Effect of residual solvent on performance of acrylamide-containing dental materials

Compatibility versus reaction diffusion: Factors that determine the heterogeneity of polymerized adhesive networks

OBJECTIVES

Heterogeneity and phase separation during network polymerization is a major issue contributing to the failure of dental adhesives. This study investigates how the ratio of hydrophobic crosslinkers to hydrophilic comonomer (C/H ratio), as well as cosolvent fraction (ethanol/water) influences the degree of heterogeneity and proclivity for phase separation in a series of model adhesive formulations.

METHODS

Twelve formulations were investigated, with 4 different C/H ratios (7:1, 2.2:1, 1:1, 0.5:1) and 3 different overall cosolvent fractions (0, 10 and 20 wt%). The heterogeneity and phase behavior were characterized using Fourier Transform Infrared Spectroscopy (FT-IR), dynamic mechanical analysis (DMA), small-angle x-ray scattering (SAXS) and atomic force microscopy (AFM).

RESULTS

In resins without cosolvent, all characterizations confirm reduced heterogeneity as C/H ratio decreases. However, when 10 or 20 wt% of cosolvent is included in the adhesive formulation, a higher degree of heterogeneity and even distinct phase separation with domains ranging from a few hundreds of nanometers to a few micrometers in size form. This is particularly noticeable at lower C/H ratios, which is surprising as HEMA is commonly considered a compatibilizer between hydrophobic crosslinkers and aqueous (co)solvents.

SIGNIFICANCE

Our experiments demonstrate that formulations with lower C/H ratio and thus a lower viscosity experience later onsets of diffusion limitations during polymerization, which favors thermodynamically driven phase separation. Therefore, to determine or predict the resulting phase structure of adhesive materials, it is necessary to consider the kinetics and diffusion constraints during the formation of the polymer network and not just the compatibility of resin constituents.

Novel hydrolytic resistant antibacterial monomers for dental resin adhesive

OBJECTIVES

To evaluate the properties of novel hydrolytic resistant antibacterial monomers and to determine the properties of resin adhesives containing these monomers.

METHODS

Methacrylamide-based QAC (Quaternary Ammonium Compound) monomers, 1-(11-Methacryla-midoundecyl)pyridine-1-ium bromide (MAUPB) and 1-(12-Methacryl-amidododecyl)pyridine-1-ium bromide (MADPB), and their methacrylate-derivatives, N-(1-Methacryloylundecanyl)pyridinium bromide (MUPB) and N-(1-Methacryloyldodecanyl)pyridinium bromide (MDPB), were synthesized and characterized. The minimum inhibitory (MIC) and bactericidal (MBC) concentrations were determined against S.mutans and E.faecalis. Cytotoxicity of unpolymerized monomers were evaluated using L-929 and MDPC-23. Each monomer was incorporated into experimental resins (BisGMA/TEGDMA/CQ/EDMAB or BisGMA/HEMA/CQ/EDMAB) at 10wt%. FTIR Spectra were collected for degree of conversion (DC%) measurement. Bacterial attachment on resin disks were determined by fluorescent microscope. Mechanical properties of experimental resins were evaluated by flexural strength & modulus and shear bond strength testing.

RESULTS

The antibacterial activity of MDPB≥MUPB>MADPB>MAUPB. The TC50 of MAUPB> MADPB>MUPB >MDPB. Incorporation of MAUPB in BisGMA/TEGDMA-based resin, had no significant effect on DC%, while significantly increase DC% in BisGMA/HEMA-based Resin. MUPB and MAUPB containing resins showed less viable bacterial attachment than pure resins. After 3-month storage, resins containing MAUPB illustrated higher flexural strength than their corresponding resins containing MUPB. BisGMA/HEMA-based resin containing MAUPB illustrated significantly higher resin-dentin shear bond strength than that of MUPB and pure resin.

CONCLUSIONS

Methacrylamide monomer containing QAC, MAUPB, possessed antibacterial properties and superior physical and mechanical properties when incorporated in resin adhesives as compared to their corresponding methacrylate monomer, MUPB.

CLINICAL SIGNIFICANCE

Methacrylamide-based QAC monomers are potentially used to formulate antibacterial hydrolytic resistant resin adhesives and enhance resin-dentin bond strength.

Influence of varying dry air temperatures on postoperative sensitivity, penetration depth, and push-out bond strength of an ethanol/water-based adhesive: An in vivo double-blind clinical trial and in vitro analysis

Background

Incomplete monomer infiltration into the etched dentin causes postoperative sensitivity (POS) with total-etch adhesives. Increasing the temperature of the air to dry the adhesive has shown to improve its infiltration into the dentin.

Aims

The aim of this research is to evaluate the effectiveness of dry air temperatures of 15°C ± 5°C and 50°C ± 5°C on the POS, depth of penetration, and bond strength of an ethanol/water-based etch-and-rinse (ER) adhesive under in vivo and in vitro conditions.

Methods

Forty-four premolars from 11 patients scheduled for orthodontic extraction were allocated into cold air (Group 1) and warm air (Group 2) groups using a split-mouth design. A 2 mm × 2 mm × 2 mm cavity was prepared on the middle third of the buccal surface of the teeth, acid etched, and two coats of an ethanol/water-based adhesive resin mixed with 0.1% rhodamine B was applied for 10 s. A dental air gun customized to deliver warm and cold air was used to dry the adhesive prior to its light polymerization. The cavities were restored with resin composite incrementally. POS was assessed using visual analog scale at 24 and 72 h using cold test. The teeth were atraumatically extracted and analyzed for depth of adhesive penetration using confocal laser scanning microscope (n = 11) and push-out bond strength (n = 11).

Statistical Analysis Used

The data were analyzed using sample t-test and Wilcoxon signed-rank test (P < 0.05).

Results

A significantly lower POS and greater adhesive penetration into the dentin was observed in the warm air group compared to cold air (P < 0.05). No significant difference could be elicited between the push-out bond strength of both the groups (P > 0.05).

Conclusions

Warm air alleviated POS and improved the penetration of an ethanol/water-based ER adhesive into the dentin.

Poly(L-lactic acid) Reinforced with Hydroxyapatite and Tungsten Disulfide Nanotubes

Poly(L-lactic acid) (PLLA) is a biocompatible, biodegradable, and semi-crystalline polymer with numerous applications including food packaging, medical implants, stents, tissue engineering scaffolds, etc. Hydroxyapatite (HA) is the major component of natural bone. Conceptually, combining PLLA and HA could produce a bioceramic suitable for implants and bone repair. However, this nanocomposite suffers from poor mechanical behavior under tensile strain. In this study, films of PLLA and HA were prepared with small amounts of nontoxic WS2 nanotubes (INT-WS2). The structural aspects of the films were investigated via electron microscopy, X-ray diffraction, Raman microscopy, and infrared absorption spectroscopy. The mechanical properties were evaluated via tensile measurements, micro-hardness tests, and nanoindentation. The thermal properties were investigated via differential scanning calorimetry. The composite films exhibited improved mechanical and thermal properties compared to the films prepared from the PLLA and HA alone, which is advantageous for medical applications.

Cytotoxic Evaluation and Determination of Organic and Inorganic Eluates from Restorative Materials

Over the last years, diverse commercial resin-based composites have dominated as dental filling materials. The purpose of the present study was to determine organic and inorganic eluates from five restorative materials using GC/MS and ICP–OES and to compare the effect on cell survival of human gingival fibroblasts of a conventional and a bioactive resin. Five commercially available restorative materials were employed for this study: Activa^TM Bioactive Restorative, ENA HRi, Enamel plus HRi Biofunction, Fuji II LC Capsule, and Fuji IX Capsule. Disks that were polymerized with a curing LED light or left to set were immersed in: 1 mL methanol or artificial saliva for GC/MS analysis, 5mL deionized water for ICP–OES, and 5mL of culture medium for cell viability. Cell viability was investigated with a modified staining sulforhodamine B assay.The following organic substances were detected: ACP, BHT, BPA, 1,4-BDDMA, CQ, DBP, DMABEE, HEMA, MCE, MeHQ, MOPA, MS, TMPTMA, and TPSb and the ions silicon, aluminum, calcium, sodium, and barium. Activa Bioactive Restorative was found to be biocompatible. Elution of organic substances depended on material’s composition, the nature of the solvent and the storage time. Ions’ release depended on material’s composition and storage time. The newly introduced bioactive restorative was found to be more biocompatible.

Selected Spectroscopic Techniques for Surface Analysis of Dental Materials: A Narrative Review

The presented work focuses on the application of spectroscopic methods, such as Infrared Spectroscopy (IR), Fourier Transform Infrared Spectroscopy (FT-IR), Raman spectroscopy, Ultraviolet and Visible Spectroscopy (UV-Vis), X-ray spectroscopy, and Mass Spectrometry (MS), which are widely employed in the investigation of the surface properties of dental materials. Examples of the research of materials used as tooth fillings, surface preparation in dental prosthetics, cavity preparation methods and fractographic studies of dental implants are also presented. The cited studies show that the above techniques can be valuable tools as they are expanding the research capabilities of materials used in dentistry.

Effect of side-group methylation on the performance of methacrylamides and methacrylates for dentin hybridization

The stability of the bond between polymeric adhesives to mineralized substrates is crucial in many biomedical applications. The objective of this study was to determine the effect of methyl substitution at the α- and β-carbons on the kinetics of polymerization, monomer hydrolytic stability, and long-term bond strength to dentin for methacrylamide- and methacrylate-based crosslinked networks for dental adhesive applications.

METHODS

Secondary methacrylamides (α-CH3 substituted=1-methyl HEMAM, β-CH3 substituted=2-methyl HEMAM, and unsubstituted=HEMAM) and OH-terminated methacrylates (α- and β-CH3 mixture=1-methyl HEMA and 2-methyl HEMA, and unsubstituted=HEMA) were copolymerized with urethane dimethacrylate. The kinetics of photopolymerization were followed in real-time using near-IR spectroscopy. Monomer hydrolysis kinetics were followed by NMR spectroscopy in water at pH 1 over 30 days. Solvated adhesives (40 vol% ethanol) were used to bond composite to dentin and microtensile bond strength (μTBS) measured after 24h and 6 months storage in water at 37°C.

RESULTS

The rate of polymerization increased in the following order: OH-terminated methacrylates≥methacrylamides>NH2-terminated methacrylates, with minimal effect of the substitution. Final conversion ranged between 79% for 1-methyl AEMA and 94% for HEMA. 1-methyl-HEMAM showed the highest and most stable μTBS, while HEMA showed a 37% reduction after six months All groups showed measurable degradation after up to 4 days in pH 1, with the methacrylamides showing less degradation than the methacrylates. Additionally, transesterification products were observed in the methacrylamide groups.

SIGNIFICANCE

Amide monomers were significantly more stable to hydrolysis than the analogous methacrylates. The addition of a α- or β-CH3 groups increased the rate of hydrolysis, with the magnitude of the effect tracking with the expected base-catalyzed hydrolysis of esters or amides, but opposite in influence. The α-CH3 substituted secondary methacrylamide, 1-methyl HEMAM, showed the most stable adhesive interface. A side reaction was observed with transesterification of the monomers studied under ambient conditions, which was not expected under the relatively mild conditions used here, which warrants further investigation.

Chemistry of novel and contemporary resin-based dental adhesives

The chemistry of resin-based dental adhesives is critical for its interaction with dental tissues and long-term bonding stability. Changes in dental adhesives composition influences the materials' key physical-chemical properties, such as rate and degree of conversion, water sorption, solubility, flexural strength and modulus, and cohesive strength and improves the biocompatibility to dental tissues. Maintaining a suitable reactivity between photoinitiators and monomers is important for optimal properties of adhesive systems, in order to enable adequate polymerisation and improved chemical, physical and biological properties. The aim of this article is to review the current state-of-the-art of dental adhesives, and their chemical composition and characteristics that influences the polymerisation reaction and subsequent materials properties and performance.

Inhibition of matrix metalloproteinases: a troubleshooting for dentin adhesion

Matrix metalloproteinases (MMPs) are enzymes that can degrade collagen in hybrid layer and reduce the longevity of adhesive restorations. As scientific understanding of the MMPs has advanced, useful strategies focusing on preventing these enzymes' actions by MMP inhibitors have quickly developed in many medical fields. However, in restorative dentistry, it is still not well established. This paper is an overview of the strategies to inhibit MMPs that can achieve a long-lasting material-tooth adhesion. Literature search was performed comprehensively using the electronic databases: PubMed, ScienceDirect and Scopus including articles from May 2007 to December 2019 and the main search terms were “matrix metalloproteinases”, “collagen”, and “dentin” and “hybrid layer”. MMPs typical structure consists of several distinct domains. MMP inhibitors can be divided into 2 main groups: synthetic (synthetic-peptides, non-peptide molecules and compounds, tetracyclines, metallic ions, and others) and natural bioactive inhibitors mainly flavonoids. Selective inhibitors of MMPs promise to be the future for specific targeting of preventing dentin proteolysis. The knowledge about MMPs functionality should be considered to synthesize drugs capable to efficiently and selectively block MMPs chemical routes targeting their inactivation in order to overcome the current limitations of the therapeutic use of MMPs inhibitors, i.e., easy clinical application and long-lasting effect.

Organic Eluates Derived from Intermediate Restorative Dental Materials

A great number of different types of materials have been used in dentistry as intermediate restoratives. Among them, new resin-based bases have been released in the dental market. The present study focuses on the identification of the organic eluates released from such materials and the study of their surface microstructure in combination with their corresponding elemental composition. For this purpose, the following materials were used:ACTIVA™BioACTIVE-BASE/LINER™, Ketac™Bond Glass Ionomer, SDR™ and Vitrebond™Light Cure Glass Ionomer Liner/Base. Methanolic leachates derived from polymerized materials were analyzed by means of gas chromatography-mass spectrometry (GC-MS). Scanning electron microscopy(SEM) was used for the surface monitoring of suitably prepared specimens. The GC-MS analysis revealed the elution of twenty different substances from the three resin-based materials, while none was eluted from the glass ionomer base. The SEM analysis for Vitrebond™ presented small pits, the one for Ketac™Bond presented elongated cracks, while no voids were present for ACTIVA™BioACTIVE-BASE/LINER™ and SDR™. Moreover, the resin matrix of some dental materials may inhibit elements' accumulation on the surface layers. Particularly, the detected organic eluents may be related to potential toxic effects.