Microstructural characterization of vocal folds toward a strain-energy model of collagen remodeling

Collagen fibrils are believed to control the immediate deformation of soft tissues under mechanical load. Most extracellular matrix proteins remain intact during frozen sectioning, which allows them to be scanned using atomic force microscopy (AFM). Collagen fibrils are distinguishable because of their periodic roughness wavelength. In the present study, the shape and organization of collagen fibrils in dissected porcine vocal folds were quantified using nonlinear laser scanning microscopy data at the micrometer scale and AFM data at the nanometer scale. Rope-shaped collagen fibrils were observed. The geometric characteristics for the fibrils were fed into a hyperelastic model to predict the biomechanical response of the tissue. The model simulates the micrometer-scale unlocking behavior of collagen bundles when extended from their unloaded configuration. Force spectroscopy using AFM was used to estimate the stiffness of collagen fibrils (1 ± 0.5 MPa). The presence of rope-shaped fibrils is postulated to change the slope of the force–deflection response near the onset of nonlinearity. The proposed model could ultimately be used to evaluate changes in elasticity of soft tissues that result from the collagen remodeling.     

Evaluation of antibacterial and remineralizing nanocomposite and adhesive in rat tooth cavity model

Antibacterial and remineralizing dental composites and adhesives were recently developed to inhibit biofilm acids and combat secondary caries. It is not clear what effect these materials will have on dental pulps in vivo. The objectives of this study were to investigate the antibacterial and remineralizing restorations in a rat tooth cavity model, and determine pulpal inflammatory response and tertiary dentin formation. Nanoparticles of amorphous calcium phosphate (NACP) and antibacterial dimethylaminododecyl methacrylate (DMADDM) were synthesized and incorporated into a composite and an adhesive. Occlusal cavities were prepared in the first molars of rats and restored with four types of restoration: control composite and adhesive; control plus DMADDM; control plus NACP; and control plus both DMADDM and NACP. At 8 or 30 days, rat molars were harvested for histological analysis. For inflammatory cell response, regardless of time periods, the NACP group and the DMADDM + NACP group showed lower scores (better biocompatibility) than the control group (p = 0.014 for 8 days, p = 0.018 for 30 days). For tissue disorganization, NACP and DMADDM + NACP had better scores than the control (p = 0.027) at 30 days. At 8 days, restorations containing NACP had a tertiary dentin thickness (TDT) that was five- to six-fold that of the control. At 30 days, restorations containing NACP had a TDT that was four- to six-fold that of the control. In conclusion, novel antibacterial and remineralizing restorations were tested in rat teeth in vivo for the first time. Composite and adhesive containing NACP and DMADDM exhibited milder pulpal inflammation and much greater tertiary dentin formation than the control adhesive and composite. Therefore, the novel composite and adhesive containing NACP and DMADDM are promising as a new therapeutic restorative system to not only combat oral pathogens and biofilm acids as shown previously, but also facilitate the healing of the dentin–pulp complex.     

Zirconia nanoparticles prepared by laser vaporization as fillers for dental adhesives

Zirconia nanoparticles prepared by laser vaporization were incorporated into the primer or into the adhesive of a commercial adhesive system in order to evaluate its effect on bond strength to dentin. Zirconia nanoparticles (20–50 nm) were prepared using a particular laser vaporization technique and incorporated into the primer (P) or into the adhesive (A) of the Adper Scotchbond Multi-Purpose (SBMP) system at 5, 10, 15 and 20 wt.% by means of mechanical mixing (stirring) and ultrasonication. Control (unfilled) and experimental groups (filled) were applied, according to the manufacturer’s instructions, onto flat mid-coronal human dentin. Composite crowns were built up, stored in distilled water for 24 h at 37 °C and cut into 0.65 ± 0.05 mm² beams following a non-trimming microtensile technique. Specimens were fractured in tension using a universal testing machine (Zwick) and examined by scanning electron microscopy for fractographic analysis. Microtensile bond strength (μTBS) data were analyzed using a two-way ANOVA and modified LSD test at α = 0.05. Analysis of the nanofiller distribution and ultramorphological characterization of the interface were performed by transmission electron microscopy (TEM). Zirconia nanoparticle incorporation into the primer or into the adhesive of SBMP significantly increased μTBS to dentin. Filler concentration only affected μTBS significantly in the P group. Statistically significant differences between groups P and A occurred only at 20 wt.% filler content, with a significantly higher μTBS in group P. TEM micrographs revealed nanoparticle deposition on top of a hybrid layer when incorporated into the primer, whereas they remained dispersed through the adhesive layer in group A. Zirconia nanoparticles incorporation into SBMP increased bond strength to dentin by reinforcing the interface adhesive layer. Nanofiller incorporation into the primer solution showed a tendency of increasing bond strength with increasing concentration. At high concentrations (20 wt.%) nanofiller incorporation was more efficient in increasing bond strength if incorporated in the primer solution. Adding nanofillers to the primer and to the adhesive solutions resulted in different particle distributions at the interface.     

Effect of solvent content on resin hybridization in wet dentin bonding

With wet bonding techniques, the channels between the demineralized dentin collagen fibrils are filled with debris, solvent, and water. Commercial adhesives include solvents such as ethanol or acetone to facilitate resin-infiltration into this wet substrate. Under in vivo conditions, the solvent may be diluted because of repeated exposure of the material to the atmosphere, or concentrated because of separation of the bonding liquids into layers within the bottle. The purpose of this study was to investigate the effect of different concentrations of ethanol (10-50%) on infiltration of the adhesive resin and collagen fibril encapsulation in the adhesive/dentin interface using light microscopy, micro-Raman spectroscopy, and scanning electron microscopy. The results indicated that under wet bonding conditions the hybridization process was highly sensitive to the initial solvent concentration in the adhesive system. The staining and scanning electron microscopy results showed that the quality of the interfacial hybrid layer was poor at the lower (10%) or higher (50%) ethanol content. Micro-Raman analysis indicated that there was a distinct difference in the degree of adhesive penetration among adhesives containing different concentrations of ethanol. Adhesives containing 10 or 50% ethanol did not realize effective penetration; the penetration of the adhesive monomers increased dramatically when the initial ethanol content was 30%. The amount of solvents are essential for achieving effective bonding to dentin.     

Dynamic shear-influenced collagen self-assembly

The ability to influence the direction of polymerization of a self-assembling biomolecular system has the potential to generate materials with extremely high anisotropy. In biological systems where highly-oriented cellular populations give rise to aligned and often load-bearing tissue such organized molecular scaffolds could aid in the contact guidance of cells for engineered tissue constructs (e.g. cornea and tendon). In this investigation we examine the detailed dynamics of pepsin-extracted type I bovine collagen assembly on a glass surface under the influence of flow between two plates. Differential Interference Contrast (DIC) imaging (60×-1.4NA) with focal plane stabilization was used to resolve and track the growth of collagen aggregates on borosilicate glass for 4 different shear rates (500, 80, 20, and 9 s^?1). The detailed morphology of the collagen fibrils/aggregates was examined using Quick Freeze Deep Etch (QFDE) electron microscopy. Nucleation of fibrils on the glass was observed to occur rapidly (～2 min) followed by continued growth of the fibrils. The growth rates were dependent on flow in a complex manner with the highest rate of axial growth (0.1 μ/s) occurring at a shear rate of 9 s^?1. The lowest growth rate occurred at the highest shear. Fibrils were observed to both branch and join during the experiments. The best alignment of fibrils was observed at intermediate shear rates of 20 and 80 s^?1. However, the investigation revealed that fibril directional growth was not stable. At high shear rates, fibrils would often turn downstream forming what we term “hooks” which are likely the combined result of monomer interaction with the initial collagen layer or “mat” and the high shear rate. Further, QFDE examination of fibril morphology demonstrated that the assembled fibrillar structure did not possess native D-periodicity. Instead, fibrils comprised a collection of generally aligned, monomers which were self-assembled to form a fibril-like aggregate. In conclusion, though constant shear-rate clearly influences collagen fibrillar alignment, the formation of highly-organized collagenous arrays of native-like D-banded fibrils remains a challenge. Modulation of shear in combination with surface energy patterning to produce a highly-aligned initial mat may provide significant improvement of both the fibril morphology and alignment.     

The inhibitory effect of polyvinylphosphonic acid on functional matrix metalloproteinase activities in human demineralized dentin

This study has examined the use of polyvinylphosphonic acid (PVPA) as a potential matrix metalloproteinase (MMP) inhibitor and how brief cross-linking of demineralized dentin matrix that did not affect its mechanical properties enhanced the anti-MMP activity of PVPA. The anti-MMP potential of five PVPA concentrations (100–3000 μg ml^–1) was initially screened using a rhMMP-9 colorimetic assay. Demineralized dentin beams were treated with the same five concentrations of PVPA to collagen and then aged for 30 days in a calcium- and zinc-containing medium. The changes in modulus of elasticity, loss of dry mass and dissolution of collagen peptides were measured via three-point bending, precision weighing and hydroxyproline assay, respectively. All tested PVPA concentrations were highly effective (P < 0.05) in inhibiting MMP-9. Ageing in the incubation medium did not significantly alter the modulus of elasticity of the five PVPA treatment groups. Conversely, aged dentin beams from the control group exhibited a significant decline in their modulus of elasticity (P < 0.05) over time. Mass loss from the dentin beams and the corresponding increase in hydroxyproline in the medium in the five PVPA treatment groups were significantly lower than for the control (P < 0.05). PVPA is a potent inhibitor of endogenous MMP activities in demineralized dentin. It may be used as an alternative to chlorhexidine to prevent collagen degradation within hybrid layers to extend the longevity of resin–dentin bonds.     

Galloyl moieties enhance the dentin biomodification potential of plant-derived catechins

Proanthocyanidin-rich plant-derived agents have been shown to enhance dentin biomechanical properties and resistance to collagenase degradation. This study systematically investigated the interaction of chemically well-defined monomeric catechins with dentin extracellular matrix components by evaluating dentin mechanical properties as well as activities of matrix metalloproteinases (MMPs) and cysteine-cathepsins (CTs). Demineralized dentin beams (n = 15) were incubated for 1 h with 0.65% (+)-catechin (C), (−)-catechin gallate (CG), (−)-gallocatechin gallate (GCG), (−)-epicatechin (EC), (−)-epicatechin gallate (ECG), (−)-epigallocatechin (EGC) and (−)-epigallocatechin-3-gallate (EGCG). The modulus of elasticity (E) and the fold increase in E were determined by comparing specimens at baseline and after treatment. Biodegradation rates were assessed by differences in percentage of dry mass before and after incubation with bacterial collagenase. The inhibition of MMP-9 and CT-B by 0.65, 0.065 and 0.0065% of each catechin was determined using fluorimetric proteolytic assay kits. All monomeric catechins led to a significant increase in E. EGCG showed the highest fold increase in E, followed by ECG, CG and GCG. EGCG, ECG, GCG and CG significantly lowered biodegradation rates and inhibited both MMP-9 and CT-B at a concentration of 0.65%. Overall, the 3-O-galloylated monomeric catechins are clearly more potent than their non-galloylated analogues in improving dentin mechanical properties, stabilizing collagen against proteolytic degradation, and inhibiting the activity of MMPs and CTs. The results indicate that galloylation is a key pharmacophore in the monomeric and likely also in the oligomeric proanthocyanidins that exhibit high cross-linking potential for dentin extracellular matrix.     

Polymerization- and solvent-induced phase separation in hydrophilic-rich dentin adhesive mimic

Current dental resin undergoes phase separation into hydrophobic-rich and hydrophilic-rich phases during infiltration of the over-wet demineralized collagen matrix. Such phase separation undermines the integrity and durability of the bond at the composite/tooth interface. This study marks the first time that the polymerization kinetics of model hydrophilic-rich phase of dental adhesive has been determined. Samples were prepared by adding varying water content to neat resins made from 95 and 99 wt.% hydroxyethylmethacrylate and 5 and 1 wt.% (2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl1]-propane prior to light curing. Viscosity of the formulations decreased with increased water content. The photopolymerization kinetics study was carried out with a time-resolved Fourier transform infrared spectrometer. All of the samples exhibited two-stage polymerization behavior which has not been reported previously for dental resin formulation. The lowest secondary rate maxima were observed for water contents of 10–30 wt.%. Differential scanning calorimetry (DSC) showed two glass transition temperatures for the hydrophilic-rich phase of dental adhesive. The DSC results indicate that the heterogeneity within the final polymer structure decreased with increasing water content. The results suggest a reaction mechanism involving both polymerization-induced phase separation and solvent-induced phase separation for the model hydrophilic-rich phase of dental resin.     

Changes in stiffness of resin-infiltrated demineralized dentin after remineralization by a bottom-up biomimetic approach

This study examined changes in elastic modulus, mineral density and ultrastructure of resin-infiltrated dentin after biomimetic remineralization. Sixty demineralized dentin beams were infiltrated with Clearfil Tri-S Bond, One-Step or Prime&Bond NT. They were immersed in simulated body fluid (SBF) for 1 week to maximize water sorption before determining the baseline elastic moduli. For each adhesive (N = 20) half of the beams remained immersed in SBF (control). The rest were immersed in a biomimetic remineralization medium. The elastic moduli were measured weekly for 15 additional weeks. Representative remineralized specimens were evaluated by X-ray microtomography and transmission electron microscopy (TEM). The elastic moduli of control resin-infiltrated dentin remained consistently low, while those immersed in the biomimetic remineralization medium increased by 55–118% after 4 months. X-ray microtomography of the remineralized specimens revealed decreases in mineral density from the beam surface to the beam core that were indicative of external mineral aggregation and internal mineral deposition. Interfibrillar and intrafibrillar remineralization of resin-sparse intertubular dentin were seen under TEM, together with remineralized peritubular dentin. Biomimetic remineralization occurs by diffusion of nanoprecursors and biomimetic analogs in completely demineralized resin-infiltrated dentin and proceeds without the contribution of materials released from a mineralized dentin base.     

The effect of BisGMA on cyclooxygenase-2 expression,PGE2 production and cytotoxicity via reactive oxygen species- and MEK/ERK-dependent and -independent pathways

After operative restoration, some monomers released from dentin bonding agents or composite resin may induce tissue inflammation and affect the vitality of dental pulp. Whether BisGMA, a major monomer of composite resin, may induce prostaglandin release and cytotoxicity to pulp cells and their mechanisms awaits investigation. We found that BisGMA induced cytotoxicity to human dental pulp cells at concentrations higher than 0.075 mm as analyzed by 3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. BisGMA (0.1 mm) also stimulated ERK phosphorylation, PGE₂ production, COX-2 mRNA and protein expression as well as ROS production (as indicated by an increase in cellular DCF fluorescence) in dental pulp cells. Catalase (500 and 1000 U/ml) and U0126 (10 and 20 μm, a MEK inhibitor) effectively prevented the BisGMA-induced ERK activation, PGE₂ production and COX-2 expression. Moreover, catalase can protect the pulp cells from BisGMA cytotoxicity, whereas aspirin and U0126 lacked of this protective activity. These results suggest that BisGMA released from composite resin may potentially affect the vitality of dental pulp and induce pulpal inflammation via stimulation of ROS production, MEK/ERK1/2 activation and subsequent COX-2 gene expression and PGE₂ production. Cytotoxicity of BisGMA to dental pulp cells is related to ROS production, but not directly mediated by MEK activation and PGE₂ production.     

High density type I collagen gels for tissue engineering of whole menisci

This study investigates the potential of high density type I collagen gels as an injectable scaffold for tissue engineering of whole menisci, and compares these results with previous strategies using alginate as an injectable scaffold. Bovine meniscal fibrochondrocytes were mixed with collagen and injected into micro-computed tomography-based molds to create 10 and 20 mg ml^?1 menisci that were cultured for up to 4 weeks and compared with cultured alginate menisci. Contraction, histological, confocal microscopy, biochemical and mechanical analysis were performed to determine tissue development. After 4 weeks culture, collagen menisci had preserved their shape and significantly improved their biochemical and mechanical properties. Both 10 and 20 mg ml^?1 menisci maintained their DNA content while significantly improving the glycosaminoglycan and collagen content, at values significantly higher than the alginate controls. Collagen menisci matched the alginate control in terms of the equilibrium modulus, and developed a 3- to 6-fold higher tensile modulus than alginate by 4 weeks. Further fibrochondrocytes were able to reorganize the collagen gels into a more fibrous appearance similar to native menisci.     

Acellular vascular grafts generated from collagen and elastin analogs

Tissue-engineered vascular grafts require long fabrication times, in part due to the requirement of cells from a variety of cell sources to produce a robust, load-bearing extracellular matrix. Herein, we propose a design strategy for the fabrication of tubular conduits comprising collagen fiber networks and elastin-like protein polymers to mimic native tissue structure and function. Dense fibrillar collagen networks exhibited an ultimate tensile strength (UTS) of 0.71 ± 0.06 MPa, strain to failure of 37.1 ± 2.2% and Young’s modulus of 2.09 ± 0.42 MPa, comparing favorably to a UTS and a Young’s modulus for native blood vessels of 1.4–11.1 MPa and 1.5 ± 0.3 MPa, respectively. Resilience, a measure of recovered energy during unloading of matrices, demonstrated that 58.9 ± 4.4% of the energy was recovered during loading–unloading cycles. Rapid fabrication of multilayer tubular conduits with maintenance of native collagen ultrastructure was achieved with internal diameters ranging between 1 and 4 mm. Compliance and burst pressures exceeded 2.7 ± 0.3%/100 mmHg and 830 ± 131 mmHg, respectively, with a significant reduction in observed platelet adherence as compared to expanded polytetrafluoroethylene (ePTFE; 6.8 ± 0.05 × 10⁵ vs. 62 ± 0.05 × 10⁵ platelets mm^–2, p < 0.01). Using a rat aortic interposition model, early in vivo responses were evaluated at 2 weeks via Doppler ultrasound and CT angiography with immunohistochemistry confirming a limited early inflammatory response (n = 8). Engineered collagen–elastin composites represent a promising strategy for fabricating synthetic tissues with defined extracellular matrix content, composition and architecture.     

Odontogenic differentiation and dentin formation of dental pulp cells under nanobioactive glass induction

Bioactive glass (BG) has been widely used in bone regeneration; however, reports on the biological effects of BG on dental pulp cells are rare. This study aims to investigate the effects of nanoscale BG (n-BG) on odontogenic differentiation and dentin formation of dental pulp cells and to compare these effects with those of microscale BG (m-BG). Human dental pulp cells (hDPCs) from third molars were cultured directly with m-BG and n-BG in vitro. The cell proliferation increased at 0.1 mg ml⁻¹ BG, which also had a chemotactic effect on hDPCs. The mineralization capacity and expression of odontogenic-related proteins and genes (dentin sialophosphoprotein, dentin matrix protein 1 and collagen type I) of hDPCs were significantly up-regulated under BG induction, and were particularly higher in the n-BG group than in the control group. m-BG and n-BG combined with pulp tissues were transplanted into the dorsum of immunodeficient mice to observe their biological effects on dental pulp cells in vivo. A continuous layer of dentin-like tissue with uniform thickness, a well-organized dentinal tubule structure and polarizing odontoblast-like cells aligned along it was generated upon the n-BG layer, whereas some irregular sporadic osteodentin-like mineralized tissues were observed in the control group. This study reveals that BG, especially n-BG, induces the odontogenic differentiation and dentin formation of dental pulp cells and may serve as a potential material for pulp repair and dentin regeneration.     

Collagen hydrogels incorporated with surface-aminated mesoporous nanobioactive glass: Improvement of physicochemical stability and mechanical properties is effective for hard tissue engineering

Collagen (Col) hydrogels have poor physicochemical and mechanical properties and are susceptible to substantial shrinkage during cell culture, which limits their potential applications in hard tissue engineering. Here, we developed novel nanocomposite hydrogels made of collagen and mesoporous bioactive glass nanoparticles (mBGns) with surface amination, and addressed the effects of mBGn addition (Col:mBG = 2:1, 1:1 and 1:2) and its surface amination on the physicochemical and mechanical properties of the hydrogels. The amination of mBGn was shown to enable chemical bonding with collagen molecules. As a result, the nanocomposite hydrogels exhibited a significantly improved physicochemical and mechanical stability. The hydrolytic and enzymatic degradation of the Col–mBGn hydrogels were slowed down due to the incorporation of mBGn and its surface amination. The mechanical properties of the hydrogels, specifically the resistance to loading as well as the stiffness, significantly increased with the addition of mBGn and its aminated form, as assessed by a dynamic mechanical analysis. Mesenchymal stem cells cultivated within the Col–mBGn hydrogels were highly viable, with enhanced cytoskeletal extensions, due to the addition of surface aminated mBGn. While the Col hydrogel showed extensive shrinkage (down to ～20% of initial size) during a few days of culture, the shrinkage of the mBGn-added hydrogel was substantially reduced, and the aminated mBGn-added hydrogel had no observable shrinkage over 21 days. Results demonstrated the effective roles of aminated mBGn in significantly improving the physicochemical and mechanical properties of Col hydrogel, which are ultimately favorable for applications in stem cell culture for bone tissue engineering.     

Effect of structural change of collagen fibrils on the durability of dentin bonding

This study investigates the effect of structural changes of collagen fibrils on the bonding durability of a total etch luting resin (Super-Bond C&B) and a self-etching luting resin (Panavia F 2.0) to dentin. An atomic force microscope (AFM) was used to observe structural changes of intact dentin collagen fibrils after acidic conditionings of two bonding systems. After 90 d water storage and 15,000 thermal cycles (TC) as artificial aging, micro-tensile bond strength (microTBS) was utilized to evaluate the bonding durability of the two bonding systems to dentin. microTBS after 1 d or 90 d water storage without TC were separately measured in control groups. A cross-banding periodicity of about 67 nm along collagen fibrils was seen on demineralized intertubular dentin surfaces in AFM images. For both luting resins, thermal cycling decreased (p < 0.05) microTBS of 1 d and 90 d, compared to controls. Scanning electron microscope and transmission electron microscopic examinations revealed that the top and bottom of hybrid layer (HL) were weak links in the bonding interface over time. The results suggest that the top of HL contains disorganized collagen fibrils from the smear layer which degrade over time. AFM results indicate that the demineralized intact collagen fibrils beneath the smear layer were not denatured during acidic conditioning. However, these collagen fibrils may be structurally unstable due to poor infiltration by resin or loss of resin protection within the HL over time, reducing the long-term microTBS. This process was accelerated by thermal fatigue cycling.     

Semi-interpenetrating networks of hyaluronic acid in degradable PEG hydrogels for cartilage tissue engineering

Hydrolytically biodegradable poly(ethylene glycol) (PEG) hydrogels offer a promising platform for chondrocyte encapsulation and tuning degradation for cartilage tissue engineering, but offer no bioactive cues to encapsulated cells. This study tests the hypothesis that a semi-interpenetrating network of entrapped hyaluronic acid (HA), a bioactive molecule that binds cell surface receptors on chondrocytes, and crosslinked degradable PEG improves matrix synthesis by encapsulated chondrocytes. Degradation was achieved by incorporating oligo (lactic acid) segments into the crosslinks. The effects of HA molecular weight (MW) (2.9 × 10⁴ and 2 × 10⁶ Da) and concentration (0.5 and 5 mg g⁻¹) were investigated. Bovine chondrocytes were encapsulated in semi-interpenetrating networks and cultured for 4 weeks. A steady release of HA was observed over the course of the study with 90% released by 4 weeks. Incorporation of HA led to significantly higher cell numbers throughout the culture period. After 8 days, HA increased collagen content per cell, increased aggrecan-positive cells, while decreasing the deposition of hypertrophic collagen X, but these effects were not sustained long term. Measuring total sulfated glycosaminoglycan (sGAG) and collagen content within the constructs and released to the culture medium after 4 weeks revealed that total matrix synthesis was elevated by high concentrations of HA, indicating that HA stimulated matrix production although this matrix was not retained within the hydrogels. Matrix-degrading enzymes were elevated in the low-, but not the high-MW HA. Overall, incorporating high-MW HA into degrading hydrogels increased chondrocyte number and sGAG and collagen production, warranting further investigations to improve retention of newly synthesized matrix molecules.     

Discovery of low mucus adhesion surfaces

Mucus secretion from the body is ubiquitous, and finding materials that resist mucus adhesion is a major technological challenge. Here, using a high throughput platform with photo-induced graft polymerization, we first rapidly synthesized, screened and tested a library of 55 different surfaces from six functional monomer classes to discover porcine intestinal low mucus adhesion surfaces using a 1 h static mucus adsorption protocol. From this preliminary screen, two chemistries, a zwitterionic ([2-(acryloyloxy)ethyl] trimethylammonium chloride) and a multiple hydroxyl (N-[tris(hydroxymethyl)methyl]acrylamide) surface, exhibited significantly low mucus adhesion from a Langmuir-type isotherm when exposed to increasing concentrations of mucus for 24 h. Apolar or hydrophobic interactions were likely the dominant attractive forces during mucus binding since many polar or hydrophilic monomers reduced mucus adhesion. Hansen solubility parameters were used to illustrate the importance of monomer polarity and hydrogen bonding in reducing mucus adsorption. For a series of polyethylene glycol (PEG) monomers with changing molecular weight from 144 g mol^?1 to 1100 g mol^?1, we observed an excellent linear correlation (R² = 0.998) between relative amount adsorbed and the distance from a water point in a specialized Hansen solubility parameter plot, emphasizing the role of surface–water interactions for PEG modified surfaces.     

Synchrotron X-ray diffraction study of load partitioning during elastic deformation of bovine dentin

The elastic properties of dentin, a biological composite consisting of stiff hydroxyapatite (HAP) nano-platelets within a compliant collagen matrix, are determined by the volume fraction of these two phases and the load transfer between them. We have measured the elastic strains in situ within the HAP phase of bovine dentine by high energy X-ray diffraction for a series of static compressive stresses at ambient temperature. The apparent HAP elastic modulus (ratio of applied stress to elastic HAP strain) was found to be 18 ± 2 GPa. This value is significantly lower than the value of 44 GPa predicted by the lower bound load transfer Voigt model, using HAP and collagen volume fractions determined by thermo-gravimetric analysis. This discrepancy is explained by (i) a reduction in the intrinsic Young’s modulus of the nano-size HAP platelets due to the high fraction of interfacial volume and (ii) an increase in local stresses due to stress concentration around the dentin tubules.     

Recent advances in the theory and mechanism of adhesive resin bonding to dentin: a critical review

Dentin bonding issues involving adhesive resins have attracted considerable research interest in recent years. An important advance due to the ongoing research is the concept of hybridization of the tissue with primer/adhesive systems. Hybridization involves permeation of primer monomer into the tissue substrate. Although the mechanism of adhesive permeation and interaction with tissue may be complex, significant advances have been made. In systems where etching precedes priming and bonding steps, the Hoy's solubility parameter compatibility of the primer formulation with that of demineralized dentin matrix may determine adhesive permeability. Monomer permeation brings the primer atoms in closer contact with the substrate atoms, leading to adhesive interactions through van der Waals, hydrogen bonding, and electrostatic interactions. In self-etch primer systems, stronger electrostatic interaction between primer monomers and hydroxyapatite has been used to explain the adhesion process. These interactions have been computer-modeled and analyzed. Such interactions and subsequent polymerization of the monomer promote improved bond strength and efficient margin sealing. Incomplete permeation of monomer into the full depth of demineralized region may, however, leave exposed collagen fibrils and cause nanoleakage of water into these regions through a 20-100 nm sized marginal gap, leading to subsequent hydrolytic degradation of these collagen fibrils and the hybrid layer. Microleakage is also a problem in some single step formulations. In this review, we analyze these current theoretical and mechanism-related issues of interest in adhesive resin bonding to dentin, and outline the continuing problems that need to be overcome in the future.