In-situ photo-crosslinkable elastomer based on polyalphaolefin/halloysite nanohybrid.

In this research, new injectable and in situ photocurable elastomeric nanohybrids have been fabricated from polyalphaolefin (PAO) resins and halloysite nanofiller. In this regard, the co-oligomerization of long α-olefin monomers (C6, C8 and C10) with alkenol counterparts was carried out via a simple cationic route to provide OH-functionalized PAOs. The newly formed PAO type copolymer resins as well as halloysite nanoclay were then equipped with photocurable CC bonds containing an acrylate moiety. After the characterization of the final chemical substances and also of the intermediate structures, experimentally and computationally by means of Density Functional Theory (DFT) calculations, the neat treated PAO and PAO/halloysite nanohybrids were subjected to a curing process by visible light irradiation (λ âˆ¼ 475 nm, blue light). The crosslinking efficiency of the neat resins and the formed nanohybrid was evaluated using shrinkage strain-time curves and equilibrium swelling method. The suggested nanohybrid is not only biocompatible (96 % in the MTT assay), and hydrophilic (with a water contact angle of 61°), but also exhibits an easy, fast and robust curing process with great potential for coating and sealing technologies for medical devices.

Effect of bioactive glass-containing dentin adhesives on microshear bond strength of composite restorations.

Background: In general, bioactive glasses (BAGs) can react with tissue minerals and promote remineralization. However, the application of BAG in bonding agents and its impact on bond strength remain uncertain due to insufficient information and limited research in this area. Materials and Methods: This study employed a randomized controlled design to assess the effects of composite-bonding agents with varying BAG contents on shear bond strength and fracture pattern in sound and demineralized teeth, with and without thermocycling. Thus, 80 healthy third molars were randomly divided into two groups: sound teeth and demineralized teeth. Five bonding agents were applied to the prepared dentin surfaces, including four experimental composite-bonding agents with varying BAG content (0, 0.2, 0.5, and 2 wt%) and the Adper Single Bond commercial bonding as control. The shear bond strength of all samples was measured using a universal tester. The type of failure of each specimen was determined using a stereomicroscope. Kruskal-Wallis nonparametric test was performed on the obtained shear bond strength data followed by Mann-Whitney post hoc test with Bonferroni correction to determine statistical significance. The level of significance was considered P ≤ 0.05 for all tests and was adjusted by Bonferroni correction. Results: Demineralization significantly decreased shear bond strength in the teeth samples. Adper Single Bond exhibited the highest shear bond strength values. The addition of BAG did not have a significant influence on shear bond strength, regardless of demineralization or thermocycling condition. Adhesive failure was the predominant type of failure in all groups. Conclusion: The incorporation of BAG filler up to 2 wt% did not result in significant changes in shear bond strength. Experimental adhesive bonding agents with 2 wt% BAG content demonstrated shear bond strengths comparable to the commercial bonding agent in sound nontreated, sound thermocycled, demineralized nontreated, and demineralized thermocycled groups.

Synthesis, characterization, photo-polymerization, hydrolytic stability, and etching behavior of new self-etch adhesive monomers.

Considering the poor hydrolytic stability of the most methacrylate-based functional monomers of self-etch dental adhesives in acidic and aqueous conditions, in this study allyl-based photo-polymerizable self-etch monomers was synthesized in order to improve the hydrolytic stability. The new self-etch monomers based on phosphonic acid functional groups were synthesized through a two-step procedure. First, phosphoric anhydride, poly-phosphoric acid, and polyethylene glycol were reacted to produce phosphate ester precursor (P-PEG-P). Next, allyl 2, 3-epoxypropyl ether was reacted with P-PEG-P to synthesize allyl self-etch monomer. Glycidyl methacrylate was also reacted with P-PEG-P to synthesize a methacrylate self-etch analogue monomer. The monomers were characterized using FTIR and 1H-NMR spectroscopy. The viscosities of monomers were measured using a rheometer. The degree photopolymerization conversion of monomers was measured using FTIR spectroscopy. The pH assay was performed by a digital pH-meter. The etching behavior of the monomers on human teeth was studied using scanning electron microscopy (SEM). Thermo-gravimetric analysis (TGA) was performed to evaluate the possible interaction of the monomers with tricalcium phosphate (TCP). The solubility of synthesized monomers was examined in ethanol, acetone, and water. The hydrolytic stability of cured resins in artificial saliva during 4 months was also surveyed. The synthesis of new self-etching monomers was successfully confirmed by spectroscopy analyses. The results represented appropriate viscosity of self-etching monomers around 1 (Pa s). The resin containing methacrylate monomer exhibited its degree of conversion is more than that of allyl monomer (p < 0.05). The allyl and methacrylate self-etch monomers exhibited pH values of 1.2 and 1.3, respectively. SEM micrograph verified that the synthesized monomers were able to suitable etching of the enamel human premolar teeth. The data obtained from TGA tests revealed that thermal stability of (TCP) containing monomers is enhanced. Also, the monomers exhibited an excellent solubility in polar solvents, but when they are mixed with TCP, they are not, anymore, dissolved in these solvents. Furthermore, the allyl monomer showed higher hydrolytic stability than the methacrylate monomer. The new photo-polymerizable acidic monomer based on allyl functionality showed enhanced hydrolytic stability compared to methacrylate-based monomer. It may be considered as a promising monomer for self-etch dental adhesives.

Solution photo-copolymerization of acrylic acid and itaconic acid: The effect of polymerization parameters on mechanical properties of glass ionomer cements.

OBJECTIVE: To synthesize a series of poly (acrylic acid-co-itaconic acid) (P(AA-co-IA)) copolymers with different molecular weights (MWs) through a facile water-based solution photopolymerization and to investigate the operational and mechanical properties of the experimental glass-ionomer (GI) cements made of the ionomers. METHODS: Thioglycolic acid (TGA) was used as a chain transfer agent to synthesize P(AA-co-IA) ionomers with different MWs through the solution photopolymerization. The chemical structure, MWs, and rheological properties of the copolymers were fully characterized. The GI cements were prepared using the ionomer solutions in different MWs and concentrations. Finally, the operating and mechanical properties of the experimental GI cements were investigated and compared with those of a commercially available GI cement. RESULTS: The synthesis and composition of the P(AA-co-IA) were approved by spectroscopy analyses. The results revealed that by increasing the TGA content, MW and polydispersity index (PDI) of the synthesized copolymers demonstrate a decreasing trend from 4.5 × 104 g/mol (PDI of 2.45) to 7.4 × 103 g/mol (PDI of 1.62). Accordingly, the viscosity of copolymers decreased with increasing the TGA concentration in the polymerization recipes. Setting times of the cements increased with reducing the MWs and ionomer concentration. The compressive and flexural strengths of GI cements were improved by increasing the MWs, ionomers concentration, and storage time. SIGNIFICANCE: The solution photopolymerization provides a facile and environmentally safe method to synthesize P(AA-co-IA) copolymers with controlled MWs. The structure-property relationships presented in the study also provide valuable information in the production and improvement of the GI cements.

Effect of zinc oxide nanoparticles on physical and antimicrobial properties of resin-modified glass ionomer cement.

BACKGROUND: To improve the limitations, many modifications in the resin-modified glass ionomer (RMGI) composition have been proposed. In this study, we evaluated the effect of different concentrations of zinc oxide (ZnO) nanoparticles incorporated into RMGI cement on its physical and antimicrobial properties. MATERIALS AND METHODS: In this in vitro study, ZnO nanoparticles with 0-4 wt.% concentrations were incorporated into RMGI. The following tests were carried out: (a) Antibacterial activity against Streptococcus mutans tested by disc diffusion method, (b) mechanical behavior assessment by measuring flexural strength (FS) and flexural modulus (FM), (c) micro-shear bond strength (µ-SBS), and (d) fluoride and zinc release. Data were analyzed using the statistical tests of ANOVA, t-test, and Tukey's HSD post hoc in SPSS V22. The level of significancy was 0.05. RESULTS: In the disc diffusion method, specimens with 2 wt.% ZnO nanoparticles showed the highest antimicrobial efficacy (P < 0.05). After 1 month of water storage, no significant difference was observed in FS and FM of the samples (P > 0.05). In 2 wt.% ZnO nanoparticles group, µSBS increased in the first 7 days but decreased by 17% after one month, which showed a significant difference with that of the control group. The fluoride release did no change in the ZnO nanoparticle-containing group compared with the control group at all time intervals. CONCLUSION: Incorporation of 2 wt.% ZnO nanoparticles into the RMGI cement adds antimicrobial activity to the cement without sacrificing FS and fluoride release properties, while decreased µSBS.

Effect of Application of a Bio-Adhesive on the Removal Torque Value and Rotational Misfit at the Implant-Abutment Junction: An In Vitro Study.

The aim of this study was to assess the effect of application of a recently developed bio-adhesive (Impladhesive) to abutment screw threads on the removal torque value and rotational misfit at the implant-abutment junction. This in vitro study evaluated 20 implant fixtures and 20 straight abutments. Specimens were randomly divided into two groups (n = 10) with/without adhesive application. In the adhesive group, the abutment was dipped in Impladhesive before torquing. In the control group, the abutment was torqued conventionally without adhesive application. The removal torque value was recorded after completion of the cyclic loading of 500,000 cycles with 2 Hz frequency and 75 N load. Rotational misfit was recorded using a video measuring machine. After applying the torque, the change in the bisector angle on the abutment hex was recorded for each implant. The biocompatibility of Impladhesive was evaluated using a MTT cell vitality assay. Normal distribution of data was assessed using the Kolmogorov-Smirnov test. Data were analyzed using a t-test and Pearson's correlation coefficient The application of Impladhesive at the implant-abutment interface resulted in significantly greater mean removal torque value compared to the control group (p = 0.008). In addition, the mean rotational misfit at the implant-abutment interface was significantly lower in the use of Impladhesive compared to the control group (p = 0.001). In addition, the cell vitality was found to be greater than 80% at all evaluated time points. It can be concluded that the application of Impladhesive on the abutment screw significantly decreased rotational misfit and increased the removal torque value. Future studies are needed to evaluate the efficacy of this bio-adhesive an in vivo setting.

Modified POSS nano-structures as novel co-initiator-crosslinker: Synthesis and characterization.

OBJECTIVE: To synthesize an amine-modified polyhedral oligomeric silsesquioxane (POSS) nano-structure as a novel co-initiator-crosslinker (co-Ini-Linker) and to determine the effect of the co-Ini-linker on the physical and mechanical behavior of an experimental dental composite. METHODS: The amine-methacrylate POSS nano-structures (AMA-POSS) were chemically synthesized by anchoring a tertiary amine functionality on the methacrylate POSS (MA-POSS) branches. Three types of AMA-POSS, having different amine branches in their structures, were synthesized through the Aza Michael reaction. The chemical structure of AMA-POSSs were evaluated by1H-NMR spectroscopy. Afterward, the AMA-POSS was incorporated into a dental resin system composed of Bis-GMA, TEGDMA, and photo-initiator. Three resin systems with different AMA-POSS types were then prepared, and their properties were compared with a resin containing DMAEMA as a conventional co-initiator. The degree of conversion evaluated by FTIR spectroscopy and the shrinkage kinetics of the resins were determined through the bonded-disk technique. The flexural properties of the photopolymerized resins were also investigated. The distribution of nano-structures in the matrix resin was analyzed using EDX analysis. RESULTS: The modified POSS structure and the number of amine branches were confirmed with1H-NMR spectroscopy. The resin containing 8 amine branches (P8) showed the same degree of conversion (DC%) as the resin containing DMAEMA (P > 0.05). Decreasing the amine branches in the POSS structure, however, revealed an increasing trend in DC%. The resin containing P8 showed the lowest shrinkage strain. By incorporating AMA-POSS into the resin system, the water sorption significantly decreased (P < 0.05). The flexural strength and modulus increased by adding P3 into the resin system (P < 0.05). EDX Si-map revealed that the co-Ini-linker was well dispersed in the resin matrix. SIGNIFICANCE: The synthesized novel amine-methacrylate POSS nanostructures not only act as an amine co-initiator but also work as a reinforcing filler and a cross-linking agent.

On the properties of nanosilicate-based filled dental adhesives: Synthesis, characterization, and optimized formulation.

OBJECTIVE: In this study, we incorporated hybrid nanoparticles (poly (acrylic acid)-grafted nanoclay/nanosilica, respectively, with platelet and spherical morphologies, abbreviated as PAA-g-NC-Sil) in different concentrations (0, 0.2, 0.5, 1, 2 and 5 wt%) to an experimental dentin bonding system and investigated the physical properties of the filled adhesive and its shear bond strength (µ-SBS) to dentin. We subsequently compared the properties of the adhesives containing PAA-g-NC-Sil with previously studied adhesives containing poly (methacrylic acid)-g-nanoclay (PMA-g-NC) (Solhi et al., 2012a), poly (acrylic acid)-g-nanoclay (PAA-g-NC) (Solhi et al., 2012b), and the hybrid poly (methacrylic acid)-grafted-nanoclay-nanosilica (PMA-g-NC-Sil) (Solhi et al., 2020). MATERIALS AND METHODS: In a set of previous publications and the present paper, we grafted poly (acrylic acid) (PAA) or poly (methacrylic acid) (PMA) onto the surface of pristine Na-MMT nanoclay (Cloisite® Na+) through free radical polymerization of monomer in an aqueous media in the presence or absence of nanosilica particles. We characterized the resulting modified nanoparticles (PMA-g-NC, PAA-g-NC, PMA-g-NC-Sil and PAA-g-NC-Sil) using GPC, FTIR, TGA, and XRD. We then incorporated the modified particles as functionalized fillers to experimental dentin adhesives in different concentrations and studied the stability of modified fillers dispersion by separation analysis. We also studied the properties of the photo-cured adhesive matrices using FTIR, TEM, SEM, EDXA, and XRD. We examined the shear bond strength of the adhesives (containing different contents of each modified filler, separately) to human premolar teeth. The results were analysed and compared statistically. RESULTS: The results confirmed that the polymers have been grafted onto the surface of nanoclay. An exfoliated structure for the nanoclay platelets in the photo-cured adhesive containing PAA-g-NC-Sil was observed. Addition of 0.5 wt% of PAA-g-NC-Sil to the experimental adhesive increased the shear bond strength and the dispersion stability in comparison to unfilled adhesive. The same trend was also observed for adhesives containing PMA-g-NC, PAA-g-NC, and PMA-g-NC-Sil. The adhesive containing PAA-g-NC-Sil showed the best dispersion stability and subsequently the highest shear bond strength in the optimal concentration among adhesives containing the four available fillers (PMA-g-NC, PAA-g-NC, PMA-g-NC-Sil and PAA-g-NC-Sil). SIGNIFICANCE: Addition of poly (acrylic acid) modified nanoparticles to the experimental dentin adhesives resulted in higher shear bond strength due to the potential interactions between the carboxylic acid functional groups on the surface of the modified particles and the dentin structure. Between the poly (acrylic acid) and poly (methacrylic acid), the former acid with higher PKa performed better. Addition of the spherical nanosilica particles to the adhesives containing platelet nanoclay helped to better exfoliate the platelets resulting in improved µ-SBS and dispersion stability.

Poly(hydroxybutyrate-co-hydroxyvalerate) Porous Matrices from Thermally Induced Phase Separation.

Thermally induced phase separation followed by freeze drying has been used to prepare biodegradable and biocompatible scaffolds with interconnected 3D microporous structures from poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) copolymers containing 5 and 12 wt % of 3-hydroxyvalerate (HV). Solutions of PHBV in 1,4-dioxane, underwent phase separation by cooling under two different thermal gradients (at -25 °C and -5 °C). The cloud point and crystallization temperature of the polymer solutions were determined by turbidimetry and differential scanning calorimetry, respectively. Parameters affecting the phase separation mechanism such as variation of both the cooling process and the composition of the PHBV copolymer were investigated. Afterwards, the influence of these variables on the morphology of the porous structure and the final mechanical properties (i.e., rigidity and damping) was evaluated via scanning electron microscopy and dynamic mechanical thermal analysis, respectively. While the morphology of the scaffolds was considerably affected by polymer crystallization upon a slow cooling rate, the effect of solvent crystallization was more evident at either high hydroxyvalerate content (i.e., 12 wt % of HV) or high cooling rate. The decrease in the HV content gave rise to scaffolds with greater stiffness because of their higher degree of crystallinity, being also noticeable the greater consistency of the structure attained when the cooling rate was higher. Scaffolds were fully biocompatible supports for cell adhesion and proliferation in 3D cultures and show potential application as a tool for tissue regeneration.

Dual modified nanosilica particles as reinforcing fillers for dental adhesives: Synthesis, characterization, and properties.

A facile procedure has been devised to develop a novel dentin bonding system containing poly (acrylic acid)-grafted-silanized fumed silica particles as reinforcing filler, with high stability of nanoparticle dispersion and enhanced bond strength and mechanical properties. In the first step, the silanization of fumed silica nanoparticles was performed in the following conditions: (i) ethanol-water solution with a pH of 5 and (ii) cyclohexane with a pH of 9 using trimethoxysilylpropyl methacrylate (γ-MPS) as a reactive silane coupling agent. FTIR and TGA analyses confirmed the presence of silane in the resultant structure and enhanced dispersion stability of modified particles was proved by a separation analyzer and also zeta potential analyses. In the second step, free radical polymerization of acrylic acid monomers in the presence of silanized nanoparticles was carried out and poly (acrylic acid) -grafted- silanized fumed silica were acquired. The flexural strength and fracture toughness of the adhesive containing 0.2 wt.% of the dual modified filler reached maximum of 70.4 MPa and 1.34 MPa m1/2, respectively, showing average improvements of 74% and 179%, respectively, in comparison with the adhesive without filler. Flexural modulus values did not significantly change with increasing the filler content except the adhesive containing 5 wt.% having the lowest flexural modulus. The highest microtensile bond strength was also observed at 0.2 wt.% filler content showing the average improvements of 197% as compared with the neat adhesive. Energy dispersive X-ray (EDX) mapping confirmed a homogenous and uniform distribution of the fillers in the adhesive matrix containing 0.2 wt.% and 0.5 wt.% of filler while incorporation of 5 wt.% led to large particle aggregates. SEM images of the fracture surface of the adhesive with different filler contents subjected to fracture toughness test showed rougher surface and longer crack path by increasing filler concentration. The adhesive containing 0.2 wt.% of filler perfectly penetrated into the dentin tubules proved by the SEM micrographs in microtensile bond strength test.

Synthesis and characterization of core-shell nanoparticles and their application in dental resins.

The aim of this study was to synthesize acrylic core-shell particles and silica-loaded core-shell hybrid particles through emulsion polymerization. Also this work examined the influence of synthesized nanoparticles loading in a Bis-GMA/TEGDMA resin matrix on some mechanical properties of the dental composite resins. Core-shell particles consisting of polybutyl acrylate (PBA) rubbery core and polymethyl methacrylate (PMMA)/polystyrene (PS) shell were synthesized by seeded emulsion polymerization. For preparing the core-shell hybrid particles, first silica particles with diameters of about 68 nm were synthesized based on the Stöber process. Then the surface of silica particles was treated with É£-MPS. Afterwards, polymeric shell was coated on silica nanoparticles through emulsion polymerization. The morphology of core-shell particles was examined by SEM/TEM. Mechanical properties (fracture toughness, flexural strength and flexural modulus) of the photo-cured Bis-GMA/TEGDMA dental resins/composites filled with different mass fractions of synthesized nanoparticles were tested, and analysis of variance (ANOVA) was used for the statistical analysis of the acquired data. Formation of glassy shell on PBA core in core-shell particles, grafting of É£ -MPS onto the silica particles and encapsulation of modified silica by polymeric shell in core-shell hybrid particles were confirmed using various analytical techniques. The results of mechanical tests showed that fracture toughness of Bis-GMA/TEGDMA dental resins improved about 35% by the inclusion of 5 wt% silica-loaded core-shell hybrid particles with little effect on flexural strength. This study shows that incorporation of proper amount of hybrid core-shell particles in dental composites can improve their fracture toughness and thus may extend their service life.

Poly (methacrylic acid) modified spherical and platelet hybrid nanoparticles as reinforcing fillers for dentin bonding systems: Synthesis and properties.

OBJECTIVE: In this study the mechanical and adhesion properties of an experimental methacrylate based dentin bonding system containing a combination of spherical and layered platelet nanoparticles were investigated. The nanoparticles were first modified through surface graft polymerization of methacrylic acid in order to make the particles surface compatible with the bonding matrix resin. MATERIALS AND METHODS: Graft free radical polymerization in aqueous media was performed to attach Poly (methacrylic acid) (PMA) chains onto the surface of Na-MMT nanoclay (Cloisite® Na+) and silica nanoparticles (Aerosil® 200). The hybrid PMA grafted nanoparticles (PMA-g-NC-Sil) were characterized using GPC, FTIR, TGA, and X-ray diffraction (XRD). Dentin adhesives containing different amounts of the hybrid modified nanoparticles were photopolymerized and their characteristics were studied using FTIR, TEM, SEM, EDXA, and XRD techniques. The adhesives containing different amounts of PMA-g-NC-Sil were applied to the conditioned human premolar dentin to bond a dental composite to the teeth. The bond strength was then measured by microshear bond strength testing method. The results were analyzed and compared statistically. The stability of PMA-g-NC-Sil dispersion in the dentin adhesive was investigated using separation analysis (LUMi Reader) techniques. RESULTS: The grafting of PMA chains onto the surface of nanoclay was confirmed by FTIR and TGA analytical techniques. The intercalated-exfoliated structure for the nanoclay platelets in the photo-cured adhesive was observed using XRD and TEM. The surface modification of the nanoparticles significantly increased the dispersion stability of the fillers in the adhesive solution. The microshear test results indicated that the incorporation of the PMA-g-NC-Sil nanoparticles significantly enhanced the bond strength to dentin with the highest shear bond strength observed at 0.5 wt%. SIGNIFICANCE: The incorporation of the PMA modified hybrid nanofillers into the dentin adhesive resulted in a dentin bonding agent with enhanced shear bond strength through reinforcing the adhesive matrix and potential interactions between their carboxylic acid groups and the tooth structure. The dispersion stability of the nanoparticles was also dramatically improved by the surface modification of the nanoparticles.

Synthesis of poly(acrylic-co-itaconic acid) through precipitation photopolymerization for glass-ionomer cements: Characterization and properties of the cements.

OBJECTIVE: The aim of this study was to synthesize poly(acrylic acid-co-itaconic acid) (PAA-co-PIA) ionomer through a novel precipitation photopolymerization technique. The ionomer was characterized and the effect of its structural parameters, such as molecular weight and copolymer composition were investigated on the mechanical properties of glass-ionomer prepared using the ionomer. METHODS: Design of experiment (DOE) was used to examine the effect of monomer ratio and the amount of chain transfer agent on the molecular weight and final conversion of the ionomers synthesized through the precipitation photopolymerization. The copolymer compositions were identified using FTIR and 1H-NMR spectroscopy. The molecular weights of the copolymers were evaluated by GPC. A series of PAA-co-PIA copolymers were then synthesized via the photopolymerization technique in three monomer ratios and two molecular weight ranges (high and low) to study the properties of the glass ionomers thereof. Experimental dental glass-ionomer cements were prepared by mixing the synthesized polymers with glass powder and their compressive properties were determined according to ISO 9917-1:2007 after storing for 0, 1, 7 and 28 days in distilled water. The scanning electron microscopy (SEM) was used to study the fracture surface morphology of the cements. RESULTS: The PAA-co-PIA polymers were synthesized by the photopolymerization method in a short time and high purity. The DOE showed that by decreasing the acrylic acid/itaconic acid ratio and increasing the amount of transfer agent, the molecular weight and final conversion decreased significantly. By increasing the itaconic acid content in the copolymer composition and increasing the molecular weight in a constant copolymer composition, the compressive strength and modulus were increased. Microstructures revealed that cements made of the higher molecular weight poly acids showed lower cracks and voids. SIGNIFICANCE: The precipitation photopolymerization technique provides a promising and facile method in the synthesis of ionomers which are used in dental cements and other application.

Synthesis of plate-like ß-tricalcium phosphate nanoparticles and their efficiency in remineralization of incipient enamel caries.

The purpose of this study was to synthesize nano-sized ß-tricalcium phosphate (nano-TCP) particles and determine its concentration-dependent properties on incipient enamel caries lesions. Nano-TCP was synthesized as a wet chemical through a method using low concentration of precursors and low addition rate of calcium nitrate tetrahydrate as a second phase. Morphology and phase composition of the particles were analyzed by SEM, XRD, and EDXA techniques. Incipient enamel lesions were created in human premolars with an acidic buffer. The teeth were then incubated in aqueous dispersions of nano-TCP as remineralization solutions. Sodium fluoride solution and deionized water were used as positive and negative control groups, respectively. The quality and thickness of the remineralized layer on enamel were investigated using SEM. The data were statistically analyzed by analysis of variance (ANOVA) and post hoc Tukey's test. The synthesized nano-TCP mostly consisted of porous platelet-like crystals of 50-100 nm thickness and pore diameters of 100-300 nm. SEM observation showed that a homogenous layer was formed on the surface of the enamels remineralized in nano-TCP solutions. The thickness of the mineralized layer was dependent on the incubation time and nano-TCP concentration.

A novel thymol-doped enamel bonding system: Physico-mechanical properties, bonding strength, and biological activity.

PURPOSE: Over the past decades, the preparation of antibacterial restorative dental adhesives has obtained increasing attention in order to prevent secondary caries. In the present study, a novel essential oil-based antibacterial resin adhesive was prepared and evaluated for dental applications. In this regards, thymol, which is a major phenolic component of thyme essential oil, was incorporated into methacrylate resin matrix and its effect on the physico-mechanical and biological properties of the experimental bonding agent was investigated. MATERIALS AND METHODS: Mechanical properties were evaluated via measuring flexural strength, flexural modulus and fracture toughness. Degree of conversion (DC%) of monomers was measured using FTIR spectroscopy. Viscoelastic properties of the samples were also determined by dynamic mechanical thermal analysis (DMTA). The bactericidal activity of composite specimens against Streptococcus mutans (ATCC 35668) was determined based on ASTM E 2180-07.MTT assay was performed to investigate the cytocompatibility of samples. Furthermore, the bonding strength of the adhesives was evaluated through microshear bond test on the caries-free extracted human premolar teeth and the mode of failure was investigated by scanning electron microscopy. RESULTS: Thymol-doped resin adhesive exhibited comparable degree of conversion to the control resin adhesive. The plasticizing behavior of thymol slightly decreased the flexural modulus and glass transition temperature of the thymol containing specimens, even though; it caused significant increases in fracture toughness of adhesive. The results represented appropriate antibacterial activity as well as suitable cytocompatibility. Furthermore, the thymol-doped resin adhesive showed comparable adhesive strength to the control. CONCLUSION: The thymol is extremely compatible with the methacrylate resin restorative system and completely fulfills all requirements of a good bactericidal component in construction of an ideal enamel bonding system.

The Effect of Calcium Hydroxide and Nano-calcium Hydroxide on Microhardness and Superficial Chemical Structure of Root Canal Dentin: An Ex Vivo Study.

INTRODUCTION: Calcium hydroxide (CH) and nano-calcium hydroxide (NCH) as intracanal medicaments may affect the physical and chemical properties of dentin. The aim of this investigation was to evaluate the effects of CH and NCH on the microhardness and superficial chemical structure of radicular dentin. METHODS: In this in vitro trial, 80 dentin discs were randomly assigned into 2 control and 2 treatment groups (n = 20). CH and NCH pastes were used in the treatment groups. In the control groups, the samples were washed with either normal saline or 2.5% sodium hypochlorite. After 1 and 4 weeks, dentin microhardness was assessed by the Vickers test, and the phosphate/amide I ratio was evaluated by the Fourier-transform infrared spectrometry test. The data were subjected to 1-way and 2-way analyses of variance, the Tukey multiple comparison test, and the Student t test. RESULTS: A significant reduction in the mean microhardness was observed in the CH group after 4 weeks, whereas the differences were not significant in the NCH and control groups. The amount of reduction induced by NCH was comparable with hypochlorite. A significant increase in the mean phosphate/amide I ratio was observed in the CH and NCH groups compared with the sodium hypochlorite and intact control groups after 1 week, which did not change significantly during the observation period. CONCLUSIONS: The use of CH as an intracanal medicament for 4 weeks reduced dentin microhardness, whereas NCH did not result in any change in the microhardness value. However, a change in the superficial chemical structure was observed earlier after 1 week and in both the CH and NCH groups.

The Effect of Incorporation of 0.5 %wt. Silica Nanoparticles on the Micro Shear Bond Strength of a Resin Modified Glass Ionomer Cement.

STATEMENT OF THE PROBLEM: The clinical success of glass ionomer restorations depends on the strength of resin-modified glass ionomer (RMGI) cement bonding to dentin and there is limited information available regarding the bond strength of resin modified glass ionomers containing silica nanoparticles to dental structures. PURPOSE: The aim of this study was to compare the microshear bond strength (µSBS) of RMGI with and without silica (SiO2) nanoparticles to dentin of permanent teeth. MATERIALS AND METHOD: In this experimental study, the occlusal surfaces of 30 freshly extracted intact third molars were ground to expose the flat dentin and after conditioning with 20% poly acrylic acid, were randomly assigned to two main groups (n=15). The first group was filled with RMGI (Fuji II LC, GC) and the second group was filled with RMGI plus 0.5%wt. silica nanoparticles. Then, each main group was divided into three subgroups, and then stored in an incubator at 37 oC with 100% humidity for 1, 7, and 30 days. The µSBS test was performed using a universal testing machine (1 mm/min). The data were analyzed by t-test, repeated measures ANOVA and Tukey test (p< 0.05). RESULTS: There were no statistically significant differences between the mean µSBS of the groups with and without nanoparticles along the different storage periods (p> 0.05). There was significant difference in µSBS values among the three different storage periods in all the tested materials (p< 0.05). CONCLUSION: Incorporation of 0.5 %wt. silica nanoparticles did not compromise the µSBS of Fuji II LC RMGI to dentin.

PMMA/double-modified organoclay nanocomposites as fillers for denture base materials with improved mechanical properties.

The aim of this study was to investigate the effect of double-modified organoclay on mechanical properties of polymethylmethacrylate (PMMA) denture base resins. An organoclay was further modified via a silanization process using 3-trimethoxysilylpropyl methacrylate (MPS) as a reactive silane coupling agent. X-ray diffraction patterns (XRD), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of MPS in the resulting double-modified nanoclay structure. Surface tension of the original and double-modified nanoclays, were determined using contact angle measurements. Results showed that double-modified nanoclay has more organophilic nature due to silanization of the clay edges. Free radical suspension polymerization of methylmethacrylate monomers in the presence of different amounts of double-modified organoclay was performed and PMMA/clay nanocomposites were obtained. Results of dynamic mechanical thermal analysis (DMTA) showed increased storage modulus and glass transition temperature for the resulting nanocomposites with respect to neat PMMA. Higher improvements in storage modulus and glass transition temperature were obtained for nanocomposites containing 0.25 and 0.5 wt% of double-modified nanoclay. TEM observations of the nanocomposite containing 0.5 wt% of double-modified nanoclay showed prevailing exfoliated morphology. Aforementioned nanocomposites with higher mechanical properties were incorporated into a denture base formulation and their mechanical properties were studied using static flexural and fracture toughness tests. According to the results, both reinforced samples showed increased flexural strength, flexural modulus and fracture toughness in comparison to the neat matrix. The average improvements in flexural strength, flexural modulus and fracture toughness were about 30%, 65.8% and 32%, respectively. Scanning electron microscopy (SEM) images of the fracture surface of the resin matrices subjected to static tests showed brittle and ductile surface fracture for neat and reinforced denture base specimens, respectively.

Structure-properties relationships in dental adhesives: Effect of initiator, matrix monomer structure, and nano-filler incorporation.

OBJECTIVES: This is a confirmatory study to evaluate the effect of photoinitiator type and concentration, matrix monomer chemical structure, and nanoparticle incorporation on the physical and mechanical properties of an experimental dentin bonding agent. MATERIALS AND METHODS: Different concentrations of camphorquinone-amine (CQ-A) system, butanedione (BD), and phenylpropanedione (PPD), as photoinitiator, BTDMA, as a comonomer containing carboxylic acid groups, and silica nanoparticles as reinforcing inorganic filler were incorporated into a methacrylate base experimental dental adhesive. The effect of these ingredients, as independent variables, on the shrinkage kinetics, flexural strength and modulus, and microshear bond strength of the adhesives were then investigated. The results were analyzed using one-way ANOVA and Tukey's post-hoc test at the significance level of 0.05. RESULTS: The results indicate that the efficiency of CQ-A initiator system is diminished in the presence of the acidic monomer BTDMA while the photopolymerization is efficiently progressed with BD as initiator. PPD shows the lowest efficiency in the photopolymerization of the adhesives. BTDMA as a monomer with the capability of interaction with tooth structure provides adhesive with improved microshear bond strength to dentin. Incorporation of silica nanoparticles at low concentrations enhances the flexural and microshear strength of the dentin bonding agent. SIGNIFICANCE: Understanding the structure-property relationship in dental adhesives may help the material selection in clinical dentistry. The study elucidates the relationship between monomer structure, initiator type, and nanofiller and physical and mechanical properties in dental adhesives.

Effects of ethanol concentrations of acrylate-based dental adhesives on microtensile composite-dentin bond strength and hybrid layer structure of a 10 wt% polyhedral oligomeric silsesquioxane (POSS)-incorporated bonding agent.

Background: The aim of this study was to assess for the first time the effects of different amounts of ethanol solvent on the microtensile bond strength of composite bonded to dentin using a polyhedral oligomeric silsesquioxane (POSS)-incorporated adhesive. Materials and Methods: This experimental study was performed on 120 specimens divided into six groups (in accordance with the ISO TR11405 standard requiring at least 15 specimens per group). Occlusal dentin of thirty human molar teeth was exposed by removing its enamel. Five teeth were assigned to each of six groups and were converted to 20 microtensile rods (with square cross-sections of 1 mm × 1 mm) per group. The "Prime and Bond NT" (as a common commercial adhesive) was used as the control group. Experimental acrylate-based bonding agents containing 10 wt% POSS were produced with five concentrations of ethanol as solvent (0, 20, 31, 39, and 46 wt%). After application of adhesives on dentin surface, composite cylinders (height = 6 mm) were bonded to dentin surface. The microtensile bond strength of composite to dentin was measured. The fractured surfaces of specimens were evaluated under a scanning electron microscope to assess the morphology of hybrid layer. Data were analyzed using one-sample t-test, one-way analysis of variance (ANOVA), and Tukey tests (α = 0.05). Results: the mean bond strength in the groups: "control, ethanol-free, and 20%, 31%, 39%, and 46% ethanol" was, respectively, 46.5 ± 5.6, 29.4 ± 5.7, 33.6 ± 4.1, 59.0 ± 5.5, 41.9 ± 6.2, and 18.7 ± 4.6 MPa. Overall difference was significant (ANOVA, P < 0.0001). Pairwise differences were all significant (Tukey P < 0.05) except those of "ethanol 0% versus 20%" and "20% versus 31%." All groups except "0% and 46% ethanol" had bond strengths above 30 MPa (t-test P < 0.05). Conclusion: Incorporation of 31% ethanol as solvent into a 10 wt% POSS-incorporated experimental dental adhesive might increase the bond strength of composite to dentin and improve the quality and morphology of the hybrid layer. However, higher concentrations of the solvent might not improve the bond strength or quality of the hybrid layer.