Ionic Dimethacrylates for Antimicrobial and Remineralizing Dental Composites.

Two ionic dimethacrylates (IDMA1 and IDMA2) intended for utilization in multifunctional, antibacterial and remineralizing dental resins and composites were synthesized by nucleophilic substitution reactions. Crude IDMAs were purified by multi-step extraction from ethanol-diethyl ether-hexane solvent system. Their structures were validated by nuclear magnetic resonance and mass spectrometry. As evidenced by the water contact angle measurements ((63.2-65.5)0), IDMAs did not affect the wettability of urethane dimethacrylate (UDMA)- based copolymers (average contact angle ((60.8±5.1)0).The attained degrees of vinyl conversion increased from 88.1% (no-IDMA control) up to 93.0% (IDMA2 series). Flexural strength (FS) of copolymers was reduced from 94.8 MPa (control) to (68.9-71.8) MPa (IDMA counterparts) independent of monomer type and/or its concentration. This reduction in FS should not disqualify IDMAs from consideration as viable antibacterial agents in multifunctional restoratives. Tested at concentrations exceeding the expected leachability of unreacted monomers from cured copolymers and/or composites, IDMAs had no deleterious effect on viability and/or metabolic activity of fibroblasts. The remineralization potential of amorphous calcium phosphate IDMA/UDMA composites was confirmed by calcium and phosphate ion release kinetic experiments. Results of this study warrant in-depth biological, physicochemical, mechanical and antibacterial assessments of IDMA resins and composites to identify prototype(s) suitable for clinical testing.

Polymerization shrinkage and stress development in amorphous calcium phosphate/urethane dimethacrylate polymeric composites.

This study explores how substituting a new high molecular mass oligomeric poly(ethylene glycol) extended urethane dimethacrylate (PEG-U) for 2-hydroxyethyl methacrylate (HEMA) in photo-activated urethane dimethacrylate (UDMA) resins affects degree of vinyl conversion (DC), polymerization shrinkage (PS), stress development (PSSD) and biaxial flexure strength (BFS) of their amorphous calcium phosphate (ACP) composites. The composites were prepared from four types of resins (UDMA, PEG-U, UDMA/HEMA and UDMA/PEG-U) and zirconia-hybridized ACP. Introducing PEG-U improved DC while not adversely affecting PS, PSSD and the BFS of composites. This improvement in DC is attributed to the long, more flexible structure between the vinyl groups of PEG-U and its higher molecular mass compared to poly(HEMA). The results imply that PEG-U has the potential to serve as an alternative to HEMA in dental and other biomedical applications.

Strong nanocomposites with Ca, PO(4), and F release for caries inhibition.

This article reviews recent studies on: (1) the synthesis of novel calcium phosphate and calcium fluoride nanoparticles and their incorporation into dental resins to develop nanocomposites; (2) the effects of key microstructural parameters on Ca, PO(4), and F ion release from nanocomposites, including the effects of nanofiller volume fraction, particle size, and silanization; and (3) mechanical properties of nanocomposites, including water-aging effects, flexural strength, fracture toughness, and three-body wear. This article demonstrates that a major advantage of using the new nanoparticles is that high levels of Ca, PO(4), and F release can be achieved at low filler levels in the resin, because of the high surface areas of the nanoparticles. This leaves room in the resin for substantial reinforcement fillers. The combination of releasing nanofillers with stable and strong reinforcing fillers is promising to yield a nanocomposite with both stress-bearing and caries-inhibiting capabilities, a combination not yet available in current materials.

AMORPHOUS CALCIUM PHOSPHATE COMPOSITES AND THEIR EFFECT ON COMPOSITE-ADHESIVE-DENTIN BONDING.

This study evaluates the bond strength and related properties of photo-polymerizable, remineralizing amorphous calcium phosphate (ACP) polymeric composite-adhesive systems to dentin after various periods of aqueous aging at 37 °C. An experimental ACP base and lining composite was made from a photo-activated resin comprising 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenyl]propane (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), 2-hydroxyethyl methacrylate (HEMA) and zirconyl dimethacrylate (ZrDMA); designated BTHZ. An experimental orthodontic composite was formulated from a photo-activated resin comprising ethoxylated bisphenol A dimethacrylate (EBPADMA), TEGDMA, HEMA and methacryloxyethyl phthalate (MEP); designated ETHM. In both composite series three fillers were compared: 1) freshly precipitated zirconium-modified ACP freshly precipitated (as-prepared Zr-ACP), 2) milled Zr-ACP and 3) an ion-leachable fluoride glass. In addition to the shear bond strength (SBS), work to fracture and failure modes of the orthodontic composites were determined. The SBS of the base and lining ACP composites appeared unaffected by filler type or immersion time. In the orthodontic ACP composite series, milled ACP composites showed initial mechanical advantages over as-prepared ACP composites, and produced higher incidence of a failure mode consistent with stronger adhesion. After six months of aqueous exposure, 80 % of specimens failed at the dentin-primer interface, with a 42 % overall reduction in bond strength. BTHZ and ETHM based ACP composites are potentially effective anti-demineralizing-remineralizing agents with possible clinical utility as protective base-liners and orthodontic cements, respectively. The analysis of the bond strength and failure modalities suggests that milled ACP composites may offer greater potential in clinical applications.

Light-cured dimethacrylate-based resins and their composites: comparative study of mechanical strength, water sorption and ion release.

This study explored how resin type affects selected physicochemical properties of complex methacrylate copolymers and their amorphous calcium phosphate (ACP)-filled and glass-filled composites. Two series of photo-polymerizable resin matrices were formulated employing 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenyl]propane (Bis-GMA) or an ethoxylated bisphenol A dimethacrylate (EBPADMA) as the base monomer, Unfilled copolymers and composites filled with a mass fraction with 40 %, 35 % and 30 %, respectively, of ACP or the un-silanized glass were assessed for biaxial flexure strength (BFS), water sorption (WS) and mineral ion release upon immersion in HEPES-buffered saline solution for up to six months. Substituting EBPADMA for Bis-GMA significantly reduced the WS while only marginally affected the BFS of both dry and wet copolymers. Independent of the filler level, both dry and wet ACP composites formulated with either BTHM or ETHM resins were mechanically weaker than the corresponding copolymers. The BFS of ACP composite specimens after 1 month in saline did not further decrease with further aqueous exposure. The BFS of glass-filled composites decreased with the increased level of the glass filler and the time of aqueous exposure. After 6 months of immersion, the BFS of glass-filled BTHM and ETHM composites, respectively, remained 58 % and 41 % higher than the BFS of the corresponding ACP composites. Ion release data indicated that a minimum mass fraction of 35 % ACP was required to attain the desired solution supersaturation with respect to hydroxyapatite for both the BTHM and ETHM derived composites.

ILLUMINATING THE ROLE OF AGGLOMERATES ON CRITICAL PHYSICOCHEMICAL PROPERTIES OF AMORPHOUS CALCIUM PHOSPHATE COMPOSITES.

Water sorption (WS), mechanical strength, and ion release of polymeric composites formulated with 40 % as-made or milled amorphous calcium phosphate (ACP) are compared after 1, 2 and 3 months of aqueous exposure. Ethoxylated bisphenol A dimethacrylate, triethylene glycol dimethacrylate, 2-hydroxyethyl methacrylate and methacryloxyethyl phthalate comprised the resin. The WS (mass %) peaked at 3 months. WS of as-made ACP composites was significantly higher than WS of milled ACP composites and copolymers. Both composite groups experienced decreases in biaxial flexural strength (BFS) with water aging, with milled ACP composites retaining a significantly higher BFS throughout immersion. Ion release was moderately reduced in milled ACP composites, though they remained superior to as-made ACP composites due to significantly lower WS and higher BFS after prolonged aqueous exposure.

Dental composites based on amorphous calcium phosphate - resin composition/physicochemical properties study.

This study explores how the resin composition/structure affects the physicochemical properties of copolymers and their amorphous calcium phosphate (ACP)-filled composites. A series of photo-polymerizable binary and ternary matrices are formulated utilizing 2,2-bis[ p-(2(')-hydroxy-3(')methacryloxypropoxy)phenyl]propane, 2,2-bis[ p-(2(')-methacryloxypropoxy)phenyl]-propane (EBPADMA), or a urethane dimethacrylate as base monomers, and triethylene glycol dimethacrylate or hexamethylene dimethacrylate (HmDMA) with or without 2-hydroxyethyl methacrylate (HEMA) as diluent monomer. Unfilled copolymers and composites filled with 40% by mass zirconia-hybridized ACP are evaluated for biaxial flexure strength (BFS), degree of conversion (DC), mineral ion release, polymerization shrinkage (PS), and water sorption (WS). The average DC values are 82-94% and 74-91% for copolymers and composites, respectively. Unrelated to the resin composition, the PS values of composites are up to 8.4 vol. % and the BFS values of wet composite specimens are on average 51 +/- 8 MPa. The maximum WS values attained in copolymers and composites reach 4.8 mass%. Inclusion of hydrophobic HmDMA monomer in the matrices significantly reduces the WS. The levels of Ca and PO(4) released from all types of composites are significantly above the minimum necessary for the re-deposition of apatite to occur. Elevated Ca, and to a lesser extent PO(4) release, is observed in HEMA-containing, ternary EBPADMA formulations. Further resin reformulations may be needed to improve the PS of composite specimens. Poor dispersion of ;as-synthesized' ACP within the composite contributes to their inferior mechanical performance.

Effect of particle size of an amorphous calcium phosphate filler on the mechanical strength and ion release of polymeric composites.

The random clustering of amorphous calcium phosphate (ACP) particles within resin matrices is thought to diminish the strength of their polymerized composites. The objective of this study was to elucidate the effect of ball-milling on the particle size distribution (PSD) of ACP fillers and assess if improved dispersion of milled ACP in methacrylate resin sufficiently enhanced filler/matrix interactions to result in improved biaxial flexure strength (BFS), without compromising the remineralizing potential of the composites. Unmilled and wet-milled zirconia-hybridized ACP (Zr-ACP) fillers were characterized by PSD analysis, X-ray diffraction, thermogravimetric and chemical analysis, infrared spectroscopy, and scanning electron microscopy. Composite specimens made from a photoactivated, ternary methacrylate resin admixed with a mass fraction of 40% of un-milled or milled Zr-ACP were evaluated for the BFS (dry and wet) and for the release of calcium and phosphate ions into saline solutions. While having no apparent effect on the structure, composition, and morphology/topology of the fillers, milling significantly reduced the average size of Zr-ACP particulates (median diameter, d(m) = 0.9 +/- 0.2 microm) and the spread of their PSD. Better dispersion of milled Zr-ACP in the resins resulted in the improved BFS of the composites, even after aqueous soaking, and also gave a satisfactory ion release profile. The demonstrated improvement in the mechanical stability of anti-demineralizing/remineralizing ACP composites based on milled Zr-ACP filler may be beneficial in potentially extending their dental utility.

Amorphous Calcium Phosphate Based Composites: Effect of Surfactants and Poly(ethylene oxide) on Filler and Composite Properties.

The uncontrolled aggregation of amorphous calcium phosphate (ACP) particulate fillers and their uneven distribution within polymer matrices can have adverse effects on the properties of ACP composites. In this paper we assessed the influence of non-ionic and anionic surfactants and poly(ethylene oxide) (PEO) introduced during the preparation of ACP on the particle size distribution and compositional properties of ACP. In addition, the mechanical strength of polymeric composites utilizing such fillers with a photo-activated binary methacrylate resin was evaluated. Zirconia-hybridized ACP (Zr-ACP) filler and its corresponding composite served as controls for this study. Surfactant- and PEO-ACPs had an average water content of 16.8 % by mass. Introduction of the anionic surfactant reduced the median particle diameter about 45 % (4.1 µm vs. 7.4 µm for the Zr-ACP control). In the presence of PEO, however, the d(m) increased to 14.1 µm. There was no improvement in the biaxial flexure strength (BFS) in any of the dry composite specimens prepared with the surfactant- and/or PEO-ACPs compared to those formulated with Zr-ACP. The BFS of wet composite specimens decreased by 50 % or more after a month-long exposure to saline solutions. Other types of surfactants and/or polymers as well as alternative surface modification protocols need to be explored for their potential to provide better dispersion of ACP into the matrix resin and better mechanical performance ACP composites.

Effect of Chemical Structure and Composition of the Resin Phase on Vinyl Conversion of Amorphous Calcium Phosphate-filled Composites.

The objective of this study was to elucidate the effect of chemical structure and composition of the polymer matrix on the degree of vinyl conversion (DC) of copolymers (unfilled resins) and their amorphous calcium phosphate (ACP) composites attained upon photo-polymerization. The DC can also be an indicator of the relative potential of these polymeric materials to leach out into the oral environment un-reacted monomers that could adversely affect their biocompatibility. The following resins were examined: 1) 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenyl]propane (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) (1:1 mass ratio; BT resin) combined with hydroxyethyl methacrylate (HEMA; BTH resin) and with HEMA and zirconyl dimethacrylate (BTHZ resin), 2) urethane dimethacrylate (UDMA)/HEMA resins, and 3) pyromellitic glycerol dimethacrylate (PMGDMA)/TEGDMA (PT resin). To make composite specimens, resins were mixed with a mass fraction of 40 % zirconia-hybridized ACP. Copolymers and their composites were evaluated by near infra-red spectroscopy for DC after 1 d and 28 d post-cure at 23 °C. Inclusion of HEMA into the BT and UDMA resins yielded copolymers and composites with the highest DCs. The significantly lower DCs of PT copolymers and their composites are attributed to the rigid aromatic core structure, tetra-vinyl functionality and limited methacrylate side-chain flexibility of the surface-active PMGDMA monomer. There was, however, an increase in the 28 d DC for the PT materials as there was for the BTHZ system. Surprisingly, the usual decrease observed in DC in going from unfilled polymer to composite was reversed for the PT system.

Nano DCPA-whisker composites with high strength and Ca and PO(4) release.

The main challenges facing composite restorations are secondary caries and bulk fracture. The objective of this study was to develop nano DCPA (dicalcium phosphate anhydrous)-whisker composites with high strength and Ca and PO(4) ion release to combat caries. Flexural strength for the nano DCPA-whisker composites at a nano DCPA:whisker mass ratio of 1:2 ranged from (148 +/- 9) MPa to (167 +/- 23) MPa, significantly higher than the (103 +/- 32) MPa of an inlay/onlay commercial control composite without Ca-PO(4) release. The nano DCPA-whisker composite released PO(4) to a concentration of (1.95 +/- 0.13) mmol/L and Ca of (0.68 +/- 0.05) mmol/L. Compared with previous conventional Ca- and PO(4)-releasing composites, the nano DCPA-whisker composites had strengths two-fold higher, and released comparable or higher levels of Ca and PO(4). In conclusion, combining nano-DCPA with whiskers yielded novel composites that released high levels of Ca and PO(4) requisite for remineralization. These high-strength composites may provide a unique combination of stress-bearing and caries-inhibiting capabilities.

Amorphous calcium phosphate composites with improved mechanical properties.

Hybridized zirconium amorphous calcium phosphate (ACP)-filled methacrylate composites make good calcium and phosphate releasing materials for anti-demineralizing/remineralizing applications with low mechanical demands. The objective of this study was to assess the effect of the particle size of the filler on the mechanical properties of these composites. Photo-curable resins were formulated from ethoxylated bisphenol A dimethacrylate, triethylene glycol dimethacrylate, 2-hydroxyethyl methacrylate and methacryloxyethyl phthalate. Camphorquinone and ethyl-4-N,N-dimethylaminobenzoate were utilized as components of the photoinitiator system. After 2 h of mechanical milling in isopropanol, an approximate 64 % reduction in the median particle diameter was observed [27.48 µm vs. 9.98 µm] for unmilled and milled wet ACP, respectively. Dry ACP showed a 43 % reduction in particle size from pre- to post-milling. As well as dry composites, those that had been immersed in aqueous media were evaluated for their Young's Modulus, water sorption, biaxial tensile, three-point flexural and diametral tensile strength. Mechanically milling the filler increased the volume of fine particles in the composite specimens, resulting in a more homogeneous intra-composite distribution of ACP and a reduction in voids. In turn, less water diffused into the milled composites upon aqueous exposure, and they showed a marked improvement in biaxial flexure strength and a moderate improvement in flexural strength over composites with unmilled ACP. The demonstrated improvement in the mechanical stability of milled Zr-ACP composites may help in extending their dental applicability.