The Effect of Parylene-C Coating on the Surface Free Energy, Water Sorption, Solubility and Staining of PMMA

Background and objective: Parylenes are conformal protective polymer coating materials utilized in various biomedical applications since they are chemically and biologically inert and stable. Acrylic resins, used in the manufacturing of different types of intra-oral prostheses, are vulnerable to fluid sorption and solubility and consequently alteration of different physical and mechanical properties. This study investigated the effect of surface coating of polymethyl methacrylate (PMMA) with Parylene-C, on surface free energy, water sorption, solubility, and staining. Methods: Specimens were fabricated using heat-polymerizing PMMA, half were randomly chosen to be coated with Parylene-C and various groups were created for testing as follows: 24 specimens were subjected to water sorption and solubility tests in distilled water according to ISO 20795-1:2008 for denture base polymers. 48 specimens underwent staining tests by soaking in distilled water or coffee solution for one week. Discoloration was measured by comparing total colour differences (ΔE) and lightness values (L*) across groups after obtaining CIE L*a*b* values of all samples using a digital camera imaging and appropriate image analysis software. A further 40 samples were subjected to contact angle readings using deionized water and glycerol followed by surface free energy calculations. Nonparametric Mann-Whitney tests were used for statistical analysis (P<0.01). Results: Coated PMMA specimens demonstrated significantly less water sorption and lower surface free energy compared to noncoated (P <0.001), while no significant difference was found in solubility (P= .028). The coating did not have a significant effect on ΔE values after the staining tests, but the L* values in coated samples were significantly higher compared to the uncoated ones. Significance: This study forms part of a series of experiments on the effect of Parylene coating of PMMA. The modified surface properties of coated PMMA may prolong the lifetime of intraoral prostheses and possibly reduce biofilm formation.