nitrate Ca(NO 3 ) 2 .4H 2 O (both from Sigma–Aldrich Co., St. Louis, MO, USA) by a sol–gel process . Equimolar solutions (0.078 mol/L) were prepared with deionized water at room temperature and mixed with a magnetic stirrer. The ammonium phosphate solution received 6.86 g (0.024 mol) of triethylene glycol dimethacrylate ( Fig. 1 , TEGDMA, Mw = 286 g/mol, Sigma–Aldrich). 400 mL of the calcium nitrate solution was added to the same volume of the ammonium phosphate/TEGDMA solution using a peristaltic pump (9 mL/min). The resulting mixture was kept under stirring for 30 min, followed by 30 min of decantation. Then, the supernatant was removed, and the gel was washed by adding deionized water and centrifuging it (1085 × g , Sorvall RC6 Plus, Thermo Fisher Scientific, Asheville, NC, USA). After centrifugation, the TEGDMA-rich supernatant was removed and a new washing cycle initiated. TEGDMA content on the functionalizing layer was modulated by varying the number of washing cycles. The first two batches were washed two and seven times, respectively, while in the last synthesis the gel was washed four times using water plus one cycle using a TEGDMA/water mixture. The pastes were freeze-dried and a white powder was obtained.
Nanoparticles were characterized according to different methods. In order to identify the calcium orthophosphate phase, X-ray diffraction (XRD) measurements were taken using nickel CuKα radiation at 40 kV and 30 mA (MultiFlex, Rigaku Corp., Tokyo, Japan). The equipment geometry was θ /2 θ . Readings were continuous from 10° to 60° at 0.05° intervals, 10 s per interval. Elemental analysis (2400 CHN Elemental Analyzer, PerkinElmer, Waltham, MA, USA) was used to determine TEGDMA content, based on the percentage of carbon detected in the sample. Approximately 1 mg of each powder was heated to 925 °C under pure oxygen. Resultant gases (CO 2 , H 2 O and N 2 ) were homogenized and quantified with a resolution of 0.3%.
Particle surface area was determined after nitrogen adsorption isotherms were obtained using a volumetric adsorption analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). In order to maintain the organic coating around the particles, samples were purged at 50 °C for 14 h. Surface area was determined according to the Brunauer, Emmet and Teller (BET) method (NOVAWin, Quantachrome Instruments, Boynton Beach, FL, USA). Size distribution of the nanoparticle agglomerates was estimated using a laser diffraction particle analyzer (Mastersizer 2000 Ver. 5.54, Malvern Instruments Ltd., Worcestershire, UK).
Seven composites were prepared, with the organic phase containing 1:1 in mols of bisphenol-A glycidyl dimethacrylate (BisGMA) and TEGDMA, plus 0.5 wt% of camphorquinone and ethyl-4-dimethylamino benzoate (EDMAB, all components from Sigma–Aldrich) as photoinitiators. All composites had a total filler of 60 vol%. Filler phase was constituted by 40, 50 or 60 vol% of silanated barium glass and 20, 10 or 0 vol% of DCPD particles. Three different DPCD particles with different TEGDMA contents (obtained as previously described) were used. Composites were mechanically mixed under vacuum (Speedmixer DAC 150.1 FVZ-K, FlackTek Inc., Landrum, SC, USA) and kept under refrigeration until 2 h before use.
Degree of conversion
Degree of conversion (n = 3) was determined by Fourier-transformed near-infrared spectroscopy (near-FTIR) . Unpolymerized composite was placed in a silicon mold (7 mm diameter and 1 mm thickness) pressed between two microscopy glass slides. The spectrum of the unpolymerized material was obtained between 4000 and 10,000 cm −1 by the co-addition of 32 scans with 4 cm −1 resolution (Vertex 70; Bruker Optik GmbH, Ettlingen, Germany). The composite was then photoactivated through the glass slide (Radii Cal, SDI, Bayswater, Australia), receiving 24 J/cm 2 (1200 mW/cm 2 for 20 s) and the whole set was dry-stored at 37 °C for 24 h. Then, a new spectrum was obtained and the degree of conversion was calculated as the ratio between the areas of the absorption band located at 6165 cm −1 (corresponding to the absorption of the C H 2 group) of the polymerized and the unpolymerized material.
Biaxial flexural test
Disk-shaped specimens (12 mm × 1 mm, n = 20) were built using a stainless steel split mold. Half of the specimens were stored in water at 37 °C for 24 h and the other half for 28 days. Photoactivation was performed by irradiating each quadrant for 10 s. After storage, the specimens were placed on a “piston on three balls” device positioned on a universal testing machine (Kratos KE series, Cotia, Sao Paulo, Brazil) at a cross-speed of 0.5 mm/min. The discs were positioned with the irradiated surface facing the flat piston and loaded until fracture. Biaxial flexural strength (BFS), in MPa, was calculated according to the following equations:
BFS = − 0.2387 P ( X − Y ) b 2
X = ( 1 − ν ) ln ( r 2 r 3 ) 2 + [ ( 1 − ν 2 ) ] ( r 2 r 3 ) 2
Y = ( 1 − ν ) [ 1 − ln ( r 1 r 2 ) 2 ] + ( 1 − ν ) ( r 1 r 3 ) 2