Elution from light-cured dental composites: comparison of trimethacrylate and dimethacrylate as base monomers

The effect of the number of functional units on a monomer on the elution property of dental composites was first studied. Elution from a composite of a trifunctional methacrylate, 1,1,1-tris[4-(2'-hydroxy-3'-methacryloyloxypropoxy)phenyl]ethane (THMPE), was compared to that of bis-GMA, a typical difunctional base monomer in current dental composites. The degrees of cure and the water solubilities of composites prepared from the two kinds of methacrylates were measured. The concentration of unreacted methacrylates (base monomer and diluent) present in the photo-cured composites as well as the quantity of the methacrylates eluted into water (or 75%, volume fraction, of ethanol in water) from the composites were determined by high-performance liquid chromatography (HPLC). THMPE-based composites showed lesser amounts of residual and eluted methacrylates when compared with bis-GMA composites. This is attributed to the higher degree of functionality and larger molecular size of THMPE compared with those of bis-GMA.     

Fabrication and characterization of bioactive and antibacterial composites for dental applications

There is an increasing clinical need to design novel dental materials that combine regenerative and antibacterial properties. In this work the characterization of a recently developed sol–gel-derived bioactive glass ceramic containing silver ions (Ag-BG) is presented. The microstructural characteristics, ion release profile, zeta potential value and changes in weight loss and pH value as a function of the immersion time of Ag-BG in Tris buffer are evaluated. Ag-BG is also incorporated into natural extracellular matrix (ECM) hydrogel to further enhance its regenerative properties. Then, the micro and macro architectures of these new composites (ECM/Ag-BG) are characterized. In addition, the antibacterial properties of these new composites are tested against Escherichia coli and Enterococcus faecalis, a bacterium commonly implicated in the pathogenesis of dental pulp infections. Cell–material interaction is also monitored in a primary culture of dental pulp cells. Our study highlights the benefits of the successful incorporation of Ag in the bioactive glass, resulting in a stable antibacterial material with long-lasting bactericidal activity. Furthermore, this work presents for the first time the fabrication of new Ag-doped composite materials, with inductive pulp-cell proliferation and antibacterial properties (ECM/Ag-BG). This advanced composite made of Ag-BG incorporated into natural ECM possesses improved properties that may facilitate potential applications in tooth regeneration approaches.     

Synthesis and evaluation of novel bifunctional oligomer-based composites for dental applications

Five novel bifunctional oligomers containing both carboxylic acid and methacrylate groups are synthesized, characterized, and used to formulate compomers by mixing with strontium fluoroaluminosilicate glass powder at a filler level of 75% (by weight). Compressive strength (CS) of the cements and viscosity of the resin liquids are used as screening tools to find the optimal formulation. Diametral tensile (DTS) and flexural strengths (FS) are also determined. Results show that the oligomers derivatized with glycerol dimethacrylate exhibit higher CS than those with 2-hydroxyethyl methacrylate. The CS increases with increasing diluent content, filler level, and light-exposure time. During aging, the cement shows an increase of strength over 24 h and then remains unaltered for up to 3 months. The experimental compomer is 45 and 69% higher in CS, 35 and 174% higher in DTS, and 39 and 170% higher in FS, respectively, as compared to Dyract and Fuji II LC.     

Preparation of low shrinkage stress Bis-GMA free dental resin composites with a synthesized urethane dimethacrylate monomer

A new urethane dimethacrylate TMA was synthesized through a typical urethane reaction. TMA was used to replace 1,6-bis(methacryloxy-2-ethoxycarbonyl- amino)-2,4,4- trimethylhexane (UDMA) in UDMA based composite partially or totally to prepare TMA containing composites. Critical properties of TMA containing composites were investigated. 2,2-bis[4(2-hydroxy-3-methacryloy- propyloy)phenyl]propane (Bis-GMA) based and UDMA based composites were used as references. FT-IR and ¹H-NMR confirmed the structure of TMA. All of experimental dental resin composites had the similar double bond conversion (p?>?0.05). With a certain amount of TMA, TMA containing composites could have lower volumetric shrinkage (p?<?0.05) and shrinkage stress (p?<?0.05) than control groups. Water sorption, solubility, flexural strength and modulus of TMA containing composites were not worse than those of control groups. All of TMA containing composites and UDMA based composite had the same fracture toughness (p?>?0.05), which was higher than that of Bis-GMA based composite (p?<?0.05). TMA has potential as Bis-GMA substitute to prepare Bis-GMA free dental resin composites with low shrinkage stress.     

Poly(propylene fumarate) reinforced dicalcium phosphate dihydrate cement composites for bone tissue engineering

Calcium phosphate cements have many desirable properties for bone tissue engineering, including osteoconductivity, resorbability, and amenability to rapid prototyping-based methods for scaffold fabrication. In this study, we show that dicalcium phosphate dihydrate (DCPD) cements, which are highly resorbable but also inherently weak and brittle, can be reinforced with poly(propylene fumarate) (PPF) to produce strong composites with mechanical properties suitable for bone tissue engineering. Characterization of DCPD-PPF composites revealed significant improvements in mechanical properties for cements with a 1.0 powder to liquid ratio. Compared with nonreinforced controls, flexural strength improved from 1.80 ± 0.19 MPa to 16.14 ± 1.70 MPa, flexural modulus increased from 1073.01 ± 158.40 MPa to 1303.91 ± 110.41 MPa, maximum displacement during testing increased from 0.11 ± 0.04 mm to 0.51 ± 0.09 mm, and work of fracture improved from 2.74 ± 0.78 J/m(2) to 249.21 ± 81.64 J/m(2) . To demonstrate the utility of our approach for scaffold fabrication, 3D macroporous scaffolds were prepared with rapid prototyping technology. Compressive testing revealed that PPF reinforcement increased scaffold strength from 0.31 ± 0.06 MPa to 7.48 ± 0.77 MPa. Finally, 3D PPF-DCPD scaffolds were implanted into calvarial defects in rabbits for 6 weeks. Although the addition of mesenchymal stem cells to the scaffolds did not significantly improve the extent of regeneration, numerous bone nodules with active osteoblasts were observed within the scaffold pores, especially in the peripheral regions. Overall, the results of this study suggest that PPF-DCPD composites may be promising scaffold materials for bone tissue engineering.     

Study of a hydraulic dicalcium phosphate dihydrate/calcium oxide-based cement for dental applications

By mixing CaHPO(4) x 2H(2)O (DCPD) and CaO with water or sodium phosphate buffers as liquid phase, a calcium phosphate cement was obtained. Its physical and mechanical properties, such as compressive strength, initial and final setting times, cohesion time, dough time, swelling time, dimensional and thermal behavior, and injectability were investigated by varying different parameters such as liquid to powder (L/P) ratio (0.35-0.7 ml g(-1)), molar calcium to phosphate (Ca/P) ratio (1.67-2.5) and the pH (4, 7, and 9) and the concentration (0-1 M) of the sodium phosphate buffer. The best results were obtained with the pH 7 sodium phosphate buffer at the concentration of 0.75 M. With this liquid phase, physical and mechanical properties depended on the Ca/P and L/P ratios, varying from 3 to 11 MPa (compressive strength), 6 to 10 min (initial setting time), 11 to 15 min (final setting time), 15 to 30 min (swelling time), 7 to 20 min (time of 100% injectability). The dough or working time was over 16 min. This cement expanded during its setting (1.2-5 % according to Ca/P and L/P ratios); this would allow a tight filling. Given the mechanical and rheological properties of this new DCPD/CaO-based cement, its use as root canal sealing material can be considered as classical calcium hydroxide or ZnO/eugenol-based pastes, without or with a gutta-percha point.     

High resolution transmission electron microscopy of age-hardenable Au-Cu-Zn alloys for dental applications

Microstructures of age-hardenable AuCu-Zn pseudobinary alloys for dental applications were studied by means of high resolution transmission electron microscopic (HRTEM) observation and X-ray diffraction study. HRTEM study revealed that the appearance frequency of antiphase boundaries (APBs) per unit volume of the AuCu II superstructure effectively increased by Zn addition to AuCu, which may be the reason for that high hardness was maintained for a long time in AuCu-Zn alloys. The disordered APBs zone in the AuCu II superstructure had wavy characteristics and fluctuated within regular range. With increasing Zn content in AuCu-Zn alloys, the fluctuation range of APBs' width became narrower, thus random APBs' spacing and irregular APBs' shape of AuCu II superstructure changed to comparatively regular APBs' spacing and shape. Due to the APBs' wavy characteristics, spacing between successive APBs, M, was not constant but scattered, and the magnitude of the scattering of M value decreased with increasing Zn content. By Zn addition to AuCu, phase transformation from a disordered alpha phase to AuCu II phase was greatly accelerated, which made it possible for the AuCu-Zn alloy to have excellent age-hardenability at relatively low temperature like intraoral temperature.     

Synthesis of a proline-modified acrylic acid copolymer in supercritical CO2 for glass-ionomer dental cement applications

Supercritical (sc-) fluids (such as sc-CO₂) represent interesting media for the synthesis of polymers in dental and biomedical applications. Sc-CO₂ has several advantages for polymerization reactions in comparison to conventional organic solvents. It has several advantages in comparison to conventional polymerization solvents, such as enhanced kinetics, being less harmful to the environment and simplified solvent removal process. In our previous work, we synthesized poly(acrylic acid-co-itaconic acid-co-N-vinylpyrrolidone) (PAA-IA-NVP) terpolymers in a supercritical CO₂/methanol mixture for applications in glass-ionomer dental cements. In this study, proline-containing acrylic acid copolymers were synthesized, in a supercritical CO₂ mixture or in water. Subsequently, the synthesized polymers were used in commercially available glass-ionomer cement formulations (Fuji IX commercial GIC). Mechanical strength (compressive strength (CS), diametral tensile strength (DTS) and biaxial flexural strength (BFS)) and handling properties (working and setting time) of the resulting modified cements were evaluated. It was found that the polymerization reaction in an sc-CO₂/methanol mixture was significantly faster than the corresponding polymerization reaction in water and the purification procedures were simpler for the former. Furthermore, glass-ionomer cement samples made from the terpolymer prepared in sc-CO₂/methanol exhibited higher CS and DTS and comparable BFS compared to the same polymer synthesized in water. The working properties of glass-ionomer formulations made in sc-CO₂/methanol were comparable and better than the values of those for polymers synthesized in water.     

Polymer--calcium phosphate cement composites for bone substitutes

The use of self-setting calcium phosphate cements (CPCs) as bioresorbable bone-replacement implant materials presently is limited to non-load-bearing applications because of their low compressive strength relative to natural bone. The present study investigated the possibility of strengthening a commercially available CPC, alpha-BSM, by incorporating various water-soluble polymers into the cement paste during setting. Several polyelectrolytes, poly(ethylene oxide), and the protein bovine serum albumin (BSA) were added in solution to the cement paste to create calcium phosphate-polymer composites. Composites formulated with the polycations poly(ethylenimine) and poly(allylamine hydrochloride) exhibited compressive strengths up to six times greater than that of pure alpha-BSM material, with a maximum value reached at intermediate polymer content and for the highest molecular weight studied. Composites containing BSA developed compressive strengths twice that of the original cement at protein concentrations of 13-25% by weight. In each case, XRD studies correlate the improvement in compressive strength with reduced crystallite dimensions, as evidenced by a broadening of the (0,0,2) reflection. This suggests that polycation or BSA adsorption inhibits crystal growth and possibly leads to a larger crystal aspect ratio. SEM results indicate a denser, more interdigitated microstructure. The increased strength was attributed to the polymer's capacity to bridge between multiple crystallites (thus forming a more cohesive composite) and to absorb energy through plastic flow.     

Polypeptide-catalyzed silica for dental applications

Polypeptides such as polylysine have been shown to catalyze the condensation and direct the structure of silica from precursor solutions under ambient conditions. Several of the reaction parameters have been shown to mediate this activity. Specifically, mechanical perturbation seems to play a role in the formation of hierarchical structures. Most studies have been conducted in solution, but biomedical and particularly dental applications will likely require control of biosilicified coatings, films or particle formation on surfaces. Tetraethylorthosilicate was reacted with polylysine and then spin coated onto a surface. The process parameters catalyst structure, pH, buffer: ethanol ratio and percentage of cocatalyst polyethyleneimine were varied to determine their effects on the formed silica. The chemical nature and morphology of the silica were investigated with FTIR and SEM, respectively and reaction rates were monitored with a colorimetric assay. Our results show that these process parameters had only minor effects on composition, but the catalyst conformation influenced the degree of hydration while the pH, choice of solvent and cocatalyst strongly influenced morphology. We also found that perturbation from spin coating significantly influences the silicification dynamics. The ability to catalyze nano- to micron-sized mineral with different morphologies using polypeptides could have numerous dental applications including, sealing of dentin tubules, in situ reinforcement of resin interfaces or preparation of implant surfaces.     

Fibre reinforced bioresorbable composites for spinal surgery

Composites containing different amounts of beta-tricalcium phosphate (beta-TCP) embedded in a poly-lactide (PLA70) matrix with and without poly-lactide (PLA96) fibre reinforcement were studied and the feasibility of using these composites in spinal fusion implants was examined. Compressive yield strength was measured in two directions: parallel to (83-97 MPa) and perpendicular to (108-123 MPa) the laminated structure of the composites. In the parallel direction, the addition of beta-TCP decreased compressive yield strength while in the perpendicular direction this was increased when compared to plain specimens (p<0.05). Fibre reinforcement had no significant effect on compressive yield strength (p<0.05), but did increase impact strength by 127-216% for notched specimens (parallel direction) and by about 65% for un-notched specimens (perpendicular direction) (p<0.05). A 24 week in vitro analysis of implant prototypes in simulated body fluid revealed a decrease in compressive yield strength, which was greater for the samples containing 50 wt.% beta-TCP than for those containing 25 wt.% beta-TCP. After 12 weeks incubation the composites retained 66-99% of their initial compressive strength, depending on composition. After 24 weeks incubation the lowest compressive strength was 51% (56 MPa: 50/50) and the highest was 94% (90 MPa: 75/25) of the initial value. Calcium phosphate precipitation on the surfaces of the materials in vitro was also observed. The initial compressive strengths of the studied composites were comparable to materials used in spinal fusion applications, but adequate strength retention behaviour needs to be confirmed before undertaking clinical experiments.     

Microporous glassy fillers for dental composites

A microporous filler giving greatly improved finish ability, systemic nontoxic X-ray opacification, low thermal expansion (27.2 x 10(-6)/degrees C), and satisfactory translucencies has been developed for dental composite resin restorations. These fillers are prepared from frits obtained by the low-temperature calcination of gelled inorganic sols followed by a pulsed high-temperature treatment. Composites prepared from these fillers are within the range of commercial products with regard to strength and setting contraction.     

Properties of an injectable low modulus PMMA bone cement for osteoporotic bone

The use of polymethylmethacrylate (PMMA) cement to reinforce fragile or broken vertebral bodies (vertebroplasty) leads to extensive bone stiffening. Fractures in the adjacent vertebrae may be the consequence of this procedure. PMMA with a reduced Young's modulus may be more suitable. The goal of this study was to produce and characterize stiffness adapted PMMA bone cements. Porous PMMA bone cements were produced by combining PMMA with various volume fractions of an aqueous sodium hyaluronate solution. Porosity, Young's modulus, yield strength, polymerization temperature, setting time, viscosity, injectability, and monomer release of those porous cements were investigated. Samples presented pores with diameters in the range of 25-260 microm and porosity up to 56%. Young's modulus and yield strength decreased from 930 to 50 MPa and from 39 to 1.3 MPa between 0 and 56% porosity, respectively. The polymerization temperature decreased from 68 degrees C (0%, regular cement) to 41 degrees C for cement having 30% aqueous fraction. Setting time decreased from 1020 s (0%, regular cement) to 720 s for the 30% composition. Viscosity of the 30% composition (145 Pa s) was higher than the ones received from regular cement and the 45% composition (100-125 Pa s). The monomer release was in the range of 4-10 mg/mL for all porosities; showing no higher release for the porous materials. The generation of pores using an aqueous gel seems to be a promising method to make the PMMA cement more compliant and lower its mechanical properties to values close to those of cancellous bone.     

Long-term quantification of the release of monomers from dental resin composites and a resin-modified glass ionomer cement

This study quantified the release of monomers from polymerized specimens of four commercially available resin composites and one glass ionomer cement immersed in water:ethanol solutions. Individual standard curves were prepared from five monomers: (1) triethylene glycol dimethacrylate (TEGDMA), (2) 2-hydroxy-ethyl methacrylate (HEMA), (3) urethane dimethacrylate (UDMA), (4) bisphenol A glycidyl dimethacrylate (BISGMA), and (5) bisphenol A. The concentration of the monomers was determined at Days 1, 7, 30, and 90 with the use of electrospray ionization/mass spectrometry. Data were expressed in mean micromol per mm(2) surface area of specimen and analyzed with Scheffe's test (p<0.05). The following monomers were found in water: monomers (1) and (2) from Delton sealant, monomer (5) from ScotchBond Multipurpose Adhesive and Delton sealant, monomer (3) from Definite and monomer (4) from Fuji II LC, ScotchBond Multipurpose Adhesive, Synergy and Definite. All these monomers increased in concentration over time, with the exception of monomer (1) from Delton sealant. Monomers (3) and (5) were found in extracts of materials despite their absence from the manufacturer's published composition. All monomers were released in significantly higher concentrations in water:ethanol solutions than in water. The greatest release of monomers occurred in the first day. The effect of the measured concentrations of monomers (1-5) on human genes, cells, or tissues needs to be considered with the use of a biological model.     

Novel diamond-coated tools for dental drilling applications

The application of diamond coatings on cemented tungsten carbide (WC-Co) tools has been the subject of much attention in recent years in order to improve cutting performance and tool life in orthodontic applications. WC-Co tools containing 6% Co metal and 94% WC substrate with an average grain size of 1 - 3 microm were used in this study. In order to improve the adhesion between diamond and WC substrates it is necessary to etch cobalt from the surface and prepare it for subsequent diamond growth. Alternatively, a titanium nitride (TiN) interlayer can be used prior to diamond deposition. Hot filament chemical vapour deposition (HFCVD) with a modified vertical filament arrangement has been employed for the deposition of diamond films to TiN and etched WC substrates. Diamond film quality and purity has been characterized using scanning electron microscopy (SEM) and micro Raman spectroscopy. The performances of diamond-coated WC-Co tools, uncoated WC-Co tools, and diamond embedded (sintered) tools have been compared by drilling a series of holes into various materials such as human tooth, borosilicate glass, and acrylic tooth materials. Flank wear has been used to assess the wear rates of the tools when machining biomedical materials such as those described above. It is shown that using an interlayer such as TiN prior to diamond deposition provides the best surface preparation for producing dental tools.     

Development of a biodegradable bone cement for craniofacial applications

This study investigated the formulation of a two-component biodegradable bone cement comprising the unsaturated linear polyester macromer poly(propylene fumarate) (PPF) and crosslinked PPF microparticles for use in craniofacial bone repair applications. A full factorial design was employed to evaluate the effects of formulation parameters such as particle weight percentage, particle size, and accelerator concentration on the setting and mechanical properties of crosslinked composites. It was found that the addition of crosslinked microparticles to PPF macromer significantly reduced the temperature rise upon crosslinking from 100.3°C ± 21.6°C to 102.7°C ± 49.3°C for formulations without microparticles to 28.0°C ± 2.0°C to 65.3°C ± 17.5°C for formulations with microparticles. The main effects of increasing the particle weight percentage from 25 to 50% were to significantly increase the compressive modulus by 37.7 ± 16.3 MPa, increase the compressive strength by 2.2 ± 0.5 MPa, decrease the maximum temperature by 9.5°C ± 3.7°C, and increase the setting time by 0.7 ± 0.3 min. Additionally, the main effects of increasing the particle size range from 0-150 μm to 150-300 μm were to significantly increase the compressive modulus by 31.2 ± 16.3 MPa and the compressive strength by 1.3 ± 0.5 MPa. However, the particle size range did not have a significant effect on the maximum temperature and setting time. Overall, the composites tested in this study were found to have properties suitable for further consideration in craniofacial bone repair applications.     

Bioglass composites: a potential material for dental application

Bioglass, a promising material for dental applications, can be reinforced with ductile stainless steel fibres. Three aspects of the fibre-reinforced bioglass composites are discussed. They are the interface between the glass and the metal fibres, the mechanical properties of the composites and their in vivo bonding behaviour. The importance of a good interfacial bond between the glass and the metal fibres is outlined. The improvement in strength and toughness, due to the fibres, is explained. The in vivo bonding behaviour of the bioglass composite is checked under statically loaded conditions.     

Degradable particulate composite reinforced with nanofibres for biomedical applications

Nanofibre-based structures and their composites are increasingly being studied for many biomedical applications, including tissue engineering scaffolds. These materials enable architectures resembling the extracellular matrix to be obtained. The search for optimized supports and carriers of cells is still a major challenge for the tissue engineering field. The main purpose of this work is to develop a novel composite structure that combines microparticles and nanofibres in reinforced polymeric microfibres. This innovative combination of materials is obtained by melting extrusion of a particulate composite reinforced with chitosan nanofibre meshes (0.05 wt.%) produced by the electrospinning technique. The reinforced microfibres were analysed by scanning electron microscopy and showed a considerable alignment of the chitosan nanofibres along the longitudinal main axis of the microfibre composite structure. The tensile mechanical properties revealed that the introduction of the nanofibre reinforcement in the particulate microfibre composite increased the tensile modulus by up to 70%. The various structures were subjected to swelling and degradation tests immersed in an isotonic saline solution at 37 °C. The presence of chitosan nanofibres in the particulate microfibres enhances the water uptake by up to 24%. The combination of good mechanical properties and enhanced degradability of the developed structures is believed to have great potential for various biomedical applications, including three-dimensional fibre mesh scaffolds to be applied in the field of bone tissue engineering.     

Elastomeric high-mineral content hydrogel-hydroxyapatite composites for orthopedic applications

The design of synthetic bone grafts that mimic the structure and composition of bone and possess good surgical handling characteristics remains a major challenge. We report the development of poly(2-hydroxyethyl methacrylate) (pHEMA)-hydroxyapatite (HA) composites termed "FlexBone" that possess osteoconductive mineral content approximating that of human bone yet exhibit elastomeric properties enabling the press-fitting into a defect site. The approach involves crosslinking pHEMA hydrogel in the presence of HA using viscous ethylene glycol as a solvent. The composites exhibit excellent structural integration between the apatite mineral component and the hydroxylated hydrogel matrix. The stiffness of the composite and the ability to withstand compressive stress correlate with the microstructure and content of the mineral component. The incorporation of porous aggregates of HA nanocrystals rather than compact micrometer-sized calcined HA effectively improved the resistance of the composite to crack propagation under compression. Freeze-dried FlexBone containing 50 wt % porous HA nanocrystals could withstand hundreds-of-megapascals compressive stress and >80% compressive strain without exhibiting brittle fractures. Upon equilibration with water, FlexBone retained good structural integration and withstood repetitive moderate (megapascals) compressive stress at body temperature. When subcutaneously implanted in rats, FlexBone supported osteoblastic differentiation of the bone marrow stromal cells pre-seeded on FlexBone. Taken together, the combination of high osteoconductive mineral content, excellent organic-inorganic structural integration, elasticity, and the ability to support osteoblastic differentiation in vivo makes FlexBone a promising candidate for orthopedic applications.