Dental bleaching with violet LED: Effects on dentin color change, resin-dentin bond strength, hybrid layer nanohardness and dentinal collagen biostability.

BACKGROUND: The purpose of this study was to investigate the effect of a novel dental bleaching technique with Violet LED on enamel color change, bond strength and hybrid layer nanomechanical properties in resin-dentin restoration, and dentin biostability. METHODS: A total of 125 bovine incisors were distributed into a control group, violet LED group (LED), and 35 % peroxide hydrogen bleaching gel (BLG) groups (n = 15). Three 45-minute sessions were performed for both bleaching procedures every week. Enamel color change (ΔE, ΔL, and Δb) was determined after every bleaching session. After color analysis, dentin was exposed for the resin-dentin bond strength analysis using microtensile test and evaluation of the nanomechanical properties at the hybrid layer (nanohardness). While half of the specimens were tested immediately, the remaining were evaluated after 10,000 thermal cycles (TC). Thirty additional teeth were used to investigate dentin ultimate tensile strength (UTS) after the bleaching treatments. UTS was evaluated before and after an enzymatic challenge. Two-way repeated measures ANOVA and Tukey's post-test were used for the statistical analysis (α = 0.05). RESULTS: Enamel bleaching effect was observed in the LED and BLG groups with significant alterations in the ΔE, ΔL, and Δb in the BLG group. No difference was observed in the resin-dentin bond strength among the groups (p > 0.05), however, TC negatively affected the bond strength values for all the groups. Nanomechanical properties ​​remained unchanged when comparing immediate and after TC results (p > 0.05). Bleaching with BLG reduced significantly the dentin UTS, while all groups showed major decrease in UTS after the enzymatic challenge. CONCLUSIONS: Although violet LED was able to promote a bleaching effect, less color changes was observed when compared to BLG. None of the bleaching techniques effected the resin-bond strength or the nanomechanics of the hybrid layer. Violet LED did negatively effect dentin biostability as observed for BLG and it may promote less changes to the organic content of dentin.

Surface Nanostructuring of a CuAlBe Shape Memory Alloy Produces a 10.3 ± 0.6 GPa Nanohardness Martensite Microstructure.

Severe plastic deformation (SPD) has led to the discovery of ever stronger materials, either by bulk modification or by surface deformation under sliding contact. These processes increase the strength of an alloy through the transformation of the deformation substructure into submicrometric grains or twins. Here, surface SPD was induced by plastic deformation under frictional contact with a spherical tool in a hot rolled CuAlBe-shape memory alloy. This created a microstructure consisting of a few course martensite variants and ultrafine intersecting bands of secondary martensite and/or austenite, increasing the nanohardness of hot-rolled material from 2.6 to 10.3 GPa. In as-cast material the increase was from 2.4 to 5 GPa. The friction coefficient and surface damage were significantly higher in the hot rolled condition. Metallographic evidence showed that hot rolling was not followed by recrystallisation. This means that a remaining dislocation substructure can lock the martensite and impedes back-transformation to austenite. In the as-cast material, a very fine but softer austenite microstructure was found. The observed difference in properties provides an opportunity to fine-tune the process either for optimal wear resistance or for maximum surface hardness. The modified hot-rolled material possesses the highest hardness obtained to date in nanostructured non-ferrous alloys.

Correlation between bond strength and nanomechanical properties of adhesive interface.

OBJECTIVES: The aims of this study were to evaluate the nanohardness and Young's modulus of the adhesive-dentin interface and to correlate them with the microtensile bond strength (µTBS) after storage in water for 24 h and 6 months. METHODS: Eighty human third molar teeth were bonded to composite resin with the Adper Scotchbond Multipurpose (SBMP), Adper Single Bond 2 (SB2), Clearfil SE Bond (CSE) or Clearfil S3 Bond (CS3) adhesive systems and stored in water for 24 h and 6 months. Three bonded teeth were selected for each group for nanoindentation methodology to obtain the nanohardness and modulus values (n = 3), while seven bonded teeth each group were used for µTBS (n = 7). Data were analyzed by two-way ANOVA and Tukey's test (α = 0.05). Spearman correlation between nanohardness and modulus and µTBS values was also calculated. RESULTS: Nanohardness and Young's modulus values for the dentin and hybrid layer were not significantly different regardless of the adhesive system or water storage periods (p > 0.05). Nanohardness and Young's modulus values for the adhesive layer were significantly higher for SB2 than for SBMP, CSE, and CS3 systems in both water storage periods. The µTBS values for SBMP and CSE were significantly higher than for SB2 and CS3 in both storage periods. An inverse correlation between Young's modulus and µTBS was observed for the adhesive layer. CONCLUSION: The water storage periods did not alter nanohardness, Young's modulus, and µTBS of adhesives. The adhesive layer of SB2 showed the highest nanohardness and Young's modulus, but the highest dentin bond strength was obtained with SBMP and CSE. Thus, a lower Young's modulus yielded high bond strength. CLINICAL RELEVANCE: The inverse correlation between the Young's modulus of adhesive systems and dentin bond strength suggests adequate resistance of the adhesive to elastic deformation under stress, which are important properties to predict the success of the dental restoration.

Nanomechanical properties, SEM, and EDS microanalysis of dentin treated with 2.5% titanium tetrafluoride, before and after an erosive challenge.

The aim was to assess the nanohardness (H) and the reduced modulus of elasticity (Er ) of 2.5% titanium tetrafluoride (TiF4 ) modified dentin, before and after an erosive challenge with 0.3% citric acid (CA). Exposed dentin surfaces were divided into two groups (n = 5): (1) Control-no dentin pretreatment with TiF4 prior to etching with CA, and (2) Experimental-dentin pretreatment with TiF4 + CA. The H and the Er of intertubular dentin were measured using a triboindenter at different time points: baseline for both groups, after using 2.5% TiF4 for the experimental group, and after using CA for both the experimental and the control groups. Scanning electron microscope and energy dispersive X-ray spectroscopy (EDS) analysis of the dentin surfaces were undertaken at the same time points for both groups. Two-way ANOVA for randomized block design was applied. There was significant interaction between the application of the TiF4 solution and different time points (p = 0.001 for H and p < 0.001 for Er ), identified by Tukey's test. Erosive challenge provided a significant decrease in H and Er mean values. The TiF4 solution caused a significant increase in H and Er values, but no significant differences were found between post-TiF4 and post-CA application. TiF4 application produced a precipitate surface layer on intertubular and intratubular dentin. EDS analysis indicated the presence of titanium. The H and Er of the dentin surface were greatly increased after application of 2.5% TiF4 . TiF4 may modify the micromorphology of the dentin surface and produces an erosive resistance surface.

Human enamel nanohardness, elastic modulus and surface integrity after beverage contact

This study evaluated nanohardness, elastic modulus and surface roughness of human enamel after contact with citric beverages. Human enamel samples were assigned to 3 groups according to the type of beverage used: carbonated drink, orange juice and tap water (control). Surface roughness was assessed using a profilometer, and nanohardness and elastic modulus were recorded using a nanoindenter. The pH of the beverages was measured before and after citric contact during 5 weeks. Means(SD) were as follows: Carbonated drink: elastic modulus decreased from 111.6(14.5) to 62.3(10.3) GPa, nanohardness decreased from 4.62(0.67) to 1.28(0.46) GPa, roughness increased from 5.30(2.39) to 6.86(2.56) mum, and the pH changed from 2.69(0.35) to 2.29(0.24); Orange juice: elastic modulus changed from 115.15(12.94) to 92.11 GPa (13.83), nanohardness from 5.54(1.48) to 3.18 GPa (0.64), roughness from 5.26(2.27) to 6.73(2.25) mum, and pH from 3.46(0.20) to 3.03(0.14); Tap water (control): elastic modulus changed from 117.87(22.3) to 107.91(20.05) GPa, nanohardness from 4.35(1.66) to 4.28(0.93) GPa, roughness from 5.76(3.11) to 6.11(2.65) mum, and pH from 7.97(0.28) to 8.11(0.21). In conclusion, soft drink exposure caused a significant decrease in nanohardness and elastic modulus. The pH of the soft drink was more acidic from 5ºC to 37ºC. Orange juice showed a similar trend but, surprisingly, it had less effect on hardness, elastic modulus and roughness of enamel than the carbonated drink.

Este estudo avaliou a nanodureza, módulo de elasticidade e rugosidade de superfície do esmalte humano após contato com bebidas ácidas. Amostras de esmalte humano foram divididas em três grupos de acordo com o tipo de bebida utilizada: refrigerante gaseificado, suco de laranja e água potável (controle). A rugosidade de superfície foi avaliada usando-se um perfilômetro e a nanodureza/módulo de elasticidade utilizando-se um nanoindentador. O pH das bebidas foi medido antes e após a exposição das amostras por 5 semanas. Os valores médios (DP) foram: Refrigerante Gaseificado: o módulo de elasticidade diminuiu de 111,6(14,5) para 62,3(10,3) GPa; a nanodureza diminuiu de 4,62(0,67) para 1,28(0,46) GPa, a rugosidade de superfície aumentou de 5,30(2,39) para 6,86(2,56) mim, e o pH sofreu uma alteração de 2,69(0,35) para 2,29(0,24); Suco de Laranja: módulo de elasticidade sofreu alteração de 115,15(12,94) para 92,11(13,82) GPa; a nanodureza de 5,54(1,48) para 3,18(0,64) GPa, a rugosidade de superfície de 5,26(2,27) para 6,73(2,25) mim, e o pH sofreu uma alteração de 3,46(0,20) para 3,03(0,14); Água Potável (Controle): módulo de elasticidade sofreu alteração de 117,87(22,3) para 107,91(20,05) GPa; a nanodureza de 4,35(1,66) para 4,28(0,93) GPa, a rugosidade de superfície de 5,76(3,11) para 6,11(2,65) mim, e o pH de 7,97(0,28) para 8,11(0,21). Como conclusão, a exposição ao refrigerante provocou uma diminuição significante na nanodureza e no módulo de elasticidade do esmalte. O pH do refrigerante foi o mais ácido entre 5ºC e 37ºC. A exposição ao suco de laranja mostrou uma tendência similar, contudo o seu efeito sobre a nanodureza, módulo de elasticidade e rugosidade de superfície foi menos pronunciado.