Mohammad Atai

Hydroxyapatite nanorods as novel fillers for improving the properties of dental adhesives: Synthesis and application.

OBJECTIVES This study evaluates the hypothesis that the incorporation of fibrous hydroxyapatite nanoparticles with high crystallinity and high aspect ratio, synthesized by hydrothermal method, into an experimental ethanol-based one-bottle dentin adhesive, improves the mechanical properties of the adhesive layer, and accordingly increases the bond strength to dentin. METHODS Hydroxyapatite nanorods were synthesized using a simple hydrothermal procedure. First, the HPO(4)(2)-containing solution was added drop-wise into the Ca(2+)-containing solution while the molar ratio of Ca/P was adjusted at 1.67. The HAp precursor was then treated hydrothermally at 200 degrees C for 60h. The resulting powder was characterized using XRD, FTIR, SEM, TEM, and EDXA. The synthesized HAp nanorods were added to an experimental one-bottle dentin adhesive followed by the characterization of the filled adhesive. The diametral tensile strength, flexural strength, flexural modulus, and the microshear bond strength to the dentin of human premolars of seven adhesive systems containing different nanorod contents were evaluated. The distribution of the filler was determined using EDX-mapping. The depth of cure was also evaluated using scraping technique. Moreover, after microshear testing, the fracture cross-section was observed using SEM to determine the mode of failure involved. The colloidal stability was studied using a separation analyzer and also zeta potential measurement. Data were analyzed using one-way analysis of variance followed by the Tukey test. RESULTS The results confirmed the high purity, high crystallinity, and high aspect ratio of synthesized HAp nanorods. The diametral tensile strength of nanorod containing adhesive system appeared to increase when 0.2-0.5wt.% HAp nanorods were incorporated (p<0.05). A similar trend was observed in the flexural test providing higher flexural strength at filler contents of 0.2-0.5wt.% while flexural modulus remained unchanged. The highest microshear bond strength was also obtained at 0.2wt.% filler content (p<0.05). The improved properties of the new adhesive system might be due to the high crystallinity and high aspect ratio of the nanorods. SEM observation of debonded surfaces revealed that most specimens showed an adhesive failure from the adhesive-dentin interface. Energy dispersive X-ray (EDX) mapping confirmed the uniform distribution of nanorods in the adhesive matrix. The colloidal stability studies indicated that synthesized hydroxyapatite nanorods have high colloidal stability in the dental adhesive solution. Indeed, the nanorods are well dispersed and protected from aggregation by their high surface charge confirmed by zeta potential measurement. SIGNIFICANCE Hydroxyapatite-based composites have shown promising bioactivity. However, the knowledge about the influence of the nano-sized HAp on the properties of the dental materials, especially dentin bonding adhesives, is yet insufficient. The nanorod containing adhesive system presented here might be considered to have practical applications in dental clinics.

PMMA-grafted nanoclay as novel filler for dental adhesives

OBJECTIVE The aim of this study was to investigate the benefits of incorporation of poly(methyl methacrylate)-grafted-nanoclay on the bond strength of an experimental one-bottle dentin bonding system. The effect of the modification on the stability of the nanoparticle dispersion in the dilute adhesive was also studied. MATERIALS AND METHODS Poly(methyl methacrylate) was grafted onto the pristine Na-MMT nanoclay (Cloisite Na(+)) through the free radical polymerization of methyl methacrylate in an aqueous media in the presence of ammonium persulfate as initiator. A reactive surfactant (AMPS) was also used in the reaction recipe to provide active sites on the surface of the nanoclay. The grafting polymerization reaction was carried out at 70 degrees C. The PMMA-g-nanoclay was then coagulated in methanol and filtered. The resulting PMMA-g-nanoclay was characterized using FTIR, TGA, X-ray diffraction (XRD) and particle size distribution analysis. The modified nanoclay was added to an experimental dentin bonding system as filler and the morphology of the nanoclay layers in the adhesive matrix was studied using TEM and XRD. Shear bond strength of the adhesives containing different filler contents was tested on the caries-free extracted human premolar teeth. The mode of failure was studied by scanning electron microscopy. The stability of the nanoclay dispersion in the dilute adhesive was also studied using a separation analyzer. The results were then statistically analyzed and compared. RESULTS The grafting of poly(methylmethacrylate) onto the nanoclay was confirmed and the results revealed a partially exfoliated structure for the PMMA-g-nanoclay. Incorporation of the modified nanoclay provided a dentin bonding system with higher shear bond strength. The dispersion stability of the modified nanoparticles in the dilute adhesive was also increased more than 40 times in comparison with the pristine nanoclay. SIGNIFICANCE The grafting modification provided nanoclay particles with higher dispersion stability than pristine Na-MMT nanoclay in a dilute dentin bonding system. Incorporation of the modified nanoclay into the bonding system provided higher shear bond strength. The finding would be beneficial in producing nano-filler containing adhesive systems.

Antibacterial, physical and mechanical properties of flowable resin composites containing zinc oxide nanoparticles.

OBJECTIVES The aim of this study is evaluating the antibacterial activity of resin composites containing ZnO nanoparticles against Streptococcus mutans and examining their physical and mechanical properties. METHODS The properties of flowable resin composites containing 0-5wt.% nano-ZnO are investigated using different tests: RESULTS Although the agar diffusion test reveals no significant difference between the groups, the direct contact test demonstrates that by increasing the nanoparticle content, the bacterial growth is significantly diminished (p<0.05). In the aging test, however, the antibacterial properties reduce significantly (p<0.05). The flexural strength and compressive modulus remains unchanged by incorporation of nanoparticles (p>0.05) while the compressive strength and flexural modulus significantly increase (p<0.05). The ZnO containing resins show significantly lower depth of cure (p<0.05), and higher bond strength (p<0.05). There is no significant difference between the degrees of conversion, measured by FTIR technique, of the groups (p>0.05). SIGNIFICANCE Production of a dental resin composite with antibacterial activity without significant sacrificing effect on the mechanical properties is desirable in dental material science.

Graphene nanoplatelets as poly(lactic acid) modifier: linear rheological behavior and electrical conductivity

Two types of graphene nanoplatelets (xGn and N02) were added to ppoly(lactic acid) (PLA) through solvent mixing to prepare PLA nanocomposites with the aim of producing electrically and thermally conductive PLA products suitable for advanced applications. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed that N02 was fully exfoliated and homogenously dispersed in the PLA matrix, while xGn was poorly dispersed with stacked multi-layer structures. Rheological tests were used to characterize the viscoelastic behavior of the nanocomposites and several models/scaling relations based on the rheological results were employed to determine the structure and properties of the percolated graphene networks. The electrical conductivity of the nanocomposites increased by more than twelve orders of magnitude after the graphene concentration reached the percolation threshold. The threshold values obtained from different models showed reasonable agreement.

Nano-porous thermally sintered nano silica as novel fillers for dental composites

OBJECTIVES The study evaluates properties of an experimental dental composite consisting of a porous thermally sintered nano-silica as filler. The properties are compared with those of an experimental composite containing micro fillers and a commercially available nano-composite, Filtek Supreme(®) Translucent. Different models are used to predict the elastic modulus and strength of the composites. METHODS Nano-silica with primary particles of 12 nm was thermally sintered to form nanoporous filer particles. The experimental composites were prepared by incorporating 70 wt.% of the fillers into a mixture of Bis-GMA and TEGDMA as matrix phase. Having added photoinitiator system the composites were inserted into the test molds and light-cured. The microfiller containing composites were also prepared using micron size glass fillers. Degree of conversion (DC%) of the composites was measured using FTIR spectroscopy. Diametral tensile strength (DTS), flexural strength, flexural modulus and fracture toughness were measured. SEM was utilized to study the cross section of the fractured specimens. The surface topography of the specimens was investigated using atomic force microscopy (AFM). The specific surface area of the sintered nano silica was measured using BET method. The data were analyzed and compared by ANOVA and Tukey HSD tests (significance level=0.05). RESULTS The results showed improvements in flexural modulus and fracture toughness of the composites containing sintered filler. AFM revealed a lower surface roughness for sintered silica containing composites. No significant difference was observed between DTS, DC%, and flexural strength of the sintered nanofiller composite and the Filtek Supreme(®). The results also showed that the modulus of the composite with sintered filler was higher than the model prediction. SIGNIFICANCE The thermally sintered nano-porous silica fillers significantly enhanced the mechanical properties of dental composites introducing a new approach to develop materials with improved properties.

Injectable in situ forming drug delivery system based on poly(ε-caprolactone fumarate) for tamoxifen citrate delivery: Gelation characteristics, in vitro drug release and anti-cancer evaluation

The present study deals with the preparation and characterization of an injectable and in situ forming drug delivery system based on photocrosslinked poly(epsilon-caprolactone fumarate) (PCLF) networks loaded with tamoxifen citrate (TC). Networks were made of PCLF macromers, a photoinitiation system (comprising initiator and accelerator) and the active ingredient N-vinyl-2-pyrrolidone (NVP) as a crosslinker and reactive diluent. Shrinkage behavior, equilibrium swelling and sol fraction ratios of photocrosslinked PCLF gels were determined as functions of NVP content. It was shown that the crosslinking is facilitated up to a certain concentration of NVP and most of NVP remained unreacted above this value. In vitro drug release, biocompatibility evaluation and activity against MCF-7 breast cancer cell line were also investigated. Accurate but simple bipartite expressions were also derived that enable rapid determination of effective diffusion coefficients of TC in photocrosslinked PCLF/NVP disks. Cytotoxicity assay showed that while the photocrosslinked PCLF network with optimum NVP content exhibits no significant cytotoxicity against MCF-7 and L929 cell lines, 40-60% of the MCF-7 cells were killed after incubation with TC-loaded devices.

Poly(acrylic acid) grafted montmorillonite as novel fillers for dental adhesives: Synthesis, characterization and properties of the adhesive

OBJECTIVE This work investigates the graft polymerization of acrylic acid onto nanoclay platelets to be utilized as reinforcing fillers in an experimental dental adhesive. Physical and mechanical properties of the adhesive and its shear bond strength to dentin are studied. The effect of the modification on the stability of the nanoparticle dispersion in the dilute adhesive is also investigated. MATERIALS AND METHODS Poly(acrylic acid) (PAA) was grafted onto the pristine Na-MMT nanoclay (Cloisite(®) Na(+)) through the free radical polymerization of acylic acid in an aqueous media. The resulting PAA-g-nanoclay was characterized using FTIR, TGA and X-ray diffraction (XRD). The modified nanoclays were added to an experimental dental adhesive in different concentrations and the morphology of the nanoclay layers in the photocured adhesive matrix was studied using TEM and XRD. Shear bond strength of the adhesives containing different filler contents was tested on the human premolar teeth. The stability of nanoclay dispersion in the dilute adhesive was also studied using a separation analyzer. The results were then statistically analyzed and compared. RESULTS The results confirmed the grafting reaction and revealed a partially exfoliated structure for the PAA-g-nanoclay. Incorporation of 0.2 wt.% of the modified nanoclay into the experimental adhesive provided higher shear bond strength. The dispersion stability of the modified nanoparticles in the dilute adhesive was also enhanced more than 25 times. SIGNIFICANCE Incorporation of the modified particles as reinforcing fillers into the adhesive resulted in higher mechanical properties. The nanofiller containing bonding agent also showed higher shear bond strength due to the probable interaction of the carboxylic acid functional groups on the surface of the modified particles with hydroxyapatite of dentin.

Evaluation of fracture toughness and mechanical properties of ternary thiol–ene–methacrylate systems as resin matrix for dental restorative composites

OBJECTIVE Study and evaluation of fracture toughness, flexural and dynamic mechanical properties, and crosslink density of ternary thiol-ene-methacrylate systems and comparison with corresponding conventional methacrylate system were considered in the present study. METHODS Urethane tetra allyl ether monomer (UTAE) was synthesized as ene monomer. Different formulations were prepared based on combination of UTAE, BisGMA/TEGDMA and a tetrathiol monomer (PETMP). The photocuring reaction was conducted under visible light using BD/CQ combination as photoinitiator system. Mechanical properties were evaluated via measuring flexural strength, flexural modulus and fracture toughness. Scanning electron microscopy (SEM) was utilized to study the morphology of the fractured specimens cross section. Viscoelastic properties of the samples were also determined by dynamic mechanical thermal analysis (DMTA). The same study was performed on a conventional methacrylate system. The data were analyzed and compared by ANOVA and Tukey HSD tests (significance level=0.05). RESULTS The results showed improvement in fracture toughness of the specimens containing thiol-ene moieties. DMTA revealed a lower glass transition temperature and more homogenous structure for thiol-ene containing specimens in comparison to the system containing merely methacrylate monomer. The flexural modulus and flexural strength of the specimens with higher thiol-ene content were lower than the neat methacrylate system. The SEM micrographs of the fractured surface of specimens with higher methacrylate content were smooth and mirror-like (shiny) which represent brittle fracture. SIGNIFICANCE The thiol-ene-methacrylate system can be used as resin matrix of dental composites with enhanced fracture toughness in comparison to the methacrylate analogous.

A novel dentin bonding system containing poly(methacrylic acid) grafted nanoclay: synthesis, characterization and properties.

OBJECTIVES Developing a novel dentin bonding system containing poly(methacrylic acid)-grafted-nanoclay (PMAA-g-nanoclay) as reinforcing filler, with high stability of nanoparticle dispersion and improved bond strength and mechanical properties were the main objectives of this study. MATERIALS AND METHODS Poly(methacrylic acid) (PMAA) was grafted onto the pristine sodium montmorrillonite (Na-MMT) nanoclay surface and characterized using FTIR, TGA, and X-ray diffraction (XRD). The PMAA-g-nanoclay was incorporated into an experimental dentin bonding system as filler in different concentrations and stability of nanoclay dispersion in the dilute adhesive, morphology of nanoclay layers in the photocured adhesive matrix, shear bond strength to caries-free extracted human premolar teeth, and mode of failure were studied. The mechanical properties including diametral tensile strength (DTS), flexural strength (FS), and flexural modulus (FM) were also investigated. The measured FM was also compared to theoretical prediction models. RESULTS The grafting of PMAA onto the nanoclay surface was confirmed and the results revealed a partially exfoliated structure for PMAA-g-nanoclay. The dispersion stability of the modified nanoparticles in the dilute adhesive increased more than 45 times in comparison with the pristine nanoclay. The incorporation of 0.5wt.% PMAA-g-nanoclay to the adhesive resulted in a significant increase in microshear bond strength, DTS, and FS. Higher PMAA-g-nanoclay contents resulted in increased flexural modulus. The experimental flexural modulus was in good agreement with the Halpin-Tsai theoretical model. SIGNIFICANCE Incorporation of PMAA-g-nanoclay particles as novel functional fillers into dental adhesive could result in the development of bonding systems with improved physical, mechanical, and adhesion properties.

Temperature rise and degree of photopolymerization conversion of nanocomposites and conventional dental composites

The aim of the study was to investigate the temperature rise of a nanocomposite and a conventional hybrid dental composite during photopolymerization when cured with halogen curing lamp (QHT) and light-emitting diode (LED). Temperature rise during photopolymerization of two commercially available composites (Filtek Supreme® and TetricCeram®) were measured using a K-type thermocouple and a digital thermometer. Different curing modes were utilized to cure the composites: a high-intensity QHT unit (Optilux 501) in two different modes (standard and ramp), a low-intensity QHT unit (Coltolux 50), and an LED unit (Ultralume-2). Total temperature rise, polymerization reaction exotherm, and irradiation-induced temperature rise of the composites were determined. Degree of conversion of the specimens was measured using FTIR spectroscopy. The results revealed that the Filtek Supreme® nanocomposite showed lower temperature rise and degree of conversion in comparison with the hybrid composite (p < 0.05). It was also found that the LED curing unit induced considerable total and irradiation temperature rise without any improvement in the degree of conversion. Ramp curing mode showed lower temperature rise and delayed gel point and was found to be more effective than QHT standard mode and LED units. Although it is claimed that the LED curing units exhibit lower temperature rise during the photopolymerization, the present study showed that the curing units have no advantage over the conventional QHT units regarding the temperature rise and degree of polymerization conversion.