Reduced shrinkage stress via photo-initiated copper(I)-catalyzed cycloaddition polymerizations of azide-alkyne resins

Utilizing a degradation prediction pathway system to understand how a novel methacrylate derivative polymer with flipped external ester groups retains physico-mechanical properties following esterase exposure

OBJECTIVE

The region of failure for current methacrylates (i.e. derivatives of acrylates) are ester bond linkages that hydrolyze in the presence of salivary and bacterial esterases that break the polymer network backbone. This effect decreases the mechanical properties of methacrylate-based materials.

METHODS

The ethylene glycol dimethacrylate (EGDMA) or novel ethylene glycol ethyl methacrylate (EGEMA) discs were prepared using 40 µL of the curing mixture containing photo/co-initiators for 40 s in a PTFE mold at 1000 mW/cm². The degree of conversion was used as a quality control measure for the prepared discs, followed by physical, mechanical, and chemical characterization of discs properties before and after cholesterol esterase treatment.

RESULTS

After 9 weeks of standardized cholesterol esterase (CEase) exposure, EGDMA discs showed exponential loss of material (p = 0.0296), strength (p = 0.0014) and increased water sorption (p = 0.0002) compared to EGEMA discs. We integrated a degradation prediction pathway system to LC/MS and GC/MS analyses to elucidate the degradation by-products of both EGEMA and EGDMA polymers. GC/MS analysis demonstrated that the esterase catalysis was directed to central polymer backbone breakage, producing ethylene glycol, for EGDMA, and to side chain breakage, producing ethanol, for EGEMA. The flipped external ester group linkage design is attributed to EGEMA showing higher resistance to esterase biodegradation and changes in mechanical and physical properties than EGDMA.

SIGNIFICANCE

EGEMA is a potential substitute for common macromer diluents, such as EGDMA, based on its resistance to biodegradation effects. This work inspires the flipped external group design to be applied to analogs of current larger, hydrophobic strength bearing macromers used in future dental material formulations.

A universal adhesive containing copper nanoparticles improves the stability of hybrid layer in a cariogenic oral environment: An in situ study

PURPOSE

To evaluate how incorporating copper nanoparticles (CuNp) into a universal adhesive affects the antimicrobial activity (AMA), bond strength (μTBS), nanoleakage (NL), elastic modulus (EM) and nanohardness (NH) of resin-dentin interfaces, at 24 h (24 h) and after in situ cariogenic challenge (CC).

METHODS

CuNp (0% [control] and 0.1 wt%) was added to an adhesive. After enamel removal, the adhesives were applied to dentine surfaces. Each restored tooth was sectioned longitudinally to obtain two hemi-teeth; one of them was evaluated after 24 h, and the other was included in one of the intra-oral palatal devices placed in the mouths of 10 volunteers for 14 days in CC. After that, each hemi-tooth was removed, and any oral biofilm that formed was collected. The AMA was evaluated against Streptococcus mutans. For the 24 h and CC groups, each hemi-tooth was sectioned in the "x" direction to obtain one slice for each EM/NH evaluation. The remains of each hemi-tooth were sectioned in the "x" and "y" directions to obtain resin-dentin beams for μTBS and NL evaluation (24 h and CC). ANOVA and Tukey's test were applied (α = 0.05).

RESULTS

The presence of CuNp significantly improved AMA as well as all of the evaluated properties (24 h; p < 0.05). Although the adhesive properties (μTBS/NL) for all groups decreased after CC (p < 0.05), the adhesive containing CuNp showed higher μTBS and lower NL as compared to the copper-free adhesive (p < 0.05). The incorporation of CuNp maintained NH/EM values after CC (p < 0.05).

CONCLUSIONS

Adding 0.1% CuNp to an adhesive may provide antimicrobial activity and increase its bonding and mechanical properties, even under a cariogenic challenge.

SIGNIFICANCE

This is the first in situ study proving that incorporating CuNp into an adhesive is an achievable alternative to provide antimicrobial properties and improve the integrity of the hybrid layer under in situ cariogenic challenge.

Metal Oxide Nanoparticles and Nanotubes: Ultrasmall Nanostructures to Engineer Antibacterial and Improved Dental Adhesives and Composites

Advances in nanotechnology have unlocked exclusive and relevant capabilities that are being applied to develop new dental restorative materials. Metal oxide nanoparticles and nanotubes perform functions relevant to a range of dental purposes beyond the traditional role of filler reinforcement-they can release ions from their inorganic compounds damaging oral pathogens, deliver calcium phosphate compounds, provide contrast during imaging, protect dental tissues during a bacterial acid attack, and improve the mineral content of the bonding interface. These capabilities make metal oxide nanoparticles and nanotubes useful for dental adhesives and composites, as these materials are the most used restorative materials in daily dental practice for tooth restorations. Secondary caries and material fractures have been recognized as the most common routes for the failure of composite restorations and bonding interface in the clinical setting. This review covers the significant capabilities of metal oxide nanoparticles and nanotubes incorporated into dental adhesives and composites, focusing on the novel benefits of antibacterial properties and how they relate to their translational applications in restorative dentistry. We pay close attention to how the development of contemporary antibacterial dental materials requires extensive interdisciplinary collaboration to accomplish particular and complex biological tasks to tackle secondary caries. We complement our discussion of dental adhesives and composites containing metal oxide nanoparticles and nanotubes with considerations needed for clinical application. We anticipate that readers will gain a complete picture of the expansive possibilities of using metal oxide nanoparticles and nanotubes to develop new dental materials and inspire further interdisciplinary development in this area.

Evaluation of a photo-initiated copper(I)-catalyzed azide-alkyne cycloaddition polymer network with improved water stability and high mechanical performance as an ester-free dental restorative

OBJECTIVE

The objective is to develop and characterize an ester-free ether-based photo-CuAAC resin with high mechanical performance, low polymerization-induced stress compared with common BisGMA/TEGDMA (70/30) resins, and improved water stability in comparison to previously developed urethane-based photo-CuAAC resins.

METHODS

Triphenyl-ethane-centered ether-linked tri-azide monomers were synthesized and co-photopolymerized with ether-linked tri-alkyne monomers under visible light irradiation using a copper(II) pre-catalyst and CQ/EDAB as the initiator. The ether-based CuAAC formulation was investigated for thermo-mechanical properties, polymerization kinetics and shrinkage stress, and flexural properties with respect to a conventional BisGMA/TEGDMA (70/30) dental resin. In addition, both the ether-based CuAAC resin and the urethane-based CuAAC resin were examined for their water stability using the BisGMA/TEGDMA (70/30) resin as a control.

RESULTS

The ether-based CuAAC network (AK/AZ-1) exhibited a slightly lower glass-transition temperature compared with the BisGMA/TEGDMA network (108 °C vs 128 °C), but because of its much sharper glass transition, the AK/AZ-1 CuAAC-network maintained storage modulus higher than 1 GPa up to 100 °C. In addition, the ether-based AK/AZ-1 network exhibited reduced shrinkage stress (0.56 MPa vs 1.0 MPa) and much higher flexural toughness (7.6 MJ/m³vs 1.6 MJ/m³) while showing slightly lower flexural modulus and slightly higher flexural strength compared with the BisGMA/TEGDMA network. Moreover, the ether-based AK/AZ-1 CuAAC network displayed comparable water stability in comparison to the BisGMA/TEGDMA network with slightly higher water sorption (46 μg/mm³vs 38 μg/mm³) and much lower water solubility (2.3 μg/mm³vs 4.4 μg/mm³).

SIGNIFICANCE

Employing the ether-based hydrophobic CuAAC formulation significantly improved the water stability of the CuAAC network compared with previously developed urethane-based CuAAC networks. Furthermore, compared with the conventionally used BisGMA/TEGDMA formulation, the reduced shrinkage stress, comparable flexural strength/flexural modulus, and the superior flexural toughness of the ether-based CuAAC network make it a promising ester-free alternative to the currently widely-used methacrylate-based dental restoratives.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Molecular Interactions between Saliva and Dental Composites Resins: A Way Forward

Dentin and enamel loss related to trauma or especially caries is one of the most common pathological issues in dentistry that requires restoration of the teeth by using materials with appropriate properties. The composite resins represent dental materials with significant importance in today’s dentistry, presenting important qualities, including their mechanical behavior and excellent aesthetics. This paper focuses on the saliva interactions with these materials and on their biocompatibility, which is continuously improved in the new generations of resin-based composites. Starting from the elements involved on the molecular landscape of the dental caries process, the paper presents certain strategies for obtaining more advanced new dental composite resins, as follows: suppression of oral biofilm acids formation, promotion of remineralization process, counteraction of the proteolytic attack, and avoidance of cytotoxic effects; the relation between dental composite resins and salivary oxidative stress biomarkers is also presented in this context.

Photopolymerization shrinkage-stress reduction in polymer-based dental restoratives by surface modification of fillers

OBJECTIVES

This research explores the use of polymer brushes for surface treatment of fillers used in polymer-based dental restoratives with focus on shrinkage stress reduction. The influence of interfacial reactive groups on shrinkage stress is explored.

METHODS

Oligomers of varying lengths and with varying number of reactive groups along the length were synthesized by modifying commercial oligomers. Surface of silica fillers (OX50) was treated with methylaminopropyltrimethoxysilane and this was further reacted with the synthesized oligomers to obtain a series of polymer brushes on the surface. Fillers modified with γ-methacryloxypropyltrimethoxysilane were used as a control. Filler surface treatment was confirmed using diffuse reflectance spectroscopy and thermogravimetric analysis. Fillers were added at 30 wt % to a resin made of BisGMA/TEGDMA and polymerization kinetics, shrinkage stress, volumetric shrinkage, flexural strength and modulus, viscosity were measured.

RESULTS

Composites with polymer brush functionalized fillers showed up to a 30 % reduction in shrinkage stress as compared to the control, with no reduction in flexural strength and modulus. Shrinkage stress reduced with increasing length of the polymer brush and increased with increase in number of reactive groups along the length of the polymer brush.

SIGNIFICANCE

The interface between inorganic fillers and an organic polymer matrix has been utilized to reduce shrinkage stress in a composite with no compromise in mechanical properties. This study gives insights into the stress development mechanism at the interface.

Light-Activated Stress Relaxation, Toughness Improvement, and Photoinduced Reversal of Physical Aging in Glassy Polymer Networks

A covalent adaptable network (CAN) with high glass transition temperature (T_g ), superior mechanical properties including toughness and ductility, and unprecedented spatio-temporally controlled dynamic behavior is prepared by introducing dynamic moieties capable of reversible addition fragmentation chain transfer (RAFT) into photoinitiated copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC)-based networks. While the CuAAC polymerization yields glassy polymers composed of rigid triazole linkages with enhanced toughness, the RAFT moieties undergo bond exchange leading to stress relaxation upon light exposure. This unprecedented level of stress relaxation in the glassy state leads to numerous desirable attributes including glassy state photoinduced plasticity, toughness improvement during large deformation, and even photoinduced reversal of the effects of physical aging resulting in the rejuvenation of mechanical and thermodynamic properties in physically aged RAFT-CuAAC networks that undergo bond exchange in the glassy state. Surprisingly, when an allyl-sulfide-containing azide monomer (AS-N₃ ) is used to form the network, the network exhibits up to 80% stress relaxation in the glassy state (T_g  - 45 °C) under fixed displacement. In situ activation of RAFT during mechanical loading results in a 50% improvement in elongation to break and 40% improvement in the toughness when compared to the same network without light-activation of RAFT during the tensile testing.

Dental composite resins with low polymerization stress based on a new allyl carbonate monomer

The objective of this study was to synthesize a diallyl carbonate monomer, allyl(2-(2-(((allyloxy)carbonyl)oxy)benzoyl)-5-methoxyphenyl) carbonate (BZ-AL), and to evaluate its effect as Bis-GMA diluent in the formulation of photopolymerizable dental composite resins. The chemical structure of BZ-AL monomer was determined by means of H¹ NMR, C¹³ NMR and FTIR spectroscopies. An experimental composite comprising a mixture of Bis-GMA and BZ-AL monomers and silanized inorganic filler was formulated. Experimental material was compared with a control composite formulated with Bis-GMA/TEGDMA. Double bond conversion, polymerization kinetics, volumetric shrinkage, polymerization stress, and flexural properties were investigated. The data were analyzed through a Student t-test (α = 0.05). Flexural strength of the experimental materials with BZ-AL monomer showed a statistically significant increase (p < 0.001). The experimental composite has a lower polymerization rate than the control composite, on the other hand, the experimental composite resin has the highest degree of double bond conversion. There are no differences in the polymerization shrinkage of the composites, however, the polymerization stress of the experimental materials was 50% lower than the control resin. Finally, the cell viability test showed that the experimental resins formulated with the BZ-AL monomer was not cytotoxic. Due to its characteristics, BZ-AL monomer is potentially useful for the formulation of composite materials with applications in dentistry.

Self-healing biomaterials: The next generation is nano

The U.S. Agency for Healthcare Research and Quality estimates that there are over 1 million total hip and total knee replacements each year in the U.S. alone. Twenty five percent of those implants will experience aseptic loosening, and bone cement failure is an important part of this. Bone cements are based on poly(methyl methacrylate) (PMMA) systems that are strong but brittle polymers. PMMA-based materials are also essential to modern dental fillings, and likewise, the failure rates are high with lifetimes of 3-10 years. These brittle polymers are an obvious target for self-healing systems which could reduce revision surgeries and visits to dentist. Self-healing polymers have been described in the literature since 1996 and examples from Roman times are known, but their application in medicine has been challenging. This review looks at the development of self-healing biomaterials for these applications and the challenges that lie between development and the clinic. Many of the most promising formulations involve introducing nanoscale components which offer substantial potential benefits over their microscale counterparts especially in composite systems. There is substantial promise for translation, but issues with toxicity, robustness, and reproducibility of these materials in the complex environment of the body must be addressed. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

Photoreactive nanogels as versatile polymer networks with tunable in situ drug release kinetics

A simple, yet powerful approach to synthesize photoreactive nanogel networks <5 nm that can swell between ~3 and ~200 times their initial radius with control over the size and surface charge via a solution polymerization reaction protocol was demonstrated. Nanogels with hydrodynamic radii from 0.9 nm to 3.2 nm and surface charges from -6.4 mV to -16.5 mV with dramatically different abilities to swell were synthesized by altering the solvent ratio before synthesis. Additionally, the control over the release kinetics of a small molecule over a period of 30 days was demonstrated by the methacrylate functionalization of the nanogels post-synthesis and the subsequent photo-aggregation of the nanogels. Thepotential to control the release of small molecule drugs via the concentration of photoreactive groups and the photo-induced aggregation of the nanogels offers the unique ability to tailor the in situ release kinetics of the delivery network.

Vinyl sulfonamide based thermosetting composites via thiol-Michael polymerization

OBJECTIVE

To assess the performance of thiol Michael photocurable composites based on ester-free thiols and vinyl sulfonamides of varying monomer structures and varied filler loadings and to contrast the properties of the prototype composites with conventional BisGMA-TEGDMA methacrylate composite.

METHODS

Synthetic divinyl sulfonamides and ester-free tetrafunctional thiol monomers were utilized for thiol-Michael composite development with the incorporation of thiolated microfiller. Polymerization kinetics was investigated using FTIR spectroscopy. Resin viscosities were assessed with rheometry. Water uptake properties were assessed according to standardized methods. Thermomechanical properties were analyzed by dynamic mechanical analysis. Flexural modulus/strength and flexural toughness were measured on a universal testing machine in three-point bending testing mode.

RESULTS

The vinyl sulfonamide-based thiol-Michael resin formulation demonstrated a wide range of viscosities with a significant increase in the functional group conversion when compared to the BisGMA-TEGDMA system. The two different types of vinyl sulfonamide under investigation demonstrated significant differences towards the water sorption. Tertiary vinyl sulfonamide did not undergo visible swelling whereas the secondary vinyl sulfonamide composite swelled extensively in water. With the introduction of rigid monomer into the polymer matrix the glass transition temperature increased and so increased the toughness. Glassy thiol-Michael composites were obtained by ambient curing.

SIGNIFICANCE

Employing the newly developed step-growth thiol-Michael resins in dental composites will provide structural uniformity, improved stability and lower water sorption.

Enhancing the toughness of composites via dynamic thiol–thioester exchange (TTE) at the resin–filler interface

An adaptive interface employing thiol-thioester exchange (TTE) at the resin-filler interface is introduced to promote interfacial stress relaxation and improve the mechanical performance of thermosetting composites.

Dynamic covalent chemistry (DCC) in dental restorative materials: Implementation of a DCC-based adaptive interface (AI) at the resin-filler interface for improved performance

OBJECTIVE

Dental restorative composites have been extensively studied with a goal to improve material performance. However, stress induced microcracks from polymerization shrinkage, thermal and other stresses along with the low fracture toughness of methacrylate-based composites remain significant problems. Herein, the study focuses on applying a dynamic covalent chemistry (DCC)-based adaptive interface to conventional BisGMA/TEGDMA (70:30) dental resins by coupling moieties capable of thiol-thioester (TTE) DCC to the resin-filler interface as a means to induce interfacial stress relaxation and promote interfacial healing.

METHODS

Silica nanoparticles (SNP) are functionalized with TTE-functionalized silanes to covalently bond the interface to the network while simultaneously facilitating relaxation of the filler-matrix interface via DCC. The functionalized particles were incorporated into the otherwise static conventional BisGMA/TEGDMA (70:30) dental resins. The role of interfacial bond exchange to enhance dental composite performance in response to shrinkage and other stresses, flexural modulus and toughness was investigated. Shrinkage stress was monitored with a tensometer coupled with FTIR spectroscopy. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine.

RESULTS

A reduction of 30% in shrinkage stress was achieved when interfacial TTE bond exchange was activated while not only maintaining but also enhancing mechanical properties of the composite. These enhancements include a 60% increase in Young's modulus, 33% increase in flexural strength and 35% increase in the toughness, relative to composites unable to undergo DCC but otherwise identical in composition. Furthermore, by combining interfacial DCC with resin-based DCC, an 80% reduction of shrinkage-induced stress is observed in a thiol-ene system "equipped" with both types of DCC mechanisms relative to the composite without DCC in either the resin or at the resin-filler interface.

SIGNIFICANCE

This behavior highlights the advantages of utilizing the DCC at the resin-filler interface as a stress-relieving mechanism that is compatible with current and future developments in the field of dental restorative materials, nearly independent of the type of resin improvements and types that will be used, as it can dramatically enhance their mechanical performance by reducing both polymerization and mechanically applied stresses throughout the composite lifetime.

Zinc oxide and copper nanoparticles addition in universal adhesive systems improve interface stability on caries-affected dentin

This study evaluated the MMP inhibition of the zinc oxide and copper nanoparticles (ZnO/CuNp), and the effects of their addition into adhesives on antimicrobial activity (AMA), ultimate tensile strength (UTS), in vitro degree of conversion (in vitro-DC), as well as, resin-dentin bond strength (μTBS), nanoleakage (NL) and in situ-DC on caries-affected dentin. Anti-MMP activity was evaluated for several MMPs. ZnO/CuNp (0% [control]; 5/0.1 and 5/0.2 wt%) were added into Prime&Bond Active (PBA) and Ambar Universal (AMB). The AMA was evaluated against Streptococcus mutans. UTS were tested after 24 h and 28d. After induced caries, adhesives and composite were applied to flat dentin surfaces, and specimens were sectioned to obtain resin-dentin sticks. μTBS, NL, in vitro-DC and in situ-DC were evaluated after 24 h. ANOVA and Tukey's test were applied (α = 0.05). ZnO/CuNp demonstrated anti-MMP activity (p < 0.05). The addition of ZnO/CuNp increased AMA and UTS (AMB; p < 0.05). UTS for PBA, in vitro-DC, in situ-DC and μTBS for both adhesives were maintained with ZnO/CuNp (p > 0.05). However, lower NL was observed for ZnO/CuNp groups (p < 0.05). The addition of ZnO/CuNp in adhesives may be an alternative to provide antimicrobial, anti-MMP activities and improves the integrity of the hybrid layer on caries-affected dentin.

Evaluation of biofilm formation on novel copper-catalyzed azide-alkyne cycloaddition (CuAAC)-based resins for dental restoratives

OBJECTIVE

For the past several decades, the resins used in dental restorations have been plagued with numerous problems, including their implication in biofilm formation and secondary caries. The need for alternative resins is critical, and evaluation of biofilm formation on these resins is essential. The aim of this study was to evaluate in vitro biofilm formation on the surface of novel copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC)-based resins and composites.

METHODS

CuAAC-based resins/composites made from varying azide monomers and different copper concentrations were compared with BisGMA-TEGDMA resins/composites that served as the control. Biofilms were formed using a mono-species model containing a luciferase-expressing strain of Streptococcus mutans. Luciferase activity was measured and the number of viable bacteria was enumerated on biofilms associated with each resin and composite.

RESULTS

A significant reduction (p<0.05) in luciferase activity, and the number of viable bacteria recovered from biofilms on CuAAC-based resins and composites was observed in comparison to biofilms associated with the BisGMA-TEGDMA controls.

SIGNIFICANCE

CuAAC-based resins do still allow for the formation of biofilms; however, the statistically significant reduction of growth that was associated with the CuAAC resin may enhance the longevity of restorations that incorporate CuAAC-based materials.

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.

Toward dental caries: Exploring nanoparticle-based platforms and calcium phosphate compounds for dental restorative materials

Millions of people worldwide suffer from a toothache due to tooth cavity, and often permanent tooth loss. Dental caries, also known as tooth decay, is a biofilm-dependent infectious disease that damages teeth by minerals loss and presents a high incidence of clinical restorative polymeric fillings (tooth colored fillings). Until now, restorative polymeric fillings present no bioactivity. The complexity of oral biofilms contributes to the difficulty in developing effective novel dental materials. Nanotechnology has been explored in the development of bioactive dental materials to reduce or modulate the activities of caries-related bacteria. Nano-structured platforms based on calcium phosphate and metallic particles have advanced to impart an anti-caries potential to restorative materials. The bioactivity of these platforms induces prevention of mineral loss of the hard tooth structure and antibacterial activities against caries-related pathogens. It has been suggested that this bioactivity could minimize the incidence of caries around restorations (CARS) and increase the longevity of such filling materials. The last few years witnessed growing numbers of studies on the preparation evaluations of these novel materials. Herein, the caries disease process and the role of pathogenic caries-related biofilm, the increasing incidence of CARS, and the recent efforts employed for incorporation of bioactive nanoparticles in restorative polymer materials as useful strategies for prevention and management of caries-related-bacteria are discussed. We highlight the status of the most advanced and widely explored interaction of nanoparticle-based platforms and calcium phosphate compounds with an eye toward translating the potential of these approaches to the dental clinical reality.

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Current progress and future applications of functional nanoparticles and remineralizing compounds incorporated in dental direct restorative materials.

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Overview of the antibacterial and remineralizing mechanisms presenting direct and indirect implications on the tooth mineral loss.

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These investigations, although in the initial phase of evidence are necessary and their results are encouraging and open the doors to future clinical studies that will allow the therapeutic value of nanotechnology-based restorative materials to be established.

Dynamic Covalent Chemistry at Interfaces: Development of Tougher, Healable Composites through Stress Relaxation at the Resin-Silica Nanoparticles Interface

The interfacial region in composites that incorporate filler materials of dramatically different modulus relative to the resin phase acts as a stress concentrator and becomes a primary locus for composite failure. A novel adaptive interface (AI) platform formed by coupling moieties capable of dynamic covalent chemistry (DCC) is introduced to the resin-filler interface to promote stress relaxation. Specifically, silica nanoparticles (SNP) are functionalized with a silane capable of addition fragmentation chain transfer (AFT), a process by which DCC-active bonds are reversibly exchanged upon light exposure and concomitant radical generation, and copolymerized with a thiol-ene resin. At a fixed SNP loading of 25 wt%, the toughness (2.3 MJ m-3) is more than doubled and polymerization shrinkage stress (0.4 MPa) is cut in half in the AI composite relative to otherwise identical composites that possess a passive interface (PI) with similar silane structure, but without the AFT moiety. In situ activation of the AI during mechanical loading results in 70% stress relaxation and three times higher fracture toughness than the PI control. When interfacial DCC was combined with resin-based DCC, the toughness was improved by 10 times relative to the composite without DCC in either the resin or at the resin-filler interface.

Bismesitoylphosphinic Acid (BAPO-OH): A Ligand for Copper Complexes and Four-Electron Photoreductant for the Preparation of Copper Nanomaterials

Bismesitoylphosphinic acid, (HO)PO(COMes)₂ (BAPO-OH), is an efficient photoinitiator for free-radical polymerizations of olefins in aqueous phase. Described here are the structures of various copper(II) and copper(I) complexes with BAPO-OH as the ligand. The complex Cu^II (BAPO-O)₂ (H₂ O)₂ is photoactive, and under irradiation with UV light in aqueous phase, it serves as a source of metallic copper in high purity and yield (>80 %). Simultaneously, the radical polymerization of acrylates can be initiated and allows the preparation of nanoparticle/polymer nanocomposites in which the metallic Cu nanoparticles are protected against oxidation. The determination of the stoichiometry of the photoreductions suggests an almost quantitative conversion from Cu^II into Cu⁰ with half an equivalent of BAPO-OH, which serves as a four-electron photoreductant.

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.

Recent advances in alkyne-based click polymerizations

The recent progress in alkyne-based click polymerizations and their application in the preparation of new functional polymers are summarized. The challenges and opportunities in this area are also briefly discussed.

Fully recoverable rigid shape memory foam based on copper-catalyzed azide-alkyne cycloaddition (CuAAC) using a salt leaching technique

This study is the first to employ the use of the copper-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization to form a tough and stiff, porous material from a well-defined network possessing a high glass transition temperature. The effect of the network linkages formed as a product of the CuAAC reaction, i.e., the triazoles, on the mechanical behavior at high strain was evaluated by comparing the CuAAC foam to an epoxy-amine-based foam, which consisted of monomers with similar backbone structures and mechanical properties (i.e., T_g of 115 °C and a rubbery modulus of 1.0 MPa for the CuAAC foam, T_g of 125 °C and a rubbery modulus of 1.2 MPa for the epoxy-amine foam). When each foam was compressed uniformly to 80% strain at ambient temperature, the epoxy-amine foam was severely damaged after only reaching 70% strain in the first compression cycle with a toughness of 300 MJ/m³. In contrast, the CuAAC foam exhibited pronounced ductile behavior in the glassy state with three times higher toughness of 850 MJ/m³ after the first cycle of compression to 80% strain. Additionally, when the CuAAC foam was heated above T_g after each of five compression cycles to 80% strain at ambient temperature, the foam completely recovered its original shape while exhibiting a gradual decrease in mechanical performance over the multiple compression cycles. The foam demonstrated almost complete shape fixity and recovery ratios even through five successive cycles, indicative of "reversible plasticity", making it highly desirable as a glassy shape memory foams.

New Resins for Dental Composites

Restorative composites have evolved significantly since they were first introduced in the early 1960s, with most of the development concentrating on the filler technology. This has led to improved mechanical properties, notably wear resistance, and has expanded the use of composites to larger posterior restorations. On the organic matrix side, concerns over the polymerization stress and the potential damage to the bonded interface have dominated research in the past 20 y, with many "low-shrinkage" composites being launched commercially. The lack of clinical correlation between the use of these materials and improved restoration outcomes has shifted the focus more recently to improving materials' resistance to degradation in the oral environment, caused by aqueous solvents and salivary enzymes, as well as biofilm development. Antimicrobial and ester-free monomers have been developed in the recent past, and evidence is mounting for their potential benefit. This article reviews literature on the newest materials currently on the market and provides an outlook for the future developments needed to improve restoration longevity past the average 10 y.

One-pot blue-light triggered tough interpenetrating polymeric network (IPN) using CuAAC and methacrylate reactions

An interpenetrating polymeric network (IPN) is formed in a one-pot blue-light activated scheme, where the step- and chain- growth polymerizations of the CuAAC and methacrylate reactions, respectively, are simultaneously triggered but proceed sequentially. The glassy IPN is polymerized under ambient conditions and is able to withstand high strain before failure owing to its significantly enhanced toughness. Additionally, this material exhibits shape memory attributes with readily tunable mechanical properties at high temperature.

Kinetics and mechanics of photo-polymerized triazole-containing thermosetting composites via the copper(I)-catalyzed azide-alkyne cycloaddition

OBJECTIVE

Several features necessary for polymer composite materials in practical applications such as dental restorative materials were investigated in photo-curable CuAAC (copper(I)-catalyzed azide-alkyne cycloaddition) thermosetting resin-based composites with varying filler loadings and compared to a conventional BisGMA/TEGDMA based composite.

METHODS

Tri-functional alkyne and di-functional azide monomers were synthesized for CuAAC resins and incorporated with alkyne-functionalized glass microfillers for CuAAC composites. Polymerization kinetics, in situ temperature change, and shrinkage stress were monitored simultaneously with a tensometer coupled with FTIR spectroscopy and a data-logging thermocouple. The glass transition temperature was analyzed by dynamic mechanical analysis. Flexural modulus/strength and flexural toughness were characterized in three-point bending on a universal testing machine.

RESULTS

The photo-CuAAC polymerization of composites containing between 0 and 60wt% microfiller achieved ∼99% conversion with a dramatic reduction in the maximum heat of reaction (∼20°C decrease) for the 60wt% filled CuAAC composites as compared with the unfilled CuAAC resin. CuAAC composites with 60wt% microfiller generated more than twice lower shrinkage stress of 0.43±0.01MPa, equivalent flexural modulus of 6.1±0.7GPa, equivalent flexural strength of 107±9MPa, and more than 10 times higher energy absorption of 10±1MJm-3 when strained to 11% relative to BisGMA-based composites at equivalent filler loadings.

SIGNIFICANCE

Mechanically robust and highly tough, photo-polymerized CuAAC composites with reduced shrinkage stress and a modest reaction exotherm were generated and resulted in essentially complete conversion.

The role of copper nanoparticles in an etch-and-rinse adhesive on antimicrobial activity, mechanical properties and the durability of resin-dentine interfaces

OBJECTIVES

To evaluate the effect of addition of copper nanoparticles at different concentrations into an etch-and-rinse adhesive (ER) on antimicrobial activity, Knoop microhardness (KHN), in vitro and in situ degree of conversion (DC), as well as the immediate (IM) and 2-year (2Y) resin-dentine bond strength (μTBS) and nanoleakage (NL).

METHODS

Seven experimental ER adhesives were formulated according to the amount of copper nanoparticles incorporated into the adhesives (0 [control], 0.0075 to 1wt.%). We tested the antimicrobial activity of adhesives against Streptococcus mutans using agar diffusion assay after IM and 2Y. The Knoop microhardness and in vitro DC were tested after IM and 2Y. The adhesives were applied to flat occlusal dentine surfaces after acid etching. After resin build-ups, specimens were longitudinally sectioned to obtain beam-like resin-dentine specimens (0.8mm²), which were used for evaluation of μTBS and nanoleakage at the IM and 2Y periods. In situ DC was evaluated at the IM period in these beam-like specimens. Data were submitted to appropriate statistical analyses (α=0.05).

RESULTS

The addition of copper nanoparticles provided antimicrobial activity to the adhesives only in the IM evaluation and slightly reduced the KHN, the in vitro and in situ DC (copper concentrations of 1wt.%). However, KHN increase for all concentrations after 2Y. After 2Y, no significant reductions of μTBS (0.06 to 1% wt.%) and increases of nanoleakage were observed for copper containing adhesives compared to the control group.

CONCLUSION

Copper nanoparticles addition up to 0.5wt.% may provide antimicrobial properties to ER adhesives and prevent the degradation of the adhesive interface, without reducing the mechanical properties of the formulations.