Synthesis and evaluation of novel dental monomer with branched carboxyl acid group

Characterization of Experimental Nanoparticulated Dental Adhesive Resins with Long-Term Antibacterial Properties

Experimental adhesives with functional nitrogen-doped titanium dioxide nanoparticles (N_TiO₂) have been shown to display improved properties. However, these materials have not been characterized regarding their degree of conversion (DC), biaxial flexure strength (BFS), surface roughness (SR), elastic modulus (EM), and long-term antibacterial functionalities. Experimental adhesives were synthesized by dispersing N_TiO₂ (10%, 20%, or 30%, v/v%) into OptiBond Solo Plus (OPTB, Kerr Corp., USA). Unpolymerized adhesives (volume = 50 μL/drop, n = 3/group) were individually placed onto a heated (37 °C) attenuated total reflectance (ATR) monolithic diamond crystal (Golden Gate, Specac). The spectra of composites were obtained with a Fourier-transform infrared (FTIR) spectrometer (Nicolet IS50; 500-4500 cm^-1; resolution = 4 cm^-1, 10 internal scans/spectrum) before and after polymerization. Disk-shaped specimens (diameter = 6.0 mm, thickness = 0.5 mm) for BFS (n = 12/group), SR and EM (n = 3/group), and for antibacterial testing (n = 18/group/time-point) were fabricated and photopolymerized (1 min each; 385-515 nm, 1000 mW/cm²; VALO). DC values (%) were calculated from pre- and post-polymerization spectra using the two-frequency method and tangent-baseline technique. BFS was assessed using a universal testing machine (Instron 68TM-5, crosshead speed = 1.27 mm/min, 25 °C). SR and EM were investigated using an atomic force microscope (Multimode 8) with aluminum-coated silicon probes (8 nm pyramidal tip, spring constant 40 N/m, Bruker). Antibacterial testing was performed by growing Streptococcus mutans biofilms (UA159-ldh, 37 °C, microaerophilic) on the surfaces of specimens for 24 h and then measuring the relative luminescence units (RLU) with a Biotek Synergy HT multi-well plate reader. Results demonstrate that experimental materials containing 10%, 20%, and 30% of N_TiO₂ displayed higher levels of DC, had better mechanical properties, and were able to exert strong and durable antibacterial properties without visible light irradiation and after extended periods of simulated shelf-life and aging in PBS. The reported experimental materials are expected to increase the service lives of polymer-based bonded restorations by decreasing the incidence of secondary caries.

Autonomous-Strengthening Adhesive Provides Hydrolysis-Resistance and Enhanced Mechanical Properties in Wet Conditions

The low-viscosity adhesive that is used to bond composite restorative materials to the tooth is readily damaged by acids, enzymes, and oral fluids. Bacteria infiltrate the resulting gaps at the composite/tooth interface, demineralize the tooth, and further erode the adhesive. This paper presents the preparation and characterization of a low-crosslink-density hydrophilic adhesive that capitalizes on sol-gel reactions and free-radical polymerization to resist hydrolysis and provide enhanced mechanical properties in wet environments. Polymerization behavior, water sorption, and leachates were investigated. Dynamic mechanical analyses (DMA) were conducted using water-saturated adhesives to mimic load transfer in wet conditions. Data from all tests were analyzed using appropriate statistical tests (α = 0.05). The degree of conversion was comparable for experimental and control adhesives at 88.3 and 84.3%, respectively. HEMA leachate was significantly lower for the experimental (2.9 wt%) compared to control (7.2 wt%). After 3 days of aqueous aging, the storage and rubbery moduli and the glass transition temperature of the experimental adhesive (57.5MPa, 12.8MPa, and 38.7 °C, respectively) were significantly higher than control (7.4MPa, 4.3 MPa, and 25.9 °C, respectively). The results indicated that the autonomic sol-gel reaction continues in the wet environment, leading to intrinsic reinforcement of the polymer network, improved hydrolytic stability, and enhanced mechanical properties.

Reconfigurable Dual Peptide Tethered Polymer System Offers a Synergistic Solution for Next Generation Dental Adhesives

Resin-based composite materials have been widely used in restorative dental materials due to their aesthetic, mechanical, and physical properties. However, they still encounter clinical shortcomings mainly due to recurrent decay that develops at the composite-tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal this interface, but the adhesive seal is inherently defective and readily damaged by acids, enzymes, and oral fluids. Bacteria infiltrate the resulting gaps at the composite-tooth interface and bacterial by-products demineralize the tooth and erode the adhesive. These activities lead to wider and deeper gaps that provide an ideal environment for bacteria to proliferate. This complex degradation process mediated by several biological and environmental factors damages the tooth, destroys the adhesive seal, and ultimately, leads to failure of the composite restoration. This paper describes a co-tethered dual peptide-polymer system to address composite-tooth interface vulnerability. The adhesive system incorporates an antimicrobial peptide to inhibit bacterial attack and a hydroxyapatite-binding peptide to promote remineralization of damaged tooth structure. A designer spacer sequence was incorporated into each peptide sequence to not only provide a conjugation site for methacrylate (MA) monomer but also to retain active peptide conformations and enhance the display of the peptides in the material. The resulting MA-antimicrobial peptides and MA-remineralization peptides were copolymerized into dental adhesives formulations. The results on the adhesive system composed of co-tethered peptides demonstrated both strong metabolic inhibition of S. mutans and localized calcium phosphate remineralization. Overall, the result offers a reconfigurable and tunable peptide-polymer hybrid system as next-generation adhesives to address composite-tooth interface vulnerability.

Chemometrics-Assisted Raman Spectroscopy Characterization of Tunable Polymer-Peptide Hybrids for Dental Tissue Repair

The interfaces that biological tissues form with biomaterials are invariably defective and frequently the location where failure initiates. Characterizing the phenomena that lead to failure is confounded by several factors including heterogeneous material/tissue interfaces. To seamlessly analyze across these diverse structures presents a wealth of analytical challenges. This study aims to develop a molecular-level understanding of a peptide-functionalized adhesive/collagen hybrid biomaterial using Raman spectroscopy combined with chemometrics approach. An engineered hydroxyapatite-binding peptide (HABP) was copolymerized in dentin adhesive and dentin was demineralized to provide collagen matrices that were partially infiltrated with the peptide-functionalized adhesive. Partial infiltration led to pockets of exposed collagen-a condition that simulates defects in adhesive/dentin interfaces. The spectroscopic results indicate that co-polymerizable HABP tethered to the adhesive promoted remineralization of the defects. The spatial distribution of collagen, adhesive, and mineral as well as crystallinity of the mineral across this heterogeneous material/tissue interface was determined using micro-Raman spectroscopy combined with chemometrics approach. The success of this combined approach in the characterization of material/tissue interfaces stems from its ability to extract quality parameters that are related to the essential and relevant portions of the spectral data, after filtering out noise and non-relevant information. This ability is critical when it is not possible to separate components for analysis such as investigations focused on, in situ chemical characterization of interfaces. Extracting essential information from complex bio/material interfaces using data driven approaches will improve our understanding of heterogeneous material/tissue interfaces. This understanding will allow us to identify key parameters within the interfacial micro-environment that should be harnessed to develop durable biomaterials.

Probing the mineralized tissue-adhesive interface for tensile nature and bond strength

The mechanical performance of the dentin-adhesive interface contributes significantly to the failure of dental composite restorations. Rational material design can lead to enhanced mechanical performance, but this requires accurate characterization of the mechanical behavior at the dentin-adhesive interface. The mechanical performance of the interface is typically characterized using bond strength tests, such as the micro-tensile test. These tests are plagued by multiple limitations including large variations in the test results. The challenges associated with conventional tensile tests limit our ability to unravel the complex relationships that affect mechanical behavior at the dentin-adhesive interface. This study used the diametral compression test to overcome the challenges inherent in conventional bond strength tests. The bovine femur cortical bone tissue was considered as a surrogate material (the mineralized tissue) for human dentin. Two different adhesive formulations, which differed by means of their self-strengthening properties, were studied. The tensile behavior of the mineralized tissue, the adhesive polymer, and the bond strength of the mineralized tissue - adhesive interface was determined using the diametral compression test. The diametral compression test improved the repeatability for both the tensile and bond strength tests. The rate dependent mechanical behavior was observed for both single material and interfacial material systems. The tensile strength and bond strength of the mineralized tissue-adhesive interface was greater for the self-strengthening formulation as compared to the control.

Sustainable Shape-Memory Polyurethane from Abietic Acid: Superior Mechanical Properties and Shape Recovery with Tunable Transition Temperatures

A new chemical architecture from abietic acid, consisting of a cycloaliphatic unsaturated terminal diisocyanate (AADI) structure, is synthesized and fully characterized. The AADI is then used to construct an amorphous and biocompatible shape-memory polyurethane (SMPU) network system with adjustable cross-linking densities over a wide range. The SMPU network exhibits good shape-memory properties with a shape fixing rate of greater than 98 % and a shape recovery rate of 95 %. In vitro hydrolytic biodegradation weight loss ratio of SMPUs reaches 71 % within eight weeks. The physical properties of these SMPUs are comparable to those reported for SMPUs obtained from commercially available petroleum-derived diisocyanates. This is the first time that multiple SMPU networks based on abietic acid have been reported. These environmentally-friendly SMPUs display a wide range of physicomechanical properties with promising hydrolytic degradability, showing good potential for practical application.

Evolution of Network Structure and Mechanical Properties in Autonomous-Strengthening Dental Adhesive

The inherent degradation property of most dental resins in the mouth leads to the long-term release of degradation by-products at the adhesive/tooth interface. The by-products increase the virulence of cariogenic bacteria, provoking a degradative positive-feedback loop that leads to physicochemical and mechanical failure. Photoinduced free-radical polymerization and sol‒gel reactions have been coupled to produce a novel autonomous-strengthening adhesive with enhanced hydrolytic stability. This paper investigates the effect of network structure on time-dependent mechanical properties in adhesives with and without autonomous strengthening. Stress relaxation was conducted under 0.2% strain for 8 h followed by 40 h recovery in water. The stress‒time relationship is analyzed by nonlinear least-squares data-fitting. The fitted Prony series predicts the sample’s history under monotonic loading. Results showed that the control failed after the first loading‒unloading‒recovery cycle with permanent deformation. While for the experimental sample, the displacement was almost completely recovered and the Young’s modulus increased significantly after the first test cycle. The experimental polymer exhibited higher degree of conversion, lower leachate, and time-dependent stiffening characteristics. The autonomous-strengthening reaction persists in the aqueous environment leading to a network with enhanced resistance to deformation. The results illustrate a rational approach for tuning the viscoelasticity of durable dental adhesives.

Sorption, solubility and cytotoxicity of novel antibacterial nanofilled dental adhesive resins

Dental adhesives hydrolyze in the mouth. This study investigated the water sorption (SOR), solubility (SOL) and cytotoxicity (CYTO) of experimental adhesives containing nitrogen-doped titanium dioxide nanoparticles (N_TiO₂). Specimens (n = 15/group [SOR, SOL]; n = 10/group [CYTO]) of unaltered Clearfil SE Protect (CSP), OptiBond Solo Plus (OSP), Adper Scotchbond (ASB) and experimental adhesives (OSP + 25% or 30% of N_TiO₂) were fabricated, desiccated (37 °C) and tested for SOR and SOL according to ISO Specification 4049 (2009). CYTO specimens were UV-sterilized (8 J/cm²) and monomer extracted in growth medium (1, 3 or 7 days). Human pulp cells were isolated and seeded (0.5 × 10⁴) for MTT assay. SOR and SOL data was analyzed using GLM and SNK (α = 0.05) and CYTO data was analyzed with Kruskal–Wallis and SNK tests (α = 0.05). SOR and SOL values ranged from 25.80 μg/mm³ (30% N_TiO₂) to 28.01 μg/mm³ (OSP) and 23.88 μg/mm³ (30% N_TiO₂) to 25.39 μg/mm³ (25% N_TiO₂). CYTO results indicated that pulp cells exposed to experimental materials displayed comparable viabilities (p > 0.05) to those of OSP. Experimental materials displayed comparable SOR, SOL and CYTO values (p > 0.05) when compared to unaltered materials. N_TiO₂ incorporation have not adversely impacted SOR, SOL and CYTO properties of unaltered adhesives.

Antimicrobial Peptide-Polymer Conjugates for Dentistry

Bacterial adhesion and growth at the composite/adhesive/tooth interface remain the primary cause of dental composite restoration failure. Early colonizers, including Streptococcus mutans, play a critical role in the formation of dental caries by creating an environment that reduces the adhesive's integrity. Subsequently, other bacterial species, biofilm formation, and lactic acid from S. mutans demineralize the adjoining tooth. Because of their broad spectrum of antibacterial activity and low risk for antibiotic resistance, antimicrobial peptides (AMPs) have received significant attention to prevent bacterial biofilms. Harnessing the potential of AMPs is still very limited in dentistry-a few studies have explored peptide-enabled antimicrobial adhesive copolymer systems using mainly nonspecific adsorption. In the current investigation, to avoid limitations from nonspecific adsorption and to prevent potential peptide leakage out of the resin, we conjugated an AMP with a commonly used monomer for dental adhesive formulation. To tailor the flexibility between the peptide and the resin material, we designed two different spacer domains. The spacer-integrated antimicrobial peptides were conjugated to methacrylate (MA), and the resulting MA-AMP monomers were next copolymerized into dental adhesives as AMP-polymer conjugates. The resulting bioactivity of the polymethacrylate-based AMP conjugated matrix activity was investigated. The antimicrobial peptide conjugated to the resin matrix demonstrated significant antimicrobial activity against S. mutans. Secondary structure analyses of conjugated peptides were applied to understand the activity differential. When mechanical properties of the adhesive system were investigated with respect to AMP and cross-linking concentration, resulting AMP-polymer conjugates maintained higher compressive moduli compared to hydrogel analogues including polyHEMA. Overall, our result provides a robust approach to develop a fine-tuned bioenabled peptide adhesive system with improved mechanical properties and antimicrobial activity. The results of this study represent a critical step toward the development of peptide-conjugated dentin adhesives for treatment of secondary caries and the enhanced durability of dental composite restorations.

Multifunctional monomer acts as co-initiator and crosslinker to provide autonomous strengthening with enhanced hydrolytic stability in dental adhesives

OBJECTIVE

The purpose of this study was to evaluate a new synthesized multifunctional monomer, aminosilane functionalized methacrylate (ASMA), containing polymerizable methacrylate, tertiary amine, and methoxysilane functionalities in dental adhesive formulations, and to investigate the polymerization kinetics, leachates, thermal and mechanical properties of copolymers.

METHODS

Adhesive contained HEMA/BisGMA (45/55, w/w) was used as a control, and mixtures based on HEMA/BisGMA/ASMA at the mass ratio of 45/(55-x)/x were used as experimental adhesive. Adhesives were characterized with regard to water miscibility, photo-polymerization behavior (Fourier transform infrared spectroscopy, FTIR), leached co-monomers (high performance liquid chromatography, HPLC), thermal properties (modulated differential scanning calorimeter, MDSC), and mechanical properties (dynamic mechanical analyzer, DMA). Stress relaxation times and the corresponding moduli, obtained from stress relaxation tests, are used in a simulated linear loading case.

RESULTS

As compared to the control, ASMA-containing adhesives showed higher water miscibility, lower viscosity, improved monomer-to-polymer conversion, significantly greater T_g and rubbery modulus. HPLC results indicated a substantial reduction of leached HEMA (up to 85wt%) and BisGMA (up to 55wt%) in ethanol. The simulation reveals that the ASMA-containing adhesive becomes substantially stiffer than the control.

SIGNIFICANCE

ASMA monomer plays multiple roles, i.e. it serves as both a co-initiator and crosslinker while also providing autonomous strengthening and enhanced hydrolytic stability in the adhesive formulations. This multifunctional monomer offers significant promise for improving the durability of the adhesive at the composite/tooth interface.

New silyl-functionalized BisGMA provides autonomous strengthening without leaching for dental adhesives

Resin-based composite has overtaken dental amalgam as the most popular material for direct restorative dentistry. In spite of this popularity the clinical lifetime of composite restorations is threatened by recurrent decay. Degradation of the adhesive leads to gaps at the composite/tooth interface-bacteria, bacterial by-products and fluids infiltrate the gaps leading to recurrent decay and composite restoration failure. The durability of resin-dentin bonds is a major problem. We address this problem by synthesizing silyl-functionalized BisGMA (e.g., silyl-BisGMA), formulating dental adhesives with the new monomer and determining the physicochemical properties and leaching characteristics of the silyl-BisGMA adhesives. Silyl-BisGMA was synthesized by stoichiometric amounts of BisGMA and 3-isocyanatopropyl trimethoxysilane (IPTMS). The control adhesive was a mixture based on HEMA/BisGMA (45/55, w/w). In the experimental formulations, BisGMA was partially or completely replaced by silyl-BisGMA. Water miscibility, polymerization behavior (Fourier transform infrared spectroscopy, FTIR), thermal property (modulated differential scanning calorimetry, MDSC), mechanical properties in dry and wet conditions (dynamic mechanical analysis, DMA), and leached species (HPLC) were investigated. Data from all tests were submitted to appropriate statistical analysis (α = 0.05). Silyl-BisGMA-containing adhesives exhibited comparable water miscibility, lower viscosities, and significantly improved degree of conversion of CC bond as compared to the control. After 4 weeks aqueous aging, the glass transition temperature and rubbery moduli of the experimental copolymers were significantly greater than the control (p < 0.05). HPLC results indicated a substantial reduction of leached HEMA (up to 99 wt%) and BisGMA (up to 90 wt%). By introducing silyl-functional group, the new BisGMA derivative exhibited potential as a monomer that can lead to dental adhesives with improved mechanical properties and reduced leaching under conditions relevant to the oral environment. STATEMENT OF SIGNIFICANCE: The low-viscosity adhesive that bonds the composite to the tooth (enamel and dentin) is intended to seal and stabilize the composite/tooth interface, but it degrades leading to a breach at the composite/tooth margin. As the most popular crosslinking monomer in adhesives, Bisphenol A-glycerolate dimethacrylate (BisGMA) has limitations, e.g. susceptible to hydrolysis and concomitant property degradation. A methoxysilyl-functionalized BisGMA derivative (silyl-BisGMA) was introduced in this work to respond to these limitations. Our results indicated that by introducing silyl-BisGMA, higher crosslinked networks were obtained without sacrificing the homogeneity, and the leached amount of HEMA was reduced up to 99%. This novel resin offers potential benefits including prolonging the functional lifetime of dental resin materials.

Fabrication of hybrid crosslinked network with buffering capabilities and autonomous strengthening characteristics for dental adhesives

Ingress of bacteria and fluids at the interfacial gaps between the restorative composite biomaterial and the tooth structure contribute to recurrent decay and failure of the composite restoration. The inability of the material to increase the pH at the composite/tooth interface facilitates the outgrowth of bacteria. Neutralizing the microenvironment at the tooth/composite interface offers promise for reducing the damage provoked by cariogenic and aciduric bacteria. We address this problem by designing a dental adhesive composed of hybrid network to provide buffering and autonomous strengthening simultaneously. Two amino functional silanes, 2-hydroxy-3-morpholinopropyl (3-(triethoxysilyl)propyl) carbamate and 2-hydroxy-3-morpholinopropyl (3-(trimethoxysilyl)propyl) carbamate were synthesized and used as co-monomers. Combining free radical initiated polymerization (polymethacrylate-based network) and photoacid-induced sol-gel reaction (polysiloxane) results in the hybrid network formation. Resulting formulations were characterized with regard to real-time photo-polymerization, water sorption, leached species, neutralization, and mechanical properties. Results from real-time FTIR spectroscopic studies indicated that ethoxy was less reactive than methoxy substituent. The neutralization results demonstrated that the methoxy-containing adhesives have acute and delayed buffering capabilities. The mechanical properties of synthetic copolymers tested in dry conditions were improved via condensation reaction of the hydrolyzed organosilanes. The leaching from methoxy containing copolymers was significantly reduced. The sol-gel reaction provided a chronic and persistent reaction in wet condition-performance that offers potential for reducing secondary decay and increasing the functional lifetime of dental adhesives.

STATEMENT OF SIGNIFICANCE

The interfacial gaps between the restorative composite biomaterial and the tooth structure contributes to recurrent decay and failure of the composite restoration. The inability of the material to increase the pH at the composite/tooth interface facilitates the outgrowth of more cariogenic and aciduric bacteria. This paper reports a novel, synthetic resin that provides buffering capability and autonomous strengthening characteristics. In this work, two amino functional silanes were synthesized and the effect of alkoxy substitutions on the photoacid-induced sol-gel reaction was investigated. We evaluated the neutralization capability (monitoring the pH of lactic acid solution) and the autonomous strengthening property (monitoring the mechanical properties of the hybrid copolymers under wet conditions and quantitatively analyzing the leachable species by HPLC). The novel resin investigated in this study offers the potential benefits of reducing the risk of recurrent decay and prolonging the functional lifetime of dental adhesives.

Probing the neutralization behavior of zwitterionic monomer-containing dental adhesive

OBJECTIVE

To investigate the polymerization kinetics, neutralization behavior, and mechanical properties of amine-functionalized dental adhesive cured in the presence of zwitterionic monomer, methacryloyloxyethyl phosphorylcholine (MPC).

METHODS

The control adhesive was a mixture based on HEMA/BisGMA/2-N-morpholinoethyl methacrylate (MEMA) (40/30/30, w/w/w). The control and experimental formulations containing MPC were characterized with regard to water miscibility of liquid resins, photopolymerization kinetics, water sorption and solubility, dynamic mechanical properties and leachables from the polymers (aged in ethanol). The neutralization behavior of the adhesives was determined by monitoring the pH of lactic acid (LA) solution.

RESULTS

The water miscibility decreased with increasing MPC amount. The water sorption of experimental copolymer specimen was greater than the control. The addition of 8wt% water led to improved photo-polymerization efficiency for experimental formulations at MPC of 2.5 and 5wt%, and significant reduction in the cumulative amounts of leached HEMA, BisGMA, and MEMA, i.e. 90, 60 and 50% reduction, respectively. The neutralization rate of MPC-containing adhesive was faster than control. The optimal MPC concentration in the formulations was 5wt%.

SIGNIFICANCE

Incompatibility between MEMA and MPC led to a decrease in water miscibility of the liquid resins. Water (at 8wt%) in the MPC-containing formulations (2.5-5wt% MPC) led to higher DC, faster RPmax and significant reduction in leached HEMA, BisGMA, and MEMA. The neutralization rate was enhanced with the addition of MPC in the amine-containing formulation. Promoting the neutralization capability of dentin adhesives could play an important role in reducing recurrent decay at the composite/tooth interface.

Biophysical characterization of functionalized titania nanoparticles and their application in dental adhesives

It is demonstrated that carboxylic acid-functionalized titanium dioxide (TiO₂) NPs produce significantly higher levels of reactive oxygen species (ROS) after visible light irradiation (400-800nm, 1600mW/cm²) in comparison to nonfunctionalized TiO₂ NPs. The level of ROS produced under these irradiation conditions was not capable of generating oxidatively induced DNA damage in a cell-free system for TiO₂ concentrations of 0.5mg/L or 5mg/L. In addition, specific incorporation of the acrylic acid-functionalized TiO₂ NPs into dental composites allowed us to utilize the generated ROS to enhance photopolymerization (curing and degree of vinyl conversion (DC)) of resin adhesives and create mechanically superior and biocompatible materials for dental applications. Incorporation of the TiO₂ NPs into selected dental composites increased the mean DC values by ≈7%. The modified TiO₂ materials and dental composite materials were extensively characterized using thermogravimetric analysis, electron microscopy, Fourier transform infrared spectroscopy, and electron paramagnetic resonance. Notably, dental adhesives incorporated with acrylic acid-functionalized TiO₂ NPs produced stronger bonds to human teeth following visible light curing in comparison to traditional dental adhesives not containing NPs with an increase in the shear bond strength of ≈29%. In addition, no leaching of the incorporated NPs was detectable from the dental adhesives after 2500 thermal cycles using inductively coupled plasma-optical emission spectroscopy, indicating that biocompatibility of the adhesives was not compromised after extensive aging. These findings suggest that NP-induced ROS may be useful to produce enhanced nanocomposite materials for selected applications in the medical device field.

STATEMENT OF SIGNIFICANCE

Titanium dioxide nanoparticles (TiO₂ NPs) have unique photocatalytic, antibacterial and UV-absorbing properties that make them beneficial additives in adhesives and composites. However, there is concern that the reactive oxygen species (ROS) produced by photoactivated TiO₂ NPs might pose toxicological risks. We demonstrate that it is possible to incorporate acid-functionalized TiO₂ NPs into dental resins which can be applied as dental adhesives to human teeth. The ROS generated by these NPs through visible-light irradiation may be utilized to increase the degree of vinyl conversion of resins, leading to adhesives that have an enhanced shear-bond strength to human teeth. Investigation into the potential genotoxicity of the NPs and their potential for release from dental composites indicated a low risk of genotoxic effects.

Degradation of poly(β-amino ester) gels in alcohols through transesterification: A method to conjugate drugs to polymer matrices

Poly(β amino ester) polymers have received growing attention in the literature, owing to their ease of synthesis, versatile co-monomer selection, and highly tunable degradation kinetics. As such, they have shown extensive potential in many biomedical applications as well. In this work, it is demonstrated for the first time that PβAE polymers containing primary and secondary amine groups can undergo degradation by primary alcohols via transesterification mechanism. While this work emphasizes an important aspect of solvent compatibility of these networks, it also represents an interesting, simple mechanism for post synthesis drug incorporation, with riboflavin conjugation being demonstrated as a model compound.

New acid BisGMA analogs for dental adhesive applications with antimicrobial activity

OBJECTIVE

To achieve bisphenol A glycerolate dimethacrylate (BisGMA) analogs with reduced viscosity to be used in the formulation of dental adhesives containing biocidal components.

METHODS

A series of low-viscosity BisGMA derivatives (η: 39-12Pas) modified with 30, 60 and, respectively 80mol% carboxylic acid units were synthesized and characterized. Hydrogen bonding interactions in our monomers, the photopolymerization behavior and implicitly the conversion degree (DC) for some experimental adhesive formulations containing acid-modified BisGMA, commercial BisGMA (only in F1-F3), triethyleneglycol dimethacrylate and 2-hydroxyethyl methacrylate were examined by FTIR spectroscopy. The water effects on the photocrosslinked networks together with the flexural strength/modulus were also investigated. The adhesive penetration into the dentin surface was surveyed by SEM analysis, and the antimicrobial activity triggered by the incorporation of 0.5wt% AgNO₃, 10wt% zinc methacrylate or 1wt% triclosan methacrylate in selected adhesive formulations on the growth of Streptococcus mutans and Candida albicans strains was evidenced.

RESULTS

The contribution of the hydrogen bonding interactions was found to be lower in BisGMA derivatives than in non-modified BisGMA, and the DC varied between 56.5 (F6) and 83.7% (F1) compared with a control formulation based on BisGMA:TEGDMA (DC=58.2%). The flexural strength and flexural modulus varied in the range 33.7MPa (F6)-54.4MPa (F8)MPa and 0.64 (F6)-1.43 (F8)GPa, respectively. SEM observation of adhesive-dentin interface revealed the formation of resin tags for the carboxyl-containing adhesive, while for the control adhesive they are hardly formed. Also, the microorganism development was inhibited, the proposed materials displaying antimicrobial activity.

SIGNIFICANCE

The experimental formulations based on carboxyl-functionalized BisGMA exhibit a similar or even improved behavior over control sample, suggesting their potential applicability as antimicrobial dental adhesives.

Mimicking nature: Self-strengthening properties in a dental adhesive

Chemical and enzymatic hydrolysis provoke a cascade of events that undermine methacrylate-based adhesives and the bond formed at the tooth/composite interface. Infiltration of noxious agents, e.g. enzymes, bacteria, and so forth, into the spaces created by the defective bond will ultimately lead to failure of the composite restoration. This paper reports a novel, synthetic resin that provides enhanced hydrolytic stability as a result of intrinsic reinforcement of the polymer network. The behavior of this novel resin, which contains γ-methacryloxyproyl trimethoxysilane (MPS) as its Si-based compound, is reminiscent of self-strengthening properties found in nature. The efforts in this paper are focused on two essential aspects: the visible-light irradiation induced (photoacid-induced) sol-gel reaction and the mechanism leading to intrinsic self-strengthening. The FTIR band at 2840cm(-1) corresponding to CH3 symmetric stretch in -Si-O-CH3 was used to evaluate the sol-gel reaction. Results from the real-time FTIR indicated that the newly developed resin showed a limited sol-gel reaction (<5%) during visible-light irradiation, but after 48h dark storage, the reaction was over 65%. The condensation of methoxysilane mainly occurred under wet conditions. The storage moduli and glass transition temperature of the copolymers increased in wet conditions with the increasing MPS content. The cumulative amounts of leached species decreased significantly when the MPS-containing adhesive was used. The results suggest that the polymethacrylate-based network, which formed first as a result of free radical initiated polymerization, retarded the photoacid-induced sol-gel reaction. The sol-gel reaction provided a persistent, intrinsic reinforcement of the polymer network in both neutral and acidic conditions. This behavior led to enhanced mechanical properties of the dental adhesives under conditions that simulate the wet, oral environment.

STATEMENT OF SIGNIFICANCE

A self-strengthening dental adhesive system was developed through a dual curing process, which involves the free radical photopolymerization followed by slow hydrolysis and condensation (photoacid-induced sol-gel reaction) of alkoxylsilane groups. The concept of "living" photoacid-induced sol-gel reaction with visible-light irradiation was confirmed in the polymer. The sol-gel reaction was retarded by the polymethacrylate network, which was generated first; the network extended the life and retained the activity of silanol groups. The self-strengthening behavior was evaluated by monitoring the mechanical properties of the hybrid copolymers under wet conditions. The present research demonstrates the sol-gel reaction in highly crosslinked network as a potentially powerful strategy to prolong the functional lifetime of engineered biomaterials in wet environments.

Probing the dual function of a novel tertiary amine compound in dentin adhesive formulations

OBJECTIVES

A novel tertiary amine compound containing three methacrylate-urethane groups was synthesized for application in dentin adhesives. The synthesis, photopolymerization kinetics, and leaching were examined in an earlier study using this novel compound as the co-initiator (0.5 and 1.75wt% based on the total resin mass). The objective of this work was to investigate the potential of TUMA (8-(2-(((2-(methacryloyloxy)ethyl)carbamoyl)oxy)propyl)-6,10-dimethyl-4,12-dioxo-5,11-dioxa-3,8,13-triazapentadecane-1,15-diyl bis(2-methylacrylate)) to serve simultaneously as a co-initiator and co-monomer (15-45wt% based on the total resin mass) in dentin adhesive formulations. The polymerization kinetics, water sorption and dynamic mechanical properties of these novel formulations were determined.

MATERIALS AND METHOD

The monomer system contained Bisphenol A glycerolate dimethacrylate (BisGMA), 2-hydroxyethylmethacrylate (HEMA) and TUMA (synthesized in our lab) at the mass ratio of 45/(55-x)/x. Two photoinitiator (PI) systems were compared. One initiator system contains three components: camphorquinone (CQ), diphenyliodonium hexafluorophosphate (DPIHP) and ethyl-4-(dimethylamino) benzoate (EDMAB) and the second initiator system contains CQ and DPIHP. The control adhesive formulations are: C0-3: HEMA/BisGMA 45/55 (w/w) and 3-component PI and C0-2: HEMA/BisGMA 45/55 (w/w) and 2-component PI. These controls were used as a comparison to the experimental adhesive resins (Ex-3 or Ex-2), in which x represents the weight percentage of synthesized co-monomer (TUMA) to replace part of BisGMA. The control and experimental adhesive formulations were photo-polymerized and compared with regard to the degree of conversion (DC), polymerization rate (Rp), water sorption and dynamic mechanical analysis (DMA) under both dry and wet conditions.

RESULTS

C0-3 and Ex-3 formulations had similar DC, while the DC of Ex-2 formulation was higher than C0-2. The DC was similar when comparing the two- component with the three-component photoinitiator system when TUMA was used at the same concentration. DMA under dry conditions shows higher rubbery storage modulus for all experimental formulations, while storage modulus at rubbery region under wet conditions was decreased as compared with control (C0-3). There was no statistically significant difference for the DMA results under both dry and wet conditions when comparing two- and three-component initiator systems with the same TUMA concentration.

SIGNIFICANCE

The newly synthesized TUMA could serve simultaneously as a co-monomer and co-initiator in the absence of commercial co-initiator. This study provides information for the future development of new co-monomer/co-initiator for dentin adhesives and dental composites.

Tris(trimethylsilyl)silane as a co-initiator for dental adhesive: Photo-polymerization kinetics and dynamic mechanical property

OBJECTIVES

The purpose of this study was to evaluate the polymerization behavior of a model dentin adhesive with tris(trimethylsilyl)silane (TTMSS) as a co-initiator, and to investigate the polymerization kinetics and mechanical properties of copolymers in dry and wet conditions.

METHODS

A co-monomer mixture based on HEMA/BisGMA (45/55, w/w) was used as a model dentin adhesive. The photoinitiator system included camphorquinone (CQ) as the photosensitizer and the co-initiator was ethyl-4-(dimethylamino) benzoate (EDMAB) or TTMSS. Iodonium salt, diphenyliodonium hexafluorophosphate (DPIHP) serving as a catalyst, was selectively added into the adhesive formulations. The control and the experimental formulations were characterized with regard to the degree of conversion (DC) and dynamic mechanical properties under dry and wet conditions.

RESULTS

In two-component photoinitiator system (CQ/TTMSS), with an increase of TTMSS concentration, the polymerization rate and DC of CC double bond increased, and showed a dependence on the irradiation time and curing light intensity. The copolymers that contained the three-component photoinitiator system (CQ/TTMSS/DPIHP) showed similar dynamic mechanical properties, under both dry and wet conditions, to the EDMAB-containing system.

SIGNIFICANCE

The DC of formulations using TTMSS as co-initiator showed a strong dependence on irradiation time. With the addition of TTMSS, the maximum polymerization rate can be adjusted and the network structure became more homogenous. The results indicated that the TTMSS could be used as a substitute for amine-type co-initiator in visible-light induced free radical polymerization of methacrylate-based dentin adhesives.

The influence of water on visible-light initiated free-radical/cationic ring-opening hybrid polymerization of methacrylate/epoxy: Polymerization kinetics, crosslinking structure and dynamic mechanical properties

The objective of this study was to determine the influence of water on the polymerization kinetics, crosslinking structure and dynamic mechanical properties of methacrylate/epoxy polymers cured by visible-light initiated free-radical/cationic ring-opening hybrid polymerization. Water-containing formulations were prepared by adding ~4-7 wt% D2O depending on the water miscibility of monomer resins. The water-containing adhesives were compared with the adhesives photo-cured in the absence of water. The results show an improved degree of conversion for both methacrylates and epoxy by adding water. The rate of the epoxy cationic ring-opening reaction is increased while the rate of free radical polymerization is decreased in the presence of water. The decreased crosslinking density noted in the presence of water suggests that the chain transfer reaction between water and epoxy competes with the hydroxyl-based chain transfer mechanism. There is potential application of this visible-light initiated hybrid polymerization in biomaterials, e.g. dental restorations and tissue engineering scaffolds.

Effect of crosslinking density of polymers and chemical structure of amine-containing monomers on the neutralization capacity of dentin adhesives

OBJECTIVES

Neutralization of the acidic micro-environment at the tooth/material interface is expected to provide enhanced durability for dental composite restorations. The objective of this study is to explore the effect of amine-containing monomer formulations and the crosslinking density of the resultant polymers on the neutralization capacity.

MATERIALS AND METHODS

The co-monomer system was varied systematically to obtain different proportions of Bisphenol A glycerolate dimethacrylate (BisGMA) and 2-hydroxyethyl methacrylate (HEMA), while maintaining a constant amount of amine-containing methacrylate monomer. A series of amine-containing monomers covering a range of pKa values were examined. Crosslinking density of formed copolymers was controlled by adjusting the weight content of the dimethacrylate monomer BisGMA. Lactic acid (LA) was used as a probe to analyze the effectiveness of the basic polymers to neutralize acid. The neutralization capacity of each amine-containing crosslinked copolymer was characterized by measuring pH as a function of time when the specimens were soaked in 1-mM LA solution, and the results were compared to the control formulations composed solely of BisGMA and HEMA. Polymer surfaces were examined using the methyl orange (MO) assay to quantify the amount of accessible amine groups.

RESULTS

For each amine-containing crosslinked co-polymer, the neutralization capacity is enhanced by decreasing crosslinking density (e.g., by reducing BisGMA concentration in the formulation). In addition, more amine groups were accessible when crosslinking density was decreased. For different amine-containing polymers with the same BisGMA concentration, the neutralization capacity is higher when the amino monomers with higher pKa values were used in the formulations.

SIGNIFICANCE

This is the first time that the neutralization capacity based on crosslinked dental polymers has been studied. The information is important for future development of durable dentin adhesives.

Compositional design and optimization of dentin adhesive with neutralization capability

OBJECTIVES

The objective of this work was to investigate the polymerization behavior, neutralization capability, and mechanical properties of dentin adhesive formulations with the addition of the tertiary amine co-monomer, 2-N-morpholinoethyl methacrylate (MEMA).

METHODS

A co-monomer mixture based on HEMA/BisGMA (45/55, w/w) was used as a control adhesive. Compared with the control formulation, the MEMA-containing adhesive formulations were characterized comprehensively with regard to water miscibility of liquid resin, water sorption and solubility of cured polymer, real-time photopolymerization kinetics, dynamic mechanical analysis (DMA), and modulated differential scanning calorimetry (MDSC). The neutralization capacity was characterized by monitoring the pH shift of 1mM lactic acid (LA) solution, in which the adhesive polymers were soaked.

RESULTS

With increasing MEMA concentrations, experimental copolymers showed higher water sorption, lower glass transition temperature and lower crosslinking density compared to the control. The pH values of LA solution gradually increased from 3.5 to about 6.0-6.5 after 90 days. With the increase in crosslinking density of the copolymers, the neutralization rate was depressed. The optimal MEMA concentration was between 20 and 40 wt%.

CONCLUSIONS

As compared to the control, the results indicated that the MEMA-functionalized copolymer showed neutralization capability. The crosslinking density of the copolymer networks influenced the neutralization rate.

Development of methacrylate/silorane hybrid monomer system: Relationship between photopolymerization behavior and dynamic mechanical properties

Resin chemistries for dental composite are evolving as noted by the introduction of silorane-based composites in 2007. This shift in the landscape from methacrylate-based composites has fueled the quest for versatile methacrylate-silorane adhesives. The objective of this study was to evaluate the polymerization behavior and structure/property relationships of methacrylate-silorane hybrid systems. Amine compound ethyl-4-(dimethylamino) benzoate (EDMAB) or silane compound tris(trimethylsilyl) silane (TTMSS) was selected as coinitiators. The mechanical properties of the copolymer were improved significantly at low concentrations (15, 25, or 35 wt %) of silorane when EDMAB was used as coinitiator. The rubbery moduli of these experimental copolymers were increased by up to 260%, compared with that of the control (30.8 ± 1.9 MPa). Visible phase separation appeared in these formulations if the silorane concentrations in the formulations were 50-75 wt %. The use of TTMSS as coinitiator decreased the phase separation, but there was a concomitant decrease in mechanical properties. In the neat methacrylate formulations, the maximum rates of free-radical polymerization with EDMAB or TTMSS were 0.28 or 0.06 s(-1) , respectively. In the neat silorane resin, the maximum rates of cationic ring-opening polymerization with EDMAB or TTMSS were 0.056 or 0.087 s(-1) , respectively. The phase separation phenomenon may be attributed to differences in the rates of free-radical polymerization of methacrylates and cationic ring-opening polymerization of silorane. In the hybrid systems, free-radical polymerization initiated with EDMAB led to higher crosslink density and better mechanical properties under dry/wet conditions. These beneficial effects were, however, associated with an increase in heterogeneity in the network structure. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 841-852, 2016.

Synthesis and evaluation of a novel co-initiator for dentin adhesives: polymerization kinetics and leachables study

A new tertiary amine co-initiator (TUMA) containing three methacrylate-urethane groups was synthesized for application in dentin adhesives. The photopolymerization kinetics and leaching of unreacted components from methacrylate-based dental polymers formulated with this new co-initiator were determined. The newly synthesize co-initiator showed good chemical stability and decreased amine release from the initiator system. This study provides important information for the future development of biocompatible dentin adhesives/composites.

Visible-Light Initiated Free-Radical/Cationic Ring-Opening Hybrid Photopolymerization of Methacrylate/Epoxy: Polymerization Kinetics, Crosslinking Structure, and Dynamic Mechanical Properties

The effects of polymerization kinetics and chemical miscibility on the crosslinking structure and mechanical properties of polymers cured by visible-light initiated free-radical/cationic ring-opening hybrid photopolymerization are determined. A three-component initiator system is used and the monomer system contains methacrylates and epoxides. The photopolymerization kinetics is monitored in situ by Fourier transform infrared-attenuated total reflectance. The crosslinking structure is studied by modulated differential scanning calorimetry and dynamic mechanical analysis. X-ray microcomputed tomography is used to evaluate microphase separation. The mechanical properties of polymers formed by hybrid formed by free-radical polymerization. These investigations mark the first time that the benefits of the chain transfer reaction between epoxy and hydroxyl groups of methacrylate, on the crosslinking network and microphase separation during hybrid visible-light initiated photopolymerization, have been determined.