Evaluation of fluid transport processes in dental enamel. Methods to assess the relevance of enamel permeability in caries prevention and etching treatments.

This thesis evaluated in vivo and in vitro enamel permeability in different physiological and clinical conditions by means of SEM inspection of replicas of enamel surface obtained from polyvinyl siloxane impressions subsequently later cast in polyether impression ma-terial. This technique, not invasive and risk-free, allows the evaluation of fluid outflow from enamel surface and is able to detect the presence of small quantities of fluid, visu-alized as droplets. Fluid outflow on enamel surface represents enamel permeability. This property has a paramount importance in enamel physiolgy and pathology although its ef-fective role in adhesion, caries pathogenesis and prevention today is still not fully under-stood. The aim of the studies proposed was to evaluate enamel permeability changes in differ-ent conditions and to correlate the findings with the actual knowledge about enamel physiology, caries pathogenesis, fluoride and etchinhg treatments. To obtain confirmed data the replica technique has been supported by others specific techniques such as Ra-man and IR spectroscopy and EDX analysis. The first study carried out visualized fluid movement through dental enamel in vivo con-firmed that enamel is a permeable substrate and demonstrated that age and enamel per-meability are closely related. Examined samples from subjects of different ages showed a decreasing number and size of droplets with increasing age: freshly erupted permanent teeth showed many droplets covering the entire enamel surface. Droplets in permanent teeth were prominent along enamel perikymata. These results obtained through SEM inspection of replicas allowed innovative remarks in enamel physiology. An analogous testing has been developed for evaluation of enamel permeability in primary enamel. The results of this second study showed that primary enamel revealed a substantive permeability with droplets covering the entire enamel sur-face without any specific localization accordingly with histological features, without changes during aging signs of post-eruptive maturation. These results confirmed clinical data that showed a higher caries susceptibility for primary enamel and suggested a strong relationship between this one and enamel permeability. Topical fluoride application represents the gold standard for caries prevention although the mechanism of cariostatic effect of fluoride still needs to be clarified. The effects of topical fluoride application on enamel permeability were evaluated. Particularly two dif-ferent treatments (NaF and APF), with different pH, were examined. The major product of topical fluoride application was the deposition of CaF2-like globules. Replicas inspec-tion before and after both treatments at different times intervals and after specific addi-tional clinical interventions showed that such globule formed in vivo could be removed by professional toothbrushing, sonically and chemically by KOH. The results obtained in relation to enamel permeability showed that fluoride treatments temporarily reduced enamel water permeability when CaF2-like globules were removed. The in vivo perma-nence of decreased enamel permeability after CaF2 globules removal has been demon-strated for 1 h for NaF treated teeth and for at least 7 days for APF treated teeth. Important clinical consideration moved from these results. In fact the caries-preventing action of fluoride application may be due, in part, to its ability to decrease enamel water permeability and CaF2 like-globules seem to be indirectly involved in enamel protection over time maintaining low permeability. Others results obtained by metallographic microscope and SEM/EDX analyses of or-thodontic resins fluoride releasing and not demonstrated the relevance of topical fluo-ride application in decreasing the demineralization marks and modifying the chemical composition of the enamel in the treated area. These data obtained in both the experiments confirmed the efficacy of fluoride in caries prevention and contribute to clarify its mechanism of action. Adhesive dentistry is the gold standard for caries treatment and tooth rehabilitation and is founded on important chemical and physical principles involving both enamel and dentine substrates. Particularly acid etching of dental enamel enamel has usually employed in bonding pro-cedures increasing microscopic roughness. Different acids have been tested in the litera-ture suggesting several etching procedures. The acid-induced structural transformations in enamel after different etching treatments by means of Raman and IR spectroscopy analysis were evaluated and these findings were correlated with enamel permeability. Conventional etching with 37% phosphoric acid gel (H3PO4) for 30 s and etching with 15 % HCl for 120 s were investigated. Raman and IR spectroscopy showed that the treatment with both hydrochloric and phosphoric acids induced a decrease in the carbonate content of the enamel apatite. At the same time, both acids induced the formation of HPO42- ions. After H3PO4 treatment the bands due to the organic component of enamel decreased in intensity, while in-creased after HCl treatment. Replicas of H3PO4 treated enamel showed a strongly reduced permeability while replicas of HCl 15% treated samples showed a maintained permeability. A decrease of the enamel organic component, as resulted after H3PO4 treatment, involves a decrease in enamel permeability, while the increase of the organic matter (achieved by HCl treat-ment) still maintains enamel permeability. These results suggested a correlation between the amount of the organic matter, enamel permeability and caries. The results of the different studies carried out in this thesis contributed to clarify and improve the knowledge about enamel properties with important rebounds in theoretical and clinical aspects of Dentistry.

Solubility, diffusivity and permeability of gases in glassy polymers

Gas separation membranes of high CO2 permeability and selectivity have great potential in both natural gas sweetening and carbon dioxide capture. Many modified PIM membranes results permselectivity above Robinson upper bound. The big problem that should be solved for these polymers to be commercialized is their aging through time. In high glassy polymeric membrane such as PIM-1 and its modifications, solubility selectivity has more contribution towards permselectivity than diffusivity selectivity. So in this thesis work pure and mixed gas sorption behavior of carbon dioxide and methane in three PIM-based membranes (PIM-1, TZPIM-1 and AO-PIM-1) and Polynonene membrane is rigorously studied. Sorption experiment is performed at different temperatures and molar fraction. Sorption isotherms found from the experiment shows that there is a decrease of solubility as the temperature of the experiment increases for both gases in all polymers. There is also a decrease of solubility due to the presence of the other gas in the system in the mixed gas experiments due to competitive sorption effect. Variation of solubility is more visible in methane sorption than carbon dioxide, which will make the mixed gas solubility selectivity higher than that of pure gas solubility selectivity. Modeling of the system using NELF and Dual mode sorption model estimates the experimental results correctly Sorption of gases in heat treated and untreated membranes show that the sorption isotherms don’t vary due to the application of heat treatment for both carbon dioxide and methane. But there is decrease in the diffusivity coefficient and permeability of pure gases due to heat treatment. Both diffusivity coefficient and permeability decreases with increasing of heat treatment temperature. Diffusivity coefficient calculated from transient sorption experiment and steady state permeability experiment is also compared in this thesis work. The results reveal that transient diffusivity coefficient is higher than steady state diffusivity selectivity.

The effect of cross-linker on endogenous enzymatic activity within radicular dentin and bond strength between intraradicular post and radicular dentin

The primary aim was to evaluate the effect of 1-ethyl-3-(3-dimethylamino-propyl) carbodiimide (EDC) on endogenous enzymatic activity within radicular dentin and push-out bond strength of adhesively luted fiber posts, at baseline and after artificial aging. Additionally, the effect of different cementation strategies on endogenous enzymatic activity and fiber post retention was evaluated. The experiment was carried out on extracted human teeth, following endodontic treatment and fiber post cementation. Three cementation strategies were performed: resin cement in combination with etch-and-rinse (EAR) adhesive system, with self-etch (SE) system and self-adhesive (SE) cement. Each of the mentioned strategies had a control and experimental (EDC) group in which root canal was irrigated with 0.3M EDC for 1 minute. The push-out bond strength test was performed 24h after cementation and after 40.000 thermocycles. In order to investigate the effect of EDC and different cementation strategies, in situ zymography analyses of the resin-dentin interfaces were conducted. Statistical analyses were conducted with the software Stata 12.0 (Stata Corp, College Station, Texas, USA) and the significance was set for p<0.05. The results of statistical analysis (ANOVA) showed that the variables “EDC”, “root region” and “artificial aging” significantly influenced fiber posts’ retention to root canal (p<0.05). The highest values were observed in coronal third. The mean values observed after artificial aging were lower when compared to baseline, however EDC was effective in preserving bond strength. The level of enzymatic activity varied between the groups and EDC had a beneficial effect on silencing the enzymatic activity. Within the limitations of the study, it was concluded that the choice of cementation strategy did not influence posts’ retention, while EDC contributed to the preservation of bond strength after artificial aging and reduced enzymatic activity within radicular dentin. In vivo trials are necessary to confirm the results of this in vitro study.

Influence of application mode of universal adhesives on bond strength performances and enzymatic activity of coronal and radicular dentin

Objective: The aims of this thesis were to analyze the application mode of the universal adhesives (UA) and to give instructions for clinical procedures. The etching mode of UA on the bond strength to dentin and on the risk of retention, marginal discoloration, marginal adaptation and post-operative sensitivity (POS) was analyzed by two systematic reviews. Three in vitro studies were conducted: 1) evaporation mode of a UA on coronal dentin; 2) cementation approach on radicular dentin; 3) adhesion of metal brackets to enamel. Materials and methods: Two systematic review were conducted firstly, then in vitro study to investigate the evaporation mode in presence or not of pulpal pressure by means of μTBS, and the enzymatic activity using in situ zymography, at T0 and T6. The cementation of a fiber into radicular dentin with different resin-cements was studied, by push-out bond strength evaluation. Orthodontic brackets were cemented according to 4 adhesive protocols and shear bond strength test was conducted. Two adhesive removal techniques were evaluated, and spectrophotometry was used. Results: The probability of POS occurrence was less in SE. SEE approach seems to perform better than SE. Air-drying resulted in higher μTBS. Suction-evaporation, aging and ER mode increased MMPs activity. Differences in NL expression were present at T0 for fiber post study, and the aging produced an increase in marginal infiltration. Brackets cemented with new universal cement with previous etchant application showed good μTBS values. Conclusion: SEE performed better than SE and TE with UA in terms of uTBS. Evaporating with air-drying is better for UA in terms of uTBS and enzymatic activity. Aging and choice of resin cement for cementation of fiber posts influenced the PBS. Brackets cementation with a new resin- cement seems to offer the highest bond strength and leaves more cement remnants after the bracket removal.

Enhancing predictive capability of models for solubility and permeability in polymers and composites

The interpretation of phase equilibrium and mass transport phenomena in gas/solvent - polymer system at molten or glassy state is relevant in many industrial applications. Among tools available for the prediction of thermodynamics properties in these systems, at molten/rubbery state, is the group contribution lattice-fluid equation of state (GCLF-EoS), developed by Lee and Danner and ultimately based on Panayiotou and Vera LF theory. On the other side, a thermodynamic approach namely non-equilibrium lattice-fluid (NELF) was proposed by Doghieri and Sarti to consistently extend the description of thermodynamic properties of solute polymer systems obtained through a suitable equilibrium model to the case of non-equilibrium conditions below the glass transition temperature. The first objective of this work is to investigate the phase behaviour in solvent/polymer at glassy state by using NELF model and to develop a predictive tool for gas or vapor solubility that could be applied in several different applications: membrane gas separation, barrier materials for food packaging, polymer-based gas sensors and drug delivery devices. Within the efforts to develop a predictive tool of this kind, a revision of the group contribution method developed by High and Danner for the application of LF model by Panayiotou and Vera is considered, with reference to possible alternatives for the mixing rule for characteristic interaction energy between segments. The work also devotes efforts to the analysis of gas permeability in polymer composite materials as formed by a polymer matrix in which domains are dispersed of a second phase and attention is focused on relation for deviation from Maxwell law as function of arrangement, shape of dispersed domains and loading.