Analysis of surface characteristics of ceramic and tooth structures by plasma treatment and its effect on adhesion

Abstract

Objective : Atmospheric pressure plasma can easily change the surface properties of substrates using a low cost and environment-friendly dry process of a simple system. For adhesion, plasma can also be used to implement chemically active species on the substrate surface by inducing polar functional groups or through ion bombardment. The purpose of this study is to evaluate the effectiveness of plasma polymer coating on the bond strength of composite resin to ceramic, as well as the direct effects of plasma on the adhesion of resin composite to tooth structures and the durability of the bond. Methods : The effect of plasma polymer coating was evaluated using shear bond strength (SBS) and micro-shear bond strength (MSBS) tests. The plasma coated surfaces were characterized using Fourier transform infra-red (FTIR) spectrophotometer in an attenuated total reflectance (ATR) mode, X-ray photoelectron spectroscopy (XPS), and contact angle measurements. The fractured surfaces were evaluated using a stereomicroscope and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and also the chemical structure and composition of the surface were verified using Focused ion beam (FIB), transmission electron microscopy (TEM) and EDS respectively. The direct effect of plasma on the adhesion of resin composite to dentin was evaluated using micro-tensile bond strength (MTBS) tests, before and after thermocycling (TC). The bond strength data were interpreted using Weibull analysis. The hybrid layer formation at the adhesive/dentin interface was characterized by micro-Raman spectroscopy and SEM analyses. Results : Among the groups treated with plasma polymer coating, the SBS of the adhesive to the ceramic surface pre-treated sequentially with water and triethyleneglycol dimethacrylate (TEGDMA) plasma in helium gas was significantly higher than that of the adhesive to the untreated surface (p < 0.05). Also, the SBS obtained with the plasma deposition of TEGDMA at the highest input voltage was statistically similar to the gold standard by HF etching and silane coupling agent coating. Plasma deposition of 1,3-butadiene was effective at flow rates of more than 2 standard cubic centimeters (sccm). The SBS obtained with the sequential plasma deposition of hexamethyldisiloxane (HMDSO) and benzene was statistically similar to the gold standard by HF etching and silane coupling agent coating. The sequential plasma deposition of TMS and benzene also showed about two times higher bond strength values than that of Z-Prime Plus. The plasma polymer deposition of benzene and 1,3-butadiene improved the MSBS of resin composite to enamel (p < 0.05). The procedure improved enamel adhesion but failed to improve durability in terms of mean bond strength. However, the plasma polymer deposition increased the Weibull modulus (m) after TC, which indicated that the scatter of the bond strength was narrowed with respect to durability. However, future studies to evaluate solubility of the plasma-deposited polymers will be necessary in the point of toxicity. In the dentin, the plasma-drying groups presented significantly higher bond strength than the wet bonding and dry bonding groups, either with or without rewetting (p < 0.05). Micro-Raman spectral analysis indicated that plasma-drying improved the penetration and polymerization efficacy of the adhesive. Even after TC, the bond strength of plasma-drying group did not reduce. Conclusion : Plasma polymer coating technique could contribute to enhance the bonding of conventional dental adhesives on the surface of ceramic and tooth structures. In particular, 1,3-butadiene or benzene enhanced the adhesion of composite resin through the simple deposition of C=C double bonds. On the dentin surface, the plasma treatment using a He gas, with or without rewetting, improved the adhesion of resin composite to dentin and its durability. Plasma application in adhesion of resin composite to the ceramic and tooth structures will be a promising method.