Dental Restorative Materials Based on Thiol-Michael Photopolymerization.

Step-growth thiol-Michael photopolymerizable resins, constituting an alternative chemistry to the current methacrylate-based chain-growth polymerizations, were developed and evaluated for use as dental restorative materials. The beneficial features inherent to anion-mediated thiol-Michael polymerizations were explored, such as rapid photocuring, low stress generation, ester content tunability, and improved mechanical performance in a moist environment. An ester-free tetrafunctional thiol and a ultraviolet-sensitive photobase generator were implemented to facilitate thiol-Michael photopolymerization. Thiol-Michael resins of varied ester content were fabricated under suitable light activation. Polymerization kinetics and shrinkage stress were determined with Fourier-transform infrared spectroscopy coupled with tensometery measurements. Thermomechanical properties of new materials were evaluated by dynamic mechanical analysis and in 3-point bending stress-strain experiments. Photopolymerization kinetics, polymerization shrinkage stress, glass transition temperature, flexural modulus, flexural toughness, and water sorption/solubility were compared between different thiol-Michael systems and the BisGMA/TEGDMA control. Furthermore, the mechanical performance of 2 thiol-Michael composites and a control composite were compared before and after extensive conditioning in water. All photobase-catalyzed thiol-Michael polymerization matrices achieved >90% conversion with a dramatic reduction in shrinkage stress as compared with the unfilled dimethacrylate control. One prototype of ester-free thiol-Michael formulations had significantly better water uptake properties than the BisGMA/TEGDMA control system. Although exhibiting relatively lower Young's modulus and glass transition temperatures, highly uniform thiol-Michael materials achieved much higher toughness than the BisGMA/TEGDMA control. Moreover, low-ester thiol-Michael composite systems show stable mechanical performance even after extensive water treatment. Although further resin/curing methodology optimization is required, the photopolymerized thiol-Michael prototype resins can now be recognized as promising candidates for implementation in composite dental restorative materials.

Ruthenium photoredox-triggered phospholipid membrane formation.

As more methodologies for generating and manipulating biomimetic cellular systems are developed, opportunities arise for combining different methods to create more complex synthetic biological constructs. This necessitates an increasing need for tools to selectively trigger individual methodologies. Here we demonstrate ruthenium tris-bipyridine mediated photoredox triggering of the copper catalyzed alkyne azide cycloaddition reaction (CuAAC), resulting in the synthesis of biomimetic phospholipids in situ, and subsequent membrane assembly. The use of a ruthenium-copper electron transport chain to trigger phospholipid assembly opens up future opportunities for spatiotemporal synthesis of membranes.

Multiple shape memory polymers based on laminates formed from thiol-click chemistry based polymerizations.

This investigation details the formation of polymer network trilayer laminates formed by thiol-X click chemistries, and their subsequent implementation and evaluation for quadruple shape memory behavior. Thiol-Michael addition and thiol-isocyanate-based crosslinking reactions were employed to fabricate each of the laminate's layers with independent control of the chemistry and properties of each layer and outstanding interlayer adhesion and stability. The characteristic features of step-growth thiol-X reactions, such as excellent network uniformity and narrow thermal transitions as well as their stoichiometric nature, enabled fabrication of trilayer laminates with three distinctly different glass transition temperatures grouped within a narrow range of 100 °C. Through variations in the layer thicknesses, a step-wise modulus drop as a function of temperature was achieved. This behavior allowed multi-step programming and the demonstration and quantification of quadruple shape memory performance. As is critical for this performance, the interface connecting the layers was evaluated in stoichiometric as well as off-stoichiometric systems. It was shown that the laminated structures exhibit strong interfacial binding and hardly suffer any delamination during cyclic material testing and deformation.

Recent advances and developments in composite dental restorative materials.

Composite dental restorations represent a unique class of biomaterials with severe restrictions on biocompatibility, curing behavior, esthetics, and ultimate material properties. These materials are presently limited by shrinkage and polymerization-induced shrinkage stress, limited toughness, the presence of unreacted monomer that remains following the polymerization, and several other factors. Fortunately, these materials have been the focus of a great deal of research in recent years with the goal of improving restoration performance by changing the initiation system, monomers, and fillers and their coupling agents, and by developing novel polymerization strategies. Here, we review the general characteristics of the polymerization reaction and recent approaches that have been taken to improve composite restorative performance.

Impact of curing protocol on conversion and shrinkage stress.

Since considerable shrinkage stress develops during the curing of dental composites, various soft-start photocuring protocols, aiming to lower stress but not compromise conversion, have been proposed. We hypothesized that utilizing soft-start photocuring will result in not only reduced stress, but also decreased conversion. We evaluated the impact of 3 protocols (soft-start, pulse, and standard) on the stress development and polymerization extent of an experimental composite. A novel set-up capable of simultaneous shrinkage stress, conversion, and temperature measurements on the same specimen was utilized. Analysis of the data shows that stress rises dramatically as a function of conversion in the vitrified state, and the utilization of soft-start or pulse curing results in specimens with reduced final conversion and shrinkage stress, compared with specimens cured according to the standard full-intensity protocol. Finally, this study demonstrates that the predominant reason for the reduced shrinkage stress attained with soft-start or pulse curing is a modest decrease in final conversion.

Probing the origins and control of shrinkage stress in dental resin-composites: I. Shrinkage stress characterization technique.

The accurate and reliable characterization of the polymerization shrinkage stress is becoming increasingly important, as the shrinkage stress still is a major drawback of current dimethacrylate-based dental materials and restricts its range of applications. The purpose of this research is to develop a novel shrinkage stress measurement device to elucidate the shrinkage stress evolution of dental restorative composites while allowing for controlled sample deformation during the polymerization. Furthermore, the device is designed to mimic the clinically relevant cusp-to-cusp displacement by systematically adjusting the instrument compliance, the bonded surface area/unbonded area by sample geometry, and the total bonded area by sample diameter. The stress measurement device based on the cantilever beam deflection theory has been successfully developed and characterized using a commercial dental composite. It was shown that this device is a highly effective, practical and reliable shrinkage stress measurement tool, which enables its facile applications to the investigation of shrinkage stress kinetics of both commercial and experimental composites, as well as for probing various aspects that dictate shrinkage stress development.

The effect of cure rate on the mechanical properties of dental resins.

OBJECTIVE: This study investigates the effect of cure rate on the mechanical properties of a common dimethacrylate dental resin formulation (75/25 wt% bis-GMA/TEGDMA). METHODS: The polymerization rate and final conversion of the exact specimens subsequently used for mechanical testing were monitored by near-infrared (near-IR) spectroscopy. The glass transition temperature (T(g)) and modulus, as a function of temperature, were determined by dynamic mechanical analysis (DMA). Iniferter initiating systems were used to create partially cured networks that did not contain any trapped radicals. By the elimination of trapped radicals from the system, the formed networks can be characterized as a function of both temperature and double bond conversion without inducing additional thermal cure during testing. RESULTS: Copolymer specimens were cured with UV and visible light initiating systems, UV light intensities that varied by over four orders of magnitude, and cure temperatures that differed by 60 degrees C. Even though the polymerization rates for these resins were vastly different, similar T(g) and modulus were measured for specimens cured to the same final double bond conversion. SIGNIFICANCE: This study shows that highly cross-linked dimethacrylate systems, such as bis-GMA/TEGDMA, exhibit similar network structure and properties as a function of double bond conversion, regardless of the method or rate of cure.

Primary cyclization in the polymerization of bis-GMA and TEGDMA: a modeling approach to understanding the cure of dental resins.

An optimal dental restorative polymeric material would have a homogeneous cross-linking density giving it consistent mechanical strength throughout the material. When multifunctional monomers are polymerized, a pendant double bond can react intramolecularly with the radical on its propagating chain to form a loop, which results in a primary cyclization reaction. Primary cyclization does not contribute to overall network structure, causes microgel formation, and leads to heterogeneity in the polymer. Knowledge of how cure conditions control the degree of primary cyclization and cross-linking in the polymer is important in developing better dental materials. To gain more understanding about the evolving polymer network, the photopolymerization of a typical dental resin (75/25 wt% bis-GMA/TEGDMA) is modeled using a first principals approach. The overall polymerization rate behavior of 75/25 wt% bis-GMA/TEGDMA is predicted using experimentally obtained propagation and termination kinetic rate constants. The effect of chain stiffness and light intensity on the polymerization kinetics is also explored. Furthermore, the model predicts the extent of cross-linking and primary cyclization in the growing polymer network. At 45% conversion, the fraction of bis-GMA and TEGDMA pendant double bonds created that have cycled is 11 and 33%, respectively. The model shows that using a stiff monomer, like bis-GMA, in dental resins diminishes the extent of cyclization and increases the cross-linking density of the polymer. Therefore, better mechanical properties are obtained than if more flexible monomers were used.

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.

The effects of light intensity, temperature, and comonomer composition on the polymerization behavior of dimethacrylate dental resins.

One of the most common combinations for the organic phase of dental restorative materials is BisGMA (2,2bis[4-(2-hydroxy-3-methacryloyloxypropoxy) phenyl]propane) and TEGDMA (triethylene glycol dimethacrylate). However, this copolymer has some drawbacks, such as volume shrinkage during cure and lack of complete double-bond conversion. If the properties of this system are to be improved, an attempt must be made to understand the underlying kinetics of the reaction. This work examines the effects of light intensity, temperature, and composition on the polymerization behavior of BisGMA/TEGDMA copolymerizations. Using differential scanning calorimetry, we monitored the rates of photopolymerization for various experimental conditions. The BisGMA/TEGDMA copolymerization behaved similarly to other dimethacrylate systems and exhibited diffusion-controlled kinetics. It was found that the maximum rate of polymerization was significantly affected by the intensity of the light, and the temperature of the polymerization affected the conversion at which the maximum rate occurred. When the composition of the mixture was varied, it was discovered that the viscosity of the system played a significant role in the polymerization rate and the onset of reaction-diffusion-controlled termination. Mixtures which contained from 50 wt% to 75 wt% BisGMA displayed the highest maximum rate. This feature suggests that TEGDMA is an excellent diluent, since it increases the mobility of the reacting medium; however, the high reactivity is due to the presence of BisGMA. Therefore, based on compositional dependence, we conclude that the BisGMA portion of the mixture largely controls the polymerization mechanisms and kinetics.

Fundamental studies of biodegradable hydrogels as cartilage replacement materials.

Through intelligent control of monomer chemistry and gelling techniques, biodegradable hydrogels with a range of mechanical strengths and degradation timescales have been constructed. A diacrylated, copoly(ethylene glycol-b-dl-lactic acid) (PEG-b-PLA) macromer was used to produce synthetic networks with equilibrium water contents (EWC) above 70% and initial compressive moduli values exceeding 1 MPa, demonstrating its viability as a cartilage replacement material. Experiments have shown that the mechanical strengths, EWCs, and useful lifetimes of these water-swellable networks are coupled to their copolymer chemistry as well as their processing conditions. A systematic study utilizing photopolymerized gels has been undertaken to elucidate the controlling factors behind the bulk-degradation process, as well as monitor changes in network structure with degradation. A statistical model will be used in conjunction with the experimental data to explain the exponential modulus decay and complex mass loss behavior observed during degradation for these hydrogels.

The influence of comonomer composition on dimethacrylate resin properties for dental composites.

During the polymerization of multifunctional monomers for dental restorations, typical final double-bond conversions range from 55 to 75%. The low conversion results in a large amount of extractable monomer, reduced adhesion to the filler, and the potential for increased swelling. In this work, the ability to increase the maximum conversion by optimizing the copolymer composition is explored. A series of multi-ethylene glycol dimethacrylate monomers of various lengths was used as a model system to determine how the copolymer composition affects the final conversion, the mechanical properties, and the predicted shrinkage. It was found that the ultimate conversion can be significantly increased, shrinkage decreased, and mechanical properties maintained. It was found that up to 30 wt% of poly(ethylene glycol) 600 dimethacrylate could be added to diethylene glycol dimethacrylate without reducing the strength and increasing the conversion. Results for other comonomer combinations were similar.