Corrosion Resistance of Graphene oxide/Silver Coatings on Ni-Ti alloy and Expression of IL-6 and IL-8 in Human Oral Fibroblasts

Abstract

Graphene based materials (GBMs) have potentials for dental and medical applications. GBMs may cause changes in the levels of cytokine released in the body. This study aimed to study the corrosion resistance of graphene oxide (GO) and GO/silver (GO/Ag) nanocomposite coated nickel-titanium (NiTi) alloy by electrophoretic deposition and to access the viability of human pulp fibroblasts, and the interleukin (IL)-6 and IL-8 expression level. The bare and coated NiTi samples were characterized by scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, surface profilometry, and X-ray diffraction (XRD). The corrosion resistance of the bare NiTi and coated NiTi samples were investigated by potentiodynamic polarization and electrochemical impedance spectroscopy in 3.5% NaCl solution. The cell viability of human pulp fibroblasts was accessed by the treated culture medium of the bare NiTi and coated NiTi alloys containing 1% fetal bovine serum. IL-6 and IL-8 expression levels were studied by human enzyme-linked immunosorbent assay (ELISA). Data were analyzed using One-way ANOVA (α = 0.05). Both the GO-coated NiTi and GO/Ag-coated NiTi alloys showed better corrosion resistance, a lower rate of corrosion, and higher protection efficiency than the bare NiTi alloy. The coated NiTi alloys were biocompatible to human pulp fibroblasts and showed upregulation of IL-6 and IL-8 levels.

Figure 1

The overview of the study.

Figure 2

(a–c) SEM images, (d–f) EDS analysis, and (g–i) AFM images of the bare NiTi, GO-coated NiTi, and GO/Ag-coated NiTi alloy, respectively.

Figure 3

X-ray diffraction patterns of the GO-coated NiTi (GO) and GO/Ag-coated NiTi (GOAg) alloy.

Figure 4

(a–c) The SEM analysis of the cross-section of the uncoated NiTi alloy, GO-coated NiTi alloy, and GO/Ag-coated NiTi alloy. A layer of GO-coatings (~1.13 µm) and GO/Ag-coatings (~1.35 µm) were evident on the surface of GO-coated NiTi alloy, and GO/Ag-coated NiTi alloy, respectively. Thickness of the coatings were analyzed from the surface profilometer. (d–f) The surface morphology of the bare NiTi GO-coated NiTi, and GO/Ag-coated NiTi alloys following the potentiodynamic polarization experiments. The arrows indicate the pitting corrosion.

Figure 5

(a) Nyquist plots with equivalent circuit model, (b) Bode plots for the bare NiTi, GO-coated NiTi (GO) and GO/Ag-coated NiTi (GOAg) alloy, and (c) Potentiodynamic polarization curves of the bare NiTi, GO-coated NiTi (GO), and GO/Ag-coated NiTi (GOAg) after immersion in 3.5% NaCl. R_s = Electrolyte solution resistance, R_ct = Charge transfer resistance of the corrosion reaction, C_dl = Capacitance, W = Warburg Impedance related to the diffusion of O₂ of the NiTi alloy, and SCE = saturated calomel electrode.

Figure 6

MTT assay of human pulpal fibroblasts. (a) Results of the standard curve of cell numbers, (b) cell viability, and (c) cell morphology visualized in the light microscope (at 20x magnification) of the bare NiTi, GO-coated NiTi (GO), and GO/Ag-coated NiTi (GOAg) alloy after incubating in the conditioned medium with 1% fetal bovine serum (FBS) for 24  hours. There was no significant difference in the viable cells among all the groups (P > 0.5).

Figure 7

(a) Standard curve of ELISA human pulp fibroblast cytokines IL-6, (b) Standard curves of ELISA human pulp fibroblast cytokines IL-8, (c) ELISA measurement of cytokines IL-6 levels, (d) ELISA measurement of cytokines IL-8 levels. *Indicates a significant difference of cytokines IL-6 and IL-8 levels of the bare NiTi, GO-coated NiTi (GO), and GO/Ag-coated NiTi (GOAg) alloy compared to the control (P < 0.001).