Development of calcium phosphate/ethylene glycol dimethacrylate particles for dental applications.

This study describes the synthesis of dicalcium phosphate dihydrate (DCPD) particles in the presence of different ethylene glycol dimethacrylates (EGDMA, ethylene glycol/EG units: 1, 2, 3 or 4) at two monomer-to-ammonium phosphate molar ratios (1:1 and 2:1), as a strategy to develop CaP-monomer particles with improved interaction with resin matrices. Particles displaying high surface areas and organic contents were added to a photocurable BisGMA-TEGDMA resin and the resulting materials were tested for degree of conversion (DC), biaxial flexural strength (BFS), flexural modulus, and ion release. Data were subjected to one-way ANOVA or Kruskal-Wallis/Dunn test (alpha: 0.05). Functionalization with EGDMA derivatives was dependent upon the length of the spacer group and monomer concentration in the synthesis. No differences in DC were observed among materials (p > 0.05). A 39% increase in BFS was obtained with the use of particles with the highest functionalization level compared to non-functionalized particles (p < 0.001). The use of functionalized DCPD reduced flexural modulus in comparison to non-functionalized particles (p < 0.001). Calcium release was similar among materials and remained constant during the experiment, while phosphate release was higher at 7 days in comparison to the remaining weeks (p < 0.001). In conclusion, diethylene glycol dimethacrylate resulted in the highest functionalization levels and the highest BFS among DCPD-containing materials. Ion release was not affected by functionalization. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2018. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 708-715, 2019.

Ion-releasing dental restorative composites containing functionalized brushite nanoparticles for improved mechanical strength.

OBJECTIVES: This study describes the synthesis of brushite nanoparticles (CaHPO4·2H2O) functionalized with triethylene glycol dimethacrylate (TEGDMA) and their application in dental restorative composites with remineralizing capabilities. METHODS: Nanoparticles were synthesized, with TEGDMA being added to one of the precursor solutions at three different molar ratios (0:1, 0.5:1 and 1:1, in relation to the ammonium phosphate precursor). Then, they were added (10 vol%) to a photocurable dimethacrylate matrix containing 50 vol% of reinforcing glass particles. The resulting composites were tested for degree of conversion, biaxial flexural strength and elastic modulus (after 24h and 28days in water), and ion release (over a 28-day period). Commercial composites (one microhybrid and one microfilled) were tested as controls. RESULTS: The final TEGDMA content in the functionalizing layer was modulated by the molar ratio added to the precursor solution. Functionalization reduced nanoparticle size, but did not reduce agglomeration. Improved mechanical properties were found for the composite containing nanoparticles with higher TEGDMA level in comparison to the composite containing non-functionalized nanoparticles or those with a low TEGDMA level. All brushite composites presented statistically significant reductions in strength after 28 days in water, but only the material with high-TEGDMA nanoparticles retained strength similar to the microhybrid commercial control. Overall, ion release was not affected by functionalization and presented steady levels for 28 days. SIGNIFICANCE: Though agglomeration was not reduced by functionalization, the improvement in the matrix-nanoparticle interface allowed for a stronger material, without compromising its remineralizing potential.

Fracture toughness and cyclic fatigue resistance of resin composites with different filler size distributions.

OBJECTIVES: To verify the influence of filler size distributions on fracture toughness (KIc), initial fracture strength (IFS) and cyclic fatigue resistance (CFR) of experimental resin composites. METHODS: Four composites were prepared with same inorganic content (78 wt%), in which 67 wt% was constituted by glass particles with d50 of 0.5; 0.9; 1.2; 1.9 µm KIc of the composites was determined by the single-edge notched beam (SENB) method. To evaluate the IFS and the CFR a biaxial bending test configuration was used. The CFR was determined under cyclic loading for 10(5) cycles using the 'staircase' approach. The fracture surfaces of IFS and CFR specimens were analyzed under scanning electron microscope (SEM). RESULTS: There was a positive linear correlation between d50 vs. KIc and statistical difference was found only between C0.5 (1.24±0.10 MPa m0.5) and C1.9 (1.41±0.17 MPa m0.5). There were no statistical differences among IFS means, which ranged from 155.4±18.8 MPa (C0.9) to 170.7±23.1 MPa (C1.2). C0.5 (93.0±18.6a MPa) showed the highest and C0.9 the lowest CFR (82.5±8.0c MPa). There was no correlation between CFR with d50 values or with KIc means. SEM images showed the morphology with brittle fracture patterns for the surfaces of IFS specimens and a more smooth fracture surface for CFR specimens. SIGNIFICANCE: Resin composites showed different failure mechanisms for quasi-static and fatigue loading. For KIc and IFS, composites with larger filler size distributions showed better results due to crack deflection; while under cyclic loading, viscous behavior was predominant and composites with smaller particles showed higher fatigue resistance.

Effects of diphenyliodonium salt addition on the adhesive and mechanical properties of an experimental adhesive.

OBJECTIVES: The aim of this study was to assess the microtensile bond strength (µTBS), nanoleakage (NL), nano-hardness (NH) and Young's modulus (YM) of resin-dentine bonding components formed by an experimental adhesive system with or without inclusion of diphenyliodonium salt (DPIH) in the camphorquinone-amine (CQ) system. METHODS: On 12 human molars, a flat superficial dentine surface was exposed by wet abrasion. A model simplified adhesive system was formulated (40 wt.% UDMA/MDP, 30 wt.% HEMA and 30 wt.% ethanol). Two initiator systems were investigated: 0.5 mol% CQ+1.0 mol% EDMAB and 0.5 mol% CQ+1.0 mol% EDMAB+0.2 mol% DPIH. Each adhesive was applied and light-cured (10s; 600 mW/cm(2)). Composite build-ups were constructed incrementally and resin-dentine specimens (0.8mm(2)) were prepared. For NL, 3 bonded sticks from each tooth were coated with nail varnish, placed in the silver nitrate, polished down with SiC papers and analysed by EDX-SEM. NH and YM were performed on the hybrid layer in 2 bonded sticks from each teeth. The remaining bonded sticks were tested on µTBS (0.5mm/min). The data from each test were submitted to a Student t-test (α=0.05). RESULTS: No significant difference was found for µTBS between groups (p>0.05). Significant lower NL and higher NH and YM were found in the hybrid layer and adhesive layer produced with the iodinium salt-containing adhesive (p<0.05). CONCLUSIONS: The inclusion of the DPIH to the traditional CQ is a good strategy to improve the adhesive and mechanical properties of a simplified etch-and-rinse adhesive system.

Subcritical crack growth and in vitro lifetime prediction of resin composites with different filler distributions.

OBJECTIVES: Verify the influence of different filler distributions on the subcritical crack growth (SCG) susceptibility, Weibull parameters (m and σ(0)) and longevity estimated by the strength-probability-time (SPT) diagram of experimental resin composites. METHODS: Four composites were prepared, each one containing 59 vol% of glass powder with different filler sizes (d(50)=0.5; 0.9; 1.2 and 1.9 µm) and distributions. Granulometric analyses of glass powders were done by a laser diffraction particle size analyzer (Sald-7001, Shimadzu, USA). SCG parameters (n and σ(f0)) were determined by dynamic fatigue (10(-2) to 10(2) MPa/s) using a biaxial flexural device (12 × 1.2 mm; n=10). Twenty extra specimens of each composite were tested at 10(0) MPa/s to determine m and σ(0). Specimens were stored in water at 37°C for 24 h. Fracture surfaces were analyzed under SEM. RESULTS: In general, the composites with broader filler distribution (C0.5 and C1.9) presented better results in terms of SCG susceptibility and longevity. C0.5 and C1.9 presented higher n values (respectively, 31.2 ± 6.2(a) and 34.7 ± 7.4(a)). C1.2 (166.42 ± 0.01(a)) showed the highest and C0.5 (158.40 ± 0.02(d)) the lowest σ(f0) value (in MPa). Weibull parameters did not vary significantly (m: 6.6 to 10.6 and σ(0):170.6 to 176.4 MPa). Predicted reductions in failure stress (P(f)=5%) for a lifetime of 10 years were approximately 45% for C0.5 and C1.9 and 65% for C0.9 and C1.2. Crack propagation occurred through the polymeric matrix around the fillers and all the fracture surfaces showed brittle fracture features. SIGNIFICANCE: Composites with broader granulometric distribution showed higher resistance to SCG and, consequently, higher longevity in vitro.

Effects of increased exposure times of simplified etch-and-rinse adhesives on the degradation of resin-dentin bonds and quality of the polymer network.

One of the reasons for resin-dentin degradation is poor polymerization of the adhesive layer. This study evaluated the effect of prolonged polymerization times on the immediate and 6-month resin-dentin bond strengths, silver nitrate uptake, and polymer quality of etch-and-rinse adhesives. Thirty extracted teeth were obtained, and a flat dentin surface was exposed on each tooth. Adhesives (Adper Single Bond 2 and One Step Plus) were applied to the dentin surface of these teeth and light-cured for 10, 20, or 40 s at 600 mW cm(-2) . Bonded sticks (0.6 mm(2) ) were tested in tension (0.5 mm min(-1) ) and analyzed, after immersion in 50% silver nitrate, using scanning electron microscopy. The polymer quality of adhesive films was evaluated using thermogravimetric analysis. Statistically higher bond strengths were observed for both adhesives when light-cured for 40 s. Degradation of dentin bonds occurred under all experimental conditions but it was less pronounced for adhesives light-cured for 40 s. Longer exposure times reduced silver nitrate uptake for Adper Single Bond 2. Solvent retention and the amount of residual monomer were statistically lower when both adhesives were light-cured for 40 s. Although longer exposure times than those recommended cannot prevent degradation of the dentin bonds, they can increase bond strength, probably because of the removal of an increased amount of solvent and the presence of a lower amount of residual monomer.

Effects of immersion time and frequency of water exchange on durability of etch-and-rinse adhesive.

This study evaluated the immediate and long-term bond strength to dentin (microtensile bond strength, µTBS) and silver nitrate uptake (SNU) of a three- and two-step etch-and-rinse adhesive under different water immersion times and frequency of water exchange. The adhesives and composite resin were applied according to the manufacturer's instructions in a flat occlusal demineralized dentin of 48 molars. Teeth were assigned to four groups of immersion time (immediate and 1, 3, and 6 months), sectioned to obtain resin-dentin beams, and then subdivided into three groups of water exchange (daily, weekly, and monthly) before being tested in tension. Two resin-dentin beams from each tooth were immersed in silver nitrate and analyzed by SEM. Significant difference in µTBS and SNU was detected for both adhesives (p ≤ 0.0001 for the cross-product interaction). For Adper Single Bond 2, the most pronounced reductions of µTBS were observed for the daily exchange groups in all times. For Adper Scotchbond Multi Purpose, 1-month immersion period was not capable to induce degradation of the dentin bonds, except when the water was exchanged daily. For both adhesives, all storage regimens showed increased SNU results only after 6-month water storage; this being more pronounced for daily water exchange groups. For both adhesives, the highest SNU was observed in the daily water exchange group. The daily water exchange is a fast and reliable in vitro aging method for testing the durability of the adhesive interface produced by adhesive systems.

Poly(ethylene-co-methyl acrylate) membranes as rate-controlling barriers for drug delivery systems: characterization, mechanical properties and permeability.

Poly(ethylene-co-methyl acrylate) (EMA) membranes with different amounts of methyl acrylate (MA) content were studied in terms of the thermal and mechanical properties, swelling and drug permeation. The increase in MA content in the copolymer significantly increased the percentage of elongation and decreased the tensile strength and modulus of elasticity of the membranes. The degree of swelling of the EMA membranes increased with the ethanol composition and MA content. The contact angle of a sessile drop (10 microL of ethanol/water solution) decreased with an increase in the ethanol fraction suggesting that the membrane wettibility increased with the ethanol content. The flux of diltiazem hydrochloride increased from 0.012 to 0.018 mg cm(-2)h(-1) with an increase in the MA content from 16.5 to 29.0%. By increasing the ethanol fraction from 0.4 to 1.0, the flux of diltiazem hydrochloride into the membranes with 29.0% MA, increased from 2.56 (+/-0.09) x 10(-3) to 18.38 (+/-0.62) x 10(-3) mg cm(-2)h(-1). The permeability coefficient increased from 5.85 x 10(-6) to 3.53 x 10(-4) cm h(-1) with an increase in the ethanol fraction. The flux can also be correlated with the drug solubility in the membrane and ethanol. For example, the solubilities of diltiazem hydrochloride, paracetamol and ibuprofen were 0.64, 6.68 and 504.48 mg cm(-3) in the membrane, respectively. Under the same conditions, the flux for the above mentioned drugs was 0.08 (+/-0.01), 0.53 (+/-0.01) and 45.11 (+/-2.00) mg cm(-2)h(-1).

Characterization and drug-permeation profiles of microporous and dense cellulose acetate membranes: influence of plasticizer and pore forming agent.

The use of pore forming agents and plasticizers are efficient ways to obtain membranes for controlled drug permeation through polymeric membranes. The challenge of the present study was to combine these two strategies to obtain cellulose acetate (CA) membranes, where poly(caprolactone triol) (PCL-T) was used as a plasticizer and water, dissolved in a casting solution, was used as a pore forming agent. First, the influence of water on membrane morphology, porosity and the permeation coefficient of a model drug (paracetamol) was analyzed. The influence of different amounts of PCL-T on the permeation coefficient of the CA membranes was then evaluated. Finally, both strategies were combined to obtain porous CA/PCL-T membranes. The membrane microstructure was analyzed using scanning electron microscopy (SEM), the CA crystallinity was determined via differential scanning calorimetry (DSC), and membrane permeability was investigated using paracetamol. The addition of water, a non-solvent, during the membrane casting process was found to be a simple and effective way to change membrane porosity and consequently the drug-permeation profile. When small quantities of non-solvent were used to obtain low porosity membranes, the presence of a plasticizer agent could be used to better modulate drug permeation. Combining the addition of PCL-T with the use of a non-solvent resulted in a series of CA membranes with paracetamol-permeation coefficient values in the range of ca. 10(-7) to 10(-5) cm s(-1).