Engineering Interfacial Integrity with Hydrolytic-Resistant, Self-Reinforcing Dentin Adhesive.

The leading cause of composite restoration failure is secondary caries, and although caries is a multifactorial problem, weak, damage-prone adhesives play a pivotal role in the high susceptibility of composite restorations to secondary caries. Our group has developed synthetic resins that capitalize on free-radical polymerization and sol-gel reactions to provide dental adhesives with enhanced properties. The resins contain γ-methacryloxypropyltrimethoxysilane (MPS) as the Si-based compound. This study investigated the properties of methacrylate-based resins containing methacryloxymethyltrimethoxysilane (MMeS) as a short-chain alternative. The degree of conversion (DC), polymerization kinetics, water sorption, mechanical properties, and leachates of MMeS- and MPS-resins with 55 and 30 wt% BisGMA-crosslinker were determined. The formulations were used as model adhesives, and the adhesive/dentin (a/d) interfaces were analyzed using chemometrics-assisted micro-Raman spectroscopy. The properties of the 55 wt% formulations were comparable. In the 30 wt% BisGMA formulations, the MMeS-resin exhibited faster polymerization, lower DC, reduced leachates, and increased storage and loss moduli, glass transition (Tg), crosslink density, and heterogeneity. The spectroscopic results indicated a comparable spatial distribution of resin, mineralized, and demineralized dentin across the a/d interfaces. The hydrolytically stable experimental short-chain-silane-monomer dental adhesive provides enhanced mechanical properties through autonomous strengthening and offers a promising strategy for the development of restorative dental materials with extended service life.

The interaction between mucin and poly(amino acid)s with controlled cationic group content in bulk phase and in thin layers.

The type and concentration of charged groups in polymers have a key role in mucoadhesive interactions. A series of cationic poly(amino acid)s with different charge densities was designed to unravel the correlation between chemical structure and mucin-polymer interactions. Colloidal interactions between the mucin protein and synthetic polyaspartamides were tested by dynamic light scattering, zeta potential measurements and turbidimetric titration as a function of polymer-to-mucin mass ratio. The mucoadhesive interactions displayed a strongly non-linear change with polymer composition. The attractive interactions between mucin and the polyaspartamides with at least 50 % cationic groups caused increased light scattering of dispersions due to the aggregation of mucin particles upon their charge reversal. Interactions were further analysed in a thin mucin layer to model life-like situations using a quartz crystal microbalance (QCM) in flow mode. Results pointed out that the fully cationic polyaspartamide is not necessarily superior to derivatives with lower cationic group content. The maximum of adsorbed mass of polymers on mucin was experienced at medium cationic group contents. This emphasizes the relevance of cationic polyaspartamides as mucoadhesive excipients due to their multiple functionalities and the possibility of fine-tuning their interactions with mucin via straightforward chemical steps.

An orthotropic continuum model with substructure evolution for describing bone remodeling: an interpretation of the primary mechanism behind Wolff's law.

We propose a variational approach that employs a generalized principle of virtual work to estimate both the mechanical response and the changes in living bone tissue during the remodeling process. This approach provides an explanation for the adaptive regulation of the bone substructure in the context of orthotropic material symmetry. We specifically focus upon the crucial gradual adjustment of bone tissue as a structural material that adapts its mechanical features, such as materials stiffnesses and microstructure, in response to the evolving loading conditions. We postulate that the evolution process relies on a feedback mechanism involving multiple stimulus signals. The mechanical and remodeling behavior of bone tissue is clearly a complex process that is difficult to describe within the framework of classical continuum theories. For this reason, a generalized continuum elastic theory is employed as a proper mathematical context for an adequate description of the examined phenomenon. To simplify the investigation, we considered a two-dimensional problem. Numerical simulations have been performed to illustrate bone evolution in a few significant cases: the bending of a rectangular cantilever plate and a three-point flexure test. The results are encouraging because they can replicate the optimization process observed in bone remodeling. The proposed model provides a likely distribution of stiffnesses and accurately represents the arrangement of trabeculae macroscopically described by the orthotropic symmetry directions, as supported by experimental evidence from the trajectorial theory.

In situ gelation of thiolated poly(aspartic acid) derivatives through oxidant-free disulfide formation for ophthalmic drug delivery.

Efficient topical treatment of ocular diseases requires a prolonged residence time of drug formulations. An in situ gelling, mucoadhesive system can provide improved residence time while keeps the installation of the formulation easy and accurate due to its low initial viscosity. We synthesized a two-component, biocompatible water-based liquid formulation showing in situ gelation upon mixing. S-protected, preactivated derivatives of thiolated poly(aspartic acid) (PASP-SS-MNA) were synthesized by coupling the free thiol groups of thiolated poly(aspartic acid) (PASP-SH) with 6-mercaptonicotinic acid (MNA). The amount of protecting groups was 242, 341, and 530 µmol/g depending on the degree of thiolation of PASP. The chemical interaction between PASP-SS-MNA and mucin was proven, indicating the mucoadhesive properties. Disulfide cross-linked hydrogels were formed in situ without an oxidizing agent by mixing the aqueous solutions of PASP-SS-MNA and PASP-SH. The gelation time was controlled between 1 and 6 min, while the storage modulus was as high as 4-16 kPa depending on the composition. Swelling experiments showed that hydrogels with no residual thiol groups are stable in phosphate-buffered saline at pH = 7.4. In contrast, the presence of free thiol groups leads to the dissolution of the hydrogel with a rate depending on the excess of thiol groups. The biological safety of the polymers and MNA was confirmed on Madin-Darby Canine Kidney cell line. Furthermore, a prolonged release of ofloxacin was observed at pH = 7.4 compared to a conventional liquid formulation, supporting the potential of the developed biopolymers in ophthalmic drug delivery.

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.

Engineering peptide-polymer hybrids for targeted repair and protection of cervical lesions.

By 2060, nearly 100 million people in the U.S. will be over age 65 years. One-third of these older adults will have root caries, and nearly 80% will have dental erosion. These conditions can cause pain and loss of tooth structure that interfere with eating, speaking, sleeping, and quality of life. Current treatments for root caries and dental erosion have produced unreliable results. For example, the glass-ionomer-cement or composite-resin restorations used to treat these lesions have annual failure rates of 44% and 17%, respectively. These limitations and the pressing need to treat these conditions in the aging population are driving a focus on microinvasive strategies, such as sealants and varnishes. Sealants can inhibit caries on coronal surfaces, but they are ineffective for root caries. For healthy, functionally independent elders, chlorhexidine varnish applied every 3 months inhibits root caries, but this bitter-tasting varnish stains the teeth. Fluoride gel inhibits root caries, but requires prescriptions and daily use, which may not be feasible for some older patients. Silver diamine fluoride can both arrest and inhibit root caries but stains the treated tooth surface black. The limitations of current approaches and high prevalence of root caries and dental erosion in the aging population create an urgent need for microinvasive therapies that can: (a) remineralize damaged dentin; (b) inhibit bacterial activity; and (c) provide durable protection for the root surface. Since cavitated and non-cavitated root lesions are difficult to distinguish, optimal approaches will treat both. This review will explore the multi-factorial elements that contribute to root surface lesions and discuss a multi-pronged strategy to both repair and protect root surfaces. The strategy integrates engineered peptides, novel polymer chemistry, multi-scale structure/property characterization and predictive modeling to develop a durable, microinvasive treatment for root surface lesions.

Rotational diffusion and rotational correlations in frictional amorphous disk packings under shear.

We show here that rotations of round particles in amorphous disk packing reveal various nontrivial microscopic features when the packing is close to rigidification. We analyze experimental measurements on disk packing subjected to simple shear deformation with various inter-particle friction coefficients and across a range of volume fractions where the system is known to stiffen. The analysis of measurements indicates that shear induces diffusive microrotation, that can be both enhanced and suppressed depending upon the volume fraction as well as the inter-particle friction. Rotations also display persistent anticorrelated motion. Spatial correlations in microrotation are observed to be directly correlated with system pressure. These observations point towards the broader mechanical relevance of collective dynamics in the rotational degree of freedom of particles.

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.

Bioinspired multifunctional adhesive system for next generation bio-additively designed dental restorations.

Resin-based composite has overtaken dental amalgam as the most popular material for the repair of lost or damaged tooth structure. In spite of the popularity, the average composite lifetime is about half that of amalgam restorations. The leading cause of composite-restoration failure is decay at the margin where the adhesive is applied. The adhesive is intended to seal the composite/tooth interface, but the adhesive seal to dentin is fragile and readily degraded by acids, enzymes and other oral fluids. The inherent weakness of this material system is attributable to several factors including the lack of antimicrobial properties, remineralization capabilities and durable mechanical performance - elements that are central to the integrity of the adhesive/dentin (a/d) interfacial seal. Our approach to this problem offers a transition from a hybrid to a biohybrid structure. Discrete peptides are tethered to polymers to provide multi-bio-functional adhesive formulations that simultaneously achieve antimicrobial and remineralization properties. The bio-additive materials design combines several functional properties with the goal of providing an adhesive that will serve as a durable barrier to recurrent decay at the composite/tooth interface. This article provides an overview of our multi-faceted approach which uses peptides tethered to polymers and new polymer chemistries to achieve the next generation adhesive system - an adhesive that provides antimicrobial properties, repair of defective dentin and enhanced mechanical performance.

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.

A new approach to airway assessment-"Line of Sight" and more. Recommendations of the Task Force of Airway Management Foundation (AMF).

Assessment of airway is recommended by every airway guideline to ensure safe airway management. Numerous unifactorial and multifactorial tests have been used for airway assessment over the years. However, there is none that can identify all the difficult airways. The reasons for the inadequacy of these methods of airway assessment might be their dependence on difficult to remember and apply mnemonics and scores, inability to identify all the variations from the "normal", and their lack of stress on evaluating the non-patient factors. Airway Management Foundation (AMF) experts and members have been using a different approach, the AMF Approach, to overcome these problems inherent to most available models of airway assessment. This approach suggests a three-step model of airway assessment. The airway manager first makes the assessment of the patient through focused history, focused general examination, and focused airway assessment using the AMF "line of sight" method. The AMF "line of sight" method is a non-mnemonic, non-score-based method of airway assessment wherein the airway manager examines the airway along the line of sight as it moves over the airway and notes down all the variations from the normal. Assessment of non-patient factors follows next and finally there is assimilation of all the information to help identify the available, difficult, and impossible areas of the airway management. The AMF approach is not merely intubation centric but also focuses on all other methods of securing airway and maintaining oxygenation. Airway assessment in the presence of contagion like COVID-19 is also discussed.

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.

Computational analysis of tensile damage and failure of mineralized tissue assisted with experimental observations.

In this study, deformation and failure mechanisms of mineralized tissue (bone) were investigated both experimentally and computationally by performing diametral compression tests on millimetric disk specimens and conducting finite element analysis in which a granular micromechanics-based nonlinear user-defined material model is implemented. The force-displacement relationship obtained in the simulation agreed well with the experimental results. The simulation was also able to capture location of the failure initiation observed in the experiment, which is inside out from the hole along the loading axis. Furthermore, propagation of micro-sized cracks into failure was observed both in the experiment using simultaneous slow-motion microscopy imaging and in the simulation analyzing the local distortion and local volume change within the specimen. The anisotropy evolution was found to be significant around the hole along the loading axis by evaluating the anisotropy index computed using finite element results. In conclusion, this work revealed that the prediction capability of granular micromechanics-based user-defined nonlinear material model (UMAT) is promising considering the match between the results and observations from the physical experiment and finite element analysis such as force-displacement relationship and failure initiation/pattern. This work has also shown that the tensile damage and failure of mineralized tissues can be characterized using diametral compression (split tension) test.

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 Tg 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.

Threats to adhesive/dentin interfacial integrity and next generation bio-enabled multifunctional adhesives.

Nearly 100 million of the 170 million composite and amalgam restorations placed annually in the United States are replacements for failed restorations. The primary reason both composite and amalgam restorations fail is recurrent decay, for which composite restorations experience a 2.0-3.5-fold increase compared to amalgam. Recurrent decay is a pernicious problem-the standard treatment is replacement of defective composites with larger restorations that will also fail, initiating a cycle of ever-larger restorations that can lead to root canals, and eventually, to tooth loss. Unlike amalgam, composite lacks the inherent capability to seal discrepancies at the restorative material/tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal the interface, but the adhesive degrades, which can breach the composite/tooth margin. Bacteria and bacterial by-products such as acids and enzymes infiltrate the marginal gaps and the composite's inability to increase the interfacial pH facilitates cariogenic and aciduric bacterial outgrowth. Together, these characteristics encourage recurrent decay, pulpal damage, and composite failure. This review article examines key biological and physicochemical interactions involved in the failure of composite restorations and discusses innovative strategies to mitigate the negative effects of pathogens at the adhesive/dentin interface. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2466-2475, 2019.

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.

Structure-property relationships for wet dentin adhesive polymers.

Dentin adhesive systems for composite tooth restorations are composed of hydrophilic/hydrophobic monomers, solvents, and photoinitiators. The adhesives undergo phase separation and concomitant compositional change during their application in the wet oral environment; phase separation compromises the quality of the hybrid layer in the adhesive/dentin interface. In this work, the adhesive composition in the hybrid layer can be represented using the phase boundaries of a ternary phase diagram for the hydrophobic monomer/hydrophilic monomer/water system. The polymer phases, previously unaccounted for, play an important role in determining the mechanical behavior of the bulk adhesive, and the chemomechanical properties of the phases are intimately related to the effects produced by differences in the hydrophobic-hydrophilic composition. As the composition of the polymer phases varies from hydrophobic-rich to hydrophilic-rich, the amount of the adsorbed water and the nature of polymer-water interaction vary nonlinearly and strongly correlate with the change in elastic moduli under wet conditions. The failure strain, loss modulus, and glass transition temperature vary nonmonotonically with composition and are explained based upon primary and secondary transitions observed in dynamic mechanical testing. Due to the variability in composition, the assignment of mechanical properties and the choice of suitable constitutive models for polymer phases in the hybrid layer are not straightforward. This work investigates the relationship between composition and chemomechanical properties of the polymer phases formed on the water-adhesive phase boundary using quasistatic and dynamic mechanical testing, mass transfer experiments, and vibrational spectroscopy.

Modulating pH through lysine integrated dental adhesives.

OBJECTIVES: The objective of this study was to explore the effect of lysine integration to dental adhesives with respect to the polymerization kinetics, neutralization capacities in the acidic microenvironment, dynamic mechanical properties, and thermal properties. MATERIALS AND METHOD: Lysine was incorporated into liquid resin formulations at 2.5 and 5.0wt % with additional water/ethanol co-solvents. The co-monomer system contained 2-hydroxyethyl-methacrylate (HEMA) and Bisphenol A glycerolate dimethacrylate (BisGMA) with a mass ratio of 45/55. The kinetics of photopolymerization, neutralization capacities, lysine-leaching, dynamic mechanical properties and thermal properties of the control and experimental adhesives were analyzed. RESULTS: The degree of conversion of the experimental adhesive was increased substantially at 2.5wt% lysine as compared to the control. The experimental polymers provided acute neutralization of the acidic microenvironment. Approximately half of the lysine was released from the polymer network within one month. Under dry conditions and physiologic temperatures, the incorporation of lysine did not compromise the storage modulus. Comparison of the thermal properties suggests that the more compact structure of the control adhesive inhibits movement of the polymer chains resulting in increased Tg. SIGNIFICANCE: Incorporating lysine in the adhesive formulations led to promising results regarding modulating pH, which may serve as one aspect of a multi-spectrum approach for enhancing the durability of composite restorations. The results provide insight and lay a foundation for incorporating amino acids or peptides into adhesive formulations for pH modulation or desired bioactivity at the interfacial margin between the composite and tooth.

Finite-temperature stress calculations in atomic models using moments of position.

Continuum modeling of finite temperature mechanical behavior of atomic systems requires refined description of atomic motions. In this paper, we identify additional kinematical quantities that are relevant for a more accurate continuum description as the system is subjected to step-wise loading. The presented formalism avoids the necessity for atomic trajectory mapping with deformation, provides the definitions of the kinematic variables and their conjugates in real space, and simplifies local work conjugacy. The total work done on an atom under deformation is decomposed into the work corresponding to changing its equilibrium position and work corresponding to changing its second moment about equilibrium position. Correspondingly, we define two kinematic variables: a deformation gradient tensor and a vibration tensor, and derive their stress conjugates, termed here as static and vibration stresses, respectively. The proposed approach is validated using MD simulation in NVT ensembles for fcc aluminum subjected to uniaxial extension. The observed evolution of second moments in the MD simulation with macroscopic deformation is not directly related to the transformation of atomic trajectories through the deformation gradient using generator functions. However, it is noteworthy that deformation leads to a change in the second moment of the trajectories. Correspondingly, the vibration part of the Piola stress becomes particularly significant at high temperature and high tensile strain as the crystal approaches the softening limit. In contrast to the eigenvectors of the deformation gradient, the eigenvectors of the vibration tensor show strong spatial heterogeneity in the vicinity of softening. More importantly, the elliptic distribution of local atomic density transitions to a dumbbell shape, before significant non-affinity in equilibrium positions has occurred.