Stability of Primary Amine Groups During Plasma Polymerization for the Surface Modification of Artificial Bones

The surface modification of calcium-phosphate artificial bones was performed with amine-containing polymer deposition by low-pressure pulsed plasmas [1-3]. It has been experimentally demonstrated to improve bone regeneration when the treated bones are implanted in animals [3]. The goal of this study is to examine the possibility of increasing the amount of amine embedded in the deposited polymer film. The experimental study of amine-containing polymer deposition showed that the ratio of the number of primary amine groups (-NH₂) to the number of carbon (C) atoms in a plasma-deposited hydrocarbon film was typically about 3 % even in the presence of a relatively high amount of hydrogen (H) atoms in the plasma. In this study, we used molecular dynamics (MD) simulations [4] to examine how amine groups can be formed in a plasma polymerization process. Considering a methane (CH₄) and nitrogen (N₂) plasma process, we examined the effects of incident CH₂ or CH₃ radicals, NH₂ radicals, and CH₂⁺ or CH₃⁺ energetic ions on the amine formation. The simulations showed that H atoms of incident NH₂ radicals tended to react with surrounding C atoms in the polymerization process, resulting in the formation of secondary amine groups (-NH). The results were thus qualitatively consistent with the experimental observation that only a small percentage of primary amines were formed in plasma deposited hydrocarbon films.