Photopolymerization kinetics of dimethacrylates using the camphorquinone/amine initiator system

Volume contraction in photocured dental resins: The shrinkage-conversion relationship revisited

Polymerization shrinkage and degree of conversion (DC) of resin composites are closely related manifestations of the same process. Ideal dental composite would show an optimal degree of conversion and minimal polymerization shrinkage. These seem to be antagonistic goals, as increased monomer conversion invariably leads to high polymerization shrinkage values.

OBJECTIVES

This paper aims at accurately determining the polymerization volume contraction of experimental neat resins and to link it to the number of actual vinyl double bonds converted in single ones instead of, as generally done, to the degree of conversion.

METHODS

Different mixtures of Bis-GMA/TEGDMA (traditionally used monomers) were analyzed. Contraction of the polymers was determined by pycnometry and the use of a density column. DC was determined by the use of Raman spectrometry.

RESULTS

An univocal relationship has been found between the volume contraction and the actual number of vinyl double bonds converted into single ones. A contraction value of 20.39 cm3/mole (of converted C=C) was deduced from 27 measurements.

SIGNIFICANCE

This relationship helps in finding solutions to the polymerization shrinkage problem. A reduction of the polymerization shrinkage due to the chemical reaction may obviously be expected from the addition of molecules allowing a decrease in the number of double bonds converted per unit volume of resin matrix, while maintaining the degree of conversion (of Bis-GMA and TEGDMA) and thus the mechanical properties. Further research will be directed at this objective.

Influence of light intensity on dentin bond strength of self-etch systems

The purpose of this study was to investigate the influence of light intensity on dentin bond strengths of four self-etch adhesive systems. The light intensities used to polymerize specimens were controlled at levels of 150, 300, 600, and 900 mW/cm2. The two-step self-etch adhesive systems Imperva Fluoro Bond and Mac Bond II, and the one-step self-etch systems Fluoro Bond Shake-One and One-Up Bond F Plus were used with their corresponding light-cured resins. Labial surfaces of lower bovine incisors were ground with #600 grit SiC paper to expose the dentin. The dentin surfaces were treated according to each manufacturer's instructions and bonded with resin composites. A shear bond strength test was performed and the data were analyzed by one-way ANOVA followed by Newman-Keuls multiple comparison at a level of 0.05. Statistical analysis of the data indicated that light intensity affected the dentin bond strengths of the adhesive systems tested. Significantly lower bond strengths were obtained by exposure to 150 mW/cm2, and there were no differences between the bond strengths obtained at 600 and 900 mW/cm2 for all the adhesive systems used. Further research will be required to clarify the irradiance-dependent properties of light-cured resin adhesive systems.

Influence of Delayed Placement of Composites Over Cured Adhesives on Dentin Bond Strength of Single-application Self-etch Systems

The delay in placement of composite over single-application self-etching adhesive systems was a crucial factor influencing dentin bond strength compared to a composite placed immediately after the polymerization of adhesives.

Effect of light dispersion of LED curing lights on resin composite polymerization

PURPOSE

This study evaluated the effect of light dispersion of halogen and LED curing lights on resin composite polymerization.

MATERIALS AND METHODS

One halogen (Optilux 501, SDS/Kerr, Orange, CA, USA) and five light-emitting diode (LED) curing lights (SmartLite iQ, Dentsply Caulk, Milford, DE, USA; LEDemetron 1, SDS/Kerr; FLASHlite 1001, Discus Dental, Culver City, CA, USA; UltraLume LED 5, Ultradent Products, South Jordan, UT, USA; Allegro, Den-Mat, Santa Maria, CA, USA) were used in this study. Specimens (8 mm diameter by 2 mm thick) were made in polytetrafluoroethylene molds using hybrid (Z100, 3M ESPE, St. Paul, MN, USA) and microfill (A110, 3M ESPE) composite resins. The top surface was polymerized for 5 seconds with the curing light guide tip positioned at a distance of 1 and 5 mm. Degree of conversion (DC) of the composite specimens was analyzed on the bottom surface using micro-Fourier Transform Infrared (FTIR) spectroscopy (Perkin-Elmer FTIR Spectrometer, Wellesley, PA, USA) 10 minutes after light activation. DC at the bottom of the 2 mm specimen was expressed as a percentage of the mean maximum DC. Five specimens were created per curing light and composite type (n=5). Percent mean DC ratios and SDs were calculated for each light under each testing condition. Data were analyzed by analysis of variance (ANOVA)/Tukey's test (alpha = .05). A beam analyzer (LBA-700, Spiricon, Logan, UT, USA) was used to record the emitted light from the curing lights at 0 and 5 mm distances (n=5). A Top Hat factor was used to compare the quality of the emitted beam profile (LBA/PC, Spiricon). The divergence angle from vertical was also determined in the x- and y-axes (LBA/PC). Mean values and SDs were calculated for each light under each testing condition (0 and 5 mm, x- and y-axes) and analyzed by a two-way ANOVA/Tukey's test (alpha = .05).

RESULTS

For DC ratios, significant differences were found based on curing light and curing distance (p < .05). At 1 mm, Optilux 501 and FLASHlite 1001 produced significantly higher DC ratios with the hybrid resin composite. No differences were found among lights with the microfill at 1 mm. At 5 mm, SmartLite iQ, FLASHlite 1001, LEDemetron 1, and UltraLume LED 5 produced significantly higher DC ratios with the hybrid resin composite, whereas LEDemetron 1 and SmartLite iQ produced significantly higher DC ratios with the microfill resin composite. The UltraLume LED 5, Allegro, and Optilux 501 had significant reductions in mean DC ratios at curing distances of 1 and 5 mm with both resin composite types. For dispersion of light, significant differences were found in Top Hat factor and divergence angle (p < .001). SmartLite iQ had overall the highest Top Hat factor and lowest divergence angle of tested lights. A linear regression analysis relating pooled DC with pooled Top Hat factors and divergence angles found a very good correlation (r2 = .86) between dispersion of light over distance and the ability to polymerize resin composite.

CLINICAL SIGNIFICANCE

The latest generation of LED curing lights provides DC ratios similar to or better than the halogen curing light at a curing distance of 5 mm. Dispersion of light plays a significant role in the DC of resin composite. To maximize curing effectiveness, light guides should be maintained in close proximity to the surface of the light-activated restorative material.

Effect of metal conditioners on polymerization behavior of bonding agents

This study aimed to determine the influence of metal conditioners on the polymerization behavior of bonding agents. Bonding agents of two-step self-etching primer systems and metal conditioners for adhesion of dental metal alloys were used. Double bond conversion was determined by Fourier transform-ation infrared spectroscopy. The percentage of residual double bonds, including pendant and monomeric double bonds, was calculated by comparing the obtained ratio with that of the uncured bonding agent. The degree of conversion of the bonding agents was obtained by subtracting the remaining double bonds from 100%. ANOVA followed by a Tukey HDS test was performed. Degree of conversion of the bonding agents ranged from 86.0 to 87.8%. When the bonding agents were mixed with metal conditioners or solvents of the metal conditioners, double bond conversion of the bonding agents tended to decrease. Within the limitations of this study, which was far removed from clinical situations, the presence of metal conditioners and remaining solvents had adverse effects on the polymerization reaction of bonding agents. Clinicians should be cognizant of the various factors that can influence bond strength of restorative resins to dentin.

Polymerization contraction of light-cured composite resins containing silica/polymethylmethacrylate bonded microstructured networks

The adsorption of methylmethacrylate polymer at silica/methylmethacrylate interfaces was determined to provide microstructured networks whose structural characteristics were determined to be controlled by the amount of polymer initially supplied to the system. First, the microstructure was investigated by determining as a function of the amount of polymer (i) the shrinking rate due to evaporation of the methylmethacrylate monomer, (ii) the rate of sedimentation of the silica/polymer complexes in the methylmethacrylate monomer, and (iii) the height of the sediment in the long term. These different characteristics were found to be strongly correlated. Second, the sedimentation characteristics were determined as a function of the amount of polymer initially supplied to the dispersion of the same silica/polymer system in the ethylene glycol dimethacrylate monomer. Then the rate of the polymerization contraction during light-curing of the resin was determined for the sediment recovered after centrifugation. The slowest polymerization contraction and the smallest contraction were obtained with the filler/polymer/resin system composed of aggregates of medium porosity and size.

Polymerization contraction of resin composite vs. energy and power density of light-cure

This study measured the polymerization contraction of a resin composite cured at three levels of energy density, each attained at six different levels of power density. The polymerization contraction of the composite was recorded by the method of the deflecting disc (n = 5) for 1 h following the start of irradiation. Power densities of 50, 100, 200, 400, 800 and 1,000 mW cm(-2), as measured on a dental radiometer, were obtained by variation of distance and supply voltage of a commercial light-curing unit. The spectral distribution at each power density was recorded using a spectrophotometer. The absorption spectrum of camphorquinone was also recorded, and the efficiency of the radiation at each power density was calculated as the integral over wavelength of the product of absorption and emission. From the slope of the contraction curves, an approximation to the initial rate of polymerization, Rp, was calculated and was taken as an alternative measure of power density. Statistical analyses showed that polymerization contraction increased significantly with increasing levels of energy density received by the resin composite, and, for each level of energy density, that the contraction decreased significantly with increasing power density.

Molar extinction coefficients and the photon absorption efficiency of dental photoinitiators and light curing units

OBJECTIVES

The light absorption of dental photoinitiators should correlate with the spectral emission profiles of dental light curing units compared on an equivalent basis. Spectral data of dental photoinitiators and light curing units can be used to define the photon absorption efficiency (PAE) obtained by integrating the product of the absorption and emission spectra in terms of photons. This parameter can be used to identify the best performance for photochemical process with specific photoinitiators.

METHODS

The efficiency of two LED and one QTH lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). Absorption and emission spectra of the photoinitiators and the LED (Ultrablue I and Ultrablue IS) and QTH (Optilux 401) LCUs were determined in the 360-550nm range.

RESULTS

CQ exhibited an absorption centered in the blue region and, although the maxima of PPD, MAPO, and BAPO were in the UV-A region, their absorption extended to the visible region. Power output maxima of the LCUs were at 467 (Ultrablue I), 454 (Ultrablue IS) and 493nm (Optilux 401), and the total power densities were 170+/-1, 470+/-4 and 444+/-4mW/cm(2), respectively.

SIGNIFICANCE

The use of the PAE allows a prediction of the most efficient photoinitiator/LCU systems. For similar photoinitiator concentrations, Lucirin and CQ are most efficiently photoinitiated by the QTH unit, whereas the high-power LED device is more efficient for Irgacure. PPD is photoactivated similarly by both LCUs.

Shear bond strength of experimental methacrylated beta-cyclodextrin-based formulations

Previous studies have shown that methacrylated beta-cyclodextrins (MCDs) can be used as comonomers in resin-based dental composites. These MCDs by virtue of having several polymerizable methacrylate groups and hydrophilic hydroxyl groups, may also promote bonding of dental composites to dentin. This study evaluated MCDs as adhesive comonomers, and optimized comonomer and polymerization initiator concentrations for maximum shear bond strength (SBS). Experimental MCD-based bonding formulations in acetone were prepared by mixing 33 mass fraction % MCDs with (10, 20, 30, 40, or 50) mass fraction % of 2-hydroxyethyl methacrylate (HEMA). The MCD/HEMA-based solutions were activated with varied amounts of camphorquinone (CQ) and ethyl 4-dimethylamino benzoate (4E). Samples for SBS were prepared by bonding a composite resin to acid-etched dentin surfaces of extracted human molars with the experimental bonding solutions. The specimens were immersed in 37 degrees C water for 24 h and bond strengths were determined in shear mode. With increasing HEMA concentration, the SBS values of MCD-bonding solutions increased to 16 MPa at a composition of 33% MCD, 30% HEMA, and 37% acetone by mass. Also, SBS values of MCD-bonding solutions varied as a function of the CQ and 4E concentrations and passed through a maximum SBS at 21 MPa, which was comparable to that of a commercial control. This preliminary study indicated that nonacidic MCD monomers could be used as an adhesion-promoting comonomer. Additional modification of MCDs having both polymerizable groups and anionic ligand groups, e.g., polymerizable acidic cyclodextrin derivatives should increase the SBS even further.

Development of highly reactive mono-(meth)acrylates as reactive diluents for dimethacrylate-based dental resin systems

Reactive diluents such as triethyleneglycol-dimethacrylate (TEGDMA) have been widely used with bisphenol-A-glycidyl-dimethacrylate (Bis-GMA) to achieve restorative resins with appropriate viscosity and higher conversion. However, additional water sorption and polymerization shrinkage were also introduced. The aim of this work is to investigate whether the cure and material properties can be improved in dental resins containing novel mono-(meth)acrylates as reactive diluents so that these Bis-GMA-based copolymers have reduced polymerization shrinkage but higher overall double bond conversion. Several ultra-high-reactivity mono-(meth)acrylates that contain secondary functionalities have been synthesized and investigated. The polymerization rate and double bond conversion were monitored using photo-FTIR. Polymerization shrinkage, dynamic mechanical analysis, and flexural strength were characterized. Compared with the Bis-GMA/TEGDMA control, the Bis-GMA/mono-methacrylate systems studied showed higher final conversions, faster curing rates, and decreased polymerization shrinkage. Our optimum system Bis-GMA/morpholine carbamate methacrylate achieved 86% final conversion (vs. 65%), a polymerization rate 3.5 times faster, and a 30% reduction in polymerization volumetric shrinkage. These results indicate that certain highly reactive, novel mono-(meth)acrylates possess very promising potential to replace TEGDMA as reactive diluents and can readily be applied to develop superior dental resins.

Real-time dimensional change in light-cured composites at various depths using laser speckle contrast analysis

Laser speckle contrast analysis is an interferometric technique that is used to measure the displacement of the rough surface of a specimen. The purpose of this study was to present a laser speckle correlation method for monitoring real-time dimensional changes of light-cured composites. Uncured composite was condensed into a glass tube and irradiated for 30 s with 600 or 200 mW cm(-2). The speckle patterns obtained from lateral and bottom composite surfaces were monitored using a speckle analyser. The speckle field is recorded in a digital frame and stored by image processing system as the carrier of information on the displacement of the tested surface. The calculated values were obtained for each pair of adjacent patterns and the changes in speckle contrast as a function time were obtained from five repeated measurements. The overall magnitude of the speckle contrasts decreased soon after the initial light exposure and gradually increased thereafter. The speckle contrasts obtained from the bottom surface were smaller than those obtained from the lateral surface. This tendency was more pronounced when the specimen was irradiated with lower power density. It can be concluded that monitoring differential shrinkage at various levels of depth can be achieved with this new technique.

Low-shrinkage dental restorative composite: Modeling viscoelastic behavior during setting

Much attention has been directed toward developing dental direct restorative composites that generate less shrinkage stress during setting. The aim of this study was to explore the viscoelastic behavior of a new class of low-shrinkage dental restorative composite during setting. The setting behavior of an experimental oxirane composite has been investigated by analyzing stress-strain data with two-parametric mechanical models. Experimental data were obtained from a dynamic test method, in which the setting light-activated composite was continuously subjected to sinusoidal strain cycles. The material parameters and the model's predictive capacity were analyzed with validated modeling procedures. The light-activated oxirane composite exhibited shrinkage delay and low polymerization shrinkage strain and stresses when compared with conventional light-activated composite. Noise in the stress data restricted the predictive ability of the Maxwell model to the elastic modulus development of the composite only. Evaluation tests of their potential as restorative material are required, to examine if the biocompatibility and mechanical properties after setting of oxirane composites are acceptable for dental use.

Influence of light-curing procedures and photo-initiator/co-initiator composition on the degree of conversion of light-curing resins

OBJECTIVE

The hypothesis that the degree and rate of conversion can be modified favourably by using different light-curing procedures and different photo initiator/co-initiator combinations was tested.

METHOD

A photo-initiator (0.02 mM/g resin); either camphorquinone (CQ) or 1-phenyl-1,2-propanedione (PPD), was mixed with bisGMA:TEGDMA (50:50 by weight). In addition, a co-initiator (0.04 mM/g resin); either N,N-dimethyl-p-aminobenzoic acid ethylester (DABE), N,N-cyanoethylmethylaniline (CEMA), or 2-dimethylaminoethyl methacrylate (DMAEMA), was added. These six combinations were subjected to three curing conditions (standard curing, soft-start curing or LED curing). The conversion levels (DC) were determined with differential scanning calorimetry (DSC). The DSC results were analysed using a general linear model (GLM) and Duncan's multiple range test and regular t-test.

RESULTS

The fastest conversion initially was obtained by standard curing, followed by LED curing and soft-start curing. After 40 s of curing, conventional curing and soft-start curing produced a higher DC than LED curing. However, strong interactions occurred between the different variables (curing method, initiator and co-initiator). Initially, CQ was more efficient than PPD, but after 40 s, this difference was insignificant.

CONCLUSION

By using soft-start curing and an appropriate photo initiator/co-initiator combination it is possible to achieve slow curing and a high DC at within a curing time of 40 s.

[Light induced polymerization of resin composite restorative materials]

INTRODUCTION

Dimensional stability of polymer-based dental materials is compromised by polymerization reaction of the monomer. The conversion into a polymer is accompanied by a closer packing of molecules, which leads to volume reduction called curing contraction or polymerization shrinkage. Curing contraction may break the adhesion between the adhesive system and hard tooth tissues forming microgaps which may result in marginal deterioration, recurrent caries and pulp injury. POLYMERIZATION SHRINKAGE OF RESIN-BASED RESTORATIVE DENTAL MATERIALS: Polymerization of the organic phase (nonomer molecules) of resin-based dental materials causes shrinkage. The space occupied by filler particles is not associated with polymerization shrinkage. However, high filler loading within certain limits, can contribute to a lesser curing contraction.

POLYMERIZATION SHRINKAGE STRESS AND STRESS REDUCTION POSSIBILITIES

Polymerization shrinkage stress of polymer-based dental resins can be controlled in various ways. The adhesive bond in tooth-restoration interface guides the contraction forces to cavity walls. If leakage occurs, complications like secondary caries and pulpal irritation may jeopardize the longevity of a restoration. Stress relieve can be obtained by modifications of the monomer and photoinitiator, or by specially designed tooth preparation and application of bases and liners of low modulus of elasticity. The polymerization contraction can be compensated by water absorption due to oral cavity surrounding. The newest approach to stress relief is based on modulation of polymerization initiation.

CONCLUSION

This work deals with polymerization contraction and how to achieve leak-proof restoration. Restorative techniques that may reduce the negative effect of polymerization shrinkage stress need further research in order to confirm up-to-date findings.

The efficiency of titanocene as photoinitiator in the polymerization of dental formulations

A comparative kinetic study of the polymerization of tetraethyleneglycol dimethacrylate (TEEGDM) under visible light irradiation was carried out in order to determine the effectiveness of titanocene as photoinitiator in this process. The pair camphorquinone/dimethylaminoethyl methacrylate, CQ/DMAEMA (0.5/0.5%, by weight), was used as a reference photoinitiatior system. The reactions were carried out both in the presence and absence of oxygen (in N2) and with variable and constant titanocene concentration and incident light intensity. It was observed that in both atmospheres the polymerization reaction commenced rapidly and its initial rate was comparable with that obtained in the presence of the classic initiation system. However, after reacting for a few seconds this process suffered abrupt deactivation, this tendency being more noticeable at low titanocene concentrations. To explain the observed kinetic behavior, the study of the photofragmentation of titanocene upon irradiation with visible light in both atmospheres was carried out. The quantum yield calculated in the presence of O2 (phifr = 8 at 465 nm) suggests the existence of a chain reaction with participation of oxygen, and this explains, in turn, the high value of the initial rate of polymerization and also the rapid disappearance of the photoinitiator. In an inert atmosphere the photofragmentation occurs in a conventional way, this being a very effective process with quantum yield of 0.7 at 465 nm. However, it seems that only a small part of the active species acts as initiating radicals and these are not sufficient to achieve the appropriate monomer conversion.

Water sorption characteristics of light-cured dental resins and composites based on Bis-EMA/PCDMA

The water uptake characteristics of resins and composites based on an ethoxylated bisphenol A glycol dimethacrylate (Bis-EMA) and a polycarbonate dimethacrylate (PCDMA) were studied in detail. Polydimethacrylate resins were prepared by photopolymerization of the neat monomers and mixtures of them with various weight ratios, using the camphoroquinone/N,N-dimethylaminoethyl methacrylate system as initiator, while the composites were prepared from the light-curing of commercial samples (Sculpt-It and Alert). Water sorption/desorption was examined both in equilibrium and dynamic conditions in two adjacent sorption-desorption cycles. The equilibrium water uptake from all resins was very small with a trend to increase as the amount of PCDMA was increased. The inverse effect was observed in the solubility values. The composites studied exhibited also very low water uptake values in comparison to other composite materials reported in the literature. It was also observed that the equilibrium uptake decreased with increasing filler loading. Slightly larger equilibrium water uptake and much smaller solubility values were obtained during the second sorption-desorption cycle in comparison to the first one. Concerning the sorption rate data, it was observed that the resin materials followed Fickian diffusion during almost the whole sorption or desorption curve, while the composites showed this behavior until only M(t)/M( infinity ) congruent with 0.5. The diffusion coefficients calculated for the resins were larger than those of the composites and always higher during desorption compared to sorption. The values of the diffusion coefficients for both resins and composites were in the same order of magnitude with the values of the corresponding materials reported in the literature.

A method to protect sensitive molecules from a light-induced polymerizing environment

Systems that can be polymerized in situ upon exposure to light radiation may have significant applications in tissue engineering and drug delivery. However, the light-induced polymerization step, which is the requisite for this technology, could be potentially deleterious to sensitive bioactive agents (e.g., enzymes, cytokines, matrix metalloproteinases) being entrapped. In this study, a method to protect sensitive molecules from a light-induced polymerizing environment is proposed. This method is based on the idea that nonaccessible substances cannot interact with the polymerizing species. To examine this concept, two model enzymes-namely, horseradish peroxidase and alpha-glucosidase-were protected by gelatin-based wet granulation and incorporated within a cured polyethylene glycol dimethacrylate, a photocurable monomer, under different conditions. Unprotected enzymes were used as controls. Enzymes were then allowed to diffuse out of the polymerized matrices. The activity and total enzyme recovered from these matrices by passive diffusion were compared to ascertain the extent of activity retention. Matrix assisted laser desorption ionization mass spectrometry combined with time of flight mass spectrometry (MALDI-TOF) was used to determine changes in enzyme molecular weight. During the first 24 h of diffusion from the polymerized matrices, unprotected enzymes consistently showed a loss of activity ranging from 10-66%, depending on the matrix composition and enzyme properties. In contrast, protected enzymes retained over 94% of their activity irrespective of the experimental setting. The loss of activity appears to be a direct consequence of the polymerizing environment.

An investigation of the cytotoxicity and histocompatibility of in situ forming lactic acid based orthopedic biomaterials

The cytotoxicity and biocompatibility of polymer networks prefabricated from multifunctional lactic acid based oligomers that are being developed for orthopedic applications were assessed through in vitro cytotoxicity analysis and subcutaneous implantation. After 7 and 14 days, no significant difference was observed in the relative viability or alkaline phosphatase activity of primary rat calvarial osteoblasts cultured in the presence or absence of degrading polymer networks, indicating that the degradation products had no detrimental effect on the function or activity of the cultured cells. The tissue response to preformed lactic acid networks implanted in rats consisted of a mild inflammatory response with an increase in fibrous capsule thickness and inflammation correlating with faster degrading polymer compositions. This relatively neutral response is indicative of a biocompatible, degradable polymer that has potential medical applications. Finally, porous scaffolds were implanted subcutaneously in rats, and vascularized fibrous tissue infiltration was highly dependent on the scaffold porosity and architecture. This finding indicates that an in situ forming porous scaffold of this composition may support the infiltration of surrounding vascularized tissue, and thus be applicable to orthopedic treatments of large bone defects.

Liquid acrylate-endcapped biodegradable poly(epsilon-caprolactone-co-trimethylene carbonate). I. Preparation and visible light-induced photocuring characteristics

Photocurable liquid biodegradable copolymers were prepared by ring-opening copolymerization of epsilon-caprolactone (CL) and trimethylene carbonate (TMC) in the presence of a multifunctional hydroxyl group-bearing substance (di-, tri-, and tetra-functional alcohol and poly(ethylene glycol) (PEG) and its four-branched derivative) as an initiator and subsequent endcapping with acryloyl chloride at their hydroxyl terminals. These multifunctional, viscous liquid copolymers (molecular weights; approximately 2 x 10(3) to 7 x 10(3) g/mol) were converted to crosslinked solids by visible-light irradiation in the presence of camphorquinone as an initiator. The photocuring rate of these copolymers was enhanced by both higher functionality and lower molecule weight of the copolymers used. The photocuring rate depended on the amount of reducing agent (methacrylic acid 2-dimethylaminoethyl ester). Upon immersion in a phosphate buffer solution (pH 7.4), hydrolysis occurred preferentially on the surface except for photocured PEG-based copolymers that were degraded faster via both surface erosion and bulk degradation than low molecular weight alcohols-based copolymers. Cylindrical photocured constructs prepared by photoirradiation to the whole body in a mold filled with the liquid copolymer was demonstrated as an example of shape fabrication of biodegradable biomedical devices.

Energy dependent polymerization of resin-based composite

OBJECTIVE

This study explores the relationship between the extent of polymerization and the radiant energy (dose) applied during the photopolymerization of resin-based composites.

METHOD

FTIR was used to measure the 5-min and 24-h conversion of four resin-based composites prepared in a thin film and polymerized under conditions of decreasing intensity and a constant exposure time (30s) using a tungsten halogen curing light. The measured conversion was obtained over a wide range of applied radiant energy. Additionally, samples for two of the materials were polymerized at various intensities and exposure times such that the dose remained constant. This process was performed at four dose levels representing approximately 75% of the conversion range.

RESULTS

The curing profiles (percent conversion versus applied radiant energy) depict a gradual decrease in conversion with decreasing energy followed by a rapid descent. Though there are differences in the maximum conversion attained between the materials, when conversion is represented as a fractional conversion relative to the maximum 24-h value, their 5-min and 24-h curing profiles appear quite similar. Additionally, very similar conversion was measured when the films were exposed using equivalent doses providing evidence for a reciprocal relationship between irradiance (power density) and exposure time. For the 24-h measurements, statistical equivalence (Fishers protected LSD at the 0.05 level) was noted for most of the combinations of exposure time and power density within a given dose. Generally, the exceptions occurred with the shortest exposure times.

SIGNIFICANCE

A reciprocal relationship between exposure time and power density adds significance to the study of conversion as a function of the total applied dose. This relationship establishes the curing profile as a universal correlation between exposure time and power density.

In situ forming lactic acid based orthopaedic biomaterials: influence of oligomer chemistry on osteoblast attachment and function

The ability of osteoblasts to attach and function normally on scaffolds fabricated from synthetic materials is essential for musculoskeletal tissue engineering applications. In this study, the osteoconductivity of polymer networks formed from multifunctional lactic acid oligomers was assessed. These oligomers form highly crosslinked networks via a photoinitiated polymerization, which provides potential advantages for many orthopaedic applications. Depending on the initial oligomer chemistry and the resultant polymer hydrophobicity, protein adsorption and osteoblast function varied significantly between the various lactic acid based polymer chemistries. Results were compared to control polymers of tissue culture polystyrene (TCPS) and 50:50 poly(lactic-co-glycolic acid) (PLGA). The viability of osteoblasts attached to poly(2EG10LA) and poly(2EG6LA) was close to the TCPS and PLGA after 7 and 14 days of culture, whereas cell viability was approximately 50% lower on poly(8EG6LA). Additionally, the alkaline phosphatase activity and mineralization of attached osteoblasts were similar on poly(2EG10LA) and PLGA, whereas these markers of bone formation were significantly lower for poly(2EG6LA) and poly(8EG6LA). For example, the alkaline phosphatase activity of rat calvarial osteoblasts attached to poly(2EG10LA) was 0.048 +/- 0.006 micromol mg(-1) protein-min, but only 0.030 +/- 0.003 micromol mg(-1) protein-min for osteoblasts attached to poly(8EG6LA) after 14 days of culture. Finally, osteoblasts were seeded onto three-dimensional scaffolds to demonstrate the applicability of the scaffolds for bone tissue engineering.

Effect of chemical structure on degree of conversion in light-cured dimethacrylate-based dental resins

In this work the room-temperature photopolymerization of Bis-GMA, Bis-EMA, urethane dimethacrylate (UDMA) and triethylene glycol dimethacrylate (TEGDMA) induced by camphoroquinone/N,N-dimethylaminoethyl methacrylate, as photo-initiator system, was followed by FT-IR. The results obtained were then fitted by a non-linear least square method to a rational function, which permitted the accurate calculation of the limiting degree of conversion. The latter was found to increase in the order Bis-GMA < Bis-EMA < UDMA < TEGDMA. This trend is discussed in connection with the chemical structure of dimethacrylates. The photopolymerization of mixtures of Bis-GMA/TEGDMA, Bis-GMA/UDMA and Bis-GMA/Bis-EMA showed a good linear relationship of degree of conversion with the mole fraction of Bis-GMA and in the case of the first pair also with the Tg of the initial monomer mixture.

Synthesis and photopolymerization of N,N'-dimethyl,-N,N'-di(methacryloxy ethyl)-1,6-hexanediamine as a polymerizable amine coinitiator for dental restorations

N,N'-dimethyl,-N,N'-di(methacryloxy ethyl)-1,6-hexanediamine (NDMH) was synthesized for the purpose of replacing both triethylene glycol dimethacrylate (TEGDMA) and the non-polymerizable amine which is added as a coinitiator in dental resin mixtures, 2,2-bis[4(2-hydroxy-3-methacryloxypropoxy)phenyl] propane (bis-GMA), camphorquinone (CQ) and ethyl-4-dimethylaminobenzoate (EDAB) were used as monomer, photoinitiator and coinitiator, respectively, in these model dental resin systems. Mixtures of bis-GMA/TEGDMA/CQ/EDAB and bis-GMA/TEGDMA/CQ/NDMH were found to reach final conversions of about 45%, slightly higher than his-GMA/NDMH/CQ (40%) under comparable visible light irradiation conditions. In addition, samples cured to these conversions were tested with dynamic mechanical analysis. The bis-GMA/TEGDMA/CQ/EDAB, his-GMA/TEGDMA/CQ/NDMH and bis-GMA/NDMH/CQ mixtures were found to have approximately the same glass transition temperature and modulus. Finally, the water sorption and solubility of bis-GMA/NDMH/CQ were higher than those of the bis-GMA/TEGDMA/CQ/EDAB, and bis-GMA/TEGDMA/CQ/NDMH. However, the values were still within the range of the ISO 9000's standards. These results suggest that NDMH is a viable alternative to conventional photocuring dental resins, serving both as a diluent and coinitiator, since there are no large differences in physical and mechanical properties when using NDMH to replace the amine coinitiator and TEGDMA diluent. The key advantage to this system is that the dimethacrylate NDMH can copolymerize with bis-GMA and TEGDMA, limiting the amount of extractable amine.

The production of reactive oxygen species by irradiated camphorquinone-related photosensitizers and their effect on cytotoxicity

Camphorquinone (CQ) is widely used as an initiator in modern light-cured resin systems but there are few reports about its effects on living cells. To clarify the mechanism of photosensitizer-induced cytotoxicity, the production of initiator radicals and subsequent reactive oxygen species (ROS) by CQ, benzil (BZ), benzophenone (BP), 9-fluorenone (9-F) in the presence of the reducing agent (2-dimethylaminoethyl methacrylate or N,N-dimethyl-p-toluidine, DMT) with visible-light irradiation was examined in a cell or cell-free system. Initiator radical production was estimated by the reduction rate of 1,1-diphenyl-2-picrylhydrazyl and by the conversion of poly-triethyleneglycol dimethacrylate; the results indicated that CQ/DMT had the highest activity among them. The cytotoxic effects of the photosensitizers on both human submandibular gland (HSG) adenocarcinoma cell line and primary human gingival fibroblast (HGF) showed that the 50% toxic concentration (TC(50)) declined in the order: CQ>BP>9-F>BZ. ROS produced in HSG or HGF cells by elicited, irradiated photosensitizers were evaluated in two different assays, one using adherent cell analysis and sorting cytometry against adherent cells and the other, flow cytometry against floating cells, with fluorescent probes. ROS production was dose- and time- dependent, and declined in the order: BZ>9-F>BP>CQ. Cytotoxic activity was correlated with the amount of ROS. Cytotoxicity and ROS generation in HGF cells was significantly lower than in HSG cells. ROS induced by aliphatic ketones (CQ) were efficiently scavenged by hydroquinone and vitamin E, whereas those by aromatic ketones (9-F) were diminished by mannitol and catalase, suggesting that OH radicals were involved in ROS derived from 9-F. A possible link between the cytotoxic activity and ROS is suggested.

Synthesis and characterization of N-isopropyl, N-methacryloxyethyl methacrylamide as a possible dental resin

In this study, N-isopropyl, N-methacryloxyethyl methacrylamide (NIMM) is proposed as a possible reactive diluent in place of triethylene glycol dimethacrylate (TEGDMA) for dental resin mixtures. Real-time infrared spectroscopy was used to monitor the double-bond conversion as a function of irradiation time, and mixtures of 50/50wt% bis-GMA/NIMM were found to reach final conversions (95%) that were 1.5 times greater than bis-GMA/TEGDMA (65%) under visible light irradiation. In addition, samples cured to these conversions were tested with dynamic mechanical analysis. The bis-GMA/NIMM mixture (100% converted) was found to have a higher glass transition temperature and modulus at body temperature than a comparable bis-GMA/TEGDMA mixture (60% converted). Finally, the water sorption and solubility of bis-GMA/NIMM were determined to be higher than the bisGMA/TEGDMA comparison, but the values were still within the range of the ISO 9000s standard. These results suggest that bis-GMA/NIMM mixtures are a viable alternative to conventional dental resins since a greater degree of monomer conversion is obtainable without sacrificing physical and mechanical properties.

Developments in Shrinkage Control of Adhesive Restoratives

PURPOSE

This article reviews material properties and application techniques important in minimizing effects of polymerization shrinkage during the curing reaction of resin composite restorative materials used in adhesive dentistry.

MATERIALS AND METHODS

Relevant scientific publications were critically reviewed.

RESULTS

Since it was recognized that shrinkage, which takes place during the curing reaction of resin composite restorative materials, may cause severe problems in adhesive dentistry, considerable effort has been put into reducing the negative effects. The most important problem is the debonding of the restoration-tooth interface, resulting in increased microleakage and, ultimately, in secondary caries. Despite all efforts, there is still no material or general application method that guarantees a leak-proof and durable restoration.

CLINICAL SIGNIFICANCE

It is of the utmost importance that dental practitioners know how to deal with the problems related to resin composite shrinkage, so that they can choose the material and procedure most likely to produce a leak-proof and durable restoration, maximizing the potential for clinical success.

Curing Dental Resins and Composites by Photopolymerization

The development and continued evolution of photopolymerizable dental materials, particularly dental composite restoratives, represent a significant, practical advance for dentistry. The highly successful integration of the light-activated curing process for dental applications is described in this review. The basic mechanisms by which the photoinitiators efficiently convert monomers into polymers are discussed along with the variety of factors that influence the photopolymerization process. The conventional camphorquinone-amine visible light photoinitiator system used in most dental restorative materials is illustrated in addition to some alternative initiator systems that have been studied for dental materials applications.

CLINICAL SIGNIFICANCE

Photopolymerization has become an integral component of the practice of dentistry. A better appreciation of the photopolymerization process as well as its potential and limitations may aid the dentist in the delivery of both esthetic and restorative dental care.